repair stratergies by k. r. thanki

REPAIR STRATERGIES

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Sub.: MAINTAINANCE & REHABILIATION OF STRUCTURES (3360605)

UNIT -II

Prepared By:

Mr. K.R.THANKI (BE Civil)

LECTURER CIVIL ENGINEERING DEPARTMENTGOVERNMENT POLY TECHNIC ,

JUNAGADH,GUJARAT-INDIA.

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CONTENTS:

Causes of distress in structure

Construction and design failures

Condition assessment and distress diagnostic techniques

Inspection and evaluating damaged structure.

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The reason for distress during service is the lack of maintenance of the building which results in deterioration/aging of materials and structural components leading to corrosion and cracking.

Buildings or structures are damaged at different grades of damage when they experience extreme loading conditions like in severe earthquakes or cyclonic storms for which they are not designed.

Factors causing Building Distress :

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They may also fail if the building including the foundation is not properly designed and constructed following the standard Codes of practice. An impression exists that taller structures are seismically unsafe in comparison with low-rise buildings.

It is to be noted that most lives were lost in Kachchh (Gujarat) earthquake of 2001 in one and two storeyed masonry buildings. Hence, all buildings have to be built safe.


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Inadequacy of design and poor quality of construction and maintenance are therefore the main reasons for the distress seen in buildings during service or under natural hazards.

This is because building codes and byelaws are not conscientiously followed in design and quality of construction, nor in maintenance.


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The current [Indian standard (I.S.)] building codes and guidelines in India have been tested and found effective in achieving safety of the residents during the last six earthquakes (Uttarkashi 1991 to J & K 2005).

Hence not following these codes in design and construction is sure recipe for distress in future.


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A The building has not shown any signs of distress and It satisfies all the safety and serviceability requirements according to relevant Codes of practice, hence no action is needed towards retrofitting.

Condition Assessment of Buildings

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B The building is seen to be deficient (or distressed) but it can be repaired and strengthened to satisfy the Codal safety requirements or performance criteria set by the user.


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C The building is badly damaged. It is to be demolished and a new building may be built, build back better.


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Main steps of condition assessment will be

a) To record the damage if any, and find out the causes for distress

b) To assess the extent of distress and to estimate the residual strengths of structural components and the system including the foundation.

c) To plan the rehabilitation and retrofitting/strengthening of the building.


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Typical visible distress detrimental to the safety of buildings

Cracks in RC beams, Columns , slabs, masonry walls (particularly if the walls are load bearing walls) , spalling of concrete, sagging of beams or slabs , and tilting of columns or RC frames (out of plumb) and major failure of structural members are the typical types of crucial damages that will require structural repairs to bring back the lost strength. Such actions will need to be done along with retrofitting if that is also decided for the building in question.


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Diagonal cracks of the beam and failure at the column top

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Cracks in column

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Cracks in masonry wall

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Cracks in good quality brick wall

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Failure of a portion of building (at expansion joint)

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Failure in column-beam joint (lack of stirrups)

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Column failure (absence of tiers, buckling of longitudinal bars)

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Column top failure (bad joint detail)

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Corrosion in RC beam

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Corrosion in RC column

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Corrosion in the RC slab (lack of control on the cover)

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What does a structural failure mean?Building failure occurs when the building

loses its ability to perform its intended (design) function. Hence, building failures can be categorized into the two broad groups of physical (structural) failures (which result in the loss of certain characteristics, e.g., strength) and performance failures (which means a reduction in function below an established acceptable limit) (Douglas and Ransom, 2007).

Building failures and causes:

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Building failures and causes:

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Failure of a structure can occur from many types of problems, mostly unique to the type of structure or to the various industries.Due to size, shape, or the choice of material, the structure is not strong and tough enough to support the load. Failure can occur when the overstressed construction reaches a critical stress level.

General causes of failure

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Instability, whether due to geometry, design or material choice, causing the structure to fail from fatigue or corrosion. These types of failure often occur at stress points, such as squared corners or from bolt holes being too close to the material's edge, causing cracks to slowly form and then progress through cyclic loading. conditions.

Manufacturing errors, may be due to improper selection of materials, incorrect sizing, improper heat treating, failing to adhere to the design, or shoddy workmanship.



Cracking Due to Expansion of Brickwork

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28Arching up and Cracking of Coping Stones of a Long Garden Wall


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29Cracking in Cladding and Cross Walls of a Framed Structure


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Horizontal Crack at the Base of Brick Masonry Parapet (Masonry cum Iron Railing) Supported on a Projecting RCC Slab


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Cracking in Top Most Story of a Load Bearing Structure


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Horizontal Cracks in a Wall at supports due to Excessive Deflation of a Slab of Large Span


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Most of the structural failures are associated with materials and are the consequence of human blunder involving a lack of know-how about materials or the combination of contrary materials.

There are structural failures that can be endorsed to irregularity in materials. Although too much reliance is given on modern structural materials yet the manufacturing or production faults may exist even in the most dependable structural materials, such as standard structural steel or centrally blended concrete

V/S

Man made v/s natural causes

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V/S

One of the major natural factors that result into building collapse is rainfall; others may include temperature, pressure, etc.

The fact remain that this is a natural factor that cannot be stopped, buildings therefore need to be constructed adequately bearing in mind such uncontrollable factors.

Man made v/s natural causes

DISTRESS OF CONCRETE STRUCTURESIf a building has given about 25v to 30 years of service without much maintenance or repair then it is reasonable to expect that it would need some repair sooner or later.CATEGORIES OF REASONS DISTRESS OF CONCRETE STRUCTURES

□ WEATHERING

□ AGEING

□ ENVIRONMENTAL EFFECTS

□ INADEQUATE MAINTENANCE

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DISTRESS OF CONCRETE STRUCTURES□ POOR DESIGNING AND CONSTRUCTION

QUALITY

□ CHANGE OF LOADING PATTERN OR NON CONVENTIONAL LOADING ON STRUCTURE

□ WATER LEAKAGE LEADING TO CORROSION OF CONCRETE STRUCTURE

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DISTRESS OF CONCRETE STRUCTURESPERFORMANCE OF CONCRETE STRUCTURE DEPENDS UPON:

1. QUALITY OF CONCRETE

2. AMBIENT CONDITIONS

EXPECTED SERVICE LIFE OF STRUCTURESLife span of RCC is generally taken as 100 years 04

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DISTRESS OF CONCRETE STRUCTURESEXPECTED CONVENTION ABOUT DESIGN LIFE SPAN Monumental structure like

temple ,mosque or church is 500 to 1000 yrs.

Steel bridges ,steel buildings or similar structures is 100 to 150 yrs.

Concrete bridges or hign rise buildings or stone bridges is 100 yrs.

A house or general building is 60 to 80 yrs.

Highways :concrete pavement for about 30 to 35 yrs and bituminous pavements for about 8 to 10 yrs.

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COMMON FAILURE MODES OF R.C.C.Cracks, potholes and erosion of the top layer of concrete, rusting of reinforcement, splitting etc.Cracking is the most common phenomenon in concrete structures.Cracking of concrete section cannot be prevented but size and location of the cracks can be limited & controlled by reinforcement, placement of control joint, and better concreting and curing

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CAUSES OF EARLY DETERIORATION OF CONCRETE STRUCTURES

Poor workmanship : untrained man power and bad quality of constructionInadequate cover to reinforcement.

Loosely fitted joints.

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Excessive water cement ratioThe IS: 456:2000 has laid down upper limits for water cement ratio, normally it need about 15 to 18 lts. Of water per 50kg bag of cement.

With this quantity of water concrete will be stiff and uncompact able so we mix additional water for workability. This extra water after evaporation leaves out minute capillary pores which lead to corrosion 0f reinforcement.

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Inadequate concrete cover Bars are not bent accurately or may not be effectively fixed to ensure that specified gap is left between bars and form work while often not only bars touch formwork but also the binding wires loose ends and the steel bars are seen at the surface of concrete.

In that case, they are prone to moisture.

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Honey comb or Un vibrated concretePorous concrete is a major source of weakness in concrete.

Un-compacted concrete results in honey combing which gives low compressive strength and permits moisture to enter, resulting in corrosion of steel bars.

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Cold joints or bad construction joints A cold joint is a joint where fresh concrete is placed against a previous un-compacted concrete which has already hardened due to lapses of time that is more than concrete setting time.

Inadequate reinforcementMany a contractor does not put enough reinforcement in concrete structural member s and hence it fails under tension.

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Wrong placing of reinforcementMany times contractors do not know the correct location of reinforcement in structural member so it is not placed at proper position.

Inadequate cement quantityIS: 456:2000 has laid down the minimum quantity in concrete.

Minimum quantity of cement is needed not only to coat the fine and coarse aggregate particles but also to fill the voids between the aggregate particles and to provide a thicker fill of cement grout for easy workability.

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Congested reinforcement bars Too many steel bars in narrow and slender RCC columns, walls or beams results in NO room for having cover in concrete nor for inserting a needle vibrator to ensure full compaction leading to honey combed concrete.

Initially rust steel bars Alkali – aggregate reactivityUnder most conditions the reaction causes excessive expansion and cracking of concrete after few months or year.

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Effect of weather and water leakageWeather causes structure to delay. Prolong exposure to polluted environment and acid rain can give weathered concrete and corrode metal ties and fastenings.

Porous cover blocks Having incorrect mix proportions and water cement ratio. These are usually fixed to the steel bars at about one meter centers and if they are porous they become the starting source of decay as they permit the ingress of moisture.

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EFFECTS OF CRACKING ON LIFE OR DURABILIY OF STRUCTURE

Understanding of cracks in concrete will help us to avpoid failures of concrete on one hand, avoidable worries and expenditure on other hand . Reduce loading capacity of structure Progressive failure Loss of appearance Leakages • Apprehension of failure in mind

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Controlling cracks

Better concrete mix design Friendly environment condition at the time of

concreting at site. Have dense concrete Use low heat of pozzolanic cement in mass

concreting.

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IDENTIFICATION OF DISTRESSED LOCATIONS ON STRUCTURES

PRELIMINARY INSPECTIONCarry out a preliminary inspection possibly with some testing in order to get characterize the nature of problem and to plan the detailed investigations.

Detailed inspection Checking the strength of concrete Chemical analysis on concrete Corrosion test

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IDENTIFICATION OF DISTRESSED LOCATIONS ON STRUCTURES

Conclusion from investigation In majority of cases ,the cause of damage can be tracked to the following singly or in combination: Low cover to steel Permeable concrete • High chloride levels

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REPAIR

Repair refers to modification of structure, damaged in its appearance or serviceability, to restore, partly or wholly, the pre-existing characteristics of serviceability, load-bearing capacity and if necessary, to improve its durability.

Repair techniques are used to restore the structural integrity and shape of a concrete element and generally include removal of damaged concrete and replacement of new concrete.

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GENERAL REPAIR PROCEDURE

Surface preparation and protection of reinforcement Chipping of defective or deteriorated concrete

should be done till all defective concrete is removed.

During repairs structural load carrying members must be supported.

Remove all rust from steel bars if it is reinforced member. In concrete rust is like cancer.

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GENERAL REPAIR PROCEDURE

Surface preparation and protection of reinforcement The old concrete of masonry surface should be

made rough to give proper mechanical bond. Wash and clean the entire surface before

taking up repairs Bonding material / epoxy should be applied

between old and new concrete. Spraying of water at regular intervals on

inclined or vertical members or we may also apply curing compound.

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MATERIALS AND METHODS FOR CRACK REPAIR

Repair materials can be of following types: Cement bound Polymer cement bound Resin hardener bound(epoxy)

Following materials are generally used in crack repair:

1. Low viscosity epoxy materials2. Epoxy modified mortar3. Polyurethane based coating over epoxy mortar4. Elastomeric concrete5. Fiber reinforced concrete6. Ferro cement

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Low viscosity epoxy materialDesigned for grouting gaps of low width of about 0.5 to 10mm. Epoxy compound like RESICRETE of SWC or CONVEXTRA EP10 of FORSROC which is normally supplied as two component product consisting of base and hardener that is filler and polymer can be used.

Epoxy modified mortarEpoxy mortar is a common material used for repair of concrete surfaces in general. It cannot be used in open surfaces as it is non UV resistant

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Polyurethane based coating over epoxy mortarAs epoxy mortars are non UV resistors ,but still have good strength so they can be used with a coating of this material.Elastomeric concreteAn epoxy material used in repairs not only in slabs and roads but also in structures and concrete surface.

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Fiber reinforced concreteIt is a concrete with small sized aggregates, cement sand mortar and some type of hair .Various types of hair can be: Steel fibers Carbon fibers Glass fibers’ Polypropylene fibers

Generally polypropylene fiber is used.

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Ferro cementTerm Ferro cement is applied to a mixture of Portland cement and sand reinforced with layers of woven or expanded steel mesh and closely spaced small diameter steel rods. Construction made with this is more resistant to earthquakes.

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SOME SPECIFIC REPAIR TECHNIQUE FOR CONCRETE SURFACE

1. Cracking of concrete surface up to certain depth:

a) Cracks up to depth of 0 to 20 mm –category Ab) Cracks between depth of 20 to 50-category Bc) Cracks to depth of more than 50mm-category C1. Weather effects on concrete or rusting of

reinforcement –category D2. Blowholes and streak marks on concrete-

category E3. Honeycombing in concrete –category F4. Holes in concrete or potholes or holes in road –

category G

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SOME SPECIFIC REPAIR TECHNIQUE FOR CONCRETE SURFACE

Repair methods-anyone of the above materials can be used with following described methods Application by hand Casting into formwork fixed into existing

concrete surface Spraying through purposely designed

equipment.

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ASSESMENT OF QUALITY OF STRUCTURE SOON AFTER ITS CONSTRUCTION

As the repair of structural element of building is over, it is desirable hat soundness of structure is assed with the help of non-destructive testing technique. Some NDTs which are available are:

Pulse velocity measurement method (PVM)

Cover meter

Rebound hammer or similar methods

Core cutting and testing of critical spots

Through these methods the strength of concrete, depth of cover, cracks, voids, if any can be found.

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REQUIREMENT FOR TRAINING FOR CONCRETE REPAIR AND CONCRETE WORKERS

To achieve good quality of work it is necessary to have skilled workers at site.

Like masons know the importance of water, right placement of reinforcement, mixing ingredients etc. Training should look that construction workers are trained for correct process and quality procedure with practical sessions and hands on work.

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K.R.

THAN

KI

(BE

Civi

l)

THANK YOU………FOR ANY OTHER INFORMATION CONTECT ME TO

Email : [email protected]

repair stratergies by k. r. thanki

Education

building distress

building codes

new building

distress b

extent of distress

signs of distress

lowrise buildings

safety of buildings