Chemical Attack in Concrete

L.CRISPUS BRIGHTSTONEM.Tech. (Structural Engineering)

BASIC TERMINOLOGIES Passive layer of concrete Corrosion

PASSIVE LAYER OF CONCRETE

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.

Corrosion

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.

Chemical attacks: Chloride attack Sulphate attack Carbonation Alkali-aggregate attack

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.

Possibilities of Chloride attack•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.

Corrosion of steel due to Carbonation

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

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

CO2 +H2O H2CO3 (Dil. Carbonic acid)

H2CO3 +Ca(OH)2 CaCO3 +H2O

Effects of Carbonation: Carbonation reduces alkalinity of

concrete-which in turn reduces the passivity of concrete

The corroded volume occupies 6 times the volume of the original volume.

This trust on concrete results in spalling,cracking or delamination of cover concrete

Spalling

SULPHATE ATTACK: Present in soil in the form of

calcium,sodium,pottasium & magnesium Ammonium sulphate- water mixed with

fertilizers and sewage from industrial effluents

Natural occurrence- Decay of vegetation and shallow lakes

Formation of sulpho-aluminatesC3A + 3 CS H2 + 26 H C3A(CS)3H32 (sulpho aluminates increases in volume)

Formation of calcium-sulphate: NS + CH + 2H NH + CS H2

Alkali-aggregate reaction Chemical reactions between

aggregate containing certain reactive constituents and alkalis (sodium and potassium salts)

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

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