The primary reasons that cause disintegration of concrete member are desribed below.
1. SULPHATE ATTACK
Soluble sulphates are present in
Soluble sulphates react with Tricalcium Aluminate of cement I n the presence of moisture and form products which occupy much bigger volume than that of original constituents and so expansive reaction results causing disintegration and weakening of concrete, masonry, plaster and formation of cracks. The reaction is very slow and cracks start coming to notice after 2 to 3 years.
These reactions to take place, the presence of following three things are necessary
As per this, the following components of building are more liable to be affected by sulphate attack.
Concrete and masonry in foundation, especially where water table is quite high and concrete and masonry remain in contact with water.
Masonry and plaster in superstructure where bricks have soluble salts and sulphates in them and wall remain damp either because of splashing of rain or leakage of water in wall from some source.
Severity of sulphate attack depends upon
Amount of Soluble Sulphates
Permeability and porosity in concrete and mortar
Proportion of C3A in cement
Presence of moisture/ dampness/ leakage of water in the particular building component
2. ALKALI AGGREGATE REACTION
In OPC, alkalies namely sodium oxide (Na2O) and potassium oxide (K2O) are present to some extent. These alkalies chemically react with certain siliceous mineral (constituents of some aggregates) and causes expansion, cracking and disintegration of concrete. Because of lowering of alkalinity, rusting of reinforcement is also promoted in presence of moisture. Like sulphate attack, this reaction is also very slow and takes number of years for developing cracks. Cracks are of map pattern.
Avoiding use of alkali reactive aggregates
Use of cement with low alkali content
Use of pozzolanic material which prevents alkali aggregate reaction by itself combining with the alkalies present in cement.
When concrete hardens due to hydration of cement, some calcium hydroxide is liberated which sets up a protective alkaline medium inhibiting galvanic cell action and preventing corrosion of steel. In course of time, free hydroxide in concrete reacts with atmospheric carbon dioxide forming calcium carbonate, resulting in shrinkage cracks. This reaction known as carbonation also lowers alkalinity of concrete and reduces its effectiveness as a protective medium for reinforcement.
In good quality dense concrete, carbonation is confined mainly to surface layers of concrete and depth of carbonation may not exceed 20 mm in 50 years. Thus, when concrete is permeable or when reinforcement is very close to surface because of inadequate cover, carbonation results in corrosion of reinforcement which eventually leads to cracking and disintegration of concrete. Carbonation is more rapid in a dry atmosphere but, since presence of moisture is necessary for galvanic action to take place, hence, for corrosion of steel, an alternating dry and wet weather is more conducive to corrosion. Cracks and voids in concrete help in early carbonation. In industrial towns, having higher percentage of carbon dioxide in the atmospheric because of pollution, cracking caused in concrete due to carbonation is comparatively much more.
4. CHLORIDE ATTACKE
Chlorides solution of high concentration can attack the cement paste of concrete and can cause disruptive action in concrete similar to sulphate attack.
5. ACID ATTACK
Free lime of cement is attacked rapidly by acids. At places of acid attack, PPC (Portland Pozzolona Cement) is preferred which has low free lime content and therefore can offer resistance to mild acids attack. The attacking acid can usually be identified by the salt of the acid deposited in concrete e.g. when H2SO4 reacts to Ca(OH)2.
H2SO4 + Ca(OH)2 → CaSO42H2O
All the cement is gradually disintegrated and leached away by acid attack. Examples of some of the acid are:
Mineral acid: Sulphuric acid, Hydrochloric acid, Nitric acid, Phosphoric acid
Organic acid: Acetic, Lactic, Tannic, and Formic
This is because of acid attack only that the walls which are used by people for urinating, start spoiling and damaging because urine is acidic in nature.