Syllabus• Concrete in Aggressive Environment: Alkali – Aggregate Reaction, Sulphate Attack, Chloride Attack, Acid Attack, Effect of Sea Water, special coating for Water Proofing, Sulphate Chloride and Acid attack, Concrete for hot liquids.
Introduction• The general environment to which concrete will be exposed during its life is classified to five levels of severity, namely, mild, moderate, severe, very severe and extensive as described. In table• The destruction action of aggressive waters on concrete is progressive. The rate of deterioration decreases as the concrete is made stronger and more impermeable, and increase as the salt content of the water increases.• Whereas structures are only partially increased or in contact with aggressive soils or waters on one side only, evaporation may cause serious concentration of salts with subsequent deterioration even where the original salt content of the soil or water is high.
Introduction• At sites where alkali concentration are high or may become very high, the ground water should be lowered by drainage so that it will not come in direct contact with the concrete• We may discuss following aggressive environments for concrete• Alkali- Aggregate Reaction• Sulphate Attack• Chloride Attack• Acid Attack• Effect of Sea Water• Effect of De-icing Salts• Efflorescence• Resistance of Concrete to Fire
Alkali-Aggregate Reaction• Normally, aggregates used in concrete are considered as inert material, but some of the aggregates contain reactive type of silica, which reacts with alkalis present in cement i.e. sodium oxide (Na2 O) and potassium oxide (K2O). As a result, the alkali silicate gels of unlimited swelling type are formed. This reaction is known as ‘ Alkali Aggregate Reaction’.• The type of rocks which contain reactive constituents include traps, andesite, rhyolites, siliceous limestone and certain types of sand stones. The reactive constituents may be in the form of opals, cherts, volcanic, glass, zeolite, chalcedony etc.• The alkali silica gel formed by alkali aggregate reaction is confined by the surrounding cement paste and internal pressure is developing leading to expansion, cracking, and disruption of cement paste. This expansion appears to be due to hydraulic pressure generated through osmosis, but can also be due to swelling pressure of the still solid products of alkali silica reaction. This indicates that the swelling of hard aggregates is most harmful to concrete. The reactivity of aggregates depends upon its particle size and porosity as these influences the area over which the reaction can take place.
Alkali-Aggregate Reaction• Factors promoting the alkali aggregate reaction:• Reactive type of aggregates.• High alkali content in cement.• Optimum Temperature• Availability of moisture• Fineness of Cement Particles.
Alkali-Aggregate Reaction• As mentioned earlier certain types of rocks like traps, andesite, rhyolites, siliceous lime stone and certain types of stones contain reactive constituents.• The aggregate derived from such rocks are reactive and may promote the alkali aggregate reaction.• The high alkali contain in cement is also an important factor contributing to the alkali aggregate reaction. To prevent the deterioration of concrete due to alkali aggregate reaction alkali content in cement should not exceed 0.6 percent.• The ideal temperature for the promotion of alkali aggregate reaction is in the range of 10 0C to 38 0 C. if the temperature is below 10 0 C or more than 38 0 C, it may not provide an ideal situation for the alkali aggregate reaction.
Measures to control alkali aggregate Reaction• Selection of non-reactive type of aggregates• By restricting alkali content in cement below 0.6 %• By controlling temperature• By controlling moisture condition• By the use of corrective admixtures such as pozzolanas• By controlling the void space in concrete.• By not using very fine ground cement.
Sulphate Attack• The sulphates of Calcium, Sodium, potassium and magnesium are present in most soils, and ground water. Agricultural soil and water contains ammonium sulphate, from the use of fertilizers or from sewage and industrial effluents. Water used in concrete cooling towers can also be a potential source of sulphate attack. In marshy land decay of organic matters leads to the formation of H2S, which is converted into sulphuric acid by bacteria.• Solid salts do not attack concrete, but when present in solution they can react with hardened cement paste. In the hardened concrete, sulphates react with the free calcium hydroxide [ Ca(OH)2] to form gypsum (Calcium Sulphate). Similarly, sulphates reacts with calcium aluminium hydrate (C- A-H) to form calcium sulphoaluminate, the volume of which is approximately 117 % of the volume of original aluminates. The produce of the reactions, gypsum and calcium sulphoaluminate have a considerable greater volume than the compounds they replace, so that the reactions with the sulphates lead to expansion and disruption of the concrete. Of all the sulphates magnesium sulphate causes maximum damage to concrete. A characteristic whitish appearance in the indication of sulphate attack.
Sulphate Attack• In addition to the concentration of the sulphate, the speed with which concrete is attacked also influence 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.• Sulphate attack is greatly accelerated if accompanied by alternate wetting and drying, which normally takes place in marine structures in the zone of tidal variations. On the other hand if the concrete is completely buried, without a channel for the ground-water, condition will be less severe.
Methods for Controlling Sulphate Attack• Use of sulphate resisting cement• Addition of Pozzolana• Quality of concrete• Use of air-entrainment• High pressure steam curing• Use of high-alumina cement• Liming of Polyethylene sheet
Chloride Attack• Chloride in Concrete:• Due to high alkalinity of concrete protective oxide film is formed on the surface of steel reinforcement. This protective layer can be lost to carbonation and presence of chloride in the concrete. The action of chloride in inducing corrosion of reinforcement is more serious than any other reasons.• Chloride enters the concrete from cement, water, admixtures and aggregate. When there is chloride in concrete, there is an risk of corrosion of embedded metal. The higher the chloride content, the greater the risk of corrosion all constituents may contain chloride and concrete may be contaminated by chlorides from external environment. To minimize the chances of deterioration of concrete from harmful chemical salts, the level of such salts in concrete coming from cement, water aggregate and admixtures should be limited.
Acid Attack• Concrete is used for the storage of many kinds of liquids, some of which are harmful to concrete. In Industrial plants, concrete floor come in contact with acids, which damage the floor.• In damp condition SO2 and CO2 and other acid fumes present in the atmosphere affect concrete by dissolving and removing part of the set concrete, This form of attack occurs in chimneys and steam railway tunnels. In fact, no Portland cement is acid resistant.• Acid attack is encountered also under industrial conditions. Concrete is also attacked by water containing free CO2. Flowing pure water formed by melting ice or by condensation and containing little CO2, also dissolves Ca(OH)2 thus causes deterioration of concrete.• In practice, acid attack occurs at value of pH below about 6.5. But the attack is severe only at pH below 5.5. At a pH value below 4.5, the attack is very severe. Under acid attack, cement compounds are eventually broken down and leached away. If the acids or salts are able to reach the reinforcing steel through cracks or porosity of concrete, corrosion of reinforcement take place.
Sea Water• Sea water contains sulphates and hence attacks concrete in a manner similar to the sulphate attack.• The deterioration of concrete in sea water is often is not characterized by the expansion, as found in concrete exposed to sulphate attack. Attack of sea water causes errosion or loss of constituents of concrete without undue expansion. Calcium Hydroxide and Calcium Sulphate (gypsum) are considerable soluble in sea water, and this will increase the leaching action.• Incase of reinforced concrete the absorption of salt results in corrosion of reinforcement. The accumulation of the corrosion product on the steel, causes rupture of the surrounding concrete. So that effect of sea water is more severe on reinforced concrete than on plain concrete.
Steps to Improve Durability of Concrete in Sea Water• The use of pozzolana or slag cement is advantageous under such condition.• Slag, broken brick bat, soft limestone, or other porous or weak aggregate shall not be used.• As far as possible, preference shall be given to precast members, plastering should be avoided• Sufficient cover to reinforcement, preferable 75 mm shall be provided• Care should be taken to protect reinforcement from exposure to saline atmosphere during storage, fabrication and use. It may be achieved by treating the surface of reinforcement with cement wash or by suitable methods.
Effect of De-icing Salts• When salts like sodium chloride or calcium chloride are used for de-icing roads in cold climatic conditions, some of these salts becomes absorbed by the upper layer of the concrete. This produces a high osmotic pressure with a consequent movement of water towards the coldest zone where freezing takes place. Deicing salts increases the severity of the freezing and thawing cycles.• The salts normally used are NaCl and CaCl2 and their repeated application with intervening periods of freezing or drying results in surface scaling of concrete. Sometimes urea is also used to remove ice; it is less deleterious and less effective in removing ice. Ammonium salts even in small concentration, are very harmful and should not be used. When concrete is exposed to relative low concentrations of salts (2 to 4 % solution) greatest damage occurs and the action is believed to be physical in nature and not chemical.• When de-icing agents are applied to concrete of few week age, damage would be severe. To protect such concrete boiled linseed oil, diluted in equal parts with kerosene or mineral spirits, are applied to the surface of concrete which must be dry, in two coats. The layer of oil slows down the ingress of the de-icer solution.• Use of de-icer also enhance the corrosion of steel. The de-icer melts the snow or ice, which is often ponded by adjacent ice. As more ice melts, the melt water becomes diluted until its freezing point rises to near the freezing point of water. Freezing then takes place. De-icers increases the number of cycles of freezing and thawing and promote corrosion of steel
Efflorescence• The water leaking through cracks, faulty joints or through the area of poorly compacted porous concrete dissolve some Ca (OH) 2 compound by leaching. After evaporation, white deposit of calcium carbonate are left on the surface of concrete. These deposits are termed as efflorescence.• The occurrence of efflorescence is greater when cool, wet weather is followed by a dry and hot spell.• When Concrete is porous near the surface, the chances of efflorescence are increased.• Unwashed seashore aggregates, gypsum, and alkaline aggregate also causes efflorescence.• It mars the appearance of concrete.• Type of formwork, degree of compaction and water/cement ratio also affects the efflorescence.• Early efflorescence can be removed with a brush and water. Heavy deposits of salts may require acid treatment of the surface of the concrete. HCl is used for this purpose, the concrete surface should be washed after acid treatment.
Resistance of Concrete to Fire• Concrete has good resistance to fire. The period of time under fire during which concrete continues to perform satisfactorily is relatively high and no toxic fumes are emitted. The length of time over which the structural concrete preserves structural action is known as fire rating. Here it is suffices to mention that sustained exposure to temperature in excess of about 35 0C under conditions such that a considerable loss of moisture from concrete is allowed leads to a decrease in strength and modulus of elasticity of concrete.• The fire resistance of concrete structure is determined by three factors namely• (1) The capacity of the concrete to withstand heat and subsequent action of water without losing strength.• (2) Concrete should not crack or spall• (3) Conductivity of the concrete to heat and coefficient of thermal expansion of concrete.
Resistance of Concrete to Fire• The thickness of concrete cover to reinforcement is very important in reinforced cement concrete. The fire introduces high temperature gradients and as a result the hot surface layers tend to separate and spall from cooler interior parts. The formation of cracks is encouraged at joints in poorly compacted parts of the concrete. The heating of reinforcement aggravate the expansion both laterally and longitudinally of the reinforcement bars resulting in loss of bond strength and cracking of concrete.• The strength of concrete is not much affected below of 250 0C. But above about 300 0 C a definite loss of strength takes place. If high temperature is of short duration, a slow recovery of strength may take place. At low temperature, the strength of concrete is higher than that at room temperature.• The loss in strength at higher temperature is greater in saturated concrete than in dry concrete. The strength of mass cured concrete beyond the age of 14 days is unaffected by temperature within the range of 20 0C to 96 0C. This behavior is probably due to an absence of a change in moisture content. Excessive moisture at the time of fire causes spalling of concrete.
Resistance of Concrete to Fire• In concrete aggregate undergo a progressive expansion on heating, while the hydrated product of the set cement, beyond the expansion, shrinks. This opposite action weakens and crack the concrete. Siliceous aggregates containing quartz, granite and sand stone expands steadily unto 573 0 C at this temperature, it undergoes a sudden expansion of 0.85 % Aggregates containing quarth as the predominant mineral, has the least fire resisting property. Amongst the igneous rocks, basalts and dolerites has the best fire resistance.• Concrete made of siliceous or limestone aggregate show a change in color with temperature. The change in color is permanent, so that the maximum temperature during a fire can be estimated a posteriori thus the residual strength can be approximately judged.. Generally concrete whose color has changed beyond pink is suspect and concrete past the grey stage is probably friable and porous.
Special Coating for Water Proofing• Specially made slurry coating can be used for the water proofing of concrete, brick masonry and cement bound surfaces. Slurry coating of specially processed hydraulic setting powder component and a liquid polymer component. These two materials when mixed in a specied manner forms a brushable slurry. Two or three coats of this slurry when applied on roof surface or on any other vertical surface in basement, water tank or sunken portion of W.C. and bathrooms, etc. form a long lasting waterproofing coat. This coating needs to cure for a week or so.• The coating so formed is elastic and abrasion resistant to some extent. To make it long lasting it may be protected by mortar screening or tiles.• The tradename of such coating are• Dichtament D.S• Brush bond by Fosroc Coy.• Xypex, etc.
Special Coating for Water Proofing• The material described above is not very elastic. Its performance in sunken portion of bathroom and such other areas where the concrete is not subjected variation in temperature will be good. But, on roof slab, due to thermal movement of concrete, it may not perform well.• The modified version of the above has been made to give a better waterproofing and abrasion resistance to the treatment. The modified version will make the coating tough and more elastic and better water proofing.
Special Coating for Water Proofing• The application of modified coating are,• Terrace gardens• Parking places• Basements• Sanitary areas• Swimming pools.• This coating also give protection to chloride, sulphates and carbonation attack on bridge, and also to protect underground structures.• Before applying the above coat of water proofing the surface should be made damp and not wet . It can be applied by brush or trowel in two coats to achieve a thickness of 2 to 4 mm. A gap of about 3- 4 hours are given between successive coats.
Questions• List the situations where concrete is subjected to aggressive environment• Explain alkali-aggregate reaction. What are the factors promoting it and how it can be controlled?• Write a short note on Sulphate Attack.• Write a short note on Acid Attack• What are the effects of de-icing on concrete?• Describe resistance of concrete to fire.
References• Concrete Technology by: R.P. Rethaliya• Concrete Technology by . M.S. Shetty• Internet websites