chemical deterioration of concrete

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Portland cement concrete is vulnerable to attack by aqueous solutions of sulphate salts that occur in some soils and ground waters. The rate and degree of attack depend upon the amount of available (soluble) sulphate, the presence of water, the composition of the cement and certain characteristics of the concrete such as permeability. As the action progresses there is a gradual loss of strength in the hardened cement paste component so that the concrete will ultimately crumble

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Page 1: Chemical Deterioration of Concrete
Page 2: 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

Page 3: Chemical Deterioration of Concrete

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

Page 4: Chemical Deterioration of Concrete

Sulphate Attack

Alkali Silica Reaction

Corrosion of Reinforcing Steel

Page 5: Chemical Deterioration of Concrete

Rate and degree of attack depends on…..

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

Page 6: Chemical Deterioration of Concrete

frequently in soils and ground waters

Surface deposits –sodium sulphates ,magnesium sulphates and calcium sulphates

marshy grounds ,sewage

Page 7: Chemical Deterioration of Concrete

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

components

Products ettringite and gypsum

Page 8: Chemical Deterioration of Concrete

• 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

Page 9: Chemical Deterioration of Concrete
Page 10: Chemical Deterioration of Concrete

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

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

Page 11: Chemical Deterioration of Concrete

aggregate and cement type

Water cement ratio moisture condition of the concrete

while in service

Page 12: Chemical Deterioration of Concrete

• 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

Page 13: Chemical Deterioration of Concrete

Use of low alkali cements

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

Page 14: Chemical Deterioration of Concrete

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

Page 15: Chemical Deterioration of Concrete

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

Page 16: Chemical Deterioration of Concrete

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

protection

Page 17: Chemical Deterioration of Concrete

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