chapter 2 seviceability and durability

SERVICEABILITY AND DURABILITY OF

CONCRETEPrepared By:Assistant Professor Ankit Patel

Prof. Ankit Patel 2

DurabilityDurability of Portland cement is defined as its ability to resist weathering action, chemical attack, abrasion or any other process of deterioration that is durable concrete will retain its original form, quality, and serviceability when expose to its environment. It is generally accepted now that in designing structures the durability characteristics of materials under consideration should be evaluated as carefully as other factors. The durability of concrete depends on its resistance to deterioration and the environment in which it is placed. The resistance of concrete to weathering, chemical attack, abrasion, frost and fire depends largely upon its quality and constituent materials. All concrete in service will be subject to chemical and physical changes. A durable concrete is one in which these changes occur at a rate, which does not detrimentally affect its performance within its intended life. Leave it to concrete alone, the material remains by and large durable, but concrete alone cannot be utilised extensively for structural applications. It is the Reinforced Concrete (RCC), a composite structural material, which is utilised for variety of structural uses.

BITS Edu Campus

Prof. Ankit Patel 3

DurabilityBut, it has been observed that RCC has not proved to be durable due to large number of factors, including variations in production, loading conditions in service life and subsequent attack by the environmental factors. However, a well constituted, properly compacted, and cured concrete used in RCC continues to be substantially water tight and durable as long as capillary pores and micro-cracks in the interior do not become interconnected pathways leading to surface of concrete. Based on this simplification of causes, few holistic models of deterioration of RCC have been illustrated, which represent a qualitative design approach for easy understanding of the contributing factors for deterioration and its mechanism.

BITS Edu Campus

Prof. Ankit Patel 4BITS Edu Campus

Prof. Ankit Patel 17

Holistic Model of Deterioration of RCCModel-1:According to this holistic model of deterioration of concrete shown in Fig, the deterioration process is considered in two stages. During the first stage, due to loading and weathering effects (e.g. cycles of wetting & drying, seasonal temperature variations, etc) the voids and micro-cracks in the interfacial zone between the cement paste and coarse aggregate or reinforcing steel become inter-linked. When the inter-linked network of micro-cracks gets connected to any cracks present at the concrete surface, this provides the primary mechanism of fluid transport into the interior of concrete. Once this happens, the penetrability of concrete increases greatly and the beginning of the second stage during which water, oxygen, carbon dioxide and acidic ions are able to penetrate easily into concrete. The presence of these elements facilitates various physical-chemical interactions as a result of which, the material eventually undergoes cracking, spalling and loss of mass resulting in partial loss of strength and stiffness

BITS Edu Campus


Holistic Model of Deterioration of RCCModel-2:According to this concept as illustrated in Fig there are three stages, namely gradual loss of water tightness, initiation of damage and propagation of damage. During the stage 1 no noticeable weakening of the material occurs but some protective barrier is being broken down, such as the depassivation of the reinforcing steel by CO2 or chloride penetration

BITS Edu Campus


Holistic Model of Deterioration of RCCModule-3:Another model on deterioration of concrete, which is dependent on the important role played by water cement ratio (W/C), is illustrated in Fig. This offers an overall view on the Co-existence of the following three principal elements.(a) Interconnected porosity of Cement paste(b) Exposure to aggressive agents/chemicals (c) Intermittent presence of waterIn absence of any of these three elements, damage to RCC will not occur. For example, even in a porous and/or micro cracked concrete, further deterioration cannot occur, in absence of Water/moisture even if there is a potential presence of environmental aggressive agent such as SO4, Cl- , and CO2. In absence of water/moisture, these aggressive ions cannot travel, through inter connected pores. Thus, neither sulphate attack on cement paste nor the corrosion of steel reinforcement can occur.

BITS Edu Campus


Holistic Model of Deterioration of RCC

BITS Edu Campus


Fresh concrete contains a considerable quantity of free water. If such concrete subject to freezing temp, discrete ice lenses are formed. Water expands about 9% in volume during freezing, so that the excess water in the cavity is expelled. The formation of ice lenses formed in the body of fresh concrete disrupt the fresh concrete causing nearly permanent damage to concrete. Such concrete will not recover the structural integrity, if later on allowed to harden at temp higher than the freezing temp. For fully hardened concrete subjected to alternate cycles of freezing and thawing the Servest conditions for frost action arise when concrete has more than one face expose to the weather and it remains wet for longer period. For example parapets, road kerb etc has been estimated that the freezing of water in hardened concrete may exert pressure of about 14Mpa. If the hydraulic pressure so generated exceeds the tensile strength of the concrete, cracks are developed in concrete.Freezing starts at the surface in the largest cavities and gradually extends to smaller ones. Gel pores are too small to get it frozen till the temp goes below -78 degree Celsius so that in practice no ice is formed in gel pores.

BITS Edu Campus


Rate of freeze-thaw deterioration: Increase porosity increase rate Increase moisture saturation increase rate Increase number of freeze cycle increase rate Air entrainment decrease rate

Preventive measures: Use of lowest practical w/c ratio Adequate air entrainment Use of durable non porous aggregate Adequate curing of concrete prior to exposure to freezing and

thawing Providing proper drainage rather than flat surfaces.

BITS Edu Campus


Sea water contains 3.5 percent of salt by weight. Its PH value varies between 7.5 to 8.4. sea water also contains some amount of C02. Concrete between the tide marks subjected to alternating weeting & drying is severely attacked , while permanently immerse concrete is attacked least. It is therefore necessary to provide a sufficient cover to reinforcement preferably 75mm.

BITS Edu Campus


CarbonationThe carbon dioxide present in atmosphere reacts in the presence of water with hydrated cement minerals, converting calcium hydroxide to calcium carbonate. The carbonation penetrates beyond the exposed surface of concrete only very slowly.The important factor affecting rate of carbonation are: Grade of concrete Relative humidity Permeability of concrete Cover of reinforcement TimeIn case of stronger concrete the rate of carbonation depth will be slower. The permeability of concrete also affects the rate carbonation. In permeable concrete carbonation penetrate at a faster rate, than in dense concrete. Concrete with higher w/c ratio is more susceptible to carbonation. The depth of carbonation can be measured by treating the freshly broken surface of concrete with a solution of phenolphthalein in diluted alcohol.BITS Edu Campus


CarbonationConcrete of good quality usually carbonates very slowly. Even after a period of 50 years carbonation is to penetrate to a depth of about 5 to 10mm. On the other hand a permeable concrete may carbonate to depth of 25mm in less than 10 years.Carbonation reduced due to reduced moisture content and reduced carbon dioxide concentration in the atmosphere.

BITS Edu Campus


Alkali Aggregate Reaction (AAR)The alkali silica gel formed by alkali aggregate reaction is confined by the surrounding cement paste and internal pressure is developed leading to expansion , cracking and disruption of cement paste. The reactivity of aggregate depends upon its particle size and porosity as these influences the area over which the reaction takes placeFactors Promoting the alkali aggregate reactions: Reactive type of aggregate High alkali content in cement Optimum temperature Availability of moistureMeasures to control alkali aggregate reactions: Selection of non reactive types of aggregates By restricting alkali content in cement below 0.6% By controlling temp By controlling moisture conditions By the use of corrective admixtures such as pozzolonas By controlling the void space in concreteBITS Edu Campus


Sulphate AttackThe sulphate of calcium, sodium, potassium, and magnesium are present in most soils and ground water. Agricultural soil and water contains ammonium sulphate from the use fertilizers or from sewage and industrial effluents. Water use in concrete cooling towers can also be a potential source of sulphate attack.Solid salt do not attack concrete, but when present in solution they can react with hardened cement paste. In hardened concrete, sulphate react with the free calcium hydroxide to form calcium sulphate. Similarly sulphate react with calcium aluminate hydrate (C-A-H) to form calcium sulphoaluminate. The produce of the reactions, gypsum and calcium sulphoaluminate have a considerably greater volume than the compound they replace, so that the reactions with the sulphate lead to expansion and disruption of the concrete . Of all the sulphate, magnesium sulphate causes maximum damage to concrete. With increase in the strength of the solution the rate of sulphate attack increases. But beyond a concentration of about 0.5 % of MGSO4 the rate of increase in the intensity of the attack becomes smaller.BITS Edu Campus


Sulphate AttackAttack of magnesium sulphate to concrete with higher w/c ratio can causes serious damage to concrete. If concrete is made with low water cement ratio the concrete can withstand the action of magnesium sulphate for 2 to 3 years. In addition to the concentration of the sulphate, the speed with which concrete is attacked also influences the rate of sulphate attack. When concrete is exposed to the pressure of sulphate bearing water on one side the rate of attack will be highest.Methods for controlling sulphate attack: Use of sulphate resisting cement Addition of pozzolana Quality of concrete Use of air entrainment Use of high alumina cement

BITS Edu Campus


Acid AttackConcrete is used for the storage of many kinds of liquids, some of which are harmful to concrete. In industrial plants, concrete floors come in contact with acids which damage the floor. In damp condition SO2 and CO2 and other acid fumes present in atmosphere affect concrete by dissolving and removing part of the set concrete. The form of attack occurs in chimney and steam railway tunnels. In fact no Portland cement is acid resistant.Acid attack is encounter also under industrial conditions. Concrete is also attacked by water containing free CO2. In practice, acid attack occurs at values of PH below about 6.5. but the attack is serve only at a PH value below 5.5. At a PH value below 4.5, the attack is serve. Under acid attack, cement compounds are eventually broken down and leached away. If acids or salts are able to reach the reinforcing steel through cracks or porosity of concrete, corrosion of reinforcement take place.

BITS Edu Campus


EfflorescenceEfflorescence, a deposit of salts on concrete surfaces, can decorate the surfaces with a new color scheme that probably won’t please the architect or owner of a structure. It causes unsightly white stains, usually near cracks or joints where water passes through concrete. There are effective methods for controlling efflorescence and, if it occurs, removing it.

BITS Edu Campus


EfflorescenceWe know that efflorescence is a fine, white, powdery deposit of water-soluble salts left on the surface of masonry as the water evaporates. These efflorescent salt deposits tend to appear at the worst times, usually about a month after the building is constructed, and sometimes as long as a year after completion. Efflorescence is not a simple subject. Three conditions must exist before efflorescence will occur. • First: There must be water-soluble salts present somewhere in the wall. • Second: There must be sufficient moisture in the wall to render the salts into a soluble solution. • Third: There must be a path for the soluble salts to migrate through to the surface where the moisture can evaporate, thus depositing the salts which then crystallize and cause efflorescence. All three conditions must exist. If any one of these conditions is not present, then efflorescence cannot occur.

BITS Edu Campus


EfflorescenceThe efflorescence process:Concrete structures endure a continuous moisture cycle of wetting and drying. Water from rain, snow, groundwater, or condensation enters the concrete. As it moves through the concrete, the water dissolves soluble compounds, bringing them in solution to the surface where they are deposited when the water evaporates. The availability of water, permeability of the concrete, and amounts of soluble compounds determine how much efflorescence will occur and when it will stop. Water temperature and hardness also have an effect Calcium hydroxide, a water-soluble cement hydration product, is the most common source of efflorescence. A cubic yard of concrete contains from 100 to 150 pounds of calcium hydroxide. Other alkalies are present in the concrete but when these highly soluble alkaline compounds come to the surface they’re washed away by rain

BITS Edu Campus


EfflorescenceControlling efflorescence :There are three ways to control efflorescence: reduce water flow through the concrete, reduce the amount of salt that’s contained in mix ingredients, or convert soluble compounds to an insoluble formRemoving efflorescence:Calcium hydroxide remains soluble for only a brief time after reaching the concrete surface. When exposed to carbon dioxide in the atmosphere it converts to calcium carbonate, which is difficult to dissolve. Consequently, it pays to try to remove efflorescence promptly while the salt deposits can still be washed off with water alone. This avoids working with acids that might etch the concrete and damage adjacent materials

BITS Edu Campus


EfflorescenceAny of several diluted solutions of acids are effective ways to remove eff l o r e s c e n c e : one part hydrochloric acid in nine to 19 parts water one part phosphoric acid in nine parts water one part phosphoric acid plus one part acetic acid in 19 parts water Good drainage also helps prevent efflorescence Use a low water-cement ratio and adequate curing to reduce the

amount of water passing through pores in the concrete. Using an integral w a t e r-repellent or waterproofed will also

reportedly reduce water movement through the concrete For existing concrete, clear coatings or sealers decrease moisture

absorption. This reduces the amount of efflorescence caused by wetting-and drying cycles.

BITS Edu Campus


Thermal properties of concreteThe important thermal properties of concrete required for design of structures: Thermal conductivity Thermal diffusivity Specific heat Coefficient of thermal expansionThermal conductivity:It is a measure of the ability of the concrete to conduct heat. It is defined as the ratio of flux of heat to temp gradient.it is measure in J/m2

The conductivity of ordinary concrete depends on its composition and when concrete is saturated the conductivity ranges generally from 1.4 to 3.6 J/m2.Conductivity of concrete depends on types of aggregate moisture content density of concrete and temp of concrete.

BITS Edu Campus


Thermal diffusivity:It is a measure of the rate at which temp changes within the mass takes place.Diffusivity =Conductivity / CPwhere C is the specific heat, and P is the�density of concrete.Specific Heat:It is defined as the quantity of heat required to raise the temp of a unit mass of concrete by 1 degree Celsius.Specific heat gets increase with an increase in temp.Coefficient of thermal expansion:It is defined as the change in length per degree change in temp.It depends upon the composition mix coefficient of expansion of cement paste coefficient of expansion of aggregate.An average value of the linear thermal coefficient ofexpansion of concrete may be taken as 9.9 x 10±6 per °C

BITS Edu Campus


Permeability of concreteThe transport of fluid through concrete depends on structure of the hydrated cement paste. The flow of fluid through concrete is referred as permeability. Permeability (Interconnected porosity) is related to : Capillary Porosity Air voids Micro cracks Macro cracks1. Capillary porosity: it has been estimated that about 23% of water by weight of cement is required for chemical reaction. However for achieving full hydration excess of water is required. This extra volume of water entrapped in the cement paste after completion of hydration leaves interconnected pores called capillaries in hardened concrete which becomes means of passage for external chemicals into the concrete. This porosity is termed as capillary porosity.

BITS Edu Campus


Permeability of concrete

2.Air voids: it is formed due to inadequate compaction in the form of discrete air bubbles of much larger size than capillary pores. These air voids may get interconnected by capillary pore system, leading to permeability of concrete.3. Micro Cracks: reinforced concrete structure are subjected to various types of loading conditions ie static loading , cyclic loading, impact loading during its service life. These structures are also exposed to extreme conditions of temp changes. These varying loading condition and exposure condition cause micro cracking in concrete. Micro cracking combined with capillary pores is generally responsible for ingress of aggressive chemicals in concrete.4. Macro cracks: Any crack width, which allows aggressive chemicals to travel freely into the concrete is termed as macro cracks.Causes of micro cracks is due to improper placement of concrete, settlement cracks of fresh concrete, alkali aggregate reaction, heat of hydration, inc volume of corroded reinforcement and excessive loading.

BITS Edu Campus


Causes of Corrosion & Remedial Measures

1. Presence of cracks in concrete: Certain amount of cracking always occurs in the tension zone of RCC depending upon the stresses in the reinforcing steel. Through these cracks, oxygen or sea water ingress into the concrete and set up a good environment for corrosion of reinforcement.

2. Presence of moisture: Presence of moisture is a precondition for corrosion to take place because concrete can act as electrolyte in electrochemical cell only if it contains some moisture in pores. Corrosion can neither occur in dry concrete nor in submerged concrete. The worst combination for corrosion to proceed is when the concrete is slightly drier than saturated about 8-9% relative humidity with low resistivity and the oxygen can still penetrate to the steel. Hence in high humidity areas like coastal India, low permeability concrete is recommended.

BITS Edu Campus


Causes of Corrosion & Remedial Measures

3. Permeability of concrete: this is also an important factor affecting corrosion of reinforcement. Ingress of moisture, sea water, oxygen, carbon dioxide is easier in porous concrete than in dense and impermeable concrete. It is worth mentioning that with each increases of W/C ratio of 0.1 permeability of concrete increases 1.5 times. Poor curing increases permeability 5 to 10 times in comparison to good cured concrete and poor compaction increases permeability 7 to 10 times in comparison to good compacted concrete. Therefore quantity of cement in concrete should not be less than 350 kg/m3. and w/c ratio should not exceed 0.55 for ordinary structures and 0.45 for marine structures.4. Carbonation5. Chloride6. Sulphate Attack7. Alkali aggregate reaction8. Inadequacy of cover

BITS Edu Campus


Difference between porosity and permeability of Concrete

The main difference between porosity and permeability is that porosity is a measurement of space between rocks whereas permeability is a measurement of how easy it is for fluids to flow between rocks.

Porosity and permeability are two distinct physical properties of solids. Porosity refers to the extent to which tiny pores or spaces exist within the solid. Permeability refers to the ability of a mass of solid to allow or restrain the passage of of fluids, that is gases or liquids, through itself. These two qualities are closely related. The fluids are able to permeate through a solid by passing through the pores it contains, and grater the number and size of pores in a given mass of solid, easier it is for the fluids to pas through. Thus in general higher porosity in a material is likely to be accompanied by higher permeability also.

Porosity is a ratio of volumes, so it has no units. Permeability has (m /s in the SI system).BITS Edu Campus


REALKALISATION

Realkalisation is a method used to stop and permanently prevent reinforcement corrosion in carbonated concrete structures by increasing their pH to a value greater than 10.5, which is sufficient to restore and maintain a passive oxide film on the steel. Realkalisation involves a technique whereby a current is passed through the concrete to the reinforcement by means of an externally applied anode, which is attached temporarily to the concrete surface. A paste of sprayed cellulous fibre with a solution of potassium or sodium carbonate is used as the electrolyte covering the concrete surface.The net effect of realkalisation includes it's effect on the concrete and on the steel reinforcement. The alkaline solution is transported into the concrete mass under the influence of the low voltage electrical current. This raises the concrete pH level to greater than 10.5. With regard to the effect on steel reinforcement, the alkalinity is increased at the steel surface by production of hydroxyl ions. This reinstates the dense passive film on the steel, which protects from further corrosion.

BITS Edu Campus


REALKALISATION

The realkalisation process requires minimal repair work of the concrete, especially the need to break out concrete behind the reinforcement, which is in itself a noisy, and expensive exercise. The realkalisation process takes approximately one week to complete.

Key features of this technology are:Environmentally friendly: Major reduction of concrete breakout.Low maintenance: In comparison to patch repair and coating.Long term global protection: Provides effective treatment for the entire area of application.Proven technology: Long history of satisfactory performance.

BITS Edu Campus

Prof. Ankit Patel

THANK YOU

107

BITS Edu Campus

chapter 2 seviceability and durability

Engineering