A Foolproof Guide to Disintegration Or Spalling of Concrete
Disintegration means the physical weakening (like scaling) or decomposition of the concrete into small portions or particles. The followings are the major reasons for concrete degradation.
The dissolvable sulphates exist in ground water, soil and clay bricks. With the existence of moisture, they react with tricalcium aluminate of cement and develop objects with greater volume as compared to the original ingredients. It leads to disintegration and concrete degradation along with masonry & plaster as well as composition of cracks. The reaction occurs gradually and the cracks are visible after 2 to 3 years.
Due to sulphate attack the concrete and masonry in foundation are significantly affected particularly where water table is relatively high and concrete and masonry get in touch with water.
Besides, the masonry and plaster in superstructure are also affected where bricks contain dissolvable salts and sulphates in them and wall is moist due to sprinkling of rain or seepage of water in wall from some source.
The intensity of sulphate attack is impacted by the following factors :-
- Volume of Soluble Sulphates
- Permeability and porosity in concrete and mortar
- Percentage of C3A in cement
- Existence of moisture/ dampness/ leakage of water in the particular building component
ALKALI AGGREGATE REACTION
In OPC, alkalies alias sodium oxide (Na2O) and potassium oxide (K2O) exist moderately and they chemically react with specific siliceous mineral (ingredients of some aggregates), consequently expansion, cracking and disintegration of concrete occur. Owing to reduction in alkalinity, corrosion of reinforcement occurs in presence of moisture. Here, the reaction takes place gradually and the cracks with map pattern form after number of years.
To get rid of the above issue, the following steps should be taken :-
- Refrain from applying alkali reactive aggregates
- Application of cement with low alkali content
- Utilization of pozzolanic material to avoid alkali aggregate reaction by itself by integrating with the alkalies present in cement.
With hydration of cement the concrete is solidified and some calcium hydroxide is released which develop a protecting alkaline medium inhibiting galvanic cell action and the corrosion of steel is resisted. Later on, free hydroxide in concrete reacts with atmospheric carbon dioxide to produce calcium carbonate that leads to shrinkage cracks. This reaction is called carbonation of concrete and it reduces alkalinity of concrete as well as its strength as a protecting medium for reinforcement.
In high-quality solid concrete, carbonation is mainly restrained to surface layers of concrete and its depth does not surpass 20 mm in 50 years. Therefore, when concrete becomes permeable or when reinforcement is within reach of surface owing to insufficient cover, carbonation creates corrosion of reinforcement that results in cracking and disintegration of concrete in due course. Carbonation is very fast in dry atmosphere but as existence of moisture is required for galvanic action to happen, that's why, for corrosion of steel, an alternating dry and wet weather is more useful for corrosion. Cracks and voids in concrete facilitate premature carbonation. Due to pollution, the percentage of carbon dioxide remains high In industrial towns and as a result cracking occurs in concrete because of high carbonation.
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Chlorides solution of high concentration can deteriorate the cement paste of concrete and can induce disrupting action in concrete close to sulphate attack.
Acids quickly damage the free lime of cement. To resist mild acid attack, it is recommended to use PPC (Portland Pozzolona Cement) as it contains low free lime content. The attacking acid is normally identified with the salt of the acid stored in concrete e.g. when H2SO4 reacts to Ca(OH)2.
H2SO4 + Ca(OH)2 → CaSO42H2O
All the cement is slowly disintegrated and leached away by acid attack. The following are some of the acid are:
Mineral acid: Sulphuric acid, Hydrochloric acid, Nitric acid, Phosphoric acid
Organic acid: Acetic, Lactic, Tannic, and Formic