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Chemical attack in concrete

Jun 14, 2015

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Engineering

er_kamal

Mechanism of different chemical attacks in a concrete like chloride attack, sulfate attack , which causes corrosion and spalling. Other reactions are alkali aggregate reaction , alkali silica reaction in concrete etc.
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Page 1: Chemical attack in concrete
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Chloride Attack

Chloride attack is one of the most important aspects while dealing with durability of concrete. It primarily causes corrosion of reinforcement. Statistics have indicated that over 40% of failure of structures is due to corrosion of steel.

Concrete and the Passive Layer

The strongly alkaline nature of Ca(OH)2 (pH of about 13) prevents the corrosion of the steel by the formation of a thin protective film of iron oxide on the metal surface. This protection is known as passivity.If the concrete is permeable to such an extend that soluble chlorides penetrate right up to the reinforcement and water & oxygen is also present, then the corrosion of steel will take place. This layer can also be lost due to carbonation.

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Chloride enters the concrete from the cement, water, aggregate and sometimes from admixtures. This can also enter by diffusion from environment if concrete is permeable.

The Bureau of Indian Standard had specified the maximum chloride content in cement as 0.1%.

The amount of chloride required for initiating corrosion is partly dependent on the pH value of the pore water in concrete. At a pH value less than 11.5 corrosion may occur without the presence of chloride.

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For corrosion to occur, these elements must be present:

~There must be at least two metals (or two locations on a single metal) at different energy levels~ an electrolyte~ a metallic connection

In reinforced concrete, the rebar may have many separate areas at different energy levels. Concrete acts as the electrolyte, and the metallic connection is provided by wire ties, chair supports, or the rebar itself.

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Anodic reactions

Fe Fe ++ + 2 e-

Fe++ + 2(OH)- Fe(OH)2 (Ferrous Hydroxide)Fe(OH)2 + 2H20 + 02 Fe(OH)3 ( Ferric hydroxide)

Cathodic reaction4e- +2H20 + O2 4(OH)-

It can be noticed that no corrosion takes place if the concrete is dry or probably below relative humidity of 60% because enough water is not there to promote corrosion. If concrete is fully submersed into water corrosion does not take place because diffusion of oxygen does not take place into the concrete. Probably the optimum relative humidity for corrosion is 70 to 80%.

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The passive layer can be lost by carbonation due to reduction in alkalinity of the concrete.In the presence of moisture and CO2 reacts as

H2C03 reacts with Ca(OH)2 and carbonation of concrete takes place. This reduce the alkalinity of concrete. When pH of concrete reduces below 8.3, then passive layer destroyed and corrosion takes place.

The products of corrosion occupy a volume as much as six times the original volume of steel. This exert thrust on cover concrete resulting in cracks, spalling or delamination of concrete. (It is a result of water entering brick, concrete or natural stone and forcing the surface to peel, pop out or flake off).

CO2 +H2O H2CO3 (Dil. Carbonic acid)

H2CO3 +Ca(OH)2 CaCO3 +H2O

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Metallurgical Methods : Steel can be made more corrosion resistant by altering its structure through metallurgical processes such as rapid quenching of the hot bars by series of water jets or keeping the hot bars for a short time in a water bath.Corrosion inhibitors : Corrosion can be prevented or delayed by chemical method by using certain corrosion inhibiting chemicals such as nitrites, phosphates, benzonates etc. Calcium nitrite is generally added to concrete during mixing of concrete.

Galvanising of reinforcement: Galvanising of reinforcement (dipping of steel in molten zinc) is also effective. The zinc surface reacts with calcium hydroxide in the concrete to form a passive layer and prevents corrosion.Cathodic protection : This is extensively used in advanced countries. Due to high cost and long term monitoring this method is not popular in India.

Coatings to reinforcement: The object of coating to steel bar is to provide a durable barrier to aggressive material, such as chloride. Fusion bonded epoxy coating is one of the effective method.

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Most soils contain sulphate in the form of calcium, sodium, potassium and magnesium. They occur in soils or ground water. Ammonium sulphate is frequently present in agricultural soil and water from use of fertilizers or from sewage and industrial effluents.

Decay of organic matters in marshy lands, shallow lakes often leads to formation of H2S which can be transformed into sulphuric acid by bacterial action.

Therefore sulphate attack is common occurrence in natural and industrial situations.

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The main reactions of sulphate attack on concrete are as under;

1. Formation of sulphoaluminates (ettringite) by reaction of sulphate salts

and the C3A - phase in cement; C3A + 3 CS H2 + 26 H C3A(CS)3H32

Increase in volume of reaction products causes expansion and spalling.

2. Formation of calcium sulphate (gypsum) in reaction with lime (CH) formed by hydration of cement

NS + CH + 2H NH + CS H2 Increase in volume of reaction products causes expansion and

spalling. 3. Magnesium sulphate is more aggressive than sodium or

calcium salts.

4. There are some other reactions also.

Sulphate Attack

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Spalling (Concrete flakes)

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Spalling

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Spalling

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Carbon dioxide in air or dissolved in water reacts with hydrated cement systems.

The main concern is the reaction of carbon dioxide with the lime (CH) - phase. It gives rise to calcium carbonate; Ca(OH)2 + CO2 = CaCO3 + H2O. In severe cases, the C-S-H phase, which gives strength, can also be attacked.

In all such reactions, OH- is consumed, thus lowering down the pH of concrete. If pH is lowered very much, protection to steel reinforcement against corrosion may be lost.

Carbonation is highest in pH between 50 to 80 percent.

Carbonation

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Carbonation

in concrete

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• Chemical reactions between aggregate containing certain reactive constituents and alkalis (sodium and potassium salts) and hydroxyl ions released by the hydration of cement can have deleterious effects on concrete.

•Granite, granite gneiss and schist, quartzite and sandstone, containing strained quartz are among the reactive rocks found in India.

Alkali-Aggregate Reaction

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Alkali-silica reaction in concrete

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The basic mechanism of corrosion of steel, is an electro-chemical phenomenon, involving an anode process and a cathode process;

      Anode: Fe 2e- + Fe2+

(Metallic iron)

    Cathode: 1/2O2 + H2O + 2e- 2(OH)-

In addition, the corrosion undergone by steel is due to

combination of iron and (OH-) ions;

Fe + 1/2O2 + H2O Fe2+ + 2(OH) iron hydroxide (rust) 

Corrosion of Reinforcement

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Corrosion of steel reinforcement in concrete structures

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Mechanism of corrosion of steel in concrete

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Main Reasons of CorrosionPresence of chloride ions – from water for

mixing as well as curing, from aggregates, other ingredients, from the service environment.

Poor quality of concrete – high permeability, no control on water/cement ratio.

Low pH value of concrete – mainly due to carbonation,

Inadequate cover thickness.

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Use of Sulphate resisting cement: Use cement with low C3A content is most effective method. So use sulphate resisting cement which contains less C3A.Quality Concrete : A well designed, placed and compacted concrete exhibit higher resistance to sulphate attack.Use of air-entrainment: Use of air-entrainment to the extend of about 6% has beneficial effect on sulphate resisting quality. This is probably due to reduction of segregation, improvement in workability, reduction in bleeding and in general reduction in permeability of concrete.Use of pozzolona : Use of pozzolanic materials reduce the permeability.High Pressure Steam Curing: This improve the resistance of concrete to sulphate attack.Use of High Alumina Cement: Use of High Alumina Cement improves the resistance of concrete to sulphate attack.