Guidelines for

IS 3370 - 2020, (2nd Revision)

Code of Practice - Concrete Structures for

Retaining Aqueous Liquids :Part 1- General Requirements,

Part 2- Reinforced Concrete Structures,Upgraded

June 2020.

A Monograph by

Lalit Kumar JainConsulting Structural Engineer

Nagpur

Guide to IS 3370 – 2020

Contents PREFACE p 3

Guide to IS 3370 Part 1 - 2020 (2nd Revision)R 0 INTRODUCTION p 5R 1 SCOPE p 5R 2 REFERENCES p 6R 3 TERMINOLOGIES (& definitions) p 6R 4 MATERIALS p 8R 5 EXPOSURE CONDITION p 9R 6 CONCRETE p 13R 7 DURABILITY p 16R 8 SITE CONDITIONS p 17R 9 CAUSES AND CONTROL OF CRACKING p 19R 10 STABILITY p 23

R 11 DESIGN, DETAILING & WORKMANSHIP AT JOINTS p 24 R 12 JOINTING MATERIALS p 32

R 13 CONSTRUCTION p 34R 14 TEST OF STRUCTURE p 35

R 15 LIGHTNING PROTECTION p 36R 16 VENTILATION p 36R 17 DESIGN REPORT AND DRAWINGS p 36 APPENDIX 1 p 37

Guide to IS 3370 Part 2 - 2019 (2nd Revision) R 0 GENERAL p 38R 1 SCOPE p 38R 2 REFERENCES p 38R 3 GENERAL REQUIREMENTS p 38R 4 DESIGN p 38R 5 FLOOR p 46R 6 WALLS p 46R 7 ROOFS p 47R 8 DETAILING p 47R ANNEX A p 51R ANNEX B p 52 ANNEX C: Concrete Finishes p 52

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Guide to IS 3370 – 2020

Guide to IS 3370 Part 1 & 2 – 2020 (2nd revision) Code of practice -

Concrete Structures for Retaining Aqueous Liquids

PREFACEAll four parts of IS 3370 are revised. In the series of IS 3370, a new part is required to deal with

construction practices, quality management and maintenance. Title of the code “retaining aqueous liquid” may be taken synonymous to “storage of or containing aqueous

liquids or its exclusion on one side”. In this guide use of terms ‘aqueous liquid’ and ‘water’ are synonymous. In the title word ‘storage’ is changed to ‘retaining’, and clarified that ‘aqueous liquids’ are dealt and liquids not in general. Here after ‘Liquid Retaining Concrete’ is abbreviated to ‘LRC’.

Code does not differentiate between “water contact” and “water retaining” members. All “water contact” members may not be “water retaining” members. Member, through which water tries to pass under hydraulic gradient over most of service life of structure, can be termed as water retaining member. A member surrounded on all sides by water (i.e. in contact with water), where liquid transportation (i.e. seepage or leakage) does not take place through the thickness member over the major part of the service life, the situation is not as severe as for a water retaining member. For members in contact with liquid (& nor retaining), the design provisions given in IS 3370 (Part 2) can be bit relaxed (such as crackwidth and grade of concrete), except the requirement of clear cover. Column inside tank is a ‘water contact’ member, similar is a baffle wall in a treatment unit having water on both faces. These standards (Parts 1 to 4) are applicable to the units of structure conveying (channels), handling (pump-houses) and treating water and waste water (sewage) for environmental engineering structures and water resource engineering structures, though not mentioned specifically. Code is mainly for aqueous tanks and all other concrete structures where water-tightness and durability are of prime importance. For structures dealing with waste water additional requirements are also needed, and some guidelines are discussed at appropriate places. For structures dealing with sewage or storing liquids which may attack concrete, requirements given in this code may not be complete. If likely chemical attack is slow (in relation to service life of structure), higher concrete grade is needed. With increase in potential of chemical attack, surface finishes, and protective coatings are needed. Linings are to be provided where chemical attack may be severe or fast.

For water conveying or cross drainage structures in water resource works (e.g. aqueducts, canal syphon, pump-house etc.) IS 3370 is traditionally referred for water retaining members, till a separate code is available for such structures. All the requirements for these types of structures are not covered in this code.

Design approach is more rationalize in present revision, while keeping issues simple as far as possible. Present revisions have very little effect on the cost economy of the liquid retaining structures.

Engineers interacting with the activity of formation of code are normally dealing with bigger size works. In India large number of works is for small water supply schemes. For these small works few common requirements have become little heavy.

The code specifies a limit of 50C. Thus hot aqueous liquids will need additional precautions. For LRC, prestressed concrete is not much used in India. However, first time part 3 is revised. Limit state

design is specified and working stress design method was deleted. The concept of prestressing in one direction only or partial prestress is also considered. Code is brought in line with the present version of IS 1343.

For better understanding of the design of concrete liquid retaining structures reference can be made to British code BS 8007:1987; EN 1992-3: 2006 Design of concrete structures - Part 3: Liquid retaining and containment structures, Committee European Normalization (CEN); ACI 350; New Zealand NZS 3106.

The guidance in this document at few places may not confirm to the Indian standard or other codes, as the guide is dealing with the subject in wider perspective. In some situations codes are silent (keeping subject brief) or not explicitly clear, however designer has to take decisions, and only few such situations are discussed in this document. Views expressed here are not necessarily ‘word to word’ interpretation of code, but be treated as guide for understanding. The guide tries to explain the provision in the codes with understanding of background information, and help reader in taking an appropriate decision.

This guide assumes that the reader is well conversant with concrete technology and reinforced concrete design as dealt in basic code (IS 456:2000)# and text books*. The guide may not cover all the requirements for large projects; however the aim is to give guidance to an average engineer for small and normal projects.

Those looking for more details may refer to specialist literature. More details can be added if readers give their demand or suggestions. A handbook or design aids may be written if readers indicate the demand for the same. Reader may communicate his opinion to the author regarding his disagreement on a specific issue, or suggestions for giving more explanation if needed on an issue. These suggestions will help in revising this guide. Readers may also give suggestions to improve the usefulness of the code or the guide.

Clause number of the Indian standard code is preceded by letter R, and subsequent text is guide, remark or commentary on the concerned clause. Commentary is not given for every clause. At few places additional remarks are given with clause numbers which do not exist in the standard. For reading this document, one has to

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keep the standard alongside, as clauses in the standards are not repeated here. The information given is as per the opinion of the author. For the purpose of any contract, the

recommendations given in this document if in variance with IS code, shall not be applicable unless the contract specifies this reference for compliance. For guidelines on supporting structure for elevated tanks, reference should be made to “Guidelines for Design & Construction of RCC Elevated Water Tanks”, ICI monograph. For guidelines on construction aspects of LRC, reference should be made to “Guidelines on Construction, Detailing, Quality Management & Maintenance of Concrete Structures for Retaining Aqueous Liquid”. This standard and guideline may not remain compatible to revised IS 456 which will be issued in near future.

--------------------------------------------------------------------------------------------------------------------------------# IS 456 -2000, Indian Standard Code of Practice for Plain and Reinforced Concrete, with 6 amendments.$ IS 3370 part 1, 2. 3 & 4 -2019, Indian Standard Code of Practice – Concrete Structures for Storage of Liquid, Part 1 General requirements & part 2 Reinforced Concrete Structures.*Suggested books – 1 Properties of Concrete by A. M. Neville; 2 Concrete microstructures, properties and materials by P. K. Mehta & P.J.M. Monteiro, Indian edition by Indian Concrete Institute ; 3 Concrete Technology by Prof. M. S. Shetty, S. Chand publishers 2005.;

Author :

L. K. JAIN36 Old Sneh Nagar, Wardha Road,NAGPUR 440 015.Phone : 0712 228 4037, fax 0712 228 3335;Email lkjain.ngp@gmail.com

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Guide to IS 3370 – 2020

Guide to IS 3370 Part 1 - 2020 (2nd Revision), Code of Practice - Concrete Structures for Retaining

Aqueous Liquids : Part 1 General Requirements

R 0 INTRODUCTION A section on ‘terminology’ is added. ‘Exposer condition’ dealt in more details. Coated steel and stainless

steel have been permitted for reinforcement. A concept of H/t ratio at construction joint has been introduced. Factor of safety against uplift is deal in little more details. Information on ‘joints’ has been expanded. More details about construction joint are added. IS 456 is still the mother code, though in some of the areas it not being referred.

For water retaining member minimum exposure taken is ‘severe’. For water contact (& nor retaining) member, the cover requirement will be governed by IS 3370, the minimum concrete grade can be bit lower (M25 in place of M30) and the crackwidth requirement can be relaxed (≤ 0.2 mm).

‘Working stress design method’ was deleted. ‘Loads’ are dealt in details, and ‘liquid load’ is defined separately, and not dealt either as dead load or live load. Provisions about limit state design are in more detailed. Partial safety factors specified for materials are as per IS 456, though those can be enhances without affecting the cost of works. Table of ‘load combinations’ is given. Crackwidth control is linked to class of water-tightness. Importance of detailing is brought out, and reinforcement detailing of right angled junction is specified.

Code now deals with the weakness at construction joint, leading to design action. Designer has to check strength capacities and crackwidth at the construction joints. Location of construction joint is to be fixed and checks for adequacy of strength and satisfactory performance, are to be applied. Detailed specifications for construction joints are given.

About PCC design, ‘not enough clarity’ continues. PCC for LRC can be designed for very small tanks.Requirement and desirability of lining, coating, concrete surface finish and plaster on concrete members is

not dealt adequately. There is no differentiation available on the requirements of surface finishes and the degree of smoothness, which in certain LRC may become more important.

Bond strength reduction for coated bars, is recommended. Notes on autogenous healing of cracks in concrete are given. For plain concrete, the permissible tension in concrete is reduced. Fibres are permitted for improving concrete performance.

Design and execution of works are to be done under of a qualified and experienced engineers.Importance of low permeability of concrete is emphasized, however recommendation on testing and control

values of permeability of concrete are not given.The requirements of IS 456 & IS 1343 as applicable, and are to be treated as part of IS 3370. Few of the

provisions of IS 456 are over-ruled by this code, and few are not applicable as specifically noted while dealing structures covered by this code.

R 1 SCOPE : The process of water transport through the concrete thickness would affect the durability and functional requirement of the member over it service life. The amount of water or pollutants passing through concrete may increases over the life. With reference to general guidance given in this standard, the design service life of 50 years may be considered for non-replaceable main structural components. This life would be approximate in view of action level of environmental remaining undefined. Planned maintenance should be envisages for items other than main structural components. Main structural components shall be expected to performance in service life without any intervention or maintenance. Maintenance may be required for movement joints, secondary items, finishes, non-structural items, colouring and cleaning. For a member surrounded on all sides by water (i.e. in contact with water), where liquid transportation does not take place through the thickness of member over the major part of the service life, the situation is not as severe as for a water retaining member. For members in contact with liquid (& nor retaining), the design provisions given in IS 3370 (Part 2) can be relaxed (such as class of water-tightness & smaller crackwidths), except the requirement of grade of concrete and clear cover. The standard is applicable for all parts of concrete structure retaining or excluding the liquid. And also apply to parts (roof members) enclosing the space above the aqueous liquid, where that space is not well ventilated (i.e. ventilation area is less than 4% of the maximum liquid surface) or height of space above liquid is less than 1.5 m. These codes IS 3370 parts 1 to 4 are applicable to the units of structure conveying (channels), handling (pump-houses) and treating water and waste water (sewage) for environmental engineering structures and water resource engineering structures.

Hot, hazardous or low viscosity liquids are excluded from code. For these, additional requirements will be necessary. Code is not applicable for cryogenic liquids and for liquids susceptible for explosions.

Code does not consider the storage at high temperature or under pressure. For liquids detrimental to concrete, precautions or protection to ensure durability of concrete are required.

Tank to store potable water, shall be provided with roof and screens to prevent contamination and to avoid entry of vermin and mosquitos. This standard does not cover the requirements for concrete structures for storage and retaining of slurries, chemicals, hazardous substances, hot liquids and liquids of low viscosity and high penetrating power like petrol,

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Guide to IS 3370 – 2020

diesel, oil, etc. This code also does not cover dams, pipes, pipelines, tunnels, lined structures and damp-proofing of basements. Special problems of shrinkage arising in the storage of non-aqueous liquids and the measures necessary where chemical attack is possible are also not dealt with.

Aqueous liquids in temperature range 1C to about 40C can be considered normal. As the temperature increases the reactivity of aqueous liquid increases, and significantly above 40C. Thus hot aqueous liquids will need additional precautions. If dealing with liquid above 40C precautions should be taken. Generally the water temperature is lower by about 15C from maximum ambient. The range of daily temperature variations (minimum to maximum) of water is less than that of air (or ambient). The recommendations, are applicable to the storage or retaining the aqueous liquids and solutions having temperature range of 1°C to 40°C, and having no detrimental action on concrete and steel, or where sufficient precautions are taken to ensure protection of concrete and steel from damage due to action of such liquids as in the case of sewage. Outside the above range of temperature, design will have additional considerations and provisions like lining/ coatings etc. For ambient temperature below 1°C designer should consider taking more design actions and precautions for durability and serviceability. Design of temperature gradient across the thickness of concrete member if persistent over long time, needs a design action. This standard does not cover all the requirements of pressurised tanks & floating structures & gas tightness. Requirements regarding coatings, linings, and retaining of hazardous materials are also not dealt. For damp-proofing treatment to a members, refer IS 6494. Enough details are not covered for precast concrete for liquid retaining components. For all types of liquid containments excluded in above, the guidelines from the code can be used, however additional criteria will also be needed. To ensure compatibility of the design assumptions as per the standard, the actual nominal maximum size of the aggregate (MSA) being used should be 16 mm or above, and normally 20 mm. Concrete with lower MSA might not support the design assumptions, e.g. aggregate interlock at construction joint, shear capacity, fracture energy, stiffness (Ec value). A small thickness (< 30 mm) of concrete with lower MSA can be placed only at horizontal construction joint to avoid segregation due to free fall of concrete pour.

1.4 The requirements of IS 456:2000 ‘Code of practice for plain and reinforced concrete (fourth revision)’ and IS 1343:2012 ‘Code of practice for prestressed concrete (second revision)’ in so far as they apply, shall be deemed to form part of this standard except where otherwise laid down in this standard. For long term performance of the structure, use of dense, nearly impermeable and durable concrete, adequate concrete cover without macro defects in cover concrete, proper detailing practices, control of cracking, effective quality assurance measures in line with IS 456 and good construction practices particularly in relation to construction joints should be ensured. Designer should pay attention to the need for chemical resistance for long term performance while dealing with aggressive liquids or sewage. Preventing contamination of retained liquid and ground water are important.

R 2 REFERENCES : List of standards referred in the code is given. While referring to a standard its latest revision with up to date amendments should be used. This information is freely available at www.bis.org.in, the web site of Bureau of Indian Standards.

R 3 TERMINOLOGIES : Terms and Definitions (In the following, few more terms are given compared to the standard, hence numbering has changed.)

R 3.1 Base of structure: Level at which the horizontal earthquake ground motions are assumed to be imparted to the structure. This level does not necessarily coincide with the ground level and generally is at foundations. R 3.2 Blinding Layer: A base concrete on which structural LRC can be laid. For laying LRC, it should not allow loss of cement paste from the fresh concrete being laid over and compacted. In many cases the foundation PCC has also to act as blinding layer. It is also called mud mat, lean concrete or PCC base. If the ground which has to receive a structural concrete member is too soft or slushy or muddy, the base can be prepared in two layers. First layer can be a layer of suitable material or lean concrete which itself is not enough to totally seal off the mud from underlying material coming over. Over this sub-base, blinding layer is required.R 3.3 Capacity: It shall be the net useful volume of liquid, the structure can hold (retain) between the full supply level (FSL) and lowest supply level (LSL) i.e. the level of the lip of the outlet. Due allowance shall be made for applying lining, coating or plastering to the surfaces from inside if any specified, while calculating the capacity. The capacity (also called as designated capacity or live capacity or useful capacity) of tank excludes dead storage, which is the quantity of liquid below LSL, and also exclude that possibly in freeboard zone. Gross capacity includes dead storage, as well as the quantity of liquid which may occupy space above FSL if specified, for design consideration.R 3.4 Cementitious material: Powdery materials having cementing value, when used in concrete with Portland cement or blended cements, such materials like flyash, other raw or calcined pozzolanas, micro-silica (silica fume), ground granulated blast-furnace slag etc., which can be used in combinations of blending. These together with cement are called as cementitious materials. Multiple blends with Portland cement can be used.R 3.5 Construction joint: It is an intentional joint introduced for convenience in construction, a partial discontinuity in the concrete and treated to ensure near monolithic behaviour under serviceability and ultimate limit states; such as between two successive wall lifts, where special measures are taken to achieve continuity

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without further relative strains. Also see R 11.2 b & R 11.5.1.R 3.6 Contraction joint: It is an intentional joint introduced as partial discontinuity in concrete, or induced by a partial groove or cut in the concrete, thus creating a weak plane. Tensile strength across the joint is reduces to induce development of a crack thus reliving stresses due temperature-shrinkage restrains. The joint will open, as concrete on the two side will contract due to shrinkage and drop in temperature of concrete. It is a type of movement joint. It shall be sealed on liquid side. Also see R 11.2 a (ii).R 3.7 Dead storage: It is the volume of liquid in the tank below normal outlet level (LSL) or below useful capacity. This volume may have provision for accumulation of grit, silt, sludge etc., which also may have higher density than the normal liquid retained. Some engineers feel that a minimum (about 50 mm) dead storage may be considered for flat bottom elevated tanks. For domed bottom tanks, minimum 300 mm dead storage is usually considered, which can be more and depends upon diameter of bell-mouth on outlet pipe and arrangement for its fixing. Floor of ground tank (i.e. on grade), is generally provided with slopes towards a pit, for draining out sludge or grit accumulation and for cleaning of tank. Normally the liquid in the sludge pit or the suction pit is not counted in live capacity.R 3.8 Designed Concrete : A concrete mix engineered and proportioned for the sample of materials (aggregates, cementitious blend, admixtures etc.) to be used for achieving the specified characteristic strength, properties as specified, additional requirements, and while keeping control over the variations in properties within a small range. R 3.9 Design Service Life : It is the period (in years) for which the structure or structural element is to be used for its intended purpose with anticipated maintenance (conservation) but without substantial repair being necessary. Normally it shall not be less than 50 years. It may also be termed as ‘design life’ or ‘service life’.

R 3.10 Durability : Ability of a structure or structural element to assure no deterioration that is harmful to required performance in the relevant environment, and over the design service life. It is the capability of structures, products or materials to fulfil the requirements defined, determined after a specified period and usage (serviceability).

R 3.11 Environmental Actions : Assembly (individually or in combination) of physical, chemical, or biological influences (actions) resulting from the atmospheric conditions or characteristics of the surroundings to the structure, which may cause restraint effects or deterioration to the materials making up the structure (concrete or reinforcement or embedded metal), which in turn may adversely affect its serviceability, safety and durability of the structure. Environmental actions are not considered as loads in structural design. Actions due to wind or waves effects are mechanical loads.R 3.12 Force actions : These include bending moments, torsion, shear forces, direct tension or compression caused externally i.e. direct or internally i.e. indirect. Indirect actions can be due to imposed deformations, environmental actions (temperature, shrinkage, moisture variations, creep etc.) or vibration (seismic) etc. These are also simply called ‘action’. R 3.13 Foundation level : It is the level of the founding soil stratum on which structure will be constructed. It is the bottom level of PCC (blinding / mud-mat) base (if being provided) on which structure is constructed. R 3.14 Freeboard : Above FSL, it is the height available up to soffit of roof. In case of open top tank it is measured up to top of wall. Freeboard accommodates the waves generated on the surface of liquid and prevents loss of liquid due to sloshing. If the normal path of overflow is blocked, free board zone will allow rise of liquid above FSL till it flows out by an alternate path. If freeboard is less than possible sloshing height, near the wall roof can be subject to upward pressure due to sloshing of liquid. At times it is measured up to the lowest point of soffit of slab or beam supporting the roof or dome. Volume of space in freeboard zone, divided by area of water surface at FSL, is call average freeboard. Normally freeboard provision can be 150 to 300 mm for roofed tanks.R 3.15 Gross capacity: It includes live capacity, dead storage, as well as the quantity of water which may occupy space above FSL (up to MTL) if specified for design consideration. R 3.16 H/t ratio: It is pressure gradient, a ratio of pressure expressed as head (H) of liquid percolating through concrete to thickness (t) of concrete. H is the difference of pressure on the two faces of concrete member. H/t influences the seepage through construction joint or cracks. The amount of leakage should be very small through a crack or a construction joint properly done (good workmanship) and will depends upon this ratio. Related to workmanship, limiting ratio can be- ordinary 20, average 25, good 30, and excellent 35. Above these limits, water-bars are required to reduce the leakage through construction joints. Limiting values of H/t ratio could be low if whole section is in tension. Also see R 11.2 (b).

R.3.17 Intervention : A general term relating to an action or series of activities taken to modify or preserve the future performance of a structure or its components.R 3.18 Joint filler : A compressible, preformed material used to fill an expansion joint or an articulation, to prevent the infiltration of debris and to provide support for sealants. The filler should be fixed to any one side (old or new concrete). Most filler materials do not provide resistance to water flow. However, some fillers also

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provide resistance to water permeation, and these should be fixed (or adhered) to concrete on both sides.R 3.19 Joint Sealant : An impermeable normally synthetic, elastomeric (ductile) material used to finish a joint and to exclude liquid and solid foreign materials. It is fixed to liquid face of the joint with adhesion to concrete, not allowing liquid to cross the joint. It should sustain the pressure of liquid, and the range of movement imposed on joint, for the temperature range, and shall not de-bond or degrade in the service environment, with acceptable life. For selecting sealant, consider the shape factor of sealant, surface preparation, and the contact bond strength between the sealant and the concrete. Need will be to inspect, repair, maintain, and reseal joints with proper joint sealants at appropriate intervals, few times during the life of structure.R 3.20 Kicker : A small (75 mm to 150 mm) lift provided as first one at bottom of column or wall over a slab, to ensure the correct location and alignment of the member. It may also be called as starter. See R 11.4.1.1 (end para) & 11.4.5.R 3.21 Leakage : Under hydraulic gradient, it is the escape of liquid through the thickness of concrete member. The amount of flow may be enough to see liquid coming out if the evaporation is less than the leakage. Appearance of only wet patch on concrete surface will not constitute it as leakage. Leakages are through joints (including construction joint), holes, cracks, interconnected pores, honeycombs or macro flaws in concrete. Leakages mean loss of liquid retained. In some situations, leakages results in risk of contamination of liquid being retained or excluded. Normally leakages are not acceptable.R 3.22 Lift : It is height of a concrete member between two successive horizontal construction joints. Vertical concrete members e.g. columns or walls are constructed in lifts. R 3.23 Liquid depth: Liquid depth in tank shall be the difference between the full supply level (FSL) or working top liquid level (WTL) of the tank, and the lowest supply level (LSL). In case of liquid being water, the term ‘water depth’ can be used. The ‘design liquid depth’ for tank can be more than the ‘liquid depth’ due to dead storage and due to some rise of liquid to be accounted above FSL (in freeboard zone), for the purpose of strength design.R 3.24 Liquid Retaining Concrete (LRC) : Concrete having liquid retaining property which prevents permeation or loss of liquid to a negligible amount under hydraulic gradient over most of service life of structures, through its thickness. Liquid may be flowing within such a structure (but not through concrete). Example– units of water treatment plants or channels in which water flow. Related members can be termed as water retaining. To avoid leakage the concrete should not have small or macro-voids and macro-defects like segregation, honeycombs, interconnected pores, and these must be eliminated. The micro-structure (pore-structure) of the concrete should also have been improved to achieve very low permeability. If problem appears it should subsequently be grouted and treated adequately.R 3.25 Liquid Retaining Structure: It is a structure having very low percolation of liquid through its members, such that the liquid loss is much smaller than that to be termed as leakage. Apart from concrete members, the leakages should be avoided from construction or movement joints, junction (connection) of members, near embedment, including pipes other embedment intersecting, piercing concrete or passing through. All liquid storage structures are also liquid retaining.

R 3.26 Overall stability : State of stable equilibrium for the whole structure as a rigid body.R 3.27 Screed Layer : It is concrete laid in profile (other than formwork or shuttering) to provide a firm base, having surface finish as required. For bringing the top to the required profile, the thickness of screed layer may vary depending upon profile and tolerance at bottom surface of this layer. In many cases the purpose of blinding and screed layer may be combined in to one layer. R 3.28 Sympathetic Cracking : Crack produced in a member, influenced or aligned by other (second) member intimately in contact (with sufficient friction) and which has a joint having movements. In the first member the crack is produced at a location just adjacent to the movement joint in second concrete. The crack opening in first concrete is sympathetic (i.e. in phase) to the joint or crack movements in second.

R 3.29 Water-Bar (Water-stop) : It is a continuous preformed strip of elastomeric rubber, Polyvinyl chloride (PVC), thermo-plastic, metal (stainless steel/ GI sheet) or other material which is impermeable, anchored on the two sides of a joint in concrete, designed and constructed such that the passage of liquid through the joint is prevented and deformations due to movement would be sustained without permanent deformations (in water-bar) in the regime of temperature changes and chemical environment met. Refer R 11.5.2 for more details.

R 3.30 Water Path : The most probable path along which water can travel through pours in concrete or joints in concrete, under hydraulic gradient. At a joint the water-bar gives obstruction to water passage by increasing the gross length of the water path (creep length).

R 3.31 Maintenance : Set of activities performed during the design service life of the structure to enable it to fulfil the performance requirements.

R3.32 Maintainability : The ability of a structure is to meet service objectives with ease and a minimum expenditure of maintenance effort under service conditions in which maintenance and repair are to be performed.

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R 4 MATERIALS R 4.1 Requirements of materials are covered by section 5 of IS 456-2000 (with 6 amendments), and additional requirements for prestressed concrete work are covered by IS 1343 (with 2 amendments). Following are additional requirements. Use of blended cement is preferable, unless 7 days strength >25 N/mm² is the target. Blended cement can reduce thermal cracking, improve durability of concrete, and are also environmental sustainability.

R 4.2 AGGREGATES: Some engineers feel that water absorption of aggregates should not be more than 3% which appears to be very stringent limit. Porous aggregate increases the permeability of concrete. If satisfactory low level of concrete permeability can be achieved, absorption of aggregate will not affect the performance. However still there is no recommendation & specification of the permeability value permissible. To offset the possible effect of higher absorption of aggregate (>5%) one may adopt a little lower limit of water-cement ratio (or concrete grade higher) than that recommended.

For components enclosing the space above liquid, the percolation of liquid through concrete is not important, but the permeability influencing the deterioration mechanism of concrete is of importance. Aggregate of higher absorption (<10%) can be used for roof concrete. However in most cases grade of concrete (strength) needed is higher.

Sand may contain shell, which are contributed by aquatic life form. These consist of mostly calcium carbonate, but being hollow or flaky, may hinder the complete compaction of concrete. Tolerance of the shell content in sand will depend upon total fines (sand + cement) in the concrete (higher shell with higher fines). In absence of trials, testing or experience, shell content 3 to 6 % may be tolerated for concrete with nominal maximum size of aggregate as 20 mm to 10 mm.

Sand dredged from sea, estuaries or from salty water may contain high amount of salt. This type of sand if used should be washed with fresh (not salty) water and should be tested for the salt content for its suitability. Limits of total chlorides as given in table 7 of IS 456, should be taken as guidance. For some components like roof of chlorine contact tank (part of water treatment plant) these limits may be suitably reduced (say by 33%).

Use of sulphate resisting cement is discouraged, when chlorides and sulphates both are present. Porous aggregates are not permitted for the components of structure in contact with the retaining aqueous liquid or enclosing the space above liquid. Limits of porosity or absorption are not specified in the code. However for roofs of tanks, if higher grade concrete is used (≥ M40) some types of light weight aggregate may be used. Alkali-aggregate reactions can cause an expansive action when reactive aggregates come in contact with alkali hydroxides in the hardened concrete. These reactions can result in long-term deterioration in the interior of the concrete. It is recommended to specify testing of aggregate, if not known for its potential of Alkali-aggregate reactions. Aggregates that do not indicate a potential for alkali reactivity or reactive constituents, may be used without further testing. Aggregates having known past history of no reactivity, can be used without testing. With reactive aggregate, use of class F (low calcium) flyash is advantageous. R 4.3 REINFORCEMENT The grade of steel refers to the characteristic strength of bars, which is the guaranteed yield (or proof) strength. Use of corrosion resistant (CR) bars, give only a little extra protect relative to service life of structure.

Where needed, for reducing the risk of corrosion of reinforcement, coated steel or stainless steel can be used. Fusion bonded epoxy coated bars (IS 13620) can be used, however the epoxy coating anywhere shall not be less than 180 μm (micron) as required by other international codes. For using these bars, procedures and precautions are necessary to avoid scratches during handling and fixing bars. Refer “Field handling techniques for epoxy coated rebar at job site” published by Concrete Reinforcing Steel Institute, USA. Coated bars cannot be handled, cut, bent etc. in normal way. Scratches are to be avoided at all stages before and after cutting, bending, during handling and fixing. For handling, surfaces of all contrivances likely to have contact with bars should have hard rubber (or other similar material) lining. Specific bar cutting and bending machines are to be used. While placing or inserting the bars in position, scratches are to be avoided. Immediately after each operation like cutting, bending or any handling, the exposed or likely scratched steel surface is to be inspected. Cut ends of bars and scratches must be covered by appropriate epoxy or polymer. Similarly scratches may be caused during further handling, placing & inserting bars, tying the cage etc., and movement of workers on the bars before and during placing of concrete. With the use of coated bars at site, adoption of a reliable quality system is very necessary though extremely difficult, to detect every scratch and repair those. If these scratches go uncoated, the protection of coating against corrosion of steel will be effectively very small, thus coating defeats the purpose.

Fusion bonded epoxy coated bars are useful when exposure to chloride is much higher during construction also, e.g. bars at site and concrete are in direct contact with saline environment (e.g. sea water, sea water spay, brackish water, de-icing salts, soil having salinity, air laden with salinity etc.) If coated bars are used, binding wire should also be coated. Different types of stainless steel (say IS 16651) or steel containing high chromium (>9%) can be used. Galvanized bars can also be used. If galvanized bars are used, ensure that the zinc coating shall be sufficiently passive to avoid chemical reactions with the cement or concrete shall be made with cement that has no

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Guide to IS 3370 – 2020

detrimental effect on the bond to the galvanised reinforcement. Natural passivation of zinc coating can be achieved by storing the galvanised bars outdoors for more than a month. Instant passivation can be achieved by dipping the coated product in passivation solution. Use of protective coatings shall not permit reduction of the specified concrete cover. The tie wire or any corrodible item shall not transgress the concrete cover space. The binding wire shall not be such that it may have bi-metallic (galvanic) reaction with reinforcement. If this is feasible, coated / insulated binding wire should be used. Compared to un-coated reinforcement, for coated reinforcement the bond strength (at limited slip) will reduce, and crack width can be higher. As reinforcement, fiber (continuous) reinforcement products (rods or mats) can be used. Such composites are of carbon, glass or aramid fibres in matrix resin. Refer ISO 14484:2019. These bars have negligible ductility, hence cannot be substituted on design force basis. These bars do not corrode, and hence can be used with small cover, at lower stress limits suited for small members. Different grades of uncoated steel should not be permitted in a RCC component, without electrically insulate from each other.

R 4.4 ADMIXTURESR 4.4.1 Mineral admixtures, i.e. pozzolanic materials like flyash, GGBS, Metakaolin, silica-fume (micro-silica) etc. as additives or supplementary cementitious materials (SCM’s) are used to reduces the permeability of concrete and also reduce early age cracking due to less heat of hydration in initial period. In most cases there may also be a small saving in cost. Use of these is advantageous for many chemical exposures. Hence it is preferable to use mineral admixtures. While SCMs are in concrete, addition of lime stone (>80% CaCO2) powder (<5μm) does further improve it. The first basic action of these additive materials to give improvement of particle packing in the range of 200 to 1 μm (micron) against the cement particle (45 to 10 μm). Additional advantage is the second stage chemical actions giving more hydrated paste further contributing strength.R 4.4.2 Use of chemical admixtures help to achieve desired workability (while keeping w/c ratio low as required), and also reduce water-cement ratio for reducing porosity. Use of permeability admixtures, shrinkage-reducing admixtures and corrosion inhibitors may also be considered. Their use in LRC appears to be necessary. Calcium chloride or admixtures containing chloride ions (other than from impurities in admixture) ingredients shall not be used. Corrosion inhibitors may show erratic variations in chloride ion penetration, if water cement ratio is not low enough (say >0.45). Hence with their use choose a proper grade of concrete which can be more than M30.

R 4.5 JOINTING MATERIALS Jointing materials are required at joints such as construction joint, movement (contraction & expansion)

joints. Most of the materials used now-a-days for the joints in water tanks, are not covered by Indian Standards. For such materials specifications should be obtained from the manufacturer or the other standards (like BS or ASTM) can be referred. Use of bitumen or bituminous preparations is not desirable for structures retaining potable water, and similarly some other materials may not also be compatible. Compatibility with potable water needs to be checked for the relevant structures. Also see 11.5.

It should be noted that life for most of the jointing material is much shorter than the service life of concrete structure. This calls for the design of joints and selection of materials for maintainability/restorability.

Some Indian Standards related to joints are given in Appendix R1, at the end of this document.

R 5 EXPOSURE CONDITIONClassification of exposure conditions is given in table 3 of IS 456. Components of LRC should be assumed to

be exposed to ‘severe’ condition on both faces for the purpose of design. Though not clarified in code, outer face of roof can be assumed to be exposed to moderate and in coastal area as severe exposure. Inner face of roof (enclosing space above liquid) has to be assumed to be exposed to severe, and if conditions demand, very severe. A face of a component may be subjected to higher exposure like very severe or extreme if liquid in contact or environment demands so. Consequently the two faces of a component may be designed for different exposures such as one severe and other very severe, however in design process it does not make much of a difference by taking different exposure condition on the two faces. As a simplification some design engineer may choose higher exposure condition for both the faces. The grade of concrete has to be chosen for higher grade of exposure. From a concrete surface, amount of clear cover over bar and limiting crackwidth are functions of exposure condition on that face. Map indicating climatic zoning can also be considered.

Exposer condition of tank can be higher than ‘sever’ for area of high air pollution, sea-face etc. Components which for most of the time during service life will be surrounded on all its side by non-injurious

liquid can be treated as exposed to moderate condition, e.g. column inside tank. These components are ‘water contact members’ rather ‘water retaining’. There is no flow (transportation) of water under hydraulic gradient through the member for the major part of service life. In most cases such members may be small and it may not be worthwhile to reduce the grade of concrete for small quantity. Aspect discussed in this paragraph is not covered by the code, but many structures of water resource engineering require this consideration.

It may be seen later that difference in exposure conditions, do not affect the design much. More specifically higher exposure conditions (very severe or extreme) calls for some type of protective surface treatment. The

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code does not specify a lower crackwidth for higher exposure condition. Take the example of filter house in a water treatment plant. There are three locations of concrete components

to be distinguished for design. (a) Floor slab & wall of filter boxes, troughs (launders/channels) are LRC. Adjoining to filter box is pipe

gallery, where water due to leakages from joints & valves come. If pipe gallery floor is suspended (not directly supported on ground), it is also designed as water retaining member. At top of filter boxes cantilever walkways are provided, which are always above water surface, however are designed as LRC.

(b) Operating platform above pipe gallery is provided. Space between pipe gallery & operating platform is well ventilated like typical building. Operating platform is designed for clear cover required for moderate exposure, and crackwidth limiting to 0.2 mm under serviceability limit state. These types of members are not dealt in the code, and designer has to take decisions. Usually grade of concrete is same as provided in other components at that level.

(c) Roof of filter house is usually ≥ 3 m above the top of filter box (i.e. walkway & operating platform level). The space below roof is well ventilated like typical building. Though roof is enclosing space above liquid, the space is large and well ventilated due to doors & windows of filter house. Roof of filter house is designed like any other building for mild or moderate exposure condition as the case may be.

Similarly situation occurs in chemical solution room, wherein solution tanks are treated as LRC and other parts as normal building work. Also consider an example of sump and pump house. Wall, floor & roof of sump are designed as LRC. Floor of pump house being roof of sump is shall be treated as LRC. Floor of the pump house has some openings for access to sump and for installation of pumps etc. Space in the pump house is well ventilated and treated like industrial building. Above floor of pump house all RCC is treated like a building only and not LRC.

The modern approach is to recognize the mechanism (or combination of mechanisms) of deterioration of concrete component, and design aim should be to achieve an expected durability for the service life.

Chlorine corrodes the concrete, reducing cement paste to powder, loosing capacity to bind aggregates. For roof of tanks enclosing water with high amount of chlorine, refer R 7.2. Corrosion of concrete by chlorine is less severe in saturated concrete. Hence underside of roof is affected much more which may not be saturated. Whereas wall in freeboard zone is saturated most of time and effect of chlorine is less than that on roof.

Tanks in which chlorine is dissolved in water (chlorine solution tank), chorine contact tanks, or tanks holding water with break-point chlorination, will have nascent chlorine in top portion, which is highly corrosive to concrete. The underside of roof shall be assumed as exposed to very severe condition. Members of such roofs shall be made with concrete having minimum M40 grade and maximum water-cement ratio as 0.40. Anti-chlorine surface coating (e.g. epoxy) should also be applied.

All tanks of water supply scheme contain water which is normally chlorinated. The dissolve chlorine may be less than break-point chlorination, after few hours of adding chlorine at treatment plant. In such cases, the quantity of chlorine evolved will be less, and corrosive action of chlorine could be less. However, for a service life of over 30 years, similar treatment should be given to the underside of roof.

The grade of concrete has to be chosen for higher level of exposure condition on any one of its surfaces. For a concrete surface, amount of clear cover over reinforcement and limiting crackwidth are functions of exposure condition on that face.

The surface treatment, its smoothness and applications of coatings also depend upon the exposure condition. R 5.1 An improved exposure classification should also be considered which is related to environmental actions causing loss of durability. The exposure classes to be selected depend on the conditions in service and place of use of the concrete. Requirements will not exclude special considerations, treatment, coatings and lining etc. Different surfaces of a concrete component may be subjected to different environmental actions. A concrete component may be subjected to more than one action. Based on environmental action responsible for durability, recommended exposure classes, are given in Table A1. One may also refer to ICI TC/08-01 handbook on durability.

Table A1- Exposure classes (based on ISO 22965-1:2006, EN 206-1 & ICI TC/08-01 duly modified)Designation

/ ClassDescription of environment related to concrete For information, examples of the exposure class, concrete would

be subjected to -1. Penetration resistance or resistance against permeability of waterP0 No risk of water contact Resistance against water permeability is not required e.g. interior

building elements remaining mostly dry & no condensationP1 Exposer to water Requiring low permeability e.g. water retaining concrete

or that exposed directly to very heavy rainfall2. No risk of corrosion or attack on reinforcement or embedded metalX0 (a) PCC (no reinforcement or embedded metal):

Exposures except freeze & thaw cycles, abrasion or chemical attack.(b) For concrete with reinforcement orembedded metal: Almost dry

Inside buildings with very low humidity in air say relative humidity RH <40%.

3. Corrosion induced by carbonation of concrete cover, concrete exposed to air & moistureMoist condition relates to concrete cover on steel bars or embedment, and also to surrounding environment, except if effective barrier between the concrete and its environment is provided.XC1 Mostly dry or saturated for service life,

Effect is very smallInside buildings - low humidity in air (RH<60%);OR Concrete permanently submerged in water.

XC2 Mostly wet, rarely dry, Long term water contact, & most Foundations

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XC3 Moderate humidity orCyclic wetting and drying

Inside buildings - high humidity in air (RH>60%);External concrete not sheltered from rain or washing action

XC4 Cyclic wet & dry, Water contact not qualifying XC2

4. Corrosion induced by chlorides other than from sea waterConcrete containing reinforcement or embedded metal, subject to contact with water containing chlorides, including de-icing salts, sources other than from sea water, the exposure classified as follows:XCl 1 Moderate humidity Concrete surfaces exposed to airborne chlorides

XCl 2 Wet, rarely dry Swimming pools ; Concrete exposed to industrial waters containing chlorides, or chlorinated water

XCl 3 Cyclic wet and dry Exposed to water having high chlorine concentration, Parts of bridges exposed to spray containing chlorides, Pavement, Car park slabs (in cold countries)

5. Corrosion induced by chlorides from sea waterConcrete containing reinforcement or other embedded metal is subject to contact with chlorides from sea water or air carrying salt originating from sea water, the exposure should be classified as follows:XCs1 Exposed to airborne salt but not in direct

contact with sea waterXCs1.0XCs1.1XCs1.2

Structures near to or on the coast, further subdivide as per distance from sea coastBeyond 50 km from coast10 to 50 km from coastCoastal area up to 10 km

XCs2 Permanently submerged in sea water Parts of marine structures or coming in contact with sea water

XCs3 Tidal, splash and spray zones Parts of marine structures

6. Sulphate attackConcrete is subject to chemical attack by sulphate from exhaust gases, industrial pollution or from ground water

XS0 No risk of sulphate SO3 < 0.2% (in soil), or SO3 < 300 ppm in waterXS1 Risk of mild sulphate attack SO3 0.2% to 0.5% (in soil), or SO3 300 to 1200 ppm in waterXS2 Risk of moderate sulphate attack SO3 0.5% to 1.0% (in soil), or SO3 1200 to 2500 ppm in waterXS3 Risk of severe sulphate attack SO3 1.0% to 2.0% (in soil), or SO3 2500 to 5000 ppm in waterXS4 Risk of very severe sulphate attack SO3 > 2.0% (in soil), or SO3 > 5000 ppm in water

7. Freezing and thawing attack on concreteExposed to significant attack by freeze/thaw cycles whilst wet, the exposure classified as follows:

XF1 Moderate water saturation, without de-icing agent

Vertical concrete surfaces exposed to rain and freezing

XF2 Moderate water saturation, with de-icing agent Vertical concrete surfaces of structuresexposed to freezing and airborne de-icing agents

XF3 High water saturation, without de-icing agent Horizontal concrete surfaces exposed to rain and freezing

XF4 High water saturation, with de-icing agent or sea water

Road and bridge decks exposed to de-icing agents, Concrete surfaces exposed to direct spray containing de-icing agents and freezing, Splash zone of marine structures exposed to freezing

8. Chemical attack on concreteExposed to chemical attack from natural soils & ground water as given in Table A2, the exposure classified as below. Classification of sea water depends on the geographical location, therefore the classification valid in the place of use of the concrete applies. Note: National level special study needed to establish relevant exposure condition where there is - limits outside of Table A2; other aggressive chemicals; chemically polluted ground or water; high water velocity in combination with the chemicals in Table A2.XA1 Slightly aggressive chemical

environment according to Table A1.1XA2 Moderately aggressive chemical environment

according to Table A1.1XA3 Highly aggressive chemical

environment according to Table A1.1

Table A1.1 - Limiting values for exposure classes due to chemical attack from natural soil & ground water

Aggressive chemical environments class based on natural soil and ground water at water/soil temperature between 5°C to.30°C and a water velocity sufficiently slow to approximate to static conditions.The most onerous value for any single chemical characteristic determines the class.Where two or more aggressive characteristics lead to the same class, the environment should be classified into the next higher class, unless a special study for this specific case proves that it is not necessary.

Chemicalcharacteristic

Reference testmethod

XA1 XA2 XA3

Ground WaterSO4

2- mg/l EN 196-2 ≥ 200 and ≤ 600 > 600 and ≤ 3000 > 300 and ≤ 6000pH ISO 4316 ≤ 6.5 and ≥ 5.5 < 5.5 and ≥ 4.5 < 4.5 and ≥ 4.0CO2 mg/l aggressive EN 13577 ≥ 15 and ≤ 40 > 40 and ≤ 100 saturatedNH4

+ mg/l ISO 7450-1/2 ≥ 15 and ≤ 30 > 30 and ≤ 60 > 60 and ≤ 100Mg2+ mg/l ISO 7980 ≥ 300 and ≤ 1000 > 1000 and ≤ 3000 > 3000 to saturationNatural SoilSO4

2- mg/kg a total EN 196-2 ≥ 2000 to ≤ 3000c > 3000c to ≤ 12000 >12000 to ≤ 24000Acidity ml/kg DIN 4030-2 >200 Beaumann Gully Not encountered in practicea. Clayey soils with a coefficient of permeability below 10-5 m/s may be moved into a lower class.b. The test method should prescribe the extraction of SO4

2 by hydrochloric acid; alternatively, water extraction may be used, if experience is available in the place of use of the concrete.c The 3000 mg/kg limit should be reduced to 2000 mg/kg, where there is a risk of accumulation of sulphate ions in the concrete

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Guide to IS 3370 – 2020 due to drying and wetting cycles or capillary suction.

5.1.1 For exposure classes given in Table A1, the concrete parameters are recommended in Table B1.Table B1 – Recommended Concrete Parameters for exposure class as per table A1

Exposure Class Minimum cement content Maximum water-cement ratio Minimum concrete grade No risk X0 260 0.60 M20

Penetration resistance or resistance against permeability of waterP1 PCC 300 0.55 M20P1 RCC 340 0.50 M25

Carbonation induced corrosion in RCCXC1 300 0.55 M20XC2 320 0.50 M25XC3 340 0.48 M30XC4 340 0.45 M35

Chloride induced corrosion : chloride other than from sea waterXCl 1 320 0.48 M30XCl 2 340 0.45 M40XCl 3 360 0.40 M45

Chloride induced corrosion : sea water actionXCs1.0 300 0.50 M25XCs1.1 320 0.45 M35XCs1.2 360 0.40 M40XCs2 350 0.42 M40XCs3 380 0.40 M45

Aggressive chemical environmentXA1 300 0.48 M35XA2* 320 0.45 M40XA3* 360 0.41 M45* When SO4

2 leads to exposure class XA2 or XA3, it is essential to use sulphate-resisting cement. If classified, high sulphate-resisting cement should be used for exposure class XA3

Freeze-thaw attackXF1 300 0.50 M30XF2 300 0.48 M35XF3 320 0.45 M40XF4 340 0.44 M40

Minimum entrain air content should be 4% for XF2 to XF4Note : Recommendations in table B1 are not same as per ISO 22965-1 or EN 206-1.

R 5.2 In construction the minimum cement content and the minimum grade of concrete shall be higher of the values as recommended from table 2 of IS 456 & table B1 above. Similarly maximum water-cement ratio should be lower of the values as recommended from Table 2 of IS 456 & table B1. The concrete characteristics shall be enveloping the requirement from different considerations.

R 6 CONCRETEPCC base (also called mud-mat concrete, lean concrete, foundation PCC or blinding layer) is a non-structural

concrete and not govern by the requirements specified in table 1, and is excluded from the following discussion. PCC in foundation is discussed in R 3.2, R3.11, R 9.2.11b (2nd para), R 11.2a, R 13.1.1, R 13.1.2,

The concrete should by itself be watertight (i.e. low permeability), and plaster should not be relied for reducing leakages, but concrete should be grouted to reduce water permeation, if necessary.R 6.1 Table 1 specifies minimum cementitious / binder (cement + pozzolanas) content, maximum free water cement ratio, and minimum grade of concrete. Table 1 (of IS 3370 part 1) is reproduced below.

Table 1 – Minimum Cement content, Maximum Water-Cement ratio & Minimum Grade of concrete Concrete Minimum Binder

contentMaximum

water/binder ratioMinimum grade

of concrete Plain Concrete 250 Kg/m³ 0.50 M 20 Reinforced Concrete 350 Kg/m³ 0.45 M 30 Prestressed Concrete 380 Kg/m³ 0.40 M 40

Cement content given is irrespective of the grades of cement and it includes mineral admixtures such as flyash or ground granulated blast furnace slag and are taken into account with respect to the cement content and water-cement ratio, and do not exceed the limit of pozzolana and slag specified in IS 1489 Part 1, IS 455 and IS 16714 respectively. With maximum size of aggregate less than 20 mm, the concrete will require higher binder (cement + mineral admixture) content.

For higher exposure conditions (very severe or extreme), the requirements of table 5 of IS 456 will also

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govern the specification of concrete.R 6.2 Concrete should satisfy all the requirements of IS 456, and specifically those in table 5 of IS 456. Grade of concrete is a main parameter for specifying concrete. Though permeability is an important parameter for LRC, specific recommendation is not available. To control permeability, in addition to minimum grade (strength), maximum water-cement ratio is also specified. R 6.2.1 Concrete as proportioned (mix designed) should have enough of workability for ease of working, in relation to the method of handling and compaction of concrete. For increasing the workability the dose of plasticizer (or superplasticizer) can be enhanced. Limit on water-cement ratio should be maintained always.R 6.2.2 For concrete mix proportion (design) exercise in laboratory, the target water-cement ratio should be taken 0.02 less than the limiting value specified in Table 1 or Table B1. (Ref. ISO 22965). This is to account for the field variations. Water from all sources including that in admixture and the surface water with aggregate shall be accounted for calculating the total water in the mix, and also for water-cement ratio.

Most water tank & LRC works are small and may not conform to note 1 of table 8 of IS 456 (amendment 4), hence target mean strength shall be fck +1.65×6 MPa i.e. for M30 grade– target mean strength will be 40 MPa. This margin of strength is required to cover variations at site and in workmanship. RMC supplier does not take this factor in to account and the average strength of concrete as supplied is much lower than the mean required.R 6.2.3 It should be noted that with the modern cement (strength >53 N/mm²) as available, for conformance of limiting maximum water-cement ratio (related to exposure condition), the achievable grade of concrete may be significantly higher than that being specified, and by mix design trials, it can be determined in laboratory. Similarly if aggregate grading is good, the strength can be higher at the specified w/c ratio. In such cases, to conform to the requirement of maximum water-cement ratio, the grade of concrete adopted in construction shall be not less than the developable strength at the limiting water-cement ratio conformed by test in laboratory. The specified compressive strength should be reasonably consistent with the w/cm required for durability. The selected water-cementitious materials ratio must be low enough, and the specified strength high enough, to satisfy both the strength criteria and the durability requirements.R 6.2.4 In the modern concrete practice, for enhancing the grade of concrete, cement content need not increase. It can be enhanced by lowering the w/c ratio and marginally increasing the plasticizer dose. Hence for enhancing the grade from M30 to M40, increase in cost is very marginal (say 2 to 4% only on basic concrete supply cost) provided the cement content (kg/m³) does not change. This can be easily verified by difference of quotation for the two grades of concrete from any ready mix concrete supplier. In general higher grade concretes are more durable and also economical in designs. Concrete grade as higher as practicable should be adopted, and still it can be economical. R 6.3 Minimum concrete grade for LRC work is M30. Because of history of constructing tanks in M20 & M25 grade and satisfactory performance of many the tanks already constructed, for small tanks up to 50 m3 in the environment of medium exposure and with H/t ratio is within 25, can be designed and constructed in M25 grade concrete, except those in coastal areas, or the area where air-pollution is high, or liquid retained is aggressive like sewage. However, minimum cement content will be 350 kg/m³.R 6.3.1 For LRC designed as PCC (9.2.1), M20 grade is permitted; however H/t ratio is ≤ 20 and minimum reinforcement is as per IS 3370; or H/t ratio is ≤ 15 and minimum reinforcement is as per IS 456. Very small tanks can be designed as PCC in M20 and can be provided with nominal reinforcement as per IS 456. Hence the code gives wide options to design tanks in different grade of concrete. For concrete M20 minimum clear cover to any reinforcement should be 50 mm and for M25 it should be 45 mm.R 6.3.2 For LRC use of mineral admixtures (i.e. supplementary cementitious powders) are advantageous. Their use reduces permeability and is favourable for durability. The use of flyash (pulverized fuel ash i.e. PFA) and/or GGBS (ground granulated blast furnace slag) in concrete or use of flyash blended cement (Portland pozzolana cement conforming to IS 1489 part1) or Portland slag cement (IS 455) are preferable. Multiple blending can give better performance of concrete, and also lower OPC content.R 6.3.3 Site mixing of mineral admixture requires very efficient and through mixing. Unless a batch mixing plant or highly efficient (pan or twin shaft) mixer is used to produce concrete, site mixing of mineral admixture in concrete may be avoided.

It may be noted that the common tilting drum mixers (0.16 to 0.2 m³) used ordinarily on construction sites, have very low efficiency of mixing, and theses should not be used to mix required for LRC with mineral admixtures (Flyash/GGBS/Metakaolin). Refer 10.3 of IS 456. If a concrete delivery is segregated or not properly mixed, it must be remixed before transporting and placing in position.R 6.3.4 Cement content should be as small as possible for better performance, but not less than the minimum specified in Table 1. The minimum limit specified is a durability requirement, and assumed to include all cementitious material (i.e. binder including mineral admixtures /additives), and excluding portion of flyash retained on 45µm sieve.

For the requirement of minimum cement content and the maximum water cement ratio, cement means either OPC or PPC (blended cement as per IS 1489 or IS 455). However while mineral admixtures (SCM’s/additions) are used, the equivalent cement content is the sum of OPC & mineral admixtures for the requirement of

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minimum cement content and the maximum water cement ratio, as per IS 456. The maximum limit of cement content 400 kg/m³, which can excludes the supplementary cementitious

materials (mineral admixtures), however it is preferable to have lower OPC content. This limit (maximum 400 kg/m³) is irrespective of the grade and type of cement. Even for blended cements, limit is same. In case of addition of mineral admixtures (pozzolanic materials like flyash, GGBS, Metakaolin, silica fume /micro-silica etc. as supplementary cementitious materials) at the concrete mixer, total binder (cement + SCM’s) content can be up to 450 Kg/m³ in which OPC is not more than 80% of cementitious content.

As per the international practice, the “cement content” can be replaced by “equivalent cement content” which is sum of cement plus k times the additive content per cubic meter of concrete. Here k has a value 0.2 to 0.4 for flyash, which can be based on past experience or the tests. Maximum limit of cement content (excluding mineral admixtures) is specified to keep a control over cracking as a result of temperature built up due to heat of hydration, and that due to shrinkage. After the rise of temperature due to heat of hydration, the subsequent cooling to ambient, causes cracking of concrete, which need to be controlled. If the cement content exceeds 400 kg/m³ (or 450 kg/m³ as total cementitious), due to heat of hydration thermal cracking can be higher requiring temperature control in construction, as well the shrinkage coefficient of concrete will increase. Increase of thermal gradient due to heat of hydration and higher shrinkage coefficient should be accounted in design while calculating temperature–shrinkage reinforcement (normally called as minimum reinforcement). The code recommendations may be assumed to be based on a cement content of about 350 to 400 kg/m³. For higher cement content the requirement of temperature-shrinkage reinforcement may increase. It is desirable to keep OPC content as low as possible.R 6.3.5 In reference to evolution of heat of hydration and temperature built up due to hydration, if for construction, the conditions appear to be significant, the heat evolution characterises of the cement could be obtained from tests. The actual maximum temperature build up (and subsequent cooling) and the time (age of concrete) of its occurrence should be estimated taking account of environment during early life of the member (ambient temperature, humidity, curing regime) and thickness of member, type of formwork (heat dissipation characteristics) etc. OPC content of concrete should be as low as permitted by substituting with supplementary cementitious materials (mineral admixtures e.g. flyash or GGBS etc.), and reducing the temperature of concrete (< 30°C), are the means to limit heat of hydration in in first three days age of concrete, which eventually reduces thermal cracks, and improve durability.R 6.3.6 If concrete is subjected to high temperature (>70°C) in its early life (curing period), delayed ettringite formation can occur in certain mixes depending upon humidity, and LRC to remain wet for most part of life. To avoid adverse effect due to this on the performance of concrete in service, the peak temperature of the concrete in early life (curing period) should not be >70°C due to heat of hydration. To control this use more of mineral admixtures in the concrete mix for thick section and apply temperature control on concrete mixed and placed, as well enhance heat dissipation during hydration period. R 6.3.7 For liquid retaining concrete, use of chemical admixtures (plasticizers) is advantageous to control water-cement ratio, to reduce permeability and to get desired workability. Admixtures containing chloride must be avoided as far as possible. A particular admixture shall be permitted only after the compatibility test with the cement sample from the specific source. The source of admixture or cement if any one changes, the compatibility test shall be carried out again.

Permeability of a concrete member can reduce with extended moist curing, and also with the use of smooth forms or proper trowelling.

Cement plaster if applied to internal surfaces of concrete, should not be treated as substitute of impermeable concrete. Normally permeability of plaster is many times more than that of concrete.

If the grade of concrete for a work is higher by 10 MPa than that required by standard, clear cover requirement can be reduced by 5 mm. Total acid soluble chloride content should not exceed 0.6 kg/m³ of concrete. For very sever environment like roof of tank storing chlorinated water, the chloride content shall not exceed 0.40 kg/m³ of concrete and also < 0.10% of cement content. R 6.4 Fibres : The fracture energy of a cement-based material generally increases in proportion to the amount of short fibres or polymer used. The fracture energy corresponds to the area below the tension softening curve consisting of the crackwidth and transferred tensile stress. For members where the occurrence and progress of cracking are dominant, giving consideration to the tension soften property may make a rational performance verification possible. The fracture energy of a cement-based material can be obtained through the test specified. For enhancing the performance of concrete, addition of fibres is permitted in concrete. Fibres like steel or synthetic/polymeric can be added. Steel fibres shall conform to ICI TC 01-03, (ISO 13270, EN 14889-1, ASTM A 820), macro polymeric/synthetic to ICI TC 01-04, or micro synthetic fibres to IS 16481 or ICI TC 01-04. For guidance on use of fibres refer ICI monograph ‘Guidelines on Selection, Specification & Acceptance of Fibre & Fibre Reinforced Concrete’. With the use of fibres the performance of concrete can improved. Fibre type having established as alkali resistant (like polypropylene and steel) can be used in concrete to control plastic shrinkage cracks, control temperature shrinkage cracks and to improve flexural strength, toughness and post cracking behaviour of concrete. Structural fibres like steel can improve the dispersion of cracks due to loads and restraints in service life, thus gives better control on cracks and reduces crackwidth. For

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any other fibre, its long term chemical stability shall be established. R 6.4.1 Micro-synthetic (polymeric) fibres are used to reduce crackwidth and cracking due to plastic shrinkage of concrete. These fibres may be monofilament or fibrillated, and typically used at dosages 1.3 to 1.8 kg/m³. Macro-synthetic fibres may be used in addition to resist shrinkage and temperature cracking. However, the temperature shrinkage reinforcement (also known as minimum reinforcement) recommendation as per code does not reduce. For control of plastic shrinkage crack, minimum dose of fibres should be such as to give average residual strength 0.30 N/mm² (test as per ICI-TC FRC 01.1 or EN 14651 part 1 or ASTM C1609); and dose should not be less than 1.0 kg/m³ for polymeric fibres. For structural purpose (i.e. to account enhanced flexural strength and toughness) the minimum fibre dose shall be such that an average residual strength 1.50 N/mm² is achieved.

R 6.4.2 At present guidelines are not available to utilize in design, the enhanced flexural strength, toughness and better crackwidth control by fibre concrete. In these regard the designer should consult specialist literature and take proper decision. Basic information on design model is given in National Building Code -2016, Part 6, section 5A, Annex A, page 73-83.

R 6.4.3 If structural fibres are used resulting in the average residual strength not less than 1.50 N/mm², in limit state of serviceability the crackwidth 0.1 mm may be deemed to be satisfactory without checking the crackwidth calculations if the design of the section confirms to all other requirements under serviceability and ultimate limit states as for reinforced concrete as per IS 3370 part 2, while steel bars are designed as reinforced concrete having stress ≤ 208 N/mm² in serviceability limit state.

R 6.5 Formwork : Depending upon type of finish specified sheets of steel, aluminum, marine plywood or plastic coated plywood may be selected for shuttering. Joints in shuttering shall be made leak-proof by using foam/ rubber strips to prevent cement slurry leakage. Use of through tie rods shall be avoided. If unavoidable, ties may be provided with creep bolt (to be removed later). Holes left in place shall be filled with mortar or grout preferably non-shrink grout or epoxy mortar. If form release agents are used in LRC for drinking water, such a coating shall be non-toxic after a specified period, usually 30 days. Formwork should be rigid enough, such that change in deflections due to movement of worker on freshly laid concrete should be very small say within 1mm (excluding concrete load & DL).

R 6.6 Curing : It has following phases.i. First phase is from placing of concrete to the time till it is finished (as needed) and arrangements made for

next phase of curing. In this phase fogging or misting is to be done to avoid plastic shrinkage cracks. If air temperature is low (<25°C) or humidity in air is high (>90%) this phase may become negligible. Requirements of misting is governed by duration of this phase.

ii. Main phase of curing during which all concrete surfaces are maintained continuously nearly saturated (i.e. RH > 96%), by spaying water or covering concrete to avoid evaporation of water from it, i.e. applying curing compound. This phase continues till desirable properties are developed in concrete.

iii. Till water is filled in tank, it should not be allowed to dry-up i.e. RH 55% to 70% can be maintained.R 6.6.1 Concrete members should be initially cured continuously (without intermittent drying time) for at least 14 days, or cured to achieve 75% of the specified strength. During summer the next phase of curing period should extent to 28 days and in other season to 21 days during which concrete it kept moist (RH > 70%) and may not be 100% saturated continuously. Thereafter, LRC should be sprayed with water at least once a day for not allowing the concrete to dry out i.e. keeping RH>55%. Active curing period may be far less if ambient temperature is below 10C or humidity in air is high (>70%). [ RH = relative humidity ] Proper curing of concrete is vital for controlling temperature-shrinkage cracks, and gaining durability. The method shall ensure that the surfaces of all concrete remain continuously moist in the curing period. With curing the temperature of concrete shall also be kept in control. Temperature shock i.e. sudden cooling of concrete surface say by continuous cold water spray immediately after de-shuttering should be avoided.

R 6.7 For pneumatically applied concrete, the designer should approve the specifications, material requirements, mix proportions, effective water cement ratio, mixing, placing, equipment to be used, and curing before the construction starts.

R 7 DURABILITYR 7.1 Durability is the ability of a structure or structural element to assure no deterioration that is harmful to required performance in the relevant environment. Durability should be satisfactory in all limit states of serviceability; and also for some parameters in ultimate limit state for LRC. On reaching an ultimate limit state LRC mat get damaged and excessive leakage may take place, but otherwise the performance can continue for short duration till intervention takes place. Durability is also a limit state. The structure shall be designed such that deterioration will not reach to a limit which can affect the serviceability during the design life with the determined plan of maintenance (conservation). Repairs as additional maintenance would be required for short fall in quality of structure. Durability is of prime consideration for LRC. Refer 8 in IS 456 for durability requirements. For durability of concrete structure, its exposure to environmental and service conditions during design life is to be considered.

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Each mechanism which may cause loss of durability need to be taken in to account and enough resistance has to be designed in the structure.R 7.1.1 The important agencies causing loss of durability are as follows. Chloride ion penetration, and increase in its concentration near the steel surface reduces the passivation of concrete towards corrosion of steel. Carbon dioxide permeation through concrete results in conversion of calcium hydroxide to calcium carbonate, thus pH reduces in the protective concrete cover over steel bar. However, in saturated concrete carbonation does not take place. Normally concrete has pH >12 (in new concrete up to 14). With reduction in pH the rate of possible corrosion of steel bars increases. With increase in permeability, calcium hydroxide comes out of concrete with water in dissolved form and deposits on the external surface of concrete. This results in more porosity, more permeability, and reduction of pH etc., all helping in loss of durability. The main consideration is the permeation of the agencies (such as water, oxygen, carbon dioxide, other gases, chloride ions, etc. causing loss of durability) through concrete, and permeability of each of these are not directly controlled; but indirectly by water cement ratio and grade of concrete. Water to cementitious material ratio (w/cm) should be as low as possible, while keeping the concrete workable using plasticisers (chemical admixtures). Bigger defects like honeycomb and segregation, are havoc for durability. These type of defects must be avoided and next to this, better control over microstructure of concrete is needed to get desired durability over the life of LRC. While mineral admixtures (like flyash or GGBS) are added the threshold limit for chloride ion concentration may reduce, which can be corrected (or compensated) by using suitable corrosion inhibitor. R 7.2 As per experience of observing behaviour of roof of tanks storing chlorinated water, the durability of underside of roof is of high importance. Hence underside of roof can be treated as subjected to ‘very severe’ exposure condition, therefore minimum grade of concrete M40 and protective treatment is needed. For durability of the underside of the roof of such tanks, anti-chlorine surface treatment like epoxy coating is required to achieve a long life. See also R5 last 5 para’s. Presence of other gases (like hydrogen sulphide in sewage treatment) may also require suitable coatings. R 7.2.1 In environment prone to chemical attack, all jointing materials should be chosen such that they are resistant to the chemical exposure as conformed by tests performed by the manufacturer or the supplier.R 7.2.3 Wherever applicable concrete should be protected against erosion damage when subjected to cavitation and abrasion. Following actions may be taken – reduce velocity of flow (<3m/sec), dissipate energy, use curves conducive of smooth flow, smooth finishes, supply air at flow boundary, consider concrete ≥M40 and w/c ratio ≤ 0.40 with hard aggregate and sand with higher siliceous content, avoid carbonate aggregates. And for additional protection apply erosion resistant coating.

R 7.2.4 Service life associated to the time-dependent material degradation are for example initiation of reinforcement corrosion, cover concrete cracking and spalling due to corrosion.

R 7.3 Clear Cover: It is the next important consideration to prevent loss of durability. Time in years taken for carbonation of concrete to reach the level of steel will depend upon the actual concrete cover over the bar. Similarly the time for chloride ion concentration at the surface of bar to become threshold, also depends on the actual concrete cover on bars. Hence, the amount of clear cover being provided is very important for the life of LRC, and further more important is the variation in cover achieved actually. Hence quality control is needed to keep tolerances (variations) in cover to be very small say within 5 mm. Based on exposure condition taken for design, the amount of nominal cover should be specified based on table 16 in IS 456. For ‘severe’ exposure condition the clear requirement is minimum 45 mm, and 50 mm for ‘very severe’.

The clear cover requirements are as per IS 456 for the exposer class to meet durability requirements. Variation of clear concrete cover on the reinforcement is a prominent factor affecting durability and performance of LRC. It is necessary to emphasize control over variation of actual concrete cover achieved in practice compared to nominal cover. Systems are required to measure and maintain documentation during construction, and to ensure the variations in the clear cover achieved to be very small and within the permitted tolerance. See IS 456, Table 16, & note 2 for tolerance. Code does not allow negative tolerance (-0 +10). Note that for a nominal cover of 45 mm with tolerance of -0 +10, the specified cover would be 50 ±5 mm. R 7.3.1 Where concrete is cast against the surface of a blinding concrete (PCC), clear cover should be increased by 5 to 10 mm depending up on the roughness of PCC. If blinding concrete is finished plain and smooth increase of cover can be 5 mm only.

If the grade of concrete for a work is higher (say M40) than that required by code (i.e. M30 for a particular exposure), clear cover requirement can be reduced by 5 mm.

The clear cover provision should increase by half the thickness that may degrade during service life, due to chemical action or erosion or abrasion. And such extra thickness should not be accounted in strength calculations.R 7.3.2 Though code does not specify, there is scope to reduce the nominal clear cover by 5 mm, if the variations in the clear cover achieved are ensured to be within a narrow range (say ± 3 mm).

Similarly, there is scope to reduce the nominal clear cover by 10 mm, if the reinforcement cage and formwork are rigid (RF2 or RF3 as per appendix) i.e. under vibrations the movements are limited (amplitude is

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less than 1mm) and intense compaction effort is applied as in case of precast in industrial environment with high level of reliability of getting a good surface finish without air bubbles. Also the clear cover achievable should be in a narrow range of variation ± 3 mm. R 7.3.3 It is necessary to take precaution that the ends of binding wire (used for tying bars) should not project out in the cover portion of concrete, and the free ends should be bent inside. Items that could be corroded by the environment should not be embedded in the cover zone. Non-corrodible items such as conduits or chairs for reinforcement may be embedded in the cover zone provided- the concrete placement is not hindered, reduction in strength of section due to embedment if any is accounted in design, and the embedment does not create a path to increase the permeation through the section. R 7.3.4 Metallic items that protrude from the concrete should be designed (thus coated) such that galvanic corrosion in the buried and exposed portions would not occur. Aluminium must be isolated from wet concrete by a moisture-proof coating, lining or gasket.R 7.3.5 Clear cover and space between bars should be compatible with the nominal maximum size of aggregates, to ensure proper encasement of reinforcement and to minimize honeycombing.

R 8 SITE CONDITIONS Considerations given here have influence on layout of structures, structural requirements and performance.

Due to constraint on site selection, if within plan area of structure at site, the soil strata changes, differential settlement may not be avoided. Where softer soil is in foundation, differential settlement may be a result of softening of soil due to heavy leakage which may be only on one side of the structure. Proper structural configuration (say dividing structure in parts each bearing on different soil strata) and proper planning of drainage of water (may be due to leakage) is required.

Structures should have enough side margins to reduce possibility of interference due to leakages and foundation behaviour of other structures.R 8 a. Chemical properties of soil and ground water may affect the grade of concrete and specification of the cement to be specified. If soil or ground water is having sulphates, refer to the requirements in Table 4 of IS 456. In addition to selection of proper type of cement, higher grade of concrete and protective treatment may also be required. If ground water is acidic, protective treatment is required.

R 8 a (i) Earth pressure - Normally design liquid pressure on a wall should not be very much reduced on account of earth pressure in opposite direction. Some engineers interpret this as a condition having maximum liquid inside and no earth (soil) assumed on outside, which is not proper. Though the earth is present outside, only its inward pressure is assumed to be very small (<50% refer R 4.2.6 in part 2) while liquid pressure is acting outwards, because for many reasons earth pressure may reduce substantially and also for the issues of stiffness of soil and wall. One of the reason for the simplification is due to the fact of cyclic loading & un-loading of water pressure, with adverse temperature shrinkage condition and soil stiffness being very small compared to concrete wall. Also due to erosion of soil and due to temporary excavation near tank wall, earth pressure may substantially reduce. Earth pressures can change due to change in moisture content and temporary stiffening of soil. Though soil pressure can be neglected as a simplification in design, in most situations some depth of soil fill is definitely needed to comply with the requirement of minimum depth of foundation for the vertical foundation pressure as may be produced on foundation soil. Designer may opt for half of active soil pressure (a minimum possible value) as pressure relieving water pressure. It is debatable whether soil pressure at rest can be estimated for reliving pressure while water pressure is acting from opposite side. R 8 a (ii) Settlement and Subsidence – Geological faults, mining, earthquakes, and existence of subsoils of varying bearing capacities may give rise to movement or subsidence of supporting strata which may result in serious cracking of structure. Effects of softening of soil due to leakages should also be considered. Special considerations should be given in the preparation of the design, to the possible effect of subsidence or movement of the foundation strata for example, subdivision of the structure into smaller compartments and provision of joints to outlet pipes and other fittings. Joints in structures in mining subsidence areas will need special considerations to provide for extra movement.R 8 b Injurious Chemicals – Chemical analysis of the soil and ground-water is essential in cases where injurious soils are expected to exist, as concrete structure may suffer severe damage in contact with such soils. Where concrete is likely to be exposed to sulphate attack, requirements specified in IS 456 shall be followed. An isolating coat of bituminous or other suitable materials may improve the protective measure.R 8 c Floatation – This condition is often known as ‘uplift’. Check against floatation (uplift) is a stability check with accounting of buoyancy. It refers to ground tanks or underground tanks only, or where tank bottom is below the highest ground water table of highest flood water level. R 8 c (i) The factor of safety (≥1.25) is the ratio of downward forces to that of uplift (upward) force. Uplift force is the product of gross volume (including air void) of structure (in m³) below design ground water level and the density of water (9810 N/m³). To counteract the uplift, in addition to the weight of members of structure, other materials or methods are also used to provide downward force, e.g. lean concrete fill, rock-chips/boulder filling, sand/soil-fill, anchor

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rods, anchor piles etc., and each one has different reliability. Hence, for calculating factor of safety against up-lift, a reduction factor to the force contribution by these may be applied as follows. Lean concrete (<M10) - 0.9; Rock/boulder filling - 0.8; Soil fill - 0.7; Anchors - 0.75, Piles - 0.8 ; Structures having unsymmetrical configuration or loading require addition check by taking moments of all the forces for equilibrium with a factor of safety ≥1.4 (& 1.25 is not enough). Under the uplift condition each member should also be designed for load combinations with groundwater pressure and earth pressure, both for ultimate and serviceability limit state. See also 10.1. To resist uplift, the frictional resistance due to soil or weight of soil failure wedge may be considered to increase the factor of safety. If the soil weight in failure wedge and its friction force is neglected, the factor of safety should not be below unity. Thus friction & soil weight can be accounted to enhance the calculated factor of safety above unity. Temporarily uplift may be caused due to heavy rain water entering at sides and bottom of ground tank.Floatation is applicable on tank partly or fully in ground, and not to tanks above ground or above flood water. Consider in the design, the possibility of sudden change in ground water table or pore pressure in soil, or sudden accumulation of water in ground even if for a small period (hours) such as due to heavy rains.

R 8 c (ii) Drainage at the site of ground water tank should be designed and provided effectively to avoid uplift. Excavated trench around the tank even if refilled, gives an easy entry to rain water and may create temporary uplift condition, till water gets drained through the soil. During construction precautions should be taken against development of such a condition. To avoid such condition during service life, surface drainage of rain water should be planned and entry of rain water in to refilled trench should be prevented even during construction of ground tank.

R 8 c (iii) At times, pressure relief valves are installed in the floor slab of tank for safety against uplift. For the pressure relief valve to function, there has to be higher pressure outside compared to inside, i.e. when pressure relief valve will actuate, there would already be some uplift pressure acting on structure and its components. Hence structure & its components are to be designed for some amount (say about 1.5 m) of uplift, and whole effect of uplift cannot be relieved by pressure relief valves.

Pressure relief valves can be used only for raw water storage, where entering of ground water in to the tank is not objectionable. Experiences indicate that pressure relief valves are not reliable enough. With age of installation, the operating pressure (i.e. differential pressure inside & outside) on the valve increases. For these valves to operate, drainage system (with filter layers) is to be provided under the floor of the tank with filter system which work over the service life of tank. This drainage system can get chocked up over the years; hence efficiency and reliability of the relief system can reduce drastically. If the system works, along with the liquid inflow, fine particulate matter (from soil beneath tank) is expected to come in. with few repetitions of this process, porosity of soil in foundation will increase, and in turn may lead to foundation settlement. Hence system has poor reliability and a possibility of settlement, therefore as far as possible the system of pressure relief valve with drainage below floor may not be provide if long-term satisfactory performance is required.

R 8 c (vi) Due to heavy leakage from tank or other sources or unusually heavy rains, water level around the tank may rise temporarily, and under such condition uplift pressure would exert on the empty tank for a small period (may be in hours) till the water around the tank gets drained. Though the ground water table does not rise, but since water reaching below the tank is more than the capacity of the soil to drain it, water accumulates for few hours and pore pressure increases. Also for such a situation it is not wise to provide drainage system under the floor of the tank. Such drainage will help in developing uplift and not delaying it. Requirement is to provide drainage around the tank to take away the water due to rain or leakage and not under the tank.

Hence the drainage system under the tank is either counterproductive or unproductive, and the author recommends it not to provide. Drainage around the tank is needed rather under it.

R 9 CAUSES AND CONTROL OF CRACKING Mainly leakages are taking place through construction joints and cracks, hence crack control is needed to

keep concrete watertight. Cracks are preferential pathways for the ingress of environmental agents which may induce loss of durability. It is a chain action of permeable cracks, seepage leading to degradation, result in more seepage through crack which can become wider. Degradation can be faster in conditions of chloride induced corrosion.

Autogenous healing reduces the leakage through cracks or construction joints by blocking the continuous chain of pores. It is mainly due to late hydration of cementitious (cement & SCMs) grain which remained un-hydrated. Of the various factors, the rate healing dependent upon the initial effective width of the crack. The smaller the width the faster the crack will seal. The effect of other factors such as the type of water and the cementitious material in the concrete, are only smaller. The rate of healing is slightly increased by reducing the differential pressure across the crack; i.e. higher pressure gradient (with high H/t ratio) slows down the healing.

First time, ensuring slow filling of the concrete tank under test, can significantly reduce the total loss of water and allow healing of cracks. The acidic nature of the water may inhibit autogenous healing. This risk may be reduced by the use of flyash or ground granulated blast-furnace slag in the concrete mix. Placing a pozzolanic material such as fly ash, in the retained water improves the autogenous healing of the concrete. The healing of cracks reduces, with pH of water leaking through it. Hence, for first test the water filled in the tank should have pH >7. Lime could be added to water to increase the pH.

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Normally cracks up to 0.2 mm wide and which are not live during healing, will autogenously seal within 21 to 28 days; cracks up to 0.1 mm wide will seal in less than 7 to 14 days. Live cracks are difficult to heal. Crack widening can be very much slowed-down by slow filling of tank first time. Thus almost steady-state condition of crack prevails, and by healing continuous core can be blocked. During testing of tank for water-tightness, the application of liquid load should be slow, to allow autogenous healing of cracks. It should be ensured that the crackwidth in concrete, during its service life, does not exceed that needed from durability and leakage requirements of reinforced concrete. Crackwidths ≤ 0.20 mm have very little effect of durability, if leakage through crack is under control.

R 9.1 Causes of cracking : A. Cracks in plastic concrete:

A.1 Plastic shrinkage cracking is due to very rapid loss of moisture from concrete surface soon after placement (& before any strength development), influenced by combination of temperature, relative humidity and wind speed. When rate of evaporation from surface is more than the amount of water which can be replenished by concrete below, the plastic shrinkage cracks appears which are shallow, but weaken the concrete near surface. After placing and till curing starts, concrete is to be protected from moisture loss by covering or maintaining high moisture over surface by fogging or misting etc.A.2 After compaction, concrete has a tendency to continue consolidation and settling of particles, which is restrained by steel bars. This results in void below the restraint to settlement i.e. bar, and adjoining concrete settles, causing a crack along and above the bar in concrete surface, called as plastic settlement cracks. To avoid these, concrete should be deposited and vibrated in lifts of about 300 mm, with some time gap (10 to 30 minutes) for next lift. Non bleeding concrete has lower chances to develop these cracks.

B. Cracks in hardened concrete: B.1 Drying shrinkage strain is due to loss of moisture from cement paste during hardening. Restrain on this drying shrinkage induces cracking. Cracks due to drying shrinkage can be minimise by avoiding moisture loss ahead for sufficient strength development in concrete. The shrinkage strain can be higher for concrete having higher paste (cement + water). LRC should not be allowed to totally dry out. Due to cracking, concrete gets weakened, but on rewetting of concrete the drying shrinkage strain reduces also. Movement joints can reduced the shrinkage cracks. Many factors influence the shrinkage stain, for which specialised literature should be referred.B.2 Temperature differences in concrete causes strains with restrain cause cracking, if stress in more than the tensile strength. Temperature difference can occur due to heat of hydration of cement, or due to environment.B.3 Deleterious chemical reactions like alkali-silica gives expansive compounds, and cause cracking. When such reactivity is low, use of mineral admixture (like flyash etc.) reduces the susceptibility to such reaction. For more details refer to specialised literature. B.4 Weathering due to freeze & thaw can cause cracking. Refer to specialised literature. B.5 Corrosion of reinforcement is an electro-chemical process, which results in iron compound having very high volume creating bursting force on concrete cover, causing cracking of concrete. Alkaline environment in concrete forms a protective film on the surface of steel. If pH (alkalinity) is reduced, or the protective film is damages (as by chloride ion threshold is reached), the corrosion of steel can take place. Cracks transverse to bars do not influence the corrosion rate, if crackwidth is small (≤0.2 mm). Corrosion of bars induces crack along the bar, and in turn the rate of corrosion gets very much enhanced. Compared to transverse cracks, longitudinal cracks are potentially harmful.

R 9.1.1 In concrete cracks are caused by the stresses due to loads, and restrain on temperature gradient due to environment or the liquid and shrinkage. For crack control load combinations at service loads are considered.

Due to heat of hydration, the temperature of concrete increases, and the peak may occur in 1 to 3 days age. Subsequent cooling of concrete having restraint will cause tension and thus cracks. In young age of concrete (while hardening), shrinkage and temperature variations causes stress under restrained condition, resulting in formation of micro-cracks. At this young age, concrete continues bear increasing shrinkage and daily temperature variations, which contribute to crack development, while it is gaining strength. These cracks relive the stresses to a significant extent and develop in to hair-cracks by repeated variations under restrain. Crackwidth for this condition is controlled by reinforcement called temperature-shrinkage reinforcement designed, or minimum reinforcement provided. Crack control due to temperature-shrinkage effect in young concrete, without combination with other load conditions, can checked as per the procedure given in code (part 2 Annex A). Under the condition of no external loads also, cracks are in concrete.

Also in hardened concrete shrinkage and temperature variation causes existing cracks to open, and to a large extent effect is relived, though not complete. The strain induced forces may get relived to level of about 20% (due to few cracks), and where steel ratio is very high, many finer cracks could form and having a much smaller crackwidth. Force relived could be up to 50%.

Under serviceability state with these restrain effects combined with loads, the allowable crackwidth (0.2 mm) can be exceeded slightly say to within 0.3 mm. For daily variations of temperature, the extra temperature stress and effect on crackwidth is reversible (not long term), however effect is small due to relaxation by existing cracks. Hence usually serviceability check is not applied for load combination with temperature-shrinkage superimposed, because lot of its effect is relived but not totally. Hence, at serviceability stage some recommends (as in New Zealand code) to superimpose the modified temperature-shrinkage effect, by multiplying by a

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reduction factor (RF say 0.20 onwards). As a simplification value RF can be read from a graph.For ultimate limit state (or limit state of collapse) crackwidth check is not expected, however sever cracking

& spalling of concrete are controlled by detailing rules as a qualitative measures. In such load combinations, the effects of temperature-shrinkage get substantially relaxed due to creep, cracking (reducing stiffness for the restrains) and elasto-plastic behaviour of concrete. The strength check at ultimate limit state is not influenced by temperature-shrinkage effects.

For temperature design Australian (AS 3735) and New Zealand (NZS 3106) standards gives the guidelines. Such design is important for roofs of large tanks and to some extent for walls subject to sunshine.

The minimum reinforcement (temperature-shrinkage steel) as recommended (in IS 3370), takes care of only chemical shrinkage which is about one-third of full shrinkage. When a tank is kept empty for a long time over which moisture from concrete dries out completely and thus maximum (full) shrinkage will take place. In such condition wider than normally expected cracks are formed giving unacceptable leakages at subsequent filling. Hence precaution is that tank should not be allowed to dry out. Alternately higher temperature-shrinkage steel is to be worked out (i.e. designed) for full shrinkage and provided accordingly. Refer Part 2 A-2.3, where in strain of 67% shrinkage should be added for condition of full shrinkage. In existing tank if such wider cracks are developed, the leakage can be reduced by grouting these cracks.

Note that more often the concrete in roof members of large tank may dry-out before the 1st filling of tank, and may cause cracks due to full drying shrinkage. Hence roof members should also be kept moist till some water is filled in the tank.R 9.1.2 Control over cracking due to variation in temperature, moisture & shrinkage, can be applied by reducing temperature gradient, moisture changes and shrinkage, as measures during construction. Design measure to reduce the crackwidth is by enough reinforcement, and further method is by introducing movement joints; combinations of these three methods do provide workable solutions. Also refer discussion in R 9.1.1.R 9.1.3 As sum-up : Control over cracking can be achieved by following-

- limiting of temperature rise (& thus its peak) due to heat of hydration,- reducing (or removing) restraints,- reducing concrete shrinkage,- use concrete having low coefficient of thermal expansion,- use concrete of high straining capacity (fibre concrete / ferro-cement).

R 9.1.4 Minimising cracking due to restrained imposed strains: It is desirable (as a better control) to minimise the development of cracks in LRC, due to restrained strains from temperature changes and shrinkage. One of the method for this is to limit the tensile stress in concrete to characteristic flexural strength (only 5% probability of being exceeded i.e. fctk,0.05 = 0.27 fck

2/3) of concrete.

Concrete grade M20 M25 M30 M35 M40fctk0.05 1.98 2.31 2.61 2.89 3.16

R 9.2 Methods of Control on CrackingR 9.2.1 PCC Design : There is no enough clarity about PCC design in the code. PCC can be designed with following different philosophies. Design method for limit state of ultimate load is not developed for PCC. Hence design method has to be by working stress. Note that though it is called as PCC design, some reinforcement is provided.

(a) Plain concrete having no reinforcement : May be feasible for units of about a meter or less, tension may be very small, and where leakage is not a considerations. Structural stability may be given by masonry or soil. This at present is ruled out.

(b) Concrete designed for structural safety by accounting tensile strength of concrete, and providing nominal (as minimum) reinforcement as per IS 456. Due to history of temperature-shrinkage variations, the actual concrete in a structure has micro-cracks, and tensile (or flexural) strength of concrete vary widely. Modern standards (e.g. EN 1992-1) as an example specify tensile strength of C20 concrete from 1.5 to 2.9 MPa (at 5% & 95% fractile). If structural failure is dependent on tensile strength, safety margin is applied to such low value (1.5 & not 2.9 MPa). It should be noted that normally for concrete design the tensile strength of concrete cannot be relied upon for any primary purpose. Hence lower permissible tensile strength should be specified as given below for achieving the reliability. Plain concrete liquid retaining members preferably be designed by allowing lower permissible direct tensile stress for M20 & M25 being 0.95 N/mm² & 1.05 N/mm², and allowable flexural stress 1.35 N/mm² & 1.45 N/mm² for working stress design method (or under limit state of serviceability). The purpose of specifying the limiting tensile stress, primarily is to keep a control on tensile strain in concrete, and it may give very small safety for strength failure. Higher values of tensile stresses are not desirable, and should be substantially lower than the values as was practiced for RCC design should be recommended. If design by limit state method is to be practiced, one has to arrive at characteristic tensile strength of concrete in structure, and also differentiate between stages of crack development and failure. For PCC design, a limit on the capacity of tank can be 10 m³. In addition, the horizontal size could be maximum 3 m; and H/t should be limited to 10 (for minimum steel is as per IS 456).

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(c) The strength design and minimum steel can be as per IS 456 and a check on tension in concrete is applied. Permissible direct tensile stress for M20 & M25 being 1.20 N/mm² & 1.30 N/mm², and allowable flexural stress 1.55 N/mm² & 1.75 N/mm² for limit state of serviceability. In addition to limit on capacity of tank as 50 m³, a more appropriate criteria can be the horizontal size of 6 m, and H/t should be limited to 20. For PCC design, the code specifies a limit on the size of tank as 25 m³. In addition, the horizontal size could be 7 m; and H/t should be limited to 15 (while minimum steel is as per IS 456).

PCC design can be considered by working stress method. For RCC design the safety is basically derived from provision of steel to take tension, and tensile stain in concrete is kept in limit to control crackwidth. Crackwidth control is necessary for keeping leakages under control. In PCC design steel provision is nominal only. The tensile stress in concrete has to control both the strength failure as well the crackwidth. Hence limiting tensile stress in PCC should be much less than that was permitted in RCC design under working stress method (or serviceability state). Thus tension in concrete should be kept low, while minimum reinforcement is to be provided as per IS 456.

Corollary is that, a member can be designed as per IS 456 only, and if tension in concrete is within specified values (for PCC), no further requirements of IS 3370 part 2 are to be checked (except concrete cover); and this will deemed to be a PCC design. Grade of concrete should conform to Table 1 for PCC, and no crackwidth check is applicable.

R 9.2.2 Shrinkage depends upon amount of paste (i.e. cementitious powder plus water) per unit concrete, and not only water. Increase of the nominal maximum size of aggregate, reduces the paste volume, and therefore reduces shrinkage. Largest size of aggregate should be used as is compatible with the cover being specified and detailing of reinforcement (space between bars) etc. To reduce shrinkage, total cementitious content should be as low as possible while meeting the strength requirement, and subject to minimum content specified in Table 1.R 9.2.3 In cases where structures under construction are exposed to high wind, high temperature and low humidity, adequate measures during the initial stages of construction shall be taken to protect its surfaces from drying, such as by covering the concrete by plastic /polyethylene or tarpaulin sheets or maintaining a cover of fog or mist. Protection from direct sun and wind also reduces evaporation. During young age (1 hour or more) reduce moisture loss and in turn plastic shrinkage cracks. The risk of cracking due to overall temperature and shrinkage effects may be minimized by limiting the changes in moisture content in concrete and temperature to which the structure as a whole is subjected. Reducing temperature gradient in immature concrete can reduce associated cracking. Curing regime involves control over variations in moisture and also temperature gradient in concrete. Avoid fast drop (steep changes) of moisture content in concrete, as well as sudden temperature drop. If changes are slow, the effect is reduced due to creep, and autogenous healing. Young concrete is more susceptible to cracking as strength is low, but creep is high which allows relaxation if changes are slow and not sudden. Restrain induces tension causing cracks, thus restrain reduction can control cracking; also by avoiding or reducing the gradient of steep changes in temperature and moisture of especially at the early age concrete. Type of shuttering, de-shuttering procedure and curing method may affect the changes in temperature and moisture. Tanks can remain wet. It will be advantageous if, during construction of such tanks, thin sections below final water level are kept damp.

Till tanks are put in to service, avoidance of drying of concrete can reduce shrinkage and associated cracking. Hence, after curing period, concrete members should be moistened at least once a day such that concrete is not allow to dry out i.e. RH not below 55%.R 9.2.4 For adequate control over cracking, if effective (reliable) and economic measures cannot be taken, movement joints can be designed to relive restrain on volumetric changes in concrete by permitting movements, which in turn could reduce cracking for large tanks. By the provision of a sliding layer, for long walls or slabs on ground, restraint can be minimized. Provided with movement joints if effective and economic means cannot otherwise be taken to avoid unacceptable cracking. Steel requirement can be economised by designing movement joints with provision of separation layer.R 9.2.5 If development of cracks or overstressing of the concrete in tension cannot be avoided, the concrete section should be suitably reinforced. In making the calculations either for ascertaining the expected expansion or contraction or for strengthening the concrete section, the coefficient of expansion of concrete shall be in accordance with the provisions given in 6.2.6 in IS 456.R 9.2.6 For first filling of the tank, the rate of filling should be slow to avoid sudden change (shock) in stress, and allowing time for creep to adjust the strains, while progressive crack development to be slow, also allowing autogenous healing of cracks, and thus marginally reduce the possible crackwidth and leakages. Hence increase in the water head per day is specified. However this does not mean that water can rise 1.2 m within few hours in a day. During a day also per hour rise may not be more than 20 cm. Also see R 9 (autogenous healing).R 9.2.7 For reducing the possible crackwidth by way of diffusion of cracks (i.e. more numbers of cracks at smaller crack spacing) use of small size reinforcement and at smaller spacing is recommended. It is always preferable to have size (diameter) of bar as small as possible without causing congestion. Spacing of bars less than 5× diameter of bar (or 80 mm) have no specific advantage. It is preferable to have clear spacing between

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bars more than 2.5× nominal maximum size of aggregate. Code does not specify any size of bar as minimum for use as main reinforcement. R 9.2.8 For ground supported tank, reinforcement in floor slab can control cracks by one of the possible two types of strategies- continuous restrain or no restrain. External restrain can be at two end, or at few locations, in between these restrains, and the member will develop tensile stress and crack. If a member is continuously retrain externally (say by ground below the member i.e. founding strata), stress will not develop in the member and cracking may not occur. R 9.2.8 a) Cracks are caused by strains due to volumetric changes (i.e. temperature, shrinkage, moisture movement etc.) when restrained. External restrain if continuous, cracking is controlled. For ground supported tanks, continuous external restraint is offered by the friction due to roughness between structural member and ground (via screed layer). However, continuous restrain may not be reliably full, or may vary slightly along length, and in such cases some stress can occur at different locations, which reinforcement can resist. Thus continuous restrain reduces the requirement of reinforcement to control crack due to temperature-shrinkage-moisture variations. Reinforcement can be taken as internal restrain controlling cracks, and external restrain reduces the reinforcement. For construction without joints, satisfactory control over cracking can be obtained by increasing the reinforcement, which provides internal restraint. Requirement of reinforcement also increases as the length increases more than 20 m. Minimum reinforcement specified in 8 of IS 3370 (Part 2) is for this combination.R 9.2.8 b) If the restrain on member is reduced, it will be free to move, stress and hence crack will not develop. To accommodate movement, movement joints are provided in the floor slab of tank. Restrain from ground below should be reduced, so that parts of slab between movement joints can move (shrink or expand). To economize reinforcement, movement (or partial movement) joints are introduced. For joints to accommodate the movement, external restraint is to be reduced by providing a separation layer between structural member and ground support. Movement joint will function only if restrain on slab is reduced. For this a separation layer should be put below slab, such that slab panel can move (shrink) on this layer. To allow slip between structural layer and supporting material below (lean concrete), the top surface of this supporting material shall be in-plane and smooth, which normally is achieved by float finish. To reduce friction and allow movement, slab bottom should have plain and smooth surface. For separation polyethylene sheet is provided of enough thickness which can allow free movement of RCC slab above. The roughness of sub-base below slab can provide interlocking friction, hence sheet thickness is proportional to the roughness, which is usually 1 mm thick as per British specifications. The required thickness can be provided by using single sheet or two sheets. The sub-base i.e. PCC layer (below LRC slab) must have smooth finish and surface in a perfect plane without projections which may otherwise act as key. However smooth surface is made, a small friction (restrain) may act, and some reinforcement is needed to control crack due to that reduced restrain. Usually bottom surface of tank floor is not plane. Floor is provided with slopes, pockets, & extra thickness at different places. All these restrict the possible movement, and therefore positions of these features govern the locations of the movement joints. For each panel between successive movement joints, the bottom surface of structural concrete (floor slab) should be plane. With introduction of joints, the requirement of reinforcement is to be estimated as per 11.3 & Annex A of IS 3370 (Part 2).

R 9.2.9 Structural Fibres : Fibres control plastic shrinkage cracks, as well temperature-shrinkage cracks in young age of concrete. Structural fibres like macro polymeric (tensile strength >450 MPa) or steel can improve the dispersion of cracks due to loads or restraint in service life, thereby reducing crackwidth and also enhance the toughness, ductility and durability. For control of plastic shrinkage crack, minimum dose of fibres should be such as to give average residual strength 0.30 N/mm² tested as per ICI-TC FRC 01.1 (or EN 14651 part 1 or ASTM C1609); and dose should not be less than 1.3 kg/m³ for polymeric fibres. For structural purpose (i.e. to account enhanced flexural strength and toughness) the minimum fibre dose shall be such that an average residual strength 1.50 N/mm² is achieved. If average residual strength (fem,150) is ≥ 1.0 N/mm², the tension stiffening can be assumed to be enhanced by 1.2 times, for calculation of crackwidth. For FRC having average residual strength (fem,150) ≥ 1.50 N/mm², the tension stiffening can be assumed to be enhanced by 1.5 times, for calculation of crackwidth. Actual benefit of fibres are much higher, which can be accounted if detailed procedure as per accepted methodology can be applied. At construction joints, the contributions of fibres shall be neglected.

R 9.2.10 Occasionally leakages may be noticed through cracks or construction joints etc., and are unacceptable. These porosities can be filled by grouting. For grouting, cement based grouts are commonly used. For grouting generally tank is to be almost emptied. However, there are methods, expertise available and specific grout materials which can be used while leakage is taking place. Grouting is a cyclic process. The first leakage appears from a path of least resistance. Once a water passage path is grouted, probably leakage may appear from other nearby path, requiring grouting of this alternate path in next cycle. Normally two to 3 cycles of grouting may be required, and grouting cycles may be more for higher H/t ratio. Depending upon severity of crack (or leakage) other grouting materials may be used. For selection of grouting material and method of grouting refer ISO 16475-2011 Guidelines for the repair of water-leakage

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cracks in concrete structures. Grouting of cracks by polyurethane foam can be done without emptying the tank, but may not give long term remedy and hence it is to be supplemented by other materials of grouting and sealing.

R 10 STABILITY General principles as per IS 456 and guidelines given here are applicable. Underground structures should

also be designed for floatation (uplift) action as applicable. See R 8 c. Design for uplift involves not only the stability check, but also the design of components for the force actions during uplift condition. Tanks which are not symmetrical may be critical for overturning check, while subject to uplift. The equilibrium and safety of structure and parts of it against sliding and overturning, especially when the structure is founded on or adjacent or sloping ground, shall also be checked. Also with earthquake induced forces, the resistance to sliding is important for sloping terrain.

R 10.1 Overturning: The stable equilibrium of a structure as a whole against overturning shall be ensured so that the restoring moment shall not be less than the sum of 1.4 times the maximum overturning moment due to the characteristic loads (including DL & imposed load & excluding earthquake/wind), and 1.2 times the maximum overturning moment due to the characteristic wind or seismic action. During overturning check, for buoyancy (uplift/ floatation), 1.25 be the load factor. In this check, load factor for liquid will be 1. Liquid (FL) in tank may be any value between empty to full capacity as may be critical, and imposed loads can be neglected unless it contributes to overturning moments. Earth pressure when contributing to de-stabilizing forces, will have a higher load factor of 1.6 times. In cases where dead load provides the restoring moment, only 0.9 times the characteristic dead load shall be considered. Restoring moment due to imposed loads shall be ignored if its contribution in overturning moment is neglected. Check may be critical for combination of wind load or seismic. Restoring action due to earth pressure should consider minimum possible, and only for active state [refer R 8.1 a (i)], with load factor 1. For stability, the load combination: (1.2 or 0.9) DL + 1.0 FL + 1.2 WL(or EqL) + 1.4 IL As per IS 1904, factor of safety against overturning is 1.5, and 2.0 while only DL, IL & EP are considered.

R 10.2 Probable Variation in Loads: To ensure stability at all times, probable variations in dead load, liquid load and earth pressures during construction, repair or other temporary measures shall be taken into account. Load factor for DL may be taken as 0.9 or 1.2, as may be more critical. Similarly water load may be nil or in part or full with load factor 1, whichever gives more critical combination. Provisional dead load may be neglected, if DL helps in stabilizing. Wind and seismic loading will be treated as overturning or de-stabilizing loads. R 10.3 Sliding: The structure shall have a safety factor against sliding of not less than 1.4 under the most adverse combinations of the applied characteristic forces. In this check only 0.9 times the characteristic dead load shall be taken into account. Structure can be assumed to be subjected to maximum earth-pressure from one side, and possible earth pressure from opposite direction can be taken as minimum possible or may be neglected. Load factors as in 10.1 can be used. The factor of safety against sliding shall not be less than 1.75 when earth pressure contributes to sliding force. As per IS 1904, factor of safety against sliding is 1.5, and 1.75 while only DL, IL & EP are considered.R 10.4 Moment Connection: In designing the framework, provisions shall be made by designing adequate moment connections or by a system of diagonal bracings to effectively transmit all the destabilizing forces to the foundations, with enough factor of safety, such that the structure will act a rigid structure. All joints and junctions (connections) of members shall be designed and detailed so as to avoid disproportionate deformations, wide cracks, and failures within junctions.R 10.5 Structural integrity: Design shall provide general structural integrity, directly or implicitly. Localized damage or deterioration shall not impair general structural integrity of the structure. Structural system shall be such that sequential failure should be avoided. Structure should be robust also. R 10.6 Buoyancy (uplift) shall be considered as per 8 c.

R 10.7 Robustness : It is the ability of the structural system to fulfil its function during events like small accidents or due to human errors. The structural system should be able to serve, without being damaged to an extent disproportional to the cause of the damage.

R 11 DESIGN, DETAILING & WORKMANSHIP AT JOINTS Joints are provided to break the structure in sections (or parts) convenient for construction, to control possible cracking resulting from excessive stresses and strains (due to shrinkage, temperature changes etc.), and to comply with design assumptions. Design joints for satisfactory performance and maintainability over service life.R 11.1 As far as possible joints can be minimized or avoided in LRC. Joints are source of weakness, leakages and also positions of maintenance. The recommendations about movement joints (expansion and contraction type) are drafted basically for ground supported tanks. For elevated tanks, usually movement joints are avoided, and the restrains for temperature-shrinkage movements are far less compared to ground supported tanks. Movement joints are planned to reduce the reinforcement requirement, and also cracking. In a structure, positions of all the proposed joints, should be structurally checked and specified by the designer. The joints shall be placed at accessible locations to permit inspection and maintenance. Most of the

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joint materials and sealants have a smaller life compared to concrete structure. Hence replacement of materials at joint, sealants, and maintenance for leakage prevention should be possible. Number of joints, including construction joints, should be as less as possible, and their total lengths should be minimised. Concreting shall be carried out continuously between the joints planned. For a joints to be added or deleted or alterations in the detailing of joint, as proposed by the construction engineer or contractor, should be reviewed by the design engineer after considerations of effects on the structure’s design, crack control and performance of the joint and such alteration will be carried out only on approval by the designer. Location of joints shall be as per drawings and positions approved by designer or a competent engineer. Joints should be designed and constructed to prevent concrete cracking, spalling, and corrosion of steel, prevent leakage and resist chemical attack. The design of movement (expansion and contraction) joints should take account of the ability of the filler, sealant, and water-bar materials to sustain cycles of deformations, hence these materials cannot be substituted without verifying the design.

R 11.2 Joint Types: Type of joint depends on the number of degrees of freedom for movements to be permitted across the joint, and restrains to be provided, as well as the design and detailing for preventing leakages through the joint. However joints can be many more types, configurations and details. Common joints can be categorized as given in following and are dealt here

R 11.2 a) Movement Joints : Movement joints provide substantial structural weakness in the member which is more than that due to construction joint. Movement joints are to be sealed, such that while accommodating & permitting movements, passage of liquid (as leakage) does not take place from one face of member to other face, and in some cases vice-versa also. The joints must be sealed on liquid face, or on both faces for underground structures, by providing sealant in a small groove. Sealant shall be bonded to the concrete on only two opposite sides; and on the third side of the groove it should not be bonded to concrete at back. Considering volume changes in concrete, joint should be designed such that it should remain functional over the service life with the desirable amount of maintenance. Different types of movement joints provide partial structural continuity (few restrains) or non across the joint. In elevated tanks the restrain to linear expansion or contraction is negligible and hence movement joints are not required. Mostly movement joints are provided in large ground tanks. At all movement joints, water-bars are required. Water-bar in most cases will be at middle of thickness. For horizontal slabs resting on grade, it is convenient to provide water-bar along the bottom surface of member. Type and detailing of movement joint depend upon number of degree of freedom for movements, some are free, some are restrained and others indirectly restrained.

Joints can be of following main types.

(i) Expansion joints are designed to accommodate both linear expansion and contraction. It is a discontinuity in the structures. All reinforcement shall be terminated at the joint, and none will pass through it. A designed gap is provided between the two parts across the joint. It may be of the order of 15 to 25 mm. In the gap a compressible preformed joint filler and a water-bar are to be provided. A sealant is bonded to the two sides. The gap is to be designed for the movement expected and the compressibility (& extensibility) of filler and sealing material. Due to movement of the structure the gap and the filler should either expand (i.e. open out) or contract. The joint may be provided with dowel bars to restrict relative movements parallel to the plane of the joint. Such dowels can resist shear across the joint and avoid faulting. Water-bar should have a central bulb to allow in plane movement. Joint shall be provided with sealant on the face of liquid retention. Joint is supposed to have no strength across it otherwise (except shear strength of dowel).

(ii) Full contraction joints have discontinuity in concrete laying and also in reinforcement which may otherwise across it, and does not have any gap. It does not provide resistance to tension across the joint also flexure. Shear strength at the joint reduces significantly. When concrete contracts, the joint will open up forming a crack, being a weak & preferred place for crack to appear. The joint is created by discontinuity in the concrete laying operations on the two sides of the joint. If at the joint, the surface of concrete cast first is not made rough, loss of strength across the joint will be almost total. To transfer shear force across the contraction joint, shear key or dowels may be required. At smooth joint very little flexural strength or no shear strength should be assumed.

All contraction joints shall be sealed at the surface, on the face retaining liquid. For shear transfer dowel bars can be provided at full or partial contraction joint. At full contractions joint with discontinuity of reinforcement water-bar is required. At induced contractions joint with full continuity of reinforcement normally water-bar is not provided. At partial contraction joint, part reinforcement (about half) may be terminated.

Induced contraction joint with discontinuity of reinforcement: Across the joint reinforcement is not continuous, however concrete laying is continuous. In fraction of thickness of member (say one fourth to one third of thickness), a discontinuity is introduced either by inserting a smooth material strip (groove former) at the time of laying concrete to break bond within concrete, or by cutting (sawing) a groove in the concrete. The groove former (material strip etc.) may be removable or to be left in position which may be

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of a type to also act as outer water bar or sealant. Depth of groove depends upon strength of sub-base and amount of reinforcement continuing. While concrete contracts due to shrinkage strains and temperature fall, the section of member at the joint being relatively weaker will attract crack. Flexural and shear strength of concrete section through joint gets reduced (by say 50 to 80%). This joint can transmit some shear force. For more shear transfer dowel bars can also be provided. At the surface on liquid side the joint should be sealed by provision of joint sealant for achieving low permeation through the joint.

On each side of the contraction joint (without water-bar), for a width not less than the thickness of member (or >150mm) waterproof coating may be applied. Full contraction joint (reinforcement not continuous) should not be provided (except floor slab on grade) where pressures are high (H/t >20) or where joint is required to be gas/air-tight.

Induced contraction joint with continuous reinforcement : At the joint location concrete laying is continuous, and also the reinforcement. In fraction of thickness of member a discontinuity is introduced by either inserting a smooth material strip at the time of laying concrete, or cutting (sawing) a groove in the concrete. Section of member at the joint becomes relatively weaker attracting crack at the section, when concrete contracts due to temperature & shrinkage strains. Flexural and shear strength of concrete section through joint reduces (by say 40 to 70%) compared to a section through monolithic member, though not as much as the type above. It should be noted that behaviour at this type of joint is similar to the construction joint in some regards. For a length of about 150 mm on each side of contraction joint, all the reinforcement bars should be coated by epoxy as a protection from corrosion of bars where it is continuing through the joint. At the surface the joint should be sealed for achieving low permeation of water across the joint.

(iii) Sliding joint significantly reduces restrain for shear across the joint. In most of sliding joint restrain for moment transfer is also very small. Typically this type of joint is used at base of wall.

Above list is not exhaustive, and other types of movement joints are also possible and in use. For contraction joint only one linear relative movement (perpendicular to the joint) is provided for. No

planning (or design) is done to reduce restrains for other movements. To avoid faulting (shear movement), dowel bars or shear key may be designed. Dowels are not required in all cases.

Figures given in the code should be treated as examples only. Modifications and alterations are possible. Alternate materials are also available and can be used. For horizontal members like slab, water-bar at the middle of thickness should be avoided. It is difficult to achieve proper workmanship for the concrete placed around the water-bar. Therefore good amount of details and sequence of construction operations are to be planned such that honeycomb and un-compacted concrete is not possible. The water-bar (wide strip type) at middle of member less than 300 (preferably 350) mm thick, will pose lot of workmanship problems, as well as it will reduce strength of section. Hence for high H/t ratio thickening of member at the joint with water-bar should be considered. If at the joint water bar, dowels or shear key are also required, the thickness requirement will be ≥ 400 mm.

Unplanned construction joint (cold joint) at the ends of water-bar should be avoided. Water-bars (or water-stops) are needed where water head is high (H/t > 30 for good workmanship, & 25 for average). There the main function of water-bar becomes the transfer the responsibility of seepage from designer to constructor. No leakage at joint because of water bar, but poor workmanship is not an acceptable situation. Hence as far as possible the use of water-bar should be avoided for medium hydraulic gradients, and seepage should be reduced by grouting the joint adequately. It is preferable to increase thickness of member at partial contraction joint, than considering the provision of water-bar. Refer R 11.5.2 for water-bars.

For movement joints in slab on grade (LRC slab on PCC base), the bottom surface of the slab shall be plane and smooth, more specifically flatness and smoothness is more important near the joint (portion >1.5 m on each side of joint). Smoothness shall be of an order which can allow the two surfaces to move (slide) relative to one another, and friction if develops may become a restrain. The slab should be de-bonded from PCC base to facilitate movement of the slab.

Where movement joints are designed for the tank bottom supported on grade, it should be ensured that bottom surface of the slab is plane, smooth and without any projection or key, pockets stiffeners or foundation thickening. The projection /local thickening /key will act as anchor and will restrict the movement, thus defeating the purpose of providing movement joint. Hence in many cases it may be advisable to avoid movement joints by providing continuous restrain by grade (layer below say base PCC), and enough reinforcement.

Each joint should be designed for the estimated movement, strains on the material used in joint, estimated life of jointing materials, and method of maintenance. Shear keys should not be provided in full contraction joint or expansion joint, however corrosion resistant smooth dowels can be designed for transfer shear. Alternately shear transfer devices, allowing expansion & contraction, can be used. Half the length of dowel on one side of joint shall be kept un-bonded to concrete allowing expansion or contraction of the joint.

With a sliding wall base joint, include a bearing pad and flexible water-bar in circular prestressed tank.

R 11.2 b) Construction Joints :Construction joint is introduced for giving a break in pouring of concrete, or for convenience in construction,

at which measures are taken to achieve some continuity without future provision for relative movement i.e. to act as monolithic member across the joint. It is also called as cold joint.

Construction joints are positions of structural weaknesses in the member, hence it is desirable to minimize

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their total length in the structure or avoid them. These should be located where the effect of weakness is least and does not affect the performance. In general, these can be located where shear stress and flexural cracking is small. In slabs and beams it can be within 0.2 to 0.33 times the clear span of member from support, i.e. nearby the point of contraflexure. Joint may be provided at one of these locations, if not specified by designer on drawings. As far as possible, construction joints should not be made at critical sections. Along the joint, direct shear strength reduces and also liquid percolation may increases, depending upon the treatment, detailing and workmanship at the joint. The position, arrangements, and the treatment of the construction joints should be specified by the designer and indicated on the drawings. At the joint location the strength adequacy should be checked by the designer. In horizontal members like slabs or beams, the joint can be nearly vertical. For vertical members like walls or columns, the joint should preferably be horizontal. Designer may specify joint at an inclination if designed. The joint should continue along the same alignment in to the adjacent panel or member to avoid additional sympathetic cracks. Time lag between the two concrete phases is an important parameter for the behaviour of the joint. It is to be measured from the instance of mixing of concrete of first phase, to the instance when second phase concrete is compacted against the first phase concrete already laid. When the time lag is less than that of 80% of the initial setting time of first phase concrete, and also while second phase concrete is still plastic, it will result in a monolithic concrete across the joint, and loss of strength at interface is significantly small. When this condition is not satisfied, the construction joint is assumed to form. The time lag may be more than an hour or may be in days. As the time lag increases, the behaviour of the two concrete are in different phases of setting, hence effects of temperature (due to heat of hydration) difference, and also differential shrinkage set in. Thus due to differential movement (strains), slip develops at the joint. The bond between the two phases of concrete will reduce, formation of crack and its development takes place at lower level of strain. Hence the design and specifications at the joint will depend upon the probable maximum time lag at the joint. More the time lag, less will be the bond between concrete of two phases. In construction joint, if along the interface shear is higher than friction capacity, or tension across is significant; and the time leg is more than the period in which >70% of hydration of old concrete has taken place, the interface should be applied with a bonding material. For bonding two types of adhesives are available: one the resin-based adhesives whose base material is epoxy resin or acrylic resin; and second cement-based adhesives such as polymeric hydraulic cement mortar. The bonding strength of an adhesive differs depending on the moisture state and roughness of the bonded parts of the existing concrete members as well as on the environmental conditions including temperature, humidity and wind. Therefore, these should be taken into consideration when selecting the material. The adhesive is required to shrink little (minimum possible) when hardened and be of high quality in terms of water-tightness, heat resistance, chemical resistance, etc. while ensuring the usable time required for application. The integrity and durability of bonding material should be ensured through the service life of structure under the actions of load and environment. The bonding property should not changes much over time and being resistant to the actions of degradation. The resin-based adhesives, are empirically known to excel in energy absorption until the deformation limit is reached, can ensure integrity in longer term than those with higher strength properties. For bonding materials (including polymer hydraulic cement mortar), the characteristic values of the material need to be determined under the temperature condition appropriate for the usage environment. Elastic modulus of bonding or inter-joint material should be lower than the concrete it is bonding, but staining capacity should be higher. Thin (<3mm) bonding material has very little influence on the stiffness of the member or the joint; whereas thicker has some. Along the plane of construction joint the permeation of water, solutions, chemicals, gases or ions can be higher, which may cause loss of durability. Hence bars lying or continuing (within or parallel to joint) in this region can be more prone to corrosion and loss of durability. Any steel bar or a ring shall not lay in a construction joint, or be parallel to it within 25 mm on either side. If a bar is in this region, it should be moved away from it, or the joint should be planned a little away from the reinforcement or joint be kept between the parallel bars. Bar in the plane of construction joint has poor bond and is susceptible to higher width of longitudinal crack along the bar. The construction joint should be grouted, and on the water face should be sealed to reduce entry of water etc. unless H/t ratio is low. Enough reinforcement should cross the joint, resisting its opening. Performance of the joint will depend upon the condition of the existing concrete regarding cleanliness, roughness. If it is a very old concrete surface, conditions regarding cracking, tensile strength, chlorides and other aggressive agents, depth of carbonation, moisture content, and temperature. Also on the required conditions of fresh concrete, such as ambient temperature, humidity, wind force, precipitation, and any temporary protection. Concrete surface preparation deals with the various operations needed to fulfil the requirements. An important aspect is the roughness of the interface, the amplitude of which should be 3 to 5 mm (for 20 mm maximum size of aggregate in concrete). This roughness locks the relative movement between the two sides of the joint.

Normally construction joints should not be made at critical sections, and if proposed at a critical section detailed checks would be needed. At construction joint, concrete section should be checked under direct shear for shear friction resistance. Construction joint at bottom end of wall, is most critical location where section is subjected to high moment, high shear and susceptible to high crackwidth, hence at this joint adequate design is to be worked out. It is more usual that construction joints are permitted where the stress due to direct shear (or punching shear) is quite low. Shear key or shear dowels can be provided where roughness is not enough for the

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required shear resistance. And these shall be provided only if designer specify it on drawing. Dowels can be of steel or other suitable materials. In old hardened concrete post-installed anchors can be provided to act as dowels.

Designers should check (during process of design) the adequacy of stiffness, strength and durability at the joint. Construction joints definitely introduce weakness (in strength and stiffness) in the structure as is known to influence the performance of TG foundations. There is experience of significant reduction of stiffness (due to poor modulus of elasticity of concrete in the region of joint) of staging for elevated water tanks; and after thoroughly grouting all the construction joints in columns, the performance (natural frequency and amplitude) improved to desirable level. The data on influence of construction joint on strength capacity and stiffness is not available. This does not absolve the designer from the responsibility of dealing with the weakness introduced by the joint. In past, research has not always preceded the engineering wisdom, and later it improved the subject from understanding to knowledge.

The account of construction joint (and stiffness reduction due to that) is normally not taken in the structural analysis. Hence during construction the full structural rigidity and continuity should be realized by providing enough roughness, good quality concrete (low porosity and water cement ratio) and by grouting. Depending upon its type and expected performance, the designer should check the structural integrity, strength and possible crackwidth at the construction joints. Shear strength reduction may be from 40 to 70% related inversely to the roughness at the joint interface, detailing and workmanship at the joint. Flexural strength reduction may be assumed as 20 to 40%. Hence at all specified positions of construction joint, shear strength must be checked. Reduction of tensile strengths (both direct & flexural) are substantial across the joint, thus the estimated crackwidth may be increase (1.2 to 1.5 times). Hence if a joint is proposed, the possible weakness should be accounted in design.

In concrete members construction joint coinciding with section where shear is high, should be avoided, and if provided should be checked by the designer for adequate resistance to shear stress. Loss of strength (due to water-bar) in the region of joint should be accounted in design if provided with water-bars, and detailing should be prepared such that it does not interfere with the reinforcement. Construction joint should have offset of minimum two times the width of beam, from beam junction.

Before the placement of second phase concrete at the joint, inspecting engineer’s approval of preparation of joint interface shall be obtained.

In wrapped prestressed structures, horizontal construction joints shall not be permitted in the core wall between the base and the top, without a water-bar.

The behaviour at this type of joint is supposed to be different than the induced (partial) contraction joint, however expected difference is small only if enough roughness is given to joint. The joint may also need sealing on liquid face, specifically where across the joint interface there is no compressive stress under the load combination, or liquid pressure is acting with high H/t ratio >25, or at the joint of wall bottom (where moment, shear and direct tension are present) if H/t ratio >20.

It is not necessary to incorporate water-bars in properly executed construction joint, unless H/t ratio is more than that in R11.2 (b) (i) depending upon possible workmanship.

R 11.2 (b) (i) H/t ratio limit for construction jointWorkmanship H/t at other joints Grouting needOrdinary 20 Occasionally one cycleAverage 25 1 or 2 cyclesGood 30 2 or 3 cyclesExcellent 35 3 or more may be needed

Note : Above limits will reduced if equivalent thickness (te) is considered in place of t. Where bending moment, shear & direct tension are present, the above H/t values can be reduced by 5, in absence of reduced equivalent thickness for accounting tension. Example of such joint is at wall bottom with floor slab of elevated tank. At important joints (having H/t >25) having high stress (example wall bottom subject to bending, shear and direct tension also), the tensile stress in steel should not be >190 MPa in limit state of serviceability. For a particular H/t value, stress (compression or tension) on the construction joint affects the seepage through joint (progressively increasing).

1. Whole section in compression, no tension.2. Tension on one face & compression block ≥ 50 mm.3. Tension on one face & compression block < 50 mm.4. Tension on whole section (i.e. on both faces).

For understanding the performance affected by H/t ratio, this ratio would be modifies for framing better criteria. Leakage through crack is also related to width of crack. Stress (compression or tensile) perpendicular to the construction joint (or a crack) has an influence of leakage. Part or full section (interface) may be subjected to compression, in presence of which the leakage will reduce. Hence an equivalent thickness can be accounted which is enhanced for part of section in compression. If N is the size of compression block (depth of neutral axis), tension part = t – N ,

Equivalent thickness = te = t + N /4 = t + 0.25 N {constant 4 can reduce up to 2}. The limiting hydraulic gradient (H/t) through monolithic member (away from any joint) can be much higher (say 2 times). Above the limiting H/t ratio, water-bar is required to reduce the leakage through construction joints. It is

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preferable to provide higher thickness, compared to providing water-bar. Refer 11.4.1 for construction details.

R 11.2 c) Temporary Open Joints (Gap Joint) :Along a member like wall or slab, a temporary gap can be kept during construction, which will be filled in by

structural concrete lately, and before the scheduled time for putting the structure in service. The width of gap should be of the order of 0.8 to 1 m. This gap facilitates to lap the reinforcement extending in to the joint from two sides. Usually there is no advantage of keeping it much wider or narrower gap. This temporary gap accommodates the contraction of adjoining concrete lengths on each side, due to temperature effect in young concrete and also partially shrinkage taking place till the time of filling the joint. Through this gap the reinforcement can act as continuous by lapping. If the length of concrete member on both side of gap is more than 14 m, it may be necessary to provide laps for all bars within the gap, to reduce the restrain on contraction of concrete. As all bars are lapped within this small region, lap length should be suitably enhanced, and distribution bars (perpendicular to lap) should about 1.3 times the minimum reinforcement. To allow the contraction of adjoining concrete to the maximum possible extent, the gap can be filled in as late as possible, and after few days of drying of the concrete on each side. The interfaces at the gap should also be treated as a set of two partial contraction joints.

R 11.3 Design and detailing of Joints : This is with reference to movement joints for ground supported tanks.

R 11.4 Spacing of Movement Joints :The concept of providing movement joint is to allow movement of concrete in a panel by substantially

reducing the friction between the concerned RCC panel and its sub-base. For permitting expansion or contraction of panel between movement joint, the top of PCC sub-base must be perfectly in a plane and smooth. Flatness and smoothness of the surface must be specified, controlled and achieved in the construction.

Even if the surface which may appears to be smooth, has very small roughness (seen under a magnifying lens) of sub-base top, the concrete above will set and friction will develop, thus resistance will be offered to free movement and the purpose of providing movement joint may be partially defeated. Hence a separation (or bond breaking) layer is put in between. The thickness of such a separation layer should allow friction-less in-plane (shear deformation) displacement between bottom and top layers. Normally 1 mm thick polyethylene sheet is specified with smooth top sub-base. Thickness of sheet is directly related to the roughness (on finer scale) of the top surface of sub-base. The quality of flatness and smoothness of surface allowing sliding should be specified.

It should be noted that projections of structural concrete below the plane interface shall lock the movement, and induce unwanted cracks. The sub-base PCC below the RCC floor slab should have specification such that it can be finished smooth. For good finishing-ability of base PCC it should have enough paste and finer aggregates.

The options for movement joint are for the cases where restrain to expansion and contraction movements are present. Following are the design options. (a) Option 1- Within the area of continuity, intermediate movement joints are not planned. Design assumes

restraint. Cracking behaviour is controlled by provision of enough reinforcement (normally high amount) which has smaller spacing with minimum possible size of bar. Construction joints will induce crack pattern, and are to be sealed. This option is preferred for tank up to 22 m size. For higher size tanks reinforcement has to be designed more cautiously.

(b) Option 2- Movement joints are introduced, therefore the amount of strain to be controlled by reinforcement reduces, and thus the requirement of reinforcement can reduce. Crack pattern is induced by spacing of movement joints.

(c) Option 3- Cracking can be controlled by close interval of movement joint, thus significantly reducing the requirement of reinforcement. Cracks in between movement joints if develop will be significantly small. Total freedom for movement is very difficult to be achieved and hence some reinforcement should also be provided.

Table 2 Column 4: In option 1, the steel ratio can be significantly higher say >1.4 times ρcrit . Calculation for ρcrit will be as per Annex A Part 2.

R 11.3.1 The steel ratio ρcrit as calculated (for 415 grade) will be as below. This refers to ground tanks only.fck M20 M25 M30 M35 M40 M45 M50 M55 M60fct 1.00 1.15 1.30 1.45 1.60 1.70 1.80 1.85 1.90

ρcrit 0.18 % 0.21 % 0.23 % 0.26 % 0.29 % 0.31 % 0.33 % 0.34 % 0.34 %Option 1 0.25 % 0.29 % 0.33 % 0.37 % 0.40 % 0.43 % 0.46 % 0.47 % 0.48 %Option 2 0.18 % 0.21 % 0.23 % 0.26 % 0.29 % 0.31 % 0.33 % 0.34 % 0.34 %Option 3 0.12 % 0.14 % 0.16 % 0.17 % 0.19 % 0.20 % 0.22 % 0.23 % 0.23 %

For elevated tank, refer steel % as for option 2 in above table. (This table in not numerically part of code, but will help in understanding the effect of concrete grade.)

[ Notes : Even if full contraction joints are provided in option 2 or 3, certain situations demand higher temperature-shrinkage reinforcement; e.g. except lightly loaded columns (supporting roof), high vertical load loads prevent free movement of floor

between two such columns, & needs more reinforcement to resist cracking. ]

R 11.4.1 Horizontal cracks in free standing wall.

R 11.4.2 Vertical restrain : Walls are considered to have reduced restraint in the vertical direction because their

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own weight helps to reduce shrinkage and temperature stresses. Hence for vertical steel (except in bottom portion) the requirement can be reduced significantly say by 33% (similar to option 3 irrespective of height of wall). Similarly spherical dome can function without cracks with reduced % of steel.

R 11.5 Making of JointsR 11.5.1 Construction Joints (Also refer 11.2 b.)

At construction joints measures are taken to achieve subsequent continuity without provision for further relative movement at the joint. Actions are required to prevent shear displacements by imparting enough roughness to the interface. Similarly enough of steel be provided across the joint to prevent the opening of joint (as crack opening or crackwidth). Measures are to be taken to minimise the weakness getting introduced at the joint affecting the strength and leakage.

Concreting operation should be carried out continuously up to the pre-planned construction joint. During construction, a joint at a specified location can be avoided. However, to introduce or shift to a new location, approval of the designer is necessary. At the proposed location, the designer should check the stresses.

In each phase compact the concrete fully to remove macro pores, reduce porosity, and without segregation, to get an impermeable joint. As the time gap between the two phase increases, the temperature–shrinkage strains in the two phases of concrete are different, and differential strain is locked forming an interface at the joint. With increasing time gap (may be in hours to days), the severity of the problems of construction joint also increases. The first phase concrete is older, and much of the hardening of concrete has taken place, and undergone contraction due to temperature fall and shrinkage strain. Over the old concrete, when the new concrete is laid, it will undergo temperature-shrinkage strains, while stiffness (E value i.e. modulus) of new concrete is very low. Thus movements take place under shear stresses, during hardening of concrete and the interface tend to become un-bonded with high permeability, and permitting high slip movement to shear stress ratio (normally interpreted as low shear strength), thus the crackwidth may be high due to shear.

At the joint, two concreting phases are with a time gap. If the time gap is less than 80% of the initial setting time of first concrete (or 30 minutes before its setting, whichever is late), the concrete can become monolithic, and cold joint or construction joint is not formed. Independently concrete in each of the phases should be fully compacted without segregation, to result in minimum possible permeation through the joint.

In first phase care is needed to fully compact the concrete, simultaneously avoiding loose aggregates at the joint surface. For horizontal joints, while ensuring full compaction, there is a possibility that due to over vibration, top few millimetre thickness of concrete at the joint may have coarse aggregate sunk and only mortar remain in top portion. In top few millimetre thicknesses water cement ratio may also be higher due to bleed water. At such horizontal surface, at an earliest, laitance, mortar layer, portion having bubbles, portions of un-compacted concrete if any, any loose material / aggregate or aggregates having cavities around them should be removed, exposing coarse aggregates. For formed surface at vertical joint, side form should be removed as soon as possible, and the joint surface should be roughened. At and near the interface the quality of concrete should be satisfactory. Preparation of surface of existing concrete is the process by which sound, clean, and suitably roughened surfaces are produced in areas to receive further concrete. It includes removal of unsound concrete material, bond-inhibiting foreign materials from the exposed concrete, and reinforcement surfaces; opening the concrete pore structure by removing surface laitance, paste or mortar layer, scaling etc.; and clean the projecting reinforcement (of unsound material or paste coating of higher w:c ratio) if required. Concrete must be removed if affected by spalling, delamination, disintegration, or portion unsound or if with severe cracking due to corrosion of reinforcing steel. In addition to unsound concrete with reduced mechanical integrity and/or contamination, a bit sound concrete can also be removed as needed to provide adequate solid surface and geometry. The effectiveness of various concrete removal techniques may differ for unsound and sound concrete, and a combination of techniques may be necessary. However, the methods used to remove the deteriorated or contaminated concrete and prepare the concrete and reinforcement to receive fresh material must not weaken the surrounding sound concrete and reinforcement. The surface region (2 to 5mm layer) may have high water: cement ratio (i.e. low strength & stiffness), more mortar and absence of enough aggregates, honeycomb etc., which is to be removed. Bond strength of joint depends on several parameters. If in existing substrate (old concrete) damaged or weak portion is removed or kept at a very low level, tensile bond strength of overlay increases with the coarseness of interface. The mechanical integrity of the existing concrete is important. Hence, damage to concrete by the impacting tools such as chipping hammers should be avoided, which may otherwise completely outweigh the benefits of an increased roughness. When using such equipment, extra steps should be taken during removal of the weakened superficial layer.

For bonding concretes on the two sides of joints adequately, the surface of the old pour should be roughened to increase the shear (friction) and to provide aggregate interlock. This can be done by applying a retarder to the concrete surface immediately after compacting the concrete in earlier phase. For vertical surface, the retarder can be applied to the formwork. However this method of using retarder is not preferred now-a-days.

At the joint larger aggregate should be exposed, leaving clean, undamaged, solid and rough concrete surface. This requires removal of some mortar from the surface which is covering coarse aggregates. The concrete surface preparation is basically to develop bond and avoid slip along the interface. The effectiveness of various concrete removal techniques may differ depending upon hardness of concrete or its mortar layer, also for portions unsound and sound concrete, and a combination of techniques may be necessary.

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If the time lag between two phases is less than the final setting time of concrete, the surface of earlier concrete can be roughened by scarification and wire brushing without dislodging or disturbing the coarse aggregates. At higher age it can be done by abrading techniques using sand blasting, shot blasting, hydro-blasting, chiselling, milling tool, small pneumatic hammers, small bush hammer, jackhammering, or by any other established method. Among the various concrete removal methods, only breakers (chipping hammers and jackhammers) and high-pressure water jets are the main options for removing concrete of significant depth. Most of the other methods are intended to remove the skin concrete and/or to texturize the surface. Small breakers (5.5 to 9 kg) are commonly specified for partial removal of unsound concrete or concrete around reinforcing steel because they cause little damage to surrounding concrete. Care should be exercised when selecting the size of breakers to minimize the damage to existing concrete and its bond to embedded reinforcing steel. Most concrete removal work is done with a pointed tool, although a relatively narrow (25mm) blade-type tool is sometimes used to remove cracked and deteriorated concrete.

Use of excessive energy, causing damage to concrete by dislodging or fracturing of aggregates or inducing cracks shall be avoided. Bruising can be minimized by exercising care in the removal process and, where possible, by avoiding the use of more detrimental techniques using high energy tools. Only hand held, or light tools should be used. System employing a small jet of water at high velocities (by pressures 70 to 310 MPa), may be used as a primary technique for removal of concrete and steel with minimize damage to the concrete remaining in place, and leaving a rough profile. Care be taken not to punch through thin concrete. Trial will be needed to choose appropriate water-jetting speed, pressure, and number of overlapping passes, depending upon hardness of surface.

The surface should have coarse aggregates projecting out from matrix. After chipping and removing mortar and loose material mechanically, the surface should be washed clean, preferably by water jet. Average roughness (i.e. half of average amplitude of roughness) about 3 mm is satisfactory, it can be up to 5 mm (for concrete MSA 20 mm), and larger roughness in usual cases is not required. Amplitude of roughness can be measured by sand patch method. However for routine jobs, visual observation is enough and experimental determination may not be done. Average roughness more than one fourth of the maximum size of aggregate in concrete, is not normally possible. Concrete surface at the joint would be prepared rough to get better interlock and to restrict relative movement (shear slip) at the interface, which will result in nearly monolithic behaviour of concrete subsequently. The abraded and prepared surface should be covered with next concrete application within 24 hours usually.

At the joint interface, in the first phase concrete, shear key or shear dowels can be provided if specified in the drawing. If the time gap at joint is very high, a bonding material can be used at the interface. (Refer R11.2b, 5th para). If a chemical bonding agent is applied (usually when existing concrete is say more than 14 days old), follow the instructions of the manufacturer. The efficacy of bond can be tested by pull out test, determining the tensile strength of the interface. It is essential to make sure that the old concrete surface is cleaned, free of contaminants, dust, fragments of concrete, or a bruised concrete layer.

Before placing fresh concrete, the old concrete should be cleaned and wetted, without leaving free water at the surface of joint i.e. it should be ‘saturated and surface dry’ (SSD) at the start of concreting in second phase. The free water at the surface, it should be removed by suction, air blow, or evaporation etc. SSD means that the porosity immediately under the surface portion (below 2 to 5 mm) is damp (to 55 to 70% RH), with no film of liquid water standing on the surface. Surface dry means saturation level <55% RH. These values of RH (relative humidity) are not sacrosanct. The aim is, not to allow old concrete to suck significant water from new concrete, whereas unsaturated pores of the surface zone allows slight penetration of cement slurry in to the old concrete, which is beneficial for bond. This condition is typically achieved in practice by soaking the substrate for a while and then allowing the surface to dry out or air blown before new concrete placement. If polymer-modified bonding materials are used, such wetting of existing concrete may not be required.

Use of bonding agent prior to the placement of overlay is to enhance the bond between the latter and the existing substrate profile. The quality of the concrete surface preparation cannot be neglected, based upon the false assumption that a poor surface quality can be compensated for by using a bonding agent. A common practice is to use thin cement slurry (say water: cement as 3:1) to wet the surface of old concrete. This practice does not serve any specific purpose (for impermeable concrete in LRC). If the slurry is very thin which does not form a layer (of any appreciable thickness), the practice is not harmful provided the water from the slurry is absorbed by the old concrete. Thick slurry will form a paste layer on the surface with high water cement ratio, and such a layer or a mortar layer is undesirable. Any mortar applied at the joint, should have very low water: cement ratio (<0.4) for enough strength and small permeability. [Only if old concrete is porous it can absorb the slurry]. If specified by designer or competent engineer, or if time gap between the two phases is more than 3 days, a bonding agent should be applied to the old concrete just before pouring fresh concrete. With the technology available today, cement based overlay having the rheological characteristics to properly wet the existing concrete substrate can easily be designed, eliminating the need of a bonding agent. Furthermore, a bonding agent may act as a bond breaker when used inappropriately.

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Application of polymeric/ epoxy based bonding agent should be as per the guidelines of manufacturer. Bonding agent should be compatible with the wet concrete, and should have tested or demonstrated capacity to improve bond. Moreover, application of a bonding agent requires a meticulous management of time; it is indeed necessary to place new concrete on the applied bonding agent while it is fresh and sticky, and before it may set. If the bonding chemical sets, it will act as debonding agent between two concretes.

Concrete of second phase as placed should be fully compacted against old concrete, without leaving any air pocket, and without causing segregation.

After the concrete of second phase is hardened, grouting of the construction joint is a good practice. Vertical construction joints in wall must be grouted with cement slurry, after a time gap as late as possible allowing part shrinkage to take place. For any grouting, cement can be mixed with fine flyash or ultra-fine GGBS. Horizontal joints can also be grouted and more specifically where H/t is high (>20) or workman ship at joint is poor.

Where ratio of pressure head of liquid to thickness of member (H/t) will be < 30, and tension across the joint will be very small or in some portion (≥50mm) compression will be present, it may not be necessary to seal the joint or incorporate water-bar in properly worked and treated rough construction joint. Water-bar need not be provided at the joint unless specified by designer in the drawing. If necessary, interface can be grouted to get a leakage free joint.

11.5.1.1 It is very difficult to get proper compaction of concrete around water-bar, and workmanship is usually poor. Use of water-bar further introduces weakness at the joint, flexural and shear strength across the joint would reduce, even if concrete around the water-bar is properly compacted. It is strongly recommended that use of water-bar should be avoided. Without use of water-bar well compacted concrete (& no honey-comb) should be obtained at the joint. If concrete at a joint is found to be porous or leaky, the joint should be grouted to make it leak-proof.

Importance of workmanship at the joint increases as the ratio H/t increases from 20 to 30. With excellent workmanship and grouting in 2 to 3 cycles for H/t ratio up to 35, use of water-bar can be avoided.

For proper workmanship at the joints with water-bar at middle, the member thickness more than 300 mm is needed. Hence first option should be, to increase member thickness at the joint rather inserting a water-bar. Better sealing options at surface are also available and may be preferred over water-bar. If H/t ratio is high (say >30 for good workmanship) water-bar will be necessary, and sufficient thickness of member should be provided to achieve proper workmanship at the position of water-bar. Where water-bar and shear-key both are required the thickness should be >350 mm for proper workmanship.

If tension across the joint is high, sealing at the surface (which is in contact with liquid under pressure) is also required.

Similar to water-bar, provision of key is associated with problems of workmanship. Provision of groove or shear key at construction joint is not required unless such shear key is designed and specified in drawings. Due to additional operations required for formation of groove or key, workmanship at the interface can remain significantly poor (incomplete compaction, high porosity, local cracking in immature concrete etc.). Good job can be done with rough joint, without key, and without water-bar. Water-bar, groove or shear key should not be provided unless designed and specified in drawing.

Normally kicker (starter) can be avoided at the base of wall having water bar. A construction joint between the base of a wall and the slab below (or foundation) may indicate the use of a kicker of wall to avoid interference between the water-bar and top reinforcement in the slab or foundation. For positioning of a water-bar an upturned keyway is not recommended because of the potential of a crack forming through the keyway width or emanating from the top edge of the water-bar (ACI 350 commentary). However better method is avoid use of water bar, and reduce H/t ratio at the joint by increasing the width of wall by providing a generous fillet.

R 11.4.1.2 Height of free fall of concrete on hard surface and segregation : There is no limit on the height of formwork, and an indirect limit is the consideration of increase in the cost of formwork. As the height increases the surface area, as well the design pressure on the formwork will also increases, thus requiring more cost. Technically the formwork should be as high as possible, to reduce number of construction joints to minimum.

There is a limit on free fall of concrete, but it depends up on the method of working and precautions for avoiding problems. At a horizontal joint, placing of the concrete of second phase involves fall of concrete. In free fall stage, concrete is susceptible to segregation. Free fall height is excluding the fall through chute or tremie pipe. Significant mortar portion may get attached to reinforcement cage and formwork, while concrete is falling (without pipe), and larger stones try to move away from each other, resulting in the segregation; which can increase with the height of free fall. While the fresh concrete falls on the hard surface, the particles of coarse aggregate rebound and collect near the surface of formwork, thus introducing more segregation. Higher is the free fall of concrete, more will be the rebound and more segregation. Thus just above the joint honey-comb is formed as seen in the cover region of concrete. After a padding layer of fresh plastic concrete or micro concrete is deposited, the aggregate from the falling concrete gets embedded in the padding concrete and further segregation is not resulted. Hence at horizontal construction joint hone-comb is seen only for few cm height if padding layer is not provided. This height of likely honey-comb depends up on the height of freefall of concrete.

The concrete placing for initial height should be done by chute /pipe without significant free-fall. This requires sufficient space between the reinforcement mesh for insertion of chute or pipe.

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For smaller members having insufficient thickness (for insertion of chute/pipe), the alternate method could be as follows. For the first pour of concrete to be placed over hard surface at a joint, maximum size of aggregate should be restricted depending upon the freefall of concrete. If the height of freefall is about a metre the maximum size of aggregate in the concrete can be 6 mm, and for 300 mm fall it can be 12 mm. This will need a concrete mix designed for the smaller aggregate size (MSA), it should be highly cohesive and should be batched and produced. However if the quantity the padding concrete needed is small, one can remove larger size aggregate fraction from the concrete supply and this modified concrete can be used. Bigger size aggregate can be removed by sieving or hand picking. If tensile stress across the joint is high, sealing at the surface (which is in contact with liquid under pressure) is required, where H/t ratio is more than 20 and water-bar is not provided in the joint. Similar to construction joint, condition may arise for load transfer across concrete to concrete interface where concrete is cast at different time, or for cases of precast elements etc., which may require shear key.

R 11.5.2 Movement Joint Joints should be designed to prevent leakage, spalling, reinforcement corrosion, and to reduce cracking due to restrained shrinkage and temperature strains, as applicable. The number, spacing, and details of joints should be designed taking full account of the physical properties and ability of the filler, sealant, and water-bar to withstand cyclic deformations as necessary. Based on the design of joint, proper material properties, and specification should be selected for joint materials e.g. filler, sealing compound etc. These should have long life, and compatible with the liquid retained. Movement joints normally have water-bar. For each type of material, follow the manufacturer’s advice, instructions and the notes of designer. Joint geometry, with the position of the reinforcement, the water-bar, and the dowel should be given in drawing. Consideration should be given to the clear distances between the water-bar, reinforcement and formwork, in view of maximum aggregate size to allow proper placing of concrete and compacting the same.

R 11.5.2.1 Contraction Joint In full contraction joint, the reduction of adhesion between the two surfaces at interface of joint may be specified. To avoid adhesion between concrete at the interface, a coating can be applied or a film/sheet is placed before pouring new concrete. However, full compaction of concrete on both sides of interface is important. For making partial contraction joint, cutting a notch or placing a groove former are common methods. Groove former (a strip) can be applied at both the faces of the slab, for enough reduction of thickness. However, top reinforcement in slab should not interfere with the depth of groove, and bottom of groove should not reach to reinforcing bar and leave it without cover. For LRC slabs are >200 mm thick, and at the location of groove the top reinforcement will have to be terminated. Hence induce contraction joints with continuity of reinforcement, are very difficult to design, detail and construct. At such joints if top steel is terminated, extra bars may be provided at about mid depth of slab. [Also acknowledged in ACI 350 commentary,] Joints must be sealed at surface. It can be provided with water-bar if total contraction expected at joint is higher (say > 0.2 mm). For water-tightness class 2, water-bar should be provided at all full contraction joint. If specified, full contraction joint shall have dowels as designed.R 11.5.2.2 Expansion Joint : Designer should design and specify initial gap, which will depend upon the expected movement (i.e. shortening of gap) and the compressibility of filler material. Filler should be fixed or adhered to concrete on at least one side. Some types of fillers are force-fit i.e. fixed in compressed form and remain in position due to friction, and these also resist the permeation of water. Water-bars are needed at all expansion joint, and dowels should be provided if designed or specified.R 11.5.2.3 Sliding Joint : For reducing the friction between two surfaces, the surfaces should be perfectly flat and smooth. Coefficient of friction should be reduced by application of suitable coating or provision of one or two layers of plastic (HDPE) sheet.

R 11.5.3 Temporary Open Joint : [ Gap joint ]

R 11.5.4 Joints in Ground Slab : The slab may have been divided in panels by use of full and partial contraction joints a per design and drawing. Between two full contraction (or expansion) joint, one partial contraction joint can be provided i.e. alternate joint can be partial contraction joint. The options of type of joints are not to be exercised during construction, but are frozen by design consideration, and in field one should follow the drawings and instructions of designer. For induced contraction joint, the work of cutting or sawing of the joints should be completed before the final setting time of concrete, as well before the temperature peak can occur due to heat of hydration. The joint cutting can start at 5 to 8 hours age and should be completed at about 12 to 20 hours of age approximately.

R 11.5.5 Joints in Wall : A kicker at bottom is not be necessary. Its provision adds one more step in the construction process. It has two main functions at bottom end of formwork- to avoid movements at the bottom location, and to reduce the possible leakage of slurry from fresh concrete during compaction. The negative aspect of kicker is to add one more construction joint. Further the quality of small quantity of concrete for kicker is normally below average and thus one ends up in poor construction. Hence without kicker, one should get good concrete by making arrangements for fixing bottom of formwork in proper position and avoid leakage of slurry.

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If kicker is provided, concrete to be poured in it can be one grade (5 MPa) higher than the specified grade. Top of kicker should be matched with the central bulb of water bar. Joints should be sealed on water face, when water could be on other face also, sealing can be applied on that face also. When water-bar is at the bottom of a lift of wall to be concreted, within the formwork it is very difficult to pour concrete, vibrate it and avoid segregation in the bottom portion, simultaneously avoiding displacement or bending of top wing of water-bar. It will involve lot of arrangements to be made for keeping water bar in position while placing, vibrating and achieving a good quality concrete, and all this will require sufficient space.

R 11.5.6 Joints in Roofs : Joints in roof can be aligned along the vertical joints if provided in walls, or else it may be constructed monolithic without joints. For ground tanks, roof may experience more temperature variations than walls. In large length roof should be designed for temperature variation and construction should be done such as to justify the assumptions made in design.

R 12 JOINTING MATERIALS Jointing materials normally used are classified as follows:

a) Joint fillers,b) Water-bars, andc) Joint sealing compounds (including primers where required).

All Jointing materials, water-bars, fillers and sealants shall not contaminate liquid retained more specifically if water is potable. As well the retained liquid should not have adverse chemically reaction on the water-bars and sealants. Bituminous filler should not be used with thermosetting, chemical curing sealant like polyurethane.

R 12.1 Joint Fillers Joint fillers are usually compressible sheet or strip materials used as spacers. Sponge filler consisting of closed-cell neoprene or rubber can be used. These can be performed. They are fixed to the face of the concrete placed earlier, and against it the concrete of second placed is cast. Fillers are detailed to remain in position, accommodate maximum joint movement and prevent restrain. Some types of fillers are pre-compressed at initial installation. Fillers can be performed. To provide a desired geometry to sealant, Backer rod (compressible closed-cell polyethylene sponge or other suitable material) can be provided to support it. Joint fillers, may themselves function as watertight at expansion joints. These give support to joint sealing compound when applied in floor and roof joints. But they can only be relied upon as spacers to provide the gap in an expansion joint, the gap being bridged by a water-bar and sealant (see Fig. 6).

R 12.2 Water-bars (Water-stop) Water-bars are preformed strips of impermeable material which are wholly or partially embedded in the concrete during construction so as to span across the joint and provide a permanent watertight seal during the whole range of joint movement. For example, water-bars may be strips with a central longitudinal corrugation (see Fig. 5A and Fig. 6A), Z shaped strips (see Fig. 6B) and a central longitudinal hollow tube (see Fig. 5B and Fig. 6C) with thin walls with stiff wings of 150 mm or more width. These are normally 150 mm to 250 mm wide, higher width required for expansion joint. Material of water-bar shall have proven or demonstrated acceptable performance under conditions of its intended use. The material used for the water-bars are PVC (poly-vinyl chloride), thermoplastic elastomeric rubber (TPE-R), metal sheet, stainless steel etc. Galvanized iron sheets may also be used with the specific permission of the engineer-in-charge provided the liquids stored or the atmosphere around the liquid retaining structure is not excessively corrosive, like sewage. GI (or other metal) sheets preferably be coated by an epoxy or polymer for better durability. Stainless steel is used where exposure to ozone is possible. While selecting the materials for water-bar, the possible corrosion aspects may be kept in mind. Chemical resistance, joint movement capacity, and design temperature range are among the criteria that should be investigated when selecting water-bars. Joint movement capacity and design temperature range are among the items that should be investigated when selecting water-bars. PVC water-bars are more common in use and have longer life, and rubber is also used. With use of water-bar, sufficient thickness of concrete members should be provided so as to ensure proper placing and compaction of concrete adjacent to water-bar in order to achieve adequate structural strength. Water-bar should not interfere with re-bars, dowels, and in between space for concrete, all of which lead to enough thickness of the member needed. It is important to ensure proper placement and compaction of the concrete around water-bar without air pockets. The bar should have such shape and width that the water path through the concrete round the bar should offer resistance to flow and should not be unduly short. The holes, sometimes provided on the wings of water-bars to tie it in position or to increase bond, shorten the water path and may be disadvantageous. The water-bar should either be placed centrally in the thickness of the wall or its distance from either face of the wall should not be less than half the width of the bar. The specified concrete cover to all reinforcement should be maintained. The strip water-bars at present available in the newer materials need to be passed through the end shutter of the first-placed concrete. It can be appreciated, however, that the use of newer materials makes possible a variety of shapes or sections. Some of these designs, for example, those with several projections (see Fig. 6D), would not need to be passed through the end shutter and by occupying a bigger proportion of the thickness of the joint would also lengthen the shortest alternative water path through the concrete.

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R 12.2.1 Small swellable strip (say 20×25 mm) or a bulb (without wings) hydrophilic water-bar can be placed at middle of thickness of concrete, which seals the space by swelling, thus reduce the leakage through a construction joint. When comes in contact with water it can swells over to 200% (in unconfined state) and seal the passage of water. Due to easy methodology of application and property to seal the joints completely, these are effective to make joint watertight. Such water-bar does not have flaps which otherwise hinder the proper workmanship. These have re-swelling capacity even where cyclic wetting & drying may take place. The swelling pressure is small & not more than 1 MPa. Swelling is slow, hence there is sufficient time for concrete weight to act on it. Small hydrophilic water-bar does not significantly reduce section strength as well does not require more width of concrete, member thickness can be 200 mm or more. These are not useful where moist condition may not be maintained. Design of reinforcement crossing the joint should take in to account the extra tension imposed on the joint due to swelling of the hydrophilic water-bar. Injectable hose water-stops can also be used as per the manufacturer’s recommendations.

R 12.3 Joint Sealants Joint sealants are impermeable ductile materials, which are required to provide a watertight seal by adhesion to the concrete throughout the range of joint movements. At a joint pressures, temperatures, movements, and chemical resistance required will govern the selection of sealants material. Surface preparation of concrete is important for bonding of sealant. It is exposed to thermal or moisture induced cyclic movements. During service life, joint movement imposes cyclic mechanical strain on the seal which, depending on the exposure conditions and the design, and sealant is subject to environmental degradation, leading to seal failure. RILEM TC 190-SBJ recommendation are for weathering test of sealant. Some test are also specified in ASTM C1589, ASTM D1435, ASTM G7 and ISO 877-2. There are differences in test procedures as per these standards. Also similar test for adhesion of sealant is required, which can be as per recommendation of RILEM technical committee. ISO 6927 gives vocabulary/ definitions about sealants. The shape factor (maximum strain) of the sealant and its bond with concrete are important for design of joint. Sealants are filled in the small groove at the joint near surface. It should be bonded to the concrete on each side of the joint. At the bottom of groove, the sealant should be free to deform and kept un-bonded. A bond breaking tape normally applied between filler and the sealant. Where depth of sealant needed is smaller than the depth of grove created, a Backer rod is placed to support the sealant. Between a filler (in expansion joint) or Backer rod, and sealant a bond breaking tape is applied usually. Backer rods are provided to support sealant and to give proper geometry to sealant. Backer rods are compressible closed-cell polyethylene or other suitable closed cell material compatible with environment. For sealant application, joint preparation should be done as per the manufacturer’s recommendations. Polymer bases sealants like polyurethane (PU) or polysulfide (IS 11433 Part 1 & IS 12118 Part 1) are popular which may be single component or two component. Silicone sealant has longer life compared to others. Sealants should be non-sagging. Mostly Polyurethane sealants are better and preferred, especially where the contact is with waste-water. During curing of polysulfide sealant, moisture has to be excluded for satisfactory adhesion. Low modulus, moisture-insensitive, epoxy sealants can be used were relative movements are very small. Epoxy has good bond with steel also. Sealants do not have a life span matching to the life of structure, and during life resealing is required at proper interval, depending upon type of sealant and severity of exposure. Other materials are based on asphalt, bitumen, or coal tar pitch with or without fillers (refer IS 1834), such as limestone or slate dust, asbestos fibre, chopped hemp, rubber or other suitable material. After construction, or just before the reservoir is put into service, these are applied by pouring in the hot or cold state, by trowelling or gunning or as preformed strips ironed into position. These may also be applied during construction such as by packing round the corrugation of a water-bar. A primer is often used to assist adhesion and some local drying of the concrete surface with the help of a blow lamp is advisable. The length of the shortest water path through the concrete should be extended by suitably painting the surface at the concrete on either side of the joint. The main difficulties experienced with this class of material are in obtaining permanent adhesion to the concrete during movement of the joint whilst at the same time ensuring that the material does not slump or is not extruded from the joint. To avoid frequent renewals, sealant should be chosen to perform over long period. However best of sealants have limited life compared to life of concrete structure. Hence joint should be designed such the repairs and maintenance can be undertaken at specified interval. The geometry of sealant as applied in position is important for design and performance. In floor joints, the sealing compound is usually applied in a chase formed in the surface of the concrete along the line of the joint (see Fig. 7A). Preparation of concrete surface to receive cleaning chemical, primer and sealant are to be worked out. To enhance the bond between concrete and sealant, primer is used. The actual minimum width will depend on the known characteristics of the material. In the case of an expansion joint, the lower part of the joint is occupied by a joint filler (see Fig. 7F). This type of joint is generally quite successful since retention of the material is assisted by gravity and, in many cases, sealing can be delayed unit just before the reservoir is put into service so that the amount of joint opening subsequently to be accommodated is quite small. The chase should not be too narrow and too deep to hinder complete filling and the length of the shortest water-path through the concrete on either side of the joint. Here, again a wider joint demands a smaller percentage distortion in the material.

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An arrangement incorporating a cover slab, similar to that shown in Fig. 7G, may be advantageous in reducing dependence on the adhesion of the sealing compound in direct tension. During concreting operation cement paste should not be able to enter into the sleeve or its caped end.

R 12.4 Dowels : To restrict shear displacement across a joints, dowels are required. Normally these are of steel having plane smooth surface with saw cut ends (having no burrs) and epoxy coated for longer durability. Bar should be cylindrical i.e. same section continuing for length, to allow the un-bonded length of bar to slip in the concrete. Cutting of bar in shear is not acceptable, as the ends portion will get distorted. These may be of galvanised steel, stainless steel (reinforcement grade) or steel bar in cased in stainless steel tube, to provide high resistance to corrosion. For epoxy coated bars at expansion joints, the coating thickness should not be less than 250 micron (0.25 mm). Proprietary systems such as plate dowels can also be used if approved by designer. On one of the side of joint the dowel bar is provided with a tight fitting plastic sleeve in which bar can slide axially without rotational play. Arrangement like greasing should be done to minimise friction to axial sliding action. For initial axial slip of 0.25 mm, the friction force should be between 0.2 to 3 kN (about 0.02 to 0.3 kg force). The sleeve should not be loose on the dowel and it should be able to accommodate a movement > 5mm.During concrete laying and compaction, at a joint alignment of all dowel bars should be kept accurately perpendicular to the joint and all bars parallel to each other, to avoid locking of the movement.

R 12.5 Bearing pads : Provided in sliding joints. These should be attached to the hardened concrete base to prevent uplift. Anchoring is not commonly done, unless designed to avoid restraint. Space or interface between bearing pads should be filled by material compatible for deformation and water-tightness (say sealant).

R 13 CONSTRUCTION R 13.1 Provisions of IS 456 are applicable with modifications as per additions given in the code IS 3370. For prestressed concrete work provisions of IS 1343 are also applicable.

During construction and the initial life, LRC members shall not be allowed to dry below 50% relative humidity to prevent shrinkage due to moisture loss.

Wearing apron should be provided to prevent abrasion of structural concrete due to fall of water from a height of >3 m.

R 13.1.1 In general PCC base in foundation (lean concrete / mud mat / blinding layer) can be in M10, and if the injurious soil or aggressive ground water is expected ≥ M15 grade. Thickness of PCC should not be less than 75 mm.

For small tanks PCC base concrete could be M10 grade, should not be porous and should have fairly plain surface. If with M10 concrete proper finishing ability is not possible, M15 could be considered for PCC. Below LRC floor slab, without a separation sheet (LDPE), the PCC should be minimum M15 grade, and if the injurious soil or aggressive ground water is expected ≥ M20 grade.

PCC should give a fairly plane, smooth and hard surface to lay further watertight concrete. Enough fines to impart ability to get finished fairly smooth, and the proportion of fine aggregate should be higher than obtained by classical mix proportioning. The base concrete should not be treated as structural concrete i.e. it may not conform to table 5 of IS 456. The top surface finish can be U1 class (see appendix).

R 13.1.2 A separating sheet between PCC base and RCC floor of a ground tank is required where movement joint are proposed in the floor. The experience of constructing large number of small tanks (< 22 m size) indicates that such a sheet and the movement joints are not required, as per option 1 in Table 2 (see 11.3). For small tanks without movement joint, continuous restrain due to roughness of PCC surface is an advantage, hence separating sheet is not required and if PCC is not porous enough preventing loss of cement paste from floor concrete above it when laid and compacted. Bond braking sheet (usually LDPE 1 to 1.1 mm thick) is required where movement joints are planned. Thinner sheet will break the bond but may not prevent friction due to roughness, thus advantage of providing movement joint will not be harnessed.

If option 2 or 3 is adopted, polyethylene (LDPE) sheet is to be provided. Polyethylene sheet of 1.1 mm thickness will have a weight of 1 kg/m². A virgin quality LDPE sheet (grade 232 of IS 2508) has a tensile strength 20 to 26 N/mm², % elongation >200% & density 0.92 to 0.93 kg/litre.

For tanks larger than 15 m size if considered in design, separating sheet can be provided to allow sliding of RCC floor with respect to base. For this it is necessary to have PCC top plain, in level and smooth for facilitating siding to relieve friction and minimize restraints.

Due to construction requirement, PCC can be laid in two layers if needed for a particular work. Fist layer is mud mat / lean concrete for filling or levelling purpose and to cover the soft soil which may become slushy when exposed or if the excavated ground is uneven. Fill for levelling can be of lean concrete (M5 / M7.5 / M10). Second layer of PCC can be M15 grade, with good ability to be finished as smooth and flat even (in a plane & level). However one may choose to provide only one grade of PCC to reduce two stage construction operations to only one if the need for extra thickness is small.

For large ground tanks with separation/sliding sheet specified, PCC top should not have slopes or surface deformations (may be to accommodate local thickening of RCC floor slab as structural foundation). At positions where PCC top varies from being flat level surface or has pockets, free sliding is not possible. Hence movement joint have to be located in the RCC floor at such positions. For top surface class of finish shall be U2 with very

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low tolerance (i.e. line level ± 2 mm, gradual irregularity ± 1 mm, abrupt irregularity < ± 0.3 mm.)R 13.1.3 Binding wires (for tying reinforcement) or any other item which can corrode, shall not lay in the concrete cover zone or have insufficient cover. After tying the bars, end of binding wire should be bent inside and should not project out in to the cover zone. Any item in the cover zone should be non-corroding including the reinforcement placing aids, and these items will not create a path for permeating the environment (causing loss of durability) or liquid through cover zone.

R 13.1.4 Metallic items that protrude from the concrete shall be detailed so that the galvanic corrosion between the buried and exposed portions will not occur. Aluminium shall be isolated from any wet concrete by a moisture-proof coating, lining or gasket. Bi-metallic corrosion should be avoided by isolating contacts between metals or metal to reinforcement contact.

R 13.1.5 In walls, horizontal construction joint is less demanding compared to vertical. Larger vertical lifts of wall should not be at the cost of introducing more vertical joints. For cylindrical tanks it is good practice not to have vertical construction joints (or have only one joint of time gap not more than few hours). All vertical joints should be sealed by sealant applied on the liquid face in a small groove (say 3 mm wide 3 mm deep).

R 13.1.6 Filling and patching of tie holes is essential for long-term durability. If to be left in concrete, form ties with creeping flange (collars) should be used for members intended to be watertight. Through form ties should be avoided, and if used the hole should be tightly filled with suitable non shrink /shrinkage compensating mortar.

R 13.1.7 Due to fall of water from a height, its impact can cause cavitation and abrasion loss of concrete. Similarly abrasion loss may be where water with debris (grit or silt) has velocity more than 2 m/sec, or without debris water velocity more than 4 m/sec. To avoid damage to the structural concrete from abrasion loss, a wearing apron should be provided at such locations. Such aprons may be provided in steel fibre reinforced concrete to have high wear resistance and thus reduces the number of replacement of apron during service life. Apron should be bonded to the structural concrete below, by laying it while concrete below has not yet set, else other method of bonding shall be used.

R 13.1.8 Dense and smooth concrete finish obtained by smooth form finish or trowelling, with extended curing etc. reduces permeability.

R 13.1.9 Enough curing is vital to developing required durability of the concrete. The method of curing shall ensure that the surface layer of all concrete remain moist at all times during the curing period or till at least 75% of the specified concrete strength is developed. LRC should not be allowed to dry-up below 55% relative humidity, to avoid drying shrinkage, till tank is filled with aqueous liquid.

R 13.2 Joints : Joints shall be constructed in accordance with requirements of 11.

R 13.3 Construction of FloorsR 13.4 Construction of WallsR 13.4.3 Wall thickness 200 or more is preferably. Minimum thickness can be 180 mm where reinforcement size is ≤ 10 mm. However, minimum thickness of wall can be 160 mm for small tanks (<3 m size or <2m water head). In treatment plants walls of channels can be 120 mm thick if single layer bar mesh is required. Partition wall & baffles not subjected to significant pressure can be 120 mm thick with single layer of reinforcement mesh (or bars).

R 13.5 Prestressed Tank R 13.6 Formwork R 13.7 Lining of Tank

R 14 TEST OF STRUCTURE : The tank for potable water should be cleaned or disinfected before it is put to use. This can be done before or after the test for water-tightness. Till test by water filling is undertaken, it is advisable to the keep the water retaining concrete components slightly moist, so as to avoid development of drying shrinkage cracks. The water-tightness test is performed for new construction and also after major repairs are undertaken to reduce leakages. For ground tanks, water-tightness test is usually performed before backfilling of soil on outside. This will help in locating the spots of leakages.

R 14.1 Test of structures specified in IS 456 (clauses 17.3 to 17.8) are not mandatory. The structural test of structures as per IS 456 can be carried out only if the need be. These tests are to be performed in case of doubt due to lapses or non-conformance noticed during inspection or operation of quality system. However the water-tightness test should be carried out necessarily in all cases. Tank shall be slowly filled for first filling. Refer R 9.2.6.

R 14.2 For water-tightness test, loss of water is measured in terms of drop in water surface. Apart from seepage additional loss is due to evaporation and sometimes a gain due to rain for open top tanks, and such loss or gain is to be corrected. To the actual water drop correction should be applied for evaporation loss or gain due to rain etc. For covered tank evaporation loss is neglected.

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R 14.2.1 After the 1st filling of tank leakages in general reduce due to autogenous healing of cracks. Hence during test, a stabilizing period is allowed. If tank does not pass the water-tightness test, there is an option to extend the stabilizing period and retest. It should be noted that only fine cracks where leakage is quite small, can get healed. The stabilizing period may be 7 days, which can be extended to 21 days if need be and where a water retaining component is designed for 0.2 mm limiting crackwidth (as per IS 3370 part 2). If tank fails in water-tightness test, the portions of concrete responsible for leakages are to be recognized. Cracks or passages where leakage is high, are to be grouted to reduce the leakage.

R 14.2.2 For the elevated tanks if leakages are not visible on the sides and bottom of a tank, it can be deemed to be watertight. The level of the water shall be recorded again at subsequent intervals of 24 hours over the period of test. The total drop in surface level over a period of last seven days shall be taken as an indication of the water-tightness of the tank. The actual drop in the surface level shall be corrected by evaporation losses or rainfall for open tanks. For tank with roof, the loss by evaporation can be taken as negligible. The permissible loss shall be 10 mm plus 0.002 times the average water depth in tank, or the limiting value as specified. For ground supported tank, having water depth less than 5m, if drop in level of water is less than 20 mm in 7 days, the tank is deemed to be watertight. If wall of tank show leakage, these spots should be rectified by proper action.

R 14.3 Roofs should also be tested for water-tightness, even if roof is sloping or having dome shape.

R 15 LIGHTNING PROTECTION

R 16 VENTILATION Area of ventilation in roof should not be less than the area of outlet pipes. For tanks storing chlorinated water, to reduce the concentration of chlorine in the air of freeboard zone, the ventilation may also be not less than 0.8% of the free surface area of water. Ventilation area should be protected by mosquito-proof net.

R 17 DESIGN REPORT AND DRAWINGS : Documentation shall be prepared which shall contain all salient features of the work, engineering data and brief maintenance scheme of the work. It shall cover the following:R 17.1 Brief data and features like description of liquid to be retained, capacity of tank (in m³ or liters), height of freeboard (in mm), liquid depth in tank. Salient levels – average GL, foundation level, LSL and FSL etc.R 17.2 Foundation investigation report and soil data, description of founding stratum, type of foundation, probable depth of foundation, ultimate, net safe and net allowable bearing capacity of founding strata. The position of ground water table- highest and lowest. Soil classification for seismic design (based on corrected standard penetration value N, refer IS 2131 and IS 1893 part 1). Data on liquefaction potential of founding soil in seismic event. R 17.3 Location of structure (e.g. polluted industrial area, sea front area, costal area, urban area, rural area etc.), and purpose of liquid retention (i.e. public water supply, fire-fighting, industrial, sewage treatment etc.). The information on pollutants, salts, sulfates if any in air, soil, ground water and liquid retained or excluded, if of significance at the concerned location. Design exposure class for tank members, & roof.R 17.4 Brief specifications of concrete and its grade(s), type of cement(s) to be used, limits of maximum and minimum cement content. Brief specifications and limits of mineral admixtures (or supplementary cementitious materials) to be used. Brief specification of reinforcement bars and grade. In case of fibre concrete, the type of fibres, its size, aspect ratio, dose etc.R 17.5 Salient features of structure and construction, method of construction, height of column lifts, height of wall lifts, and guidance on release of form work. For specialized form-work, the design and drawing of the formwork shall be given. Any constraint on construction assumed in the design. Specify surfaces acceptable as form-finished. Specify finishes separately on different formed and unformed surfaces. R 17.6 Special protective coatings if being specified, and its brief specifications.R 17.7 Clear cover of concrete on reinforcement bars for various members at different locations. Guidance on locations of laps in reinforcement shall be given. Table of lap length in reinforcement of different members for each size of bars (diameter wise) shall be given. R 17.8 General locations, specifications and treatment of construction joints should be specified. R 17.9 The curing procedure in brief and its duration should be specified.R 17.10 References of codes, standards, and guidance for construction. R 17.11 Design loads : Density of concrete, liquid, soil, masonry etc.; provisional loads of rendering, finishing, flooring, coating, lining etc. as applicable, loads of railing, parapets, masonry wall etc.; imposed loads on roof, balcony, walkways, platform etc.; load of ladder, stair, pipelines, valves etc.; load of permanent equipment (/pumps) if any; provisional load of any process operation, other provisional loads, construction loads; any other loads. Magnitude and location of prestressing force, stressing sequence.Seismic zone, zone factor, response reduction factor, importance factor, critical damping factor, soil factor (see IS 1893 part 1 & 2).Basic wind speed, terrain category, factors k1, k2, k3, k4, Kd, Ka, Kc, (see IS 875 part 3); Load of equipment if any etc.Blast or explosion load if to be considered, any other loads considered. R 17.12 Design report containing basis of design, water-tightness class (refer table 4 in part 2), method of structural analysis, structural configuration and the assumptions made, detailed loading computations, structural analysis, design calculations with sizes of members and reinforcement, checks required as per codes. In some cases design is influenced by method of construction and formwork details. Such cases should be covered by design.R 17.12.1 Documented computer output is acceptable in lieu of sheet of calculations. The extent of input and output information required will vary, according to the specific requirements of individual proof checking engineer. When a well-known computer program has been used by the designer, only skeleton data may normally be required. This should consist of sufficient input and output data and other information to allow the checking engineer to perform a detailed review and make comparisons using another program or manual calculations. Input data should be identified as to member designation, applied loads, and span lengths. The related output data should include member designation and the shears, moments, and reactions at key points in the span. For column design, it is desirable to include moment magnification factors (or additional moments) in the output where applicable.R 17.13 Drawings with reinforcement detailing, junction detailing, brief specifications, instructions and notes. Detailing at construction joints and movement joint should be specified on the drawing with their locations and treatment.R 17.14 Design drawings, typical details, and specifications for all concrete construction shall bear the dated signature and seal of a design professional.R 17.15 Guide for completion drawing, and completion report for record. This will help in compiling completion record.R 17.16 Prepare a design brief, which is to be shared with construction manager, for preparing construction brief, which is key to quality

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Guide to IS 3370 – 2020assurance.R 17.17 Framework of quality management, its manual to be prepared by agency supervising the construction. Completion record will also have Record of quality of construction to be compiled by the contractor or producer or supplier.R 17.18 Proposed scheme of condition survey and maintenance (conservation) of structure.

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APPENDIX 1 Some of the available Indian Standards related to joints and jointing materials are listed below. These standards are for building & pavement work, and may not be relevant to water retaining structures. Some Indian Standards refer to dams & similar massive works of water resource engineering.IS 1580: 1991 Specification for bituminous compound for water proofing and caulking purposes.IS 1834:1984 Specification for hot applied sealing compound for joints in concrete. 1st Revision.IS 1838 part 1 -1983 Specification for preformed fillers for expansion joint in concrete pavement and structures

(non extruding and resilient type): part 1 Bitumen impregnated fibre (1st revision).IS 1838 part 2 -1983 Specification for preformed fillers for expansion joint in concrete pavement and structures

(non extruding and resilient type): part 2 CNSL Aldehyde resin and coconut pith.IS 1838 part 3 - 2011 Specification for preformed fillers for expansion joint in concrete pavements and

structures (non extruding and resilient type) Polymer basedIS 3414:1968 Code of practice for design & installation of joints in buildings. (Scope specifies that it does not cover water retaining structures)IS 4461:1998 Code of practice for joints in surface hydroelectric power station.IS 5256:1992 Code of practice for sealing expansion joints in concrete lining of canals.IS 6494:1988 Code of practice for water-proofing of underground water reservoirs and swimming pools.IS 6509:1985 Code of practice for installation of joints in concrete paving & structural construction (1st revision).IS 10566:1983 Method of test for preformed fillers for expansion joint in concrete paving and structural construction.IS 10957:1999 / ISO 2444:1988 Glossary of terms applicable for joints in buildings (1st revision)IS 10958:1999 / ISO 3447:1975 General check list of functions of joints in buildings.IS 10959:1984 / ISO 6927:1981 Glossary of terms for sealants for building purpose.IS 11433 part 1 : 1985 Specification for one part gun-grade polysulphide base- joint sealant, part 1 general requirements.IS 11433 part 2 – 1985 Specification for one part grade polysulphide base joint sealant, part 2 method of test.IS 11817:1986 / ISO 7727 Classification of joints in building for accommodation of dimensional deviation

during constructionIS 11818:1986 / ISO 6589 Method of test for laboratory determination of air permeability of joints in buildingIS 12118 part 1 : 1985 Specification for two parts polysulphide based sealant, part 1 general requirements.IS 12118 part 2 : 1985 Specification for two parts polysulphide based sealant, part 2 method of test.IS 12200:2001 Code of practice for water-stops at transverse contraction joints in masonry and concrete dams.IS 13055:1991 Method of sampling and test for anaerobic adhesives and sealants.IS 13143:1991 Joints in concrete lining of canals – sealing compound – specifications. IS 13184:1991 Mastic filler – epoxy based – specifications.IS 16354:2015 Metakaolin for Use in Cement, Cement Mortar and Concrete - Specification

There are many IRC and MORTH specification also.

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Guide to IS 3370 – 2020

Guide on IS 3370 Part 2 – 2020 (2nd Revision), Code of Practice - Concrete Structures for Retaining

Aqueous Liquids : Part 2 - Reinforced Concrete Structures

R 0 GENERAL Limit state design approach is improved, and working stress method is now deleted. The requirement of

crackwidth calculation is amplified.For ground tanks, concept of designing temperature-shrinkage reinforcement related to spacing of movement

joint has been clarified, and that is not applicable to elevated tanks.Code is drafted for retaining aqueous liquids. For retaining other liquids concrete shrinkage will be higher

and most other liquid may require lining of the tanks.Also refer to the comments under ‘General’ & ‘Scope” on IS 3370 part 1. Some guidelines are given here, which are not dealt in the code.There is experience of designs with earlier code, which have behaved satisfactorily. Hence there is no need to

increase reinforcement by way of minimum % of steel for small tanks, except large (size >22 m) ground tanks.In general deflections are not taken important for LRC members except roof slab. However check for

deflections as per IS 456 is retained. Clause 23.2 (b) of IS 456 will not apply. Design base should be such that only limited periodic maintenance may be required for serviceability over

the design life. And also an uncontrolled, rapid loss of the liquid retained would not occur in extreme events such as a major earthquake. The structure should be safe and serviceable in in extreme case repairable.Following are the significant modifications incorporated in this revision:

a) Liquid load has been defined separately.b) For circular wall liquid pressure will be assumed to act at centre of wall. c) Only limit state design method is specified; d) Table of load combinations has been specified. e) For member subjected to direct tension, reduction of shear strength of concrete, is specified. f) At construction joint, design actions are specified. g) Detailing at junctions (connections) of wall is recommended.

R 1 SCOPE : (Also refer to scope in part 1)The code is also referred for structures dealing with sewage. Components of sewage treatment having liquid

of low pH (<6) or materials which can attack concrete, will require additional protection in form of coatings or lining.

The code does not deal with ‘ferro-cement’ or ‘fibre concrete’, for which specialized literature should be referred. Use of synthetic/polymeric (say polypropylene) fibres at a dosages of 0.15 to 0.2% by volume (1.3 to 1.8 kg/m³) of concrete, are useful in controlling plastic shrinkage cracks. As well fibres are useful in dispersion of cracks, thus reducing crackwidth.

R 2 REFERENCES : List of standards referred in the code is given. While referring to a standard its latest revision with up to date amendments should be used. This information is freely available at www.bis.org.in , the web site of Bureau of Indian Standards.

R 3 GENERAL REQUIREMENTSCommon general requirements are covered in part 1 of the code. The requirements of IS 456 will also govern

the design & construction of LRC, unless a requirement is overruled or in conflict to that in IS 3370.

R 4 DESIGNAim of design is the performance of structure over the designed service life, with minimum maintenance and

avoiding repairs, rehabilitation and intervention. Adequate foundation support, robustness, stability, constructability, maintainability and restorability of structure should also be considered in design. Design should be such that construction is traditional and easy, or guided by enough details to avoid shortfall in quality of construction. Details should also be such that the planned maintenance would be possible in future for retaining the serviceability of structure. In case of slip in quality, repairs and restoration should be conveniently possible.

Structure should be designed for durability, and serviceability limit state by keeping check on water tightness, deflections, crackwidths, spalling, and excessive vibrations. Construction joints and movement joints shall be designed such that these are restorable few times during service life of structure.

Structure should also be designed for ultimate limit state by keeping check on stress limit, & strength resistance capacity. At ultimate limit state LRC may be partially damaged, leakage may take place, but should be usable temporarily, even in design wind or seismic condition, and should remain repairable. It should not become a mechanism arriving at collapse stage. Ample warning in terms of time, deflection or local damage shall precede any possible failure.

For proper functioning and least maintenance, monolithic construction is preferred. Keep joints to the minimum. For monolithic structure, effects of continuity and restrains should be worked out for determining forces (or stresses) at all critical sections of members.

The design should consider social, environmental and economic sustainability. Low carbon footprint and low consumption of materials are the present day needs.

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In future, design requirements will also need consideration of demolition (dismantling).R 4.1 In general the word “forces” mean ‘force actions’ such as direct force (as tension or compression), shears, bending moments and torsion acting on member at a section/ position. Concentrated or distributed mechanical forces acting on/in a structure are direct actions. Deformations imposed on the structure or contained within it or environmental deformations imposed on the structure or contained within it are indirect actions or environmental actions.R 4.1.1 For structural reinforced concrete members and junctions (connections), design approach based on strut-and-tie models is permissible.R.4.1.2 Sizes (geometrical values) of members to be taken for design should be the characteristic value, which should account variation possible in construction. The design dimension can be modified by half the possible variation. Such correction is more important in cantilever members and thin slabs or wall (<180 mm).R 4.1.3 Where thin shells (e.g. floor & roof domes or shaft staging) are provided, failure by buckling mode in all load combinations (including vertical & horizontal earthquake effects) shall be avoided.

R 4.2 Loads : Loads are dealt by IS 456 as dead load (DL), imposed load (IL also popularly known as live load), wind (WL), earthquake or seismic load (EL). Liquid (FL or fluid load/pressure) and earth pressure (EP) do not fall in the classification either as DL or IL. Loads (actions) almost constant or monotonically approaching a limiting value during a reference period, are permanent actions. Load (action) which is not permanently acting, not constant or not monotonically changing during a reference period are variable actions.R 4.2.1 Consideration should also be given to loads in construction. R 4.2.2 DL : One should differentiate permanent DL and provisional DL (DLp) which may or may not be present at all time.R 4.2.3 IL : Differentiation should be made for normal IL, ILs of temporary storage, and ILp provisional imposed or equipment load.

Imposed loads (live loads) for equipment and process area shall take into account weights of fixed equipment, loads of material stored, and normal live loads due to personnel and other transient loads. Imposed loads should account for installation, operation, and maintenance of equipment, and possible modifications or changes in use. With equipment, its allowance for impact or its action under dynamic condition shall also be accounted.

During installation or maintenance, portions of equipment may be laid down at various locations on the floor. For example, heavy equipment (pump-sets) may be temporarily placed near the centre span of a floor during installation or maintenance, even though its final location may be near support locations. Weights of concrete bases for equipment may also be included in the loads, and consideration should be given to weights of piping, valves, and other equipment accessories that may be supported by the floor slab and beams. Consequently, conservative uniform live loads may be applied as an alternate.

Information on equipment weights and their bases should be obtained for design and a margin be kept for variation in the load of equipment from different suppliers. During assembly and installation of large equipment, area adjoining to its final location may experience loads of pieces, and such load provision should be considered.

Where applicable snow load shall be considered.R 4.2.4 EL : Earthquake or seismic base shear shall be worked out for an inertial load combination -

1 DL + 1 FL + 0 IL + 1 ILs + 0.7 ILp

Where, ILs is imposed load due to storage, and ILp is provisional imposed or equipment load. In above, water load (FL) can be considered full or part or tank empty case (FL 0 to 1) separately as may be taken in a particular load combination with seismic load. The seismic base shear thus obtained shall be multiplied by an appropriate load factor for a load combination as given in Table 1. R 4.2.4.1 For earthquake analysis, the water mass should be idealized by two-mass model, wherein the water mass should be divided as convective mass and impulsive mass. The impulsive mass of water, with the mass of container and the equivalent mass of the staging (for ESR) together shall be accounted as total impulsive mass. The convective mass of water shall be separately accounted. Effect of both of these shall be combined as per details in IS 1893-2. For h/d up to 0.64, impulsive mass can be taken equal to total mass of water less convective mass calculated.

R 4.2.4.2 The two mass model is more appropriate compared one mass model, and in most cases the two mass model can gives a smaller design base shear in staging, but higher overturning moments. All containers must be designed or checked for the actions (base shear & over-turning moment) induced due to two mass model.

R 4.2.4.3 The container may have columns and braces inside tank or baffle walls, which provide obstruction to the convective water, thus reducing convective mass and increasing impulsive mass of water. For this approximate correction can be applied to increase impulsive mass for estimating seismic effect. The amount of impulsive mass thus increased should be deducted from convective mass.

R 4.2.4.4 As per present design approach, the response reduction Ri is taken from IS 1893 Part 2 Table 2 & 3 for

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impulsive modes. For convective mode Rc value should be lower or may be taken as unity (1), but in absence of clarity it can be taken as 2 at present. For seismic coefficient due to convective mode [ (Ah)c ] the critical damping for water shall be taken as 0.5% (against 5% for concrete). For 5% critical damping in concrete, Sa/g is obtained from 6.4.2 of IS 1893 part 1. This value shall be multiplied by 1.75 to obtain Sa/g for 0.5% damping. It is further recommended here that (Ah)c should not be taken less than 0.04 .

R 4.2.4.5 For elevated tank, mass of staging (Ms) is taken as a uniformly distributed over the height hs (from top of structural foundation to bottom of container). Height hm is up to C (i.e. CG of water in container). Equivalent mass of staging (Mse) is the mass applied at C which will give same horizontal deflection at C, as the distributed mass over height hs will give. See Fig. 25. As an approximation Mse = Ms /3 may be taken for design.

R 4.2.4.6 For vertical seismic, Sa/g should be taken as 2.5, and value of Ri should be smaller than that for horizontal seismic. However at present, the value can be taken as 2.5 for ESR and 1.5 for GSR. For vertical seismic, convective mass may be neglected, and total water mass taken as impulsive. For ESR Av = Z/2 ; for GSR Av = Z/1.2 .

R 4.2.4.7 For design of staging of tank less than 50 m³ having maximum horizontal spread of water less than 7 m at FSL, and founding soil is not soft, simplification by considering one mass model wherein total water is treated as impulsive mass only, can be an option of designer. However such simplification cannot be binding on any designer.

R 4.2.4.8 For estimating the natural period of vibration of staging, its stiffness for horizontal deflection need not be reduced on account of cracking or creep. Stiffness of un-cracked members should be accounted i.e. taking gross concrete area in to account. Correction to deflections (sway) due to P-δ should be neglected, for estimating natural period.

R 4.2.4.9 If specified by owner or decided by designer a lower value of R can be applied for the seismic design, than that given in IS 1893-2.

R 4.2.4.10 If imposed loads are other than live loads on roof, and of nature like a process or operations or equipment (ILp) or storage, an appropriate part (may be 0.5 to 0.7) of such ILp excluding impact allowance should be accounted for estimating base shear in R.4.2.4.10.

R 4.2.4.11 In any design, Sa/g should not be taken less than 0.40; nor should the seismic base shear be less than that given in table below.

Seismic zone ratio in %II 0.015 1.5 %III 0.020 2.0 %IV 0.027 2.7 %V 0.036 3.6 %

[ e.g. for zone III, base shear not less than 0.020 (i.e. 2.0%) of the total weight. ]

R 4.2.4.13 If the tank or staging do not have main members as overhanging or cantilever (Other than cantilever walkway), the effect of vertical seismic can be neglected for tanks in zone II and III. Otherwise, in all other cases, horizontal and vertical seismic effects should be combined. All overhanging or cantilever members (even in zone II or III), shall be designed for vertical seismic also. For the purpose of this clause cantilever walkway (≤ 1.2 m cantilever span) will not classify the tank as having cantilever members. However the cantilever walkway or gallery shall be designed considering vertical seismic.

R 4.2.4.14 Mass of pipeline, stair, ladder etc. should also be estimated for arriving at total horizontal seismic action for elevated tank.

R 4.2.4.15 For Industrial water tower of high importance or tower in highly populated urban area (where collapse of tower may damage many other buildings or structures), the importance factor may be taken higher than 1.5 (can be 1.75 to 2.0), if specified by the owner. Also refer IS 1893 part 4.

R 4.2.4.16 For tank having slab supported on ground, it acts as a diaphragm for transferring the earthquake forces from tank to ground. Hence connection of the ‘slab on grade’ to vertical members shall be detailed for the seismic load path. Movement joints may interfere with the adequacy of action as diaphragm.

R 4.2.4.17 Members of tank participating in lateral load resisting fame (i.e. floor beams) shall be designed conforming to ductility requirements of IS 13920. If flat slab is provided for floor of an elevated tank it should be designed for high degree of ductility (norms are not available in the standards).

R 4.2.5 WL : Wind load. As per section 5 to 7 of IS 875-3, wind should be accounted as pseudo-static force.

R 4.2.6 Earth Pressure (EP): The lateral pressure from earth backfill, may be symmetrical or asymmetrical. Including those caused by unequal backfill or surcharge, net lateral loads shall be determined by rational methods of soil mechanics based on soil and foundation investigations. In most cases wall does not deflect enough as required for active state of earth pressure. Hence coefficient of earth pressure could be slightly higher (may be 25 to 33%). Earth-pressure coefficient would be in between at-rest and active state, and the coefficient shall not be less than 0.50.

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If EP is definitely to remain always, it may counteract the water pressure from opposite direction. Such reliving earth pressure should not be more than half of the active state. [Also refer R8.1 a (i) in part 1].

Earth covering on reservoir roof may be taken as dead load, but due account should be taken of construction loads from machinery and heaped earth which may exceed the intended design load. Allowance should be made for the effects of any adverse soil pressures (EP) on the walls, according to the surcharge and process of compaction of the soil and the condition of the structure during construction and in service. The lateral pressure from earth backfill, may be symmetrical or asymmetrical. Including those caused by unequal backfill, net lateral loads shall be determined by rational methods of soil mechanics based on foundation and soils investigations.

R 4.2.8 Liquid load (FL) : FL should account for the actual density of the contained liquid. Density of plain water can be taken as 9810

N/m³. Aqueous solutions or suspensions can have higher densities. In some cases accumulated sludge, deposited silt, grit, lime etc. will add to the load. Liquid load includes dead storage wherever it exists.

In any combination, FL may be accounted at zero or partial liquid load or full liquid load as may make the combination more critical. The arrangement of FL should be such as to cause the most critical effects at a position of a member. The term liquid load also includes the liquid pressure and both static and dynamic effect of liquid mass. Each tank shall be designed and checked also for tank-empty condition.

For serviceability condition liquid is to be assumed to be up to normal working top liquid level (WTL) or the overflow level. This level is usually referred to as full supply level (FSL) in tanks. Occasionally liquid may rise above WTL (/FSL). A small rise as heading is required for overflowing. For overflow to match the rate of incoming liquid, the heading of liquid above WTL is usually of the order of 20 to 50 mm. Such a small heading of liquid is neglected. To keep a control over maximum FL, overflow arrangement should be designed for a discharge rate not less than the maximum filling rate of tank; an overflow pipe or weir arrangement of adequate size shall be provided to prevent overfilling of the tank. If over flow is chocked, or for any other reason liquid level rises above WTL (/FSL), liquid load will be higher under such a condition, which is unusual and its occurrence may be rare. Such maximum rise is to be estimated, which may be up to other alternate path for overflow or up to top of wall, and be limited by incoming source, this may be termed as maximum top liquid level (MTL). Above FSL liquid may be considered to rise by amount as specified (need not be up to freeboard full), and level may be taken as MTL. In many cases height of MTL above WTL (/FSL) is specified between 150 (for smaller tanks <100 m³) to 300 mm (for tanks >1000 m³). With this rise (i.e. MTL), FL is accounted only in ultimate load combination (limit state of collapse). However for all other combinations with wind/seismic, FL up to FSL/WTL only is considered. Where freeboard available is very high, a suitable MTL shall be decided from hydrostatic design considerations.

Except for pressurised tanks, vent(s) shall be provided in roof to regulate the internal pressure in space above liquid, while tank is being filled or emptied.

R 4.2.8.1 Internal pressure of liquid shall be assumed to act at the centre of thickness of the liquid retaining circular wall. If lining or impermeable treatment is applied to the inner surface, or internal pressure is due to granular material or soil, the inner surface of wall may be assumed to be the point of application of force. The external pressure of liquid will be assumed to act at outer surface of structure, which may be in combination with earth-pressure. Weight of liquid on a member (e.g. slabs) shall be up to the internal surface (i.e. top face) of member.

R 4.2.9 Other load actions which are significant for serviceability, strength and stability of the structure or its members as applicable, including the following shall be taken into account.

(a) Construction load, (b) ground movement, (c) thermal effect on roof.

R 4.2.10 An underground structure subject to groundwater pressures should be designed for floatation (uplift). Design of each member shall account the pressure due to groundwater in suitable critical load combinations. See IS 3370 part 1.R 4.2.11 If concrete is allowed to dry, the moisture dependent drying shrinkage will take place. This can happen where tank is provided with impermeable lining. Normal recommendation in this standard (for temperature-shrinkage reinforcement) does not account this moisture related drying shrinkage. Hence this extra shrinkage if restrained will need more reinforcement to control crackwidth. See also R9.1.B.2 & R9.1.1 of part 1.R 4.2.12 Loads should be grouped as ‘permanent loads’, ‘provisional loads’, ‘variable loads’ and ‘construction loads’. Environmental actions (physical, chemical or biological) are also types of loads. R 4.2.13 Junctions (say connections) of members should be designed and detailed for giving rigidity and satisfactory crack control throughout the service life of structure. R 4.3 Method of Design The level of accuracy of various physical parameters of design should be refined by devoting more efforts for analyses, so as to arrive at improved accuracy in the behaviour and strength provided by design. In regard to a specific criterion, inadequate performance in a considered to be a failure in that regard. In addition to serviceability and strength, degree of water tightness, absence of leakages and avoidance corrosion of reinforcement are prominent performance requirements amongst others.

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R 4.3.1 Components of structure adjoining to LRC may not be designed as per IS 3370. However, continuity of these with LRC may need considerations of restrains imposed on LRC. R 4.3.2 While analysing LRC members, redistribution of moments and forces (in frames or plates) on account of plasticity in general are not permitted. Use of simplifying assumptions or elaso-plastic behaviour of structure can be made (with ductility and limiting plastic strain considered), only when it is proved by long experience or technical literature or tests that it can work without undue problems in performance at or near junctions (connections) of members, and in estimation of crackwidth. Because the control on possible crackwidth during service is required, adjustments on account of plasticity (say redistribution of moment) can be done only if crackwidth can be predicted for such situations within acceptable accuracy. For plates (slabs and wall panels) having two way spans, moment coefficients from table 26 of IS 456-2000, which are based on modified (by 1.33 times) yield line theory, with readjustments are normally acceptable as proven by experience. In such plates permitting the elasto-plastic behaviour of concrete and reduction of peak moments, in serviceability state for -ve BM tensile stress in steel should be ≤190 N/mm² or crackwidth be ≤ 0.15 mm. For rectangular wall plate subjected liquid load, elasto-plastic analysis can be done wherein peak moments are reduced by ≤ 20%. For moments in other plate shapes and loading, results of elastic analysis can be used. Flat slab design by direct method (31.4 of IS 456) is not applicable. For flat slab design, high ductility is to be imparted. Norms for achieve high ductility for column width of flat slab are not available at present. Under combination of earthquake action, the flat slab participates in the frame action, demanding very high ductility. Where flexural moments are calculated by a method of linear elastic analysis of frame, the redistribution of maximum -ve BM in a continuous span can be done to a limited extent. Follow procedure as per 37.1.1 of IS 456, except that limit of reduction shall be 20% in place of 30%. A reduction of critical moment up to 20% can be done (as permitted by ACI 350) if strain is substantial (i.e. moment is a peak or maximum value in the member) but limited, at the section where redistribution is permitted. For under-reinforced sections the limiting strain limits are satisfactory. Such reduced BM shall be used for calculating redistributed moments at other sections of the member by principles of static equilibrium. For sway frames no redistribution on account of plasticity will be permitted. For flat slab, the details of design for ductility of column width for combination under earthquake action are not recommended at present. The floor member of tank, which is part of lateral action resisting frame has to be conforming to IS 13920. For LRC members, the ductility requirement is small, and need not conform to IS 13920.

R 4.4 Limit State Design (LSD) Modern approach require considerations of structural safety (ultimate bearing resistance to actions), serviceability, and sustainability. R 4.4.1.1 Limit State of Collapse (Ultimate limit state): Load combinations are given in Table 1.

R 4.4.1.2 Limit State of Serviceability : R 4.4.1.2 a) Deflection check is required as per 23.2a of IS 456, and b is not applicable. For deflection check only 70% of FL can be treated as long term load (accounting creep coefficient) and remaining 30% as short term (no creep). For tanks which may remain filled up for a long time (say the provisional storage for firefighting or units of treatment plants remaining full most of time) 100% FL should be treated as long term. Earth load shall also be a long term load. For deflection due to long term loads, effective modulus of elasticity shall be obtained by short-term modulus of elasticity (Ec) divided by (1+θ). Creep coefficient θ be taken 2.2 for DL & FL, and zero for other loads. (As per IS 456).

R 4.4.1.2 b) At the concrete surface, estimated crackwidth (i.e. calculated by the procedure specified) due to the restraining effects on temperature and shrinkage (length change) should not exceed 0.2 mm. Also at the concrete surface crackwidth is estimated (i.e. calculated by the procedure specified) for the serviceability loads (1 DL + 1 FL + 1 IL only). Here the effects of temperature & shrinkage is not taken additive to the loads. The temperature-shrinkage effect or load effect should be taken independently for crackwidth check, and will not be combined.

Author recommends that for the bottom face of roof of tanks storing chlorinated water or for very severe or extreme exposure, crackwidth limit 0.1 mm should be adopted. For members in contact with sewage (in STP) also the crackwidth limit 0.1 mm should be adopted. For guidance on reducing the limiting crackwidth, refer the optional requirement given in 4.4.3.2 & 4.4.3.3.

It should be noted that in IS 3370, while ‘severe’ exposure condition shall be considered, the limiting crackwidth is 0.2 mm, and not 0.1 mm as specified in IS 456 clause 35.3.2. In this regard the provision of IS 456 is supposed to be over-ruled by the provision of IS 3370. This code (IS 3370) has not given the limiting crackwidth for ‘very severe’ and ‘extreme’ exposure, however a lower value say 0.1 mm as limiting crackwidth may be taken relevant. The international trend is to assume that there is no significant relationship between crackwidth below 0.2 mm and the durability of concrete member. However, for more severe environment, for better control on permeability, lower crackwidth can be preferred.

R 4.4.1.2 c) In addition to crackwidth check, the general recommendation (internationally) is also to limit the tensile stress in steel and compressive stress in concrete while checking crackwidth. For these limiting values refer the clauses and discussion at RB-1 for section in flexure, and RB-4 for member in direct tension.

R 4.4.1.2 d) Limit state of ‘Maintainability or Restorability’ need considerations. Some functional requirements

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like performance of movement joints need maintenance and restorability. (It is also called conservation.) It may not be possible to design these joints for service life of structure. These joints should be designed such that without structural damage or modification, joints can be restored to perform. Normally limit states of fatigue strength and fire resistance etc. are not required in LRC structures.

R 4.4.1.3 Partial Safety Factors : Partial material factors (γm) as per IS 456 are 1.5 for concrete and 1.15 for steel. Designer may take higher γm for LRC. Increase in these factors by 10 to 15%, enhances the reliability without significantly increase in cost.

R 4.4.1.4 Load Combinations :Load combinations with partial load factors are given in Table 1 (reproduced below).

Table 1 Load Combination and Load FactorsCase Ultimate Limit State Limit State of Serviceability

DL FL EP IL WL/EL* DL FL EP IL WL1 2 2 4 5 6 7 8 9 10 11

1 1.5 1.5 1.5 1.5 0 1.0 1.0 1.0 1.0 02 1.2 1.0 1.0 0 1.4 1.0 1.0 0.7 0 0.32a 0.9 1.0 1.0 0 1.42b 1.4 0 1.0 0 1.43 1.2 1.2 1.2 1.2 1.2

*Consider WL or EL (Seismic) each separately.

Notes :1 Wind and earthquake load should be considered one at a time and not simultaneously. 2 For any combination, the partial safety factor for liquid load (FL) may be further reduced if it is expected

to give more critical design action at a section of a member. Liquid load may vary from zero (tank empty) to 1.5 in a combination. Same holds true for earth pressure and IL in load combinations. 3 Earthquake base shear shall be worked as per R 4.2.4. The earthquake base shear (or force action) as determined shall be multiplied by the load factor as given in Table 1, for further combination with other loads.

While applying the load combinations, care must also be taken to account loadings in design for transient, short duration or in service conditions that may arise during the construction or the operation of the a structural unit. Conditions to be considered may arise when the flow of the liquid may lead to unequal hydraulic pressures at different locations of the structure. Similarly differential earth pressures, with presence and absence of soil or other fill material or deposits, may act for some duration during construction, which may produce critical design actions at some locations, need to be considered.

R 4.4.1.5 Integrity and robustness are also important attributes of the structure.

R 4.4.1.6 Due to shear, crackwidth is not checked. If shear reinforcement is provided for serviceability state, the gross nominal shear τc shall not exceed τmax given in Table 24 of IS 456.

R 4.4.2 Basis of Design :Junctions (connections) of members should be assumed rigid. For frame analysis, and for analysis of

continuous plates (slabs or wall) centre to centre span should be considered. Frame analysis can be done on the basis of relative stiffness of members or frames can be analysed by stiffness method or moment distribution (or other established method).

R 4.4.2.1 Redistribution of moment on account of significant plasticity (as dealt in 22.7 & 37.1.1 of IS 456) is not permitted. [ACI 350 permits up to 20% of BM to be reduced on account of plastic redistribution with some limitations.]

Flat slab design as per 31.4 of IS 456 is based on plastic redistribution of moments, and cannot be permitted for suspended floor slab of elevated tanks, because of inadequate control on ductility and possible under estimate of crackwidth. Flat slab design as per 31.4 of IS 456 may be permitted for roof slab of tanks and also for floor slab (slab on grade) of ground tanks. Flat slab analysis if done by finite element method will be acceptable for design in all cases with provision of high degree of ductility imparted to it. [ACI 350 permits the flat slab design based on direct coefficient method. However minimum reinforcement requirement is also quite high.]

Also simplified estimate by coefficients (as in 22.5 of IS 456) cannot be permitted. For rectangular two-way slab subject to uniformly distributed load, bending moment coefficients are given

in Table 26 of IS 456. These coefficients obtained from modified yield line (multiplied by 4/3 times) are acceptable for design of liquid retaining concrete including crack control. Use of elasto-plastic behaviour of structure can be made, for members with ductility and limiting plastic strain considered. Also see R 4.3.2.

However it should be noted that tables for bending moment coefficients for triangular liquid pressure (say for walls) are generally based on elastic analysis or linear finite element analysis. For a loading configuration, uniform pressure and hydrostatic pressure (triangular) can be added. But using the coefficient tables, difference of the two cannot be permitted. In other words BM worked out by table 26 (of IS 456) for uniformly distributed load and BM for triangular load, cannot be deducted from each other.

The sections designed in flexure shall be under-reinforced (i.e. less than balanced wherein compression governing failure is avoided) not be over-reinforced.

R 4.4.2.2 For slabs and walls, if the shear stress calculated exceeds the permissible value, shear reinforcement should be provided. The shear reinforcement should be designed for a shear capacity Vu – 0.7 τc bd , (refer 40.4

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of IS 456). For arriving at the value of τc , correction due to direct compression (40.2.2 of IS 456) or tension (4.4.2 a of IS 3370-2) as applicable shall be applied.

R 4.4.2.3 Permissible bond stress depends upon the permissible slip which has to be low for LRC. Hence permissible bond stress has to be little lower for LRC. However no change is recommended for deformed reinforcement. Bond strength shall be reduced at least by a multiplying factor as follows: Fusion bonded epoxy coated deformed bars 0.80, Plain bars (not deformed) 0.625, Plain coated bars 0.50.

For member (also reinforcement) is in direct tension, the bond stress or lap length should be modified as per clause 26.2.5.1 c of IS 456, which specifies lap length as 2 Ld .

Following are additional considerations for shear and tension design.(a) Shear strength reduction due to tension in member (ultimate or factored Pu) should be accounted.(b) Shear strength of a member at construction joint reduces compared to monolithic concrete, depends largely on interface roughness and cleanliness of the surface at the time of placing second phase concrete. (c) Half the average height of valley to peak, can be measured as roughness. Roughness more than 3mm gives satisfactory performance if shear is not high. For higher shear resistance, combination of diagonal reinforcement, dowels and shear key can be designed to meet the strength demand in shear. (d) In absence of better estimate, shear strength of concrete (as diagonal tension) can be assumed to be two-third, and strength in direct shear can be assumed to be one half at construction joint. No reduction shall apply on the contribution by reinforcement. At construction joint permissible shear strength is reduced, to reduce slip (hence crackwidth).(e) Area of reinforcement for hoop tension or direct tension shall be calculated assuming all tension to be taken by reinforcement and tension in concrete is neglected i.e. concrete is assumed cracked.

R 4.4.2.4 If for floor beams of tank, the depth to span (c/c) ratio is >0.40, it requires reduction of lever-arm for calculations of tensile steel.R 4.4.2.5 Critical section for design of a member shall be at the face of other connecting member, except wall to wall junction (if continuous) having tension at the inside corner (opening type junction), for which design BM shall be that at centre of junction of members.R 4.4.2.6 For elastic stability and prevention of buckling of shells of container should be adequately assessed.

Check : ( R / t ) < 120 √( Ec / p) ; hereR = radius of curvature of shell in m, t = thickness of shell in m, Ec = Elastic modulus of concrete in N/mm² ( Ec = 5000 √fck ) , p = pressure normal to surface of shell (superimposed loading including the component due to self-weight) in N/m² (un-factored).Note: Constant ‘120’ may change from 100 to 150. Higher value can be taken where tolerance in deviation of the middle surface from as

required is very tight and measured. If there are no specifications applied for tolerance, the constant should be reduced to 100.In case of roof dome, load normal to surface shall be due to DL + (IL or that due to wind or EL).

R 4.42.7 For thin shells (e.g. dome), the direct compressive stress due to factored membrane compression shall not exceed 0.24 fck .R 4.4.3 Crackwidth :

Estimate of minimum reinforcement, crack spacing and crackwidth due to temperature and shrinkage effect in early age (immature) concrete is given in Annex A of the code. Estimate of crackwidth in mature concrete is dealt in Annex B of the code.

The calculated crackwidth is assumed to have an acceptable probability of not being exceeded. If little wider cracks (say up to 0.3mm) are noticed in the completed structure the matter should be under observation and may be investigated. If crackwidth reduces with time and settles within permissible while liquid load is full, the situation can be treated as acceptable. Few occasional wider cracks (>0.3mm) noticed in the structure will not make structure unacceptable, if the design calculations and construction are proper. However these wider cracks should be cement grouted and sealed as may be desirable for the situation of the cracks, if investigation does not indicate design or strength deficiency. Structure with wider crack may become unacceptable as a result of investigation if local damage, serious workmanship flaw, or leakage is also noticed along with a tendency for the crackwidth to increase. Such cracks shall be treated (grouted by low viscosity epoxy or polymer) and sealed.

Satisfactory behaviour with regard to crackwidth could be achieved by properly placing adequate reinforcement at suitable smaller spacing. The reinforcement required to control cracking in immature concrete may also be totally accounted for crack control for service loads. This means the requirement of minimum steel for crack control in immature concrete, and amount of steel required for service load, are not additive. Similarly effect of load and effect of temperature during service are also not taken as additive.

The underside of roof members remains 100% humid most of the time. Hence these shall also be checked for the crackwidth limit similar to the liquid retaining members.

Before applying crackwidth check, section of member is already designed, i.e. member size and amount of steel on each face of member is determined. Next the crackwidth checked. If it is found more than the specified limit (as say 0.20mm), designer has following options.

(a) Reduce the steel bar size and the spacing of bars, however it is advisable to have spacing not less than 80 mm c/c, nor less than 5× diameter of bar for slabs & wall.

(b) Increase the area of steel, i.e. reduce deign stress in steel, may be bar size increased.(c) Increase the section size say depth/ thickness of member and re-proportion the reinforcement.

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After these modification, crackwidth check is to be applied again.

R 4.4.3.1 Crackwidth may be deemed to be satisfactory if service stress in steel does not exceed the enveloping values given in Table 2. This table is applicable for reinforcement in all members like wall, slab or beams. These limits are irrespective of strain in concrete and for spacing of bars being not more than 300 mm c/c. If stress in steel is more than these limits, crackwidth check shall to be applied by detailed calculations.

Table 2 - Maximum tensile stress in steel reinforcement under limit state of serviceabilityTension in steel in limit state of serviceability

Limiting crackwidth Plain round bars Deformed bars0.10 mm 85 N/mm² 100 N/mm²0.15 mm 95 N/mm² 110 N/mm²0.20 mm 115 N/mm² 130 N/mm²

Tensile stress higher than that in Table 2 (deemed to limits) is acceptable, if detailed check for crackwidth is carried out and found to be within limit.

Similarly Table 3 gives limiting stress in steel and the spacing for ‘deemed to’ criteria for 0.2 mm crackwidth. If the criteria is fulfilled, no further detailed check for crackwidth is required.

A member in combined bending and compression, if has compression on the two extreme fibres (i.e. neutral axis is outside the section or eccentricity of compression is small), it can be said to be case of compression predominant. In such a case no tension develops, hence no cracking and crackwidth check is required.

For a member in combined bending and axial force (tension or compression), if tension develops on one face and compression on another (i.e. neutral axis is inside the section) it can be said to be case of bending predominant. In such a case crackwidth check is to be applied as a flexural member.

For a member in axial tension with or without combined bending, such that depth of neutral axis is less than 50 mm or it is outside the section (i.e. tension on both faces), the section will be termed as predominantly in direct tension, and calculations will be done as a member in direct tension.

Section size and reinforcement on each face as arrived at, is analysed for stresses under service load, which is same as working stress method. For this modular ratio shall be as per IS 456, annex B. The crackwidth formulae are adopted from British code, wherein higher value of modular ratio (m) is specified. Hence it can be (recommended by author though not in code) that in place of ‘m’, a modified value of ‘1.5×m’ may be taken in to account (i.e. ‘m’ value enhanced by 50%). Thus the calculation involves grade of concrete, however its effect on crackwidth is very small.

Section analysis for depth of neutral axis requires solution of cubic equation, which can be done by a computer programme. For a depth of neutral axis, the maximum compressive stress in extreme fibre and tensile stress on tension steel will be calculated, and further calculate crackwidth.

In serviceability state, it is prudent to limit both the compressive stress in concrete to 0.36 fck and tension in steel to 0.53 fy (220 N/mm² for 415 grade) for long term crack control. In British practice (refer Design of Liquid Retaining Concrete structures by R.D. Anchor) the stress in steel is also limited to a lower amount. Also recommends that in serviceability state, for in direct tension the steel stress should be limited to 0.5 fy (208 N/mm² for 415 grade steel).

From stress, strain in steel can be calculated, and reducing it for concrete stiffening, crackwidth can be calculated using equations given. The estimated tension stiffening is the maximum capacity possible, however this maximum value may be mobilised in all the cases. The tension stiffening of concrete can further reduce at the section of construction joint, or at the section of curtailment of bars.

At any section, the strain reduction due to tension stiffening cannot be more than 2/3rd of the strain in steel. At a construction (or partial contraction) joint, the tension stiffening reduces (say by 1/3rd of its value), hence crackwidth will be higher.

It should be noted that as per the procedure estimated crackwidth value is almost unaffected by the grade of concrete, its modulus of elasticity and its tensile strength. While the grade of concrete increases, the modular ratio (m) decreases, which has a very small effect on the calculated crackwidth. Hence the equations will need corrections when high strength concrete (say grade > M35) or concrete with higher flexural strength (> 4.2 MPa) is used. Hence the procedure cannot be applied to fibre concrete or ferro-cement.

It should also be noted that in some cases, if steel on compression side of section is accounted the estimated crackwidth will be slightly higher, but it may be permissible to neglect steel in compression in such case.

R 4.4.3.2 A liquid retaining member may be classified in to water-tightness class, as susceptible to the possible leakage related to crackwidth recommended. Note that concrete always permits the passage of small quantities of aqueous liquids by permeation and diffusion.

Classification of Water-Tightness - Giving limiting crackwidth in mmTightness

CassRequirement for leakage{ H/t is between 20 to 30 }

[ # whichever is more ]

Cracks through, no compression block (direct tension)

Compression block < 0.2 t or 50mm #

Compression block ≥ 0.2 t or 50mm #

1 Leakage to be limited to a small amount. Some surface staining or damp patches acceptable 0.15 mm 0.20 mm 0.20 mm

2 Leakage to be minimal, Appearance not to be impaired by staining. 0.10 mm 0.15 mm 0.20 mm

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Guide to IS 3370 – 2020

3 No leakage permitted. Special measures such as Prestressing or impermeable liner required.

Crackwidth nil with prestressed. OR - Leakage prevented by lining (and in concrete limiting crackwidth as for class 1).

Notes : 1. No compression block means member section with neutral axis outside section (or eccentricity of tension small) or member in direct tension or hoop.

2. The tightness classes of the wall and floor of a tank can be different.

R 4.4.3.3 Recommendation about crackwidth related to water-tightness class is given in table above. Most LRC can be assumed to be of class 1. Where aesthetics is important or the passage of pollution through concrete is important, the tightness class 2 can be applied. Tightness class 3 is where no permeation of liquid through concrete or wetness is permitted. Where appearance of wet patches are not acceptable in addition lining is necessary.

R 4.4.3.4 The limiting crackwidth values as recommended in table above (related to water-tightness class) may exceed by 0.05 mm, if H/t ratio is ≤ 20. In no case crackwidth shall exceed 0.2 mm. If H/t ratio is ≥ 30, the limiting crackwidth shall reduce by 0.05 mm.

R 4.4.3.5 The crackwidth at construction joint can be calculated by reducing the tension stiffening by concrete by 33.3%. However, crackwidth shall be assumed to be not more than 0.05 mm, nor more than 50% of the crackwidth without reducing tension stiffening i.e. the crackwidth estimate for monolithic section.

R 4.4.3.6 For crackwidth enhancement due to shear, at that section the flexural moment in slab shall be enhanced by equivalent moment due to shear force (Mes) for crackwidth check only. Mes = SF × (D/3),

D is overall depth of slab at the section considered. If shear reinforcement is provided, enhancement by the equivalent moment need not be considered.

R 4.4.3.7 The estimated crackwidth has a probability of not being exceeded. An occasional wider crack in a structure should not necessarily be regarded as evidence of local damage unless leakage is unacceptable. Reduce leakage by rectification say grouting. Even if crackwidth appear to be within permissible limit, but unacceptable leakage takes place which does not appear to reduce, it should be controlled by grouting.

R 4.5. Stresses Due to Moisture or Temperature ChangesR 4.5.1 The clause is applicable if the tank is to be used only for the storage of water or aqueous liquids at or near ambient temperature and the concrete never dries out; and adequate precautions are taken to avoid drying and hence cracking of the concrete during the construction period and until the tank is put into use. In cases of calculating stresses due to shrinkage, assume shrinkage coefficient as 300×10-6 for concrete not having cement (OPC) more than 350 kg/m³ or total cementitious content not more than 400 kg/m³. For higher cement content the shrinkage coefficient will be higher.R 4.5.2 If tank can remain empty for more than a month, or where impermeable coating or lining is applied, the concrete can dry out and total shrinkage can be much higher, and the requirement of temperature-shrinkage (i.e. minimum) reinforcement would be higher (33% to 50%).

R 4.6 Between various members (e.g. between wall & floor or wall & wall) junctions are intended to be rigid. The junctions (connection) should be designed accordingly and effect of continuity should be analysed and accounted in design and detailing of junction and each member. For LRC members the capacity of a junction to resist force actions (moment, shear etc.) should not be less than the maximum estimated actions within the junction.R 4.7 Temperature and Shrinkage Effects

Experience has shown that minimum of 0.22% (434 mm² for 200 mm thickness) has given satisfactory performance without any movement joint for ground tanks up to 16 m size. For ground tanks of size about 22 m, above minimum steel is found inadequate. Hence for small tanks minimum steel should not be increased. Minimum steel should not be increased for vertical direction, unless tank height is more than 15 m or it is restrained vertically to other source.

Though not clarified in the code, it can be stated that horizontal minimum reinforcement may be higher depending up on the horizontal size of structure (continuous construction), and also with larger spacing of movement joints. For elevated tanks which are restrained by structural features or by other structure, minimum reinforcement will be similar to ground tanks. Otherwise elevated tanks normally have very little restrains against linear temperature, moisture & shrinkage movements, hence requirement of minimum reinforcement does not increase substantially with the size of structure. Alternately minimum steel shall be calculated as per A-1.2 in code. For a tank if roof is free to slide and is not rigid with the walls, % minimum steel in roof can be reduced to that in one lower range of size.

Continuous construction without movement joints (as per option 1) can be done for ground tanks normally up to 30 m size. Above 30 m size provision of expansion joint is a normal solution, though tanks of more length (say 50 m) can be designed without movement joints.

Designer has an option to calculate the minimum steel as per the provisions of the code, depending upon the spacing of movement joints. Where concrete grade is higher than M30, designer must calculate the minimum steel required as critical steel ratio.

To economize on the minimum steel for ground tanks, top of foundation PCC should be a flat and have smooth surface with bond breaking sheet (as per 11.3.1.2 of IS 3370 part 1) to facilitate sliding and thus reducing the restrain. The thickness of such bond breaking sheet will depend upon the roughness of the top of PCC base. For PCC having plane and fairly smooth surface, about 1mm thick LDPE (polyethylene) sheet is recommended by British practice. Also see R 9.2.8 b.

Options (as per Table 2 in part 1) may be used to design movement joints at closer interval and also design the temperature shrinkage steel. For the continuous construction (Option 1), PCC top may have slight slope and gradual thickening, and also bond braking layer is not required. Where tanks are small, the horizontal tank size can be treated as spacing of movement joint and minimum steel can be smaller (as per option (Table 2 in part 1).

Temperature-shrinkage effects are in two groups, one the linear effect (in the plane of member), and second the gradient across the thickness of section of the member. All these effects are relaxed by cracking. The effect can also be subdivided as long term and short term. Long term effect will also have further relaxation by creep of concrete.

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Guide to IS 3370 – 2020R 4.7.1 Shrinkage Coefficient : In reinforced concrete (different from Prestressed), the effect of temperature & shrinkage get relaxed (reduced) due to creep and cracking of concrete. While accounting temperature fall (from peak due to heat of hydration at about 1 to 3 days), shrinkage can be assumed to be negligible in the immature concrete. In the calculation method given in annex B, a reduction of strain (100×10–6) is suggested on account of creep.

Relaxation due to cracking can be assumed to be included in the value specified (300×10–6). Shrinkage has two components, one the irreversible (physio-chemical), other the reversible (i.e. moisture dependent), total shrinkage if not known can be assumed to be 300×10–6. For M30 grade concrete typical total shrinkage is much higher compared to 300×10–6, which is a reduced value possibly due to absence of moisture dependant component and relaxation due to cracking.

While the component is in contact with aqueous liquid, only chemical shrinkage i.e. irreversible part (33 to 40 % of total) will be considered, and this gets almost compensated by creep. Hence in combination with liquid load, shrinkage may be neglected.

It should be noted that when cementitious content increases (> 400 kg/m³) the shrinkage will be higher and higher minimum steel will be needed.

R 4.7.2 For tanks protected by internal impermeable lining the design strain will higher due to possible drying of concrete. Hence design has to consider higher strain by about 150×10–6 (say total 450×10–6), and permit higher crackwidth if crack bridging property of the system of lining can be assured.

R 5 FLOOR R 5.1 Provision of movement joint is linked to the basis of minimum steel. This is explained in Part 1. R 5.2 Floor can be assumed to rest on ground if proper foundation conditions are met with. If subjected to uplift, it should be designed for bending due to net upward pressure,R 5.3 Floors not supported are also called suspended slabs, as are required for elevated tanks. For floor slabs up to 200 mm thick, in the region of +ve bending moment, reinforcement can be provided at the bottom face of the slab only (no steel at top face); and in such a case the bottom reinforcement shall conform to the minimum steel requirement for the total slab thickness. For slab thickness more than 200 mm, at least one third of minimum steel should be placed at top face, and total steel of top and bottom face together should conform to minimum steel requirement. In the region of -ve bending moment, the top reinforcement should not be less than half the minimum for gross thickness. R 5.4 In most cases for floor beams of tank, the depth to span ratio is >0.40, which requires reduction of lever-

arm for calculations of tensile steel.

R 6 WALLS For the monolithic construction of RCC floor and wall, sympathetic vertical cracks can develop at the locations of movement joints in the floor. Hence walls should also be provided with movement joints similar to floor. Where a pipe passes through a wall or floor, it is preferable to cast the pipe when member is concreted. Alternately a box-out may be kept with additional precautions. In all cases pipe should not be very near to a joint being provided in concrete. The joint between member concrete and box-out should be treated as a construction joint and should be grouted adequately. At the position of pipe the thickness of concrete should be increased and extra reinforcement provided to take care of stress concentration and extra stresses due to force/ restrain to movement on pipe.

R 7 ROOFS Roofs should be designed with sufficient slope or camber to ensure adequate drainage accounting for any long-term deflection of the roof due to the dead loads, or the loads should be increased to account for all likely accumulations of water due to long term deflection by accumulated water itself. If deflection of roof members may result in ponding of water accompanied by increased deflection and additional ponding, the design must ensure that this process is self-limiting. For roof slabs up to 200 mm thick, in the region of +ve bending moment, reinforcement can be provided at the bottom face of the slab only (no steel at top face); and in such a case the bottom reinforcement shall conform to the minimum steel requirement for the total slab thickness. For slab thickness more than 200 mm, at least one third of minimum steel should be placed at top face, and total steel of top and bottom face together should conform to minimum steel requirement. In the region of -ve bending moment, the top reinforcement should not be less than half the minimum for gross thickness. For large roof slab having multiple panels, exposed directly to external environment (i.e. without and soil cover or insulating cover), the top reinforcement near the support (for 0.1 × clear span) shall not be less than 0.2% of gross section of the slab; unless slab is specifically designed for temperature and shrinkage requirement (duly relaxed by creep & cracking) superimposed with DL.

R 8 DETAILING The dimensions of structural components in local areas of the structure, and specifying the structural details are the parts of structural detailing. Detailing is to be done on drawings.R 8.1 Minimum Reinforcement Minimum reinforcement is also called as temperature-shrinkage reinforcement which can take care of normal cracking due to same and avoid repeated calculations for normal effects of temperature and shrinkage.With the revision of codes this minimum reinforcement is being increased. In general the requirement of temperature-shrinkage steel increases with the size of structure and also with the degree of restrain. For large size tanks there are always some cases where need of steel may be more than the minimum specified. However, the recommended minimum steel is higher than the need for small works. In India there are very

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many small water tanks for rural water supply scheme. For the small tanks which have behave satisfactorily for 5 decades, cannot be overburdened with higher minimum steel, due to bad experience of poor construction or for higher need felt for larger work.

[ As per Table 2 in IS 3370 part 1, where joint are at smaller spacing (option 3) recommended steel is 2/3rd. Thus in small tanks ≤ 7 m minimum reinforcement recommended can be reduced. ]

It should also be noted that, container of elevated tank has much smaller restrain and will need smaller temperature-shrinkage steel. Floor slab of elevated tank need more steel for BM due load, compared to a floor slab of ground tank. That cannot be an argument for higher minimum steel for elevated tanks. Hence the author recommends the minimum steel as per table below. {ACI requirement is >1.35× than below.}

Type of reinforcement Elevated Tank Ground supported Tank≤ 10 m 20 m 30 m ≤ 7 m 14 m 22 m 30 m

Plain Mild Steel -Grade 250 0.33 % 0.50 % 0.60 % 0.33 % 0.50 % 0.60 % 0.66 %

High yield strength deformed – grade 415 0.20 % 0.30 % 0.36 % 0.20 % 0.30 % 0.36 % 0.40 %

NOTES: 1. For ground tanks, spacing of movement joints will control the minimum reinforcement.2. For intermediate size, interpolation can be done.3. Length is counted along the direction of steel provision.4. Designer has to take decision for lengths higher than 30 m.5. Above minimum is valid if tank does not dry-out completely, or else higher steel will be required.6. For bars of grade higher than 415, the amount of minimum steel can be taken inversely proportioned to the characteristic

strength (in N/mm²) of bars. For slabs on grade (resting on PCC & in turn on ground without separation sheet) less than 300 thick, steel may not be provided at bottom face. On the assumption that sub-base and ground below will provide friction (as continuous retrain) and hence need of steel on bottom face is eliminated. This is for continuous construction without bond braking layer put on PCC. Where bond braking layer is provided and spacing of movement joints is designed, the reinforcement in bottom layer can be half that in top layer (and need not be zero). Though not clarified in the code, it should be noted that in ground supported tanks, it is preferable to give expansion joints at 30 to 40 m spacing. However for such a large spacing of expansion joint, proper design of minimum (temperature/moisture/shrinkage) reinforcement be provided.

Minimum reinforcement in floor slab on grade (without separation layer) should be by % of the surface zone specified for each face as follows. D = thickness of slab.

Slab Thickness less than 300 mm 300 to 500 mm > 500 mm Top zone in mm D/2 D/2 250 only Bottom zone in mm Nil 100 only 100 only

Minimum % steel specified will apply to all members. For the floor slab on grade (i.e. continuously supported by ground), for which it will be in % of the surface zones thickness specified.

R 8.2.1 Size (i.e. diameter) of bar and spacing of bars (in slabs and walls) could be small as practically possible without causing congestion of steel or difficulty in placing and vibrating concrete. In walls and slab minimum preferable c/c spacing can be nearly equal to 5× diameter of bar or 75 mm whichever is higher; reducing spacing below such a limit has no significant advantage. Maximum permissible spacing is 300 c/c. This limit on spacing is also applicable for minimum steel or distribution steel. For beams the minimum clear distance between bars can be smaller. However in beams one of the clear space between horizontal bars should not be less than 75 mm for pouring the concrete and for insertion of needle vibrator.

R 8.2.3 Condition of ‘spacing of bars not more than thickness of member’ should not be applicable to lightly loaded members provided with thickness in excess of the requirement. This spacing limit can be treated as a requirement for highly stressed members requiring much higher area of steel, and do not apply to small members. There is already a limit of 3× the effective depth of member. For small tanks and small members with thickness up to 200 mm, this criterion unnecessary requires steel much more than the minimum. The amount of steel (in mm²/m) required would be more as the thickness of member decreases. This requirement is penalty on small works. This limit on spacing may be complied only if bars are >10φ, or reinforcement required is >450 mm²/m on a face of a member. It is preferable to keep spacing small (say ≤ 150c/c) where reinforcement on a face of member required is >450 mm²/m. On the side face of a beam in region of designed shear reinforcement (high shear near support), vertical space between longitudinal bars if more than 400 mm, minimum longitudinal 1×8φ skin /surface reinforcement as should be provided in between.

R 8.2.4 As far as possible minimum size (i.e. diameter) of bar should be as below.Footing – 10φ; small footings (<1.2 m) – 8φ; Beams longitudinal anchor /corner bars 10φ; column longitudinal bars 12φ; slab, wall & shells 8φ. For small tanks (< 50 m³), or designed as PCC bars can be smaller.

R 8.3 Junction (connections) : This applies to opening type right angled (‘L’) junctions of wall to wall or wall to slab, and bending tension is on inside face. Note that the tension bars of a member shall be bent as hairpin and come out on other face for at least d/2, where d is effective depth. The fillet bars (at 45 to main tension bars)

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should have area of steel not be less than 40% of main tensile steel requirement. See Fig 17. The two members meeting at a junction may not have same thickness, as well main steel also may not be same. Filet bars can be omitted if main bars are 40% more than the requirement. The reinforcement design at the junction shall consider the BM at the point of meeting of the two members, and not at face of junction.

R 8.3.1 At the inside face of wall (subject to tension, bending & shear), at bottom end (rigid junction with floor slab) of an elevated tank, the minimum steel should be 0.2% of the wall thickness.

R 8.4 Any steel bar or a ring shall not lay in a construction joint, or parallel to it within 25 mm on either side. If a bar is in this region, it should be moved away from construction joint.

R 8.5 Cover blocks shall be of non-corrodible material. These can be made of concrete of ≤ 8 mm maximum aggregate size, with water-cement ratio ≤ 0.4 and strength grade ≥ M40. These should be made by casting in suitable moulds and compacting by vibration and/or pressed (under pressure) or hammered under wooden mallet, and well cured in pond for >14 days. For adequate durability, uniformly the specified cover should be achieved in RCC at all places.

R 8.6 Within ‘T’ or ‘L’ junction diagonal tension is produced. In these junctions the tension bars changes its direction by right angle. The tension reinforcement shall be well anchored beyond the position of maximum stress, which is nearly at middle of the curved portion of the bar. (See Fig 17 to 19).

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Guide to IS 3370 – 2020

R 8.7 CONTAINER : Typical detailing for members of container are given in Fig 23 & 24.

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Guide to IS 3370 – 2020

8.8 Curtailment of bars : Note that at the position of curtailment of bars, the crackwidth enhances depending up on the ratio of area of bars curtailed to total bars (including that continue). Hence bars should not be curtailed at construction joints, as well as at sections where crackwidth estimated are critical. Bars should be curtailed away from such critical sections. For many bars to be curtailed, curtailment position should be staggered.

R ANNEX AR A-1.2 Crackwidth check is required on a face of concrete member, hence for the steel ratio ρcrit is based on the gross concrete area (i.e. no deduction of steel area) in a surface zone of member under consideration. Below is a table for fct & critical steel ratio ρcrit, for steel grade 415 (= fy).

fck M20 M25 M30 M35 M40 M45 M50 M55 M60fct 1.00 1.15 1.30 1.45 1.60 1.70 1.80 1.85 1.90

ρcrit 0.18 % 0.21 % 0.23 % 0.26 % 0.29 % 0.31 % 0.33 % 0.34 % 0.34 %Option 1 0.25 % 0.29 % 0.33 % 0.37 % 0.40 % 0.43 % 0.46 % 0.47 % 0.48 %Option 2 0.18 % 0.21 % 0.23 % 0.26 % 0.29 % 0.31 % 0.33 % 0.34 % 0.34 %Option 3 0.12 % 0.14 % 0.16 % 0.17 % 0.19 % 0.20 % 0.22 % 0.23 % 0.23 %

Options are as per Table 2 in IS 3370 part 1.

R A-1.4 The percentage of steel ρ is based on concrete area in the respective surface zone. Estimated shrinkage εcs can be assumed as 300×10–6.

In equation 2, ρ is steel ratio based on area of surface zone. [As per IS 456 the bond strength for deformed bar is 1.6 times that for plain bars. Hence, for deformed bars fct / fb ratio will be 0.625 in place of 2/3].

Equation 5 & 6 are for calculating crackwidth, as taken from BS 8007. BS code in figure A.3 gives locations

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Guide to IS 3370 – 2020

where the calculated crackwidth can be further assumed to reduce due be reduction in actual restrain. Estimate of strains are for the case where concrete has restrains (e.g. ground tank restrained by foundation). Corollary is that these calculations are not applicable for elevated tank having very low restrain.

R ANNEX B R B-1 The method of estimation of crackwidth is based on the assumption that the strain level in steel and concrete are not high enough. Crackwidth check is a serviceability limit state.

For the for serviceability limit state, the tensile stress in reinforcement should be limited to 0.6 fy (249 N/mm² for 415 grade), and concrete stress limited to 0.40 fck (12 N/mm² for M30 grade).

Code gives the guideline for crack due to flexure and separately for crack due to direct tension. However in most cases members are subjected to combined bending moment and direct tension. While neutral axis is within the section (i.e. one face of member is in tension & other in compression), the calculation can be done as for the case of flexural. If the both the faces of a member are in tension (i.e. NA outside section or depth of neutral axis <50 mm), the calculation can be done as for the case of direct tension. (NA = neutral axis)

It should be noted that the code does not differentiate between the monolithic section and the construction joint (or a partial movement joint). Compared to a section where member is monolithic, the section at a construction joint (or partial movement joint) will develop a little larger crackwidth under similar conditions otherwise. Refer R 4.4.3.5.

R B-3 For calculating tension stiffening based on crackwidth, the multiplying constants are as follows: For crackwidth 0.2 mm – 1.0 ; 0.15 mm – 1.2 ; for 0.10 mm – 1.5 .Tension stiffening dealt by equation 8 & 9 are for deformed reinforcement (uncoated). For other bars the tension stiffening strain be multiplied by following factors:

Fusion bonded epoxy coated bares 0.80 , plain bars 0.625 , plain coated bars 0.50 .R B-4 For members in direct tension, the tensile stress in reinforcement should be limited to 0.50 fy (i.e. 208

N/mm² for 415 grade). Limiting crackwidth is specified in clause 4.4.1.2 b & 4.4.3.2 of IS 3370 part 2.R B-6 For calculating tension stiffening based on crackwidth, the multiplying constants are as follows: For crackwidth 0.2 mm – 2 ; 0.15 mm – 2.4 ; for 0.10 mm – 3.0 , (replacing constant 2.) For 0.15 mm crackwidth equation will become –

0.8 bt Dε2 = ---------

Es As

R B-6.1 Where limiting crackwidth required is 0.1 mm, it is also desirable to keep the tension in concrete ≤ 0.27 fck

2/3 = fctk0.05

Concrete grade M20 M25 M30 M35 M40fctk0.05 1.98 2.31 2.61 2.89 3.16

Appendix C: Concrete Finishes [Not part of Standard]

Formed SurfacesSurface finish Type F1The main requirement is that of dense well compacted concrete. No treatment is required except repair of defective areas, filling all form tie holes and cleaning up of loose or adhering debris. For surfaces below grade which will receive waterproofing treatment the concrete shall be free of surface irregularities which would interfere with proper and effective application of waterproofing material specified for use.Surface Finish Type F2The appearance shall be that of a smooth dense, well- compacted concrete showing the slight marks of well fitted shuttering joints. The Contractor shall make good any blemishes.Surface Finish Type F3This finish shall give an appearance of smooth, dense, well-compacted concrete with no shutter marks, stain free and with no discoloration, blemishes, arises, air holes etc. Only lined or coated plywood with very tight joints shall be used to achieve this finish. The panel size shall be uniform and as large as practicable. Any minor blemishes that might occur shall be made good by the Contractor.

Unformed Surfaces : Finishes to unformed surfaces of concrete shall be classified as U1, U2, and U3, ‘spaded or bonded concrete’. Where the class of finish is not specified the concrete shall be finished to Class U1.Class U1 finish is the first stage for Class U2 and U3 finishes and for a bonded concrete surface. Class U1 finish shall be a levelled and screened, uniform plain or ridged finish which (unless it is being converted to Class U2, U3, or bonded concrete) shall not be disturbed in any way after the initial set and during the period of curing, surplus concrete being struck off immediately after compaction.Where a bonded concrete surface is specified, the laitance shall be removed from the Class U1 finished surface and the aggregate exposed while the concrete is still green.

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A spaded finish shall be a surface free from voids and brought to a reasonably uniform appearance by the use of shovels as it is placed in the Works.Class U2 finish shall be a wood float finish. Floating shall be done after the initial set of the concrete has taken place and the surface has hardened sufficiently. The concrete shall be worked no more than is necessary to produce a uniform surface free from screed marks.Class U3 finish shall be a hard smooth steel-trowel led finish. Trowelling shall not commence until the moisture film has disappeared and the concrete has hardened sufficiently to prevent excess laitance from being worked into the surface. The surfaces shall be trowelled under firm pressure and left free from trowel marks.The addition of dry cement, mortar or water shall not be permitted during any of the above operations.Tolerances in Concrete Surfaces Concrete surfaces for the various classes of unformed and formed finishes specified in various clauses shall comply with the tolerances shown in Table 1 hereunder, except where different tolerances are expressly required by the Specification or shown on the Drawings. In Table 1 ‘line and level’ and ‘dimension’ shall mean the lines, levels and cross-sectional dimensions shown on the Drawings. Surface irregularities shall be classified as ‘abrupt ‘or ‘gradual’. Abrupt irregularities include, but shall not be limited to; off-sets and fins caused by displaced or misplaced formwork, loose knots and other defects in formwork materials, and shall be tested by direct measurement. Gradual irregularities shall be tested by means of a straight template for plane surfaces or its suitable equivalent for curved surfaces, the template being 3 m long for unformed surfaces and 1.5 m long for formed surfaces.

Table 1 Maximum tolerance (mm) in:Class of Finish Line and level Abrupt

irregularityGradual

irregularityDimension

U1 ± 12 ± 6 ± 6 -U2 ± 6 ± 3 ± 3 -U3 ± 6 ± 3 ± 3 -F1 ± 12 ± 5 ± 6 +12, -6F2 ± 6 ± 3 ± 4 +12, -6F3 ± 3 ± 1 ± 2 +6, -2

RIGIDITY OF FORM WORK : (Rigid formwork are called moulds.)RF1 : Highly rigid mould (or forms) are very rigid as required for casting concrete samples for testing, say mould for cube or beam. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 0.2 mm. Tolerance for dimensions of mould should be within ± 0.2 mm.RF2 : Very rigid form as are used in precast factory. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 0.5 mm. Tolerance for dimensions of mould should be within ± 0.5 mm or smaller if in the specification.RF3 : Rigid form as are used for precast products cast on site but sensitive to tolerance. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 1 mm. Tolerance for dimensions of formwork should be within ± 1 mm or smaller if in the specification.RF4 : Normal formworks as are used for in-situ works. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 3 mm. Tolerance for dimensions of mould should be within ± 3 mm or smaller if in the specification.

June 2020 upgraded.

It is proposed that this document will be upgraded from time to time. Hence send your comments to -

Er. L. K. JAIN , Consulting Engineer,36 Old Sneh Nagar, Wardha Road,NAGPUR 440 015, IndiaEmail : lkjain.ngp@gmail.comPhone +91 712 228 4037 , fax +91 712 228 3335

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