Concrete in Aggressive Environment

Concrete in Aggressive

Environment

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 exposedduring its life is classified to five levels of severity, namely,mild, moderate, severe, very severe and extensive asdescribed. In table

• The destruction action of aggressive waters on concrete isprogressive. The rate of deterioration decreases as theconcrete is made stronger and more impermeable, andincrease as the salt content of the water increases.

• Whereas structures are only partially increased or in contactwith aggressive soils or waters on one side only,evaporation may cause serious concentration of salts withsubsequent deterioration even where the original saltcontent of the soil or water is high.

Environmental Exposure Conditions

Introduction

• At sites where alkali concentration are high or may becomevery high, the ground water should be lowered by drainageso that it will not come in direct contact with the concrete

• We may discuss following aggressive environments forconcrete

• 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, butsome of the aggregates contain reactive type of silica, which reacts withalkalis present in cement i.e. sodium oxide (Na2 O) and potassium oxide(K2O). As a result, the alkali silicate gels of unlimited swelling type areformed. 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 thesurrounding cement paste and internal pressure is developing leading toexpansion, cracking, and disruption of cement paste. This expansionappears to be due to hydraulic pressure generated through osmosis, but canalso be due to swelling pressure of the still solid products of alkali silicareaction. This indicates that the swelling of hard aggregates is most harmfulto concrete. The reactivity of aggregates depends upon its particle size andporosity as these influences the area over which the reaction can take place.


• Factors promoting the alkali aggregate

reaction:

• Reactive type of aggregates.

• High alkali content in cement.

• Optimum Temperature

• Availability of moisture

• Fineness of Cement Particles.


• As mentioned earlier certain types of rocks like traps, andesite,rhyolites, siliceous lime stone and certain types of stones containreactive constituents.

• The aggregate derived from such rocks are reactive and maypromote the alkali aggregate reaction.

• The high alkali contain in cement is also an important factorcontributing to the alkali aggregate reaction. To prevent thedeterioration of concrete due to alkali aggregate reaction alkalicontent in cement should not exceed 0.6 percent.

• The ideal temperature for the promotion of alkali aggregate reactionis in the range of 10 0C to 38 0 C. if the temperature is below 10 0 Cor more than 38 0 C, it may not provide an ideal situation for thealkali 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.

Alkali Silica Reactions

Alkali Silica Reactions

Sulphate Attack

• The sulphates of Calcium, Sodium, potassium and magnesium are presentin most soils, and ground water. Agricultural soil and water containsammonium sulphate, from the use of fertilizers or from sewage andindustrial effluents. Water used in concrete cooling towers can also be apotential source of sulphate attack. In marshy land decay of organic mattersleads to the formation of H2S, which is converted into sulphuric acid bybacteria.

• Solid salts do not attack concrete, but when present in solution they canreact with hardened cement paste. In the hardened concrete, sulphates reactwith the free calcium hydroxide [ Ca(OH)2] to form gypsum (CalciumSulphate). Similarly, sulphates reacts with calcium aluminium hydrate (C-A-H) to form calcium sulphoaluminate, the volume of which isapproximately 117 % of the volume of original aluminates. The produce ofthe reactions, gypsum and calcium sulphoaluminate have a considerablegreater volume than the compounds they replace, so that the reactions withthe sulphates lead to expansion and disruption of the concrete. Of all thesulphates magnesium sulphate causes maximum damage to concrete. Acharacteristic 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

Sulphate Attack

Sulphate Attack

Chloride Attack

• Chloride in Concrete:

• Due to high alkalinity of concrete protective oxide film is formed onthe surface of steel reinforcement. This protective layer can be lostto carbonation and presence of chloride in the concrete. The actionof chloride in inducing corrosion of reinforcement is more seriousthan any other reasons.

• Chloride enters the concrete from cement, water, admixtures andaggregate. When there is chloride in concrete, there is an risk ofcorrosion of embedded metal. The higher the chloride content, thegreater the risk of corrosion all constituents may contain chlorideand concrete may be contaminated by chlorides from externalenvironment. To minimize the chances of deterioration of concretefrom harmful chemical salts, the level of such salts in concretecoming from cement, water aggregate and admixtures should belimited.

Chloride Attack

Acid Attack

• Concrete is used for the storage of many kinds of liquids, some ofwhich are harmful to concrete. In Industrial plants, concrete floorcome in contact with acids, which damage the floor.

• In damp condition SO2 and CO2 and other acid fumes present in theatmosphere affect concrete by dissolving and removing part of theset concrete, This form of attack occurs in chimneys and steamrailway tunnels. In fact, no Portland cement is acid resistant.

• Acid attack is encountered also under industrial conditions. Concreteis also attacked by water containing free CO2. Flowing pure waterformed 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. Butthe 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 areeventually broken down and leached away. If the acids or salts areable to reach the reinforcing steel through cracks or porosity ofconcrete, corrosion of reinforcement take place.

Acid Attack

Acid Attack

Sea Water

• Sea water contains sulphates and hence attacks concrete in amanner similar to the sulphate attack.

• The deterioration of concrete in sea water is often is notcharacterized by the expansion, as found in concreteexposed to sulphate attack. Attack of sea water causeserrosion or loss of constituents of concrete without undueexpansion. Calcium Hydroxide and Calcium Sulphate(gypsum) are considerable soluble in sea water, and this willincrease the leaching action.

• Incase of reinforced concrete the absorption of salt results incorrosion of reinforcement. The accumulation of thecorrosion product on the steel, causes rupture of thesurrounding concrete. So that effect of sea water is moresevere 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 orweak aggregate shall not be used.

• As far as possible, preference shall be given to precastmembers, plastering should be avoided

• Sufficient cover to reinforcement, preferable 75 mm shallbe provided

• Care should be taken to protect reinforcement fromexposure to saline atmosphere during storage, fabricationand use. It may be achieved by treating the surface ofreinforcement with cement wash or by suitable methods.

Sea Water Attack

Effect of De-icing Salts

• When salts like sodium chloride or calcium chloride are used for de-icing roads incold climatic conditions, some of these salts becomes absorbed by the upper layerof the concrete. This produces a high osmotic pressure with a consequentmovement of water towards the coldest zone where freezing takes place. Deicingsalts increases the severity of the freezing and thawing cycles.

• The salts normally used are NaCl and CaCl2 and their repeated application withintervening 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 inremoving ice. Ammonium salts even in small concentration, are very harmful andshould 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 physicalin nature and not chemical.

• When de-icing agents are applied to concrete of few week age, damage would besevere. To protect such concrete boiled linseed oil, diluted in equal parts withkerosene or mineral spirits, are applied to the surface of concrete which must bedry, 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 orice, which is often ponded by adjacent ice. As more ice melts, the melt waterbecomes 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 andthawing 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.

Efflorescence

Efflorescence

Resistance of Concrete to Fire

• Concrete has good resistance to fire. The period of time under fireduring which concrete continues to perform satisfactorily isrelatively high and no toxic fumes are emitted. The length of timeover which the structural concrete preserves structural action isknown as fire rating. Here it is suffices to mention that sustainedexposure to temperature in excess of about 35 0C under conditionssuch that a considerable loss of moisture from concrete is allowedleads to a decrease in strength and modulus of elasticity of concrete.

• The fire resistance of concrete structure is determined by threefactors namely

• (1) The capacity of the concrete to withstand heat and subsequentaction of water without losing strength.

• (2) Concrete should not crack or spall

• (3) Conductivity of the concrete to heat and coefficient of thermalexpansion of concrete.


• The thickness of concrete cover to reinforcement is very important inreinforced cement concrete. The fire introduces high temperature gradientsand as a result the hot surface layers tend to separate and spall from coolerinterior parts. The formation of cracks is encouraged at joints in poorlycompacted parts of the concrete. The heating of reinforcement aggravatethe expansion both laterally and longitudinally of the reinforcement barsresulting in loss of bond strength and cracking of concrete.

• The strength of concrete is not much affected below of 250 0C. But aboveabout 300 0 C a definite loss of strength takes place. If high temperature isof short duration, a slow recovery of strength may take place. At lowtemperature, the strength of concrete is higher than that at roomtemperature.

• The loss in strength at higher temperature is greater in saturated concretethan in dry concrete. The strength of mass cured concrete beyond the age of14 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 moisturecontent. Excessive moisture at the time of fire causes spalling of concrete.


• 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 stoneexpands steadily unto 573 0 C at this temperature, it undergoes asudden expansion of 0.85 % Aggregates containing quarth as thepredominant mineral, has the least fire resisting property. Amongstthe igneous rocks, basalts and dolerites has the best fire resistance.

• Concrete made of siliceous or limestone aggregate show a change incolor with temperature. The change in color is permanent, so that themaximum temperature during a fire can be estimated a posteriorithus the residual strength can be approximately judged.. Generallyconcrete whose color has changed beyond pink is suspect andconcrete 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 ofconcrete, brick masonry and cement bound surfaces. Slurry coatingof specially processed hydraulic setting powder component and aliquid polymer component. These two materials when mixed in aspecied manner forms a brushable slurry. Two or three coats of thisslurry when applied on roof surface or on any other vertical surfacein basement, water tank or sunken portion of W.C. and bathrooms,etc. form a long lasting waterproofing coat. This coating needs tocure for a week or so.

• The coating so formed is elastic and abrasion resistant to someextent. To make it long lasting it may be protected by mortarscreening or tiles.

• The tradename of such coating are

• Dichtament D.S

• Brush bond by Fosroc Coy.

• Xypex, etc.


• The material described above is not very elastic.Its performance in sunken portion of bathroomand such other areas where the concrete is notsubjected variation in temperature will be good.But, on roof slab, due to thermal movement ofconcrete, it may not perform well.

• The modified version of the above has been madeto give a better waterproofing and abrasionresistance to the treatment. The modified versionwill make the coating tough and more elastic andbetter 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 andcarbonation attack on bridge, and also to protect undergroundstructures.

• Before applying the above coat of water proofing the surface shouldbe made damp and not wet . It can be applied by brush or trowel intwo coats to achieve a thickness of 2 to 4 mm. A gap of about 3- 4hours 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

Thanks

Engineering

Concrete in Aggressive Environment