chemical deterioration of concrete

Although reinforced concrete has become the material of choice for general construction, there is a worldwide concern about its durability. Good concrete is relatively durable under different exposure conditions

A durable concrete which can withstand the conditions without deterioration is the need of the day

Concrete being a heterogeneous material, its behavior under stresses and performance with time is highly dependent on individual properties and the environment to which it is exposed in its life time. Serious deterioration of concrete is mainly due to chemical attack CHEMICAL ATTACK on concrete

Sulphate Attack

Alkali Silica Reaction

Corrosion of Reinforcing Steel

Rate and degree of attack depends on…..

available sulphate presence of water composition of the cement characteristics of concrete

frequently in soils and ground waters

Surface deposits –sodium sulphates ,magnesium sulphates and calcium sulphates

marshy grounds ,sewage

Chemical reactionsulphate ion reacts with hydrated calcium aluminates or the calcium hydroxide

components

Products ettringite and gypsum

• ResultDevelopment of stresses

Magnesium sulphate is more aggressive than sodium sulphate and it operates in separate reaction that can be destructive

This possible second reaction may account for cases of deterioration

During this process, expansive compounds form – over long periods of time

Expansion –fissures, intensify the action by weathering and freeze thaw cycles

aggregate and cement type

Water cement ratio moisture condition of the concrete

while in service

• Reaction – siliceous minerals in aggregates and OH- ions in the Cement paste

• Alkali's ( from cement , mixing water environment) increase OH- ion concentration in the concrete

• Gel – expands ,generating pressures – cracking

• Damage – invisible to the naked eye for years

Use of low alkali cements

Use of Blended cements –fly ash, natural pozzolans, calcined clay , silica fume ,slag,lithium admixtures

Ca(OH)2 –pH 13 ,prevents the corrosion forms a thin protective film of iron oxide

Passive iron oxide layer – destroyed - ph 11and carbonation lowers pH about 9

Corrosion – expansion, cracking and eventually Spalling of the cover

Electro-chemical action ,when two dissimilar metals are in electrical contact in the presence of moisture and oxygen

The process takes place in steel alone, because of differences in electro chemical potential on the surface

permeability of concrete low water cement ratioadequate cementaggregate size and gradinguse of admixtures reinforcing bar coatings and cathode

protection

Controlling the risk of concrete deterioration by chemical attack may be achieved in number of ways i.e. by limiting the content of reactive alkalis, reduce water cement ratio and permeability, use of admixtures, reducing chloride content