chemical deterioration of concrete
DESCRIPTION
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 crumbleTRANSCRIPT
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