The primary causes that lead to the dissolution of a solid member are described below.
soluble sulfates are present
- ground water
- clay bricks
Soluble sulfates react with the tricalcium aluminate of cement in the presence of moisture and form products that contain much greater amounts than the original components and so an elaborate reaction results in the dissolution and weakening of concrete, masonry, plaster and the formation of cracks. cause to be. The reaction is very slow and cracks start appearing after 2 to 3 years.
For these reactions to occur the following three things are necessary:
- soluble sulfate
- tricalcium aluminate
According to this the following components of the building are more affected by sulfate attack.
Concrete and masonry in foundations, especially where the water table is high enough and concrete and masonry remain in contact with water.
The superstructure consists of masonry and plaster where the bricks contain soluble salts and sulfates and the wall remains moist either due to rain splashes or water seepage into the wall from some source.
Severity of sulfate attack depends on
- amount of soluble sulfates
- Permeability and porosity in concrete and mortar
- C . ratio of3A in cement
- Presence of moisture/moisture/water seepage in the particular component of the building
In OPC, alkali i.e. sodium oxide (Na.)2O) and potassium oxide (K.)2o) are present to some extent. These alkalis chemically react with some siliceous minerals (components of some aggregates) and cause expansion, cracking and dissolution of concrete. Due to the low alkalinity, corrosion of reinforcement is also promoted in the presence of moisture. Like the sulfate attack, this reaction is also very slow and takes many years for cracks to develop. The cracks are of map pattern.
- Avoiding the use of alkali reactive aggregates
- Use of cement with low alkali content
- The use of pozzolanic materials that inhibit the alkali aggregate reaction by combining with the alkalis present in the cement.
When concrete hardens due to the hydration of the cement, some calcium hydroxide is liberated which sets up a protective alkaline medium that inhibits galvanic cell action and prevents corrosion of the steel. Over time, the free hydroxide in concrete reacts with atmospheric carbon dioxide to form calcium carbonate, resulting in shrinkage cracks. This reaction, known as carbonation, also lowers the alkalinity of concrete and reduces its effectiveness as a protective medium for reinforcement.
In good quality dense concrete, the carbonation is mainly confined to the surface layers of the concrete and the carbonation depth cannot exceed 20 mm over 50 years. Thus, when the concrete is permeable or when the reinforcement is too close to the surface due to insufficient casing, carbonation results in corrosion of the reinforcement which eventually leads to cracking and dissolution of the concrete. Carbonation occurs more rapidly in dry environments, but since the presence of moisture is necessary for galvanic action to take place, an alternating dry and wet season is more conducive to corrosion, as for steel corrosion. Cracks and voids in the concrete help with the initial carbonation. In industrial cities, the percentage of carbon dioxide in the atmosphere due to pollution is high, the cracking of concrete due to carbonation is comparatively much more.
High concentration chloride solutions can attack the cement paste of concrete and have a disintegrating action in concrete similar to sulfate attack.
Free lime of cement is rapidly attacked by acids. In acid attack sites, PPC (Portland Pozzolona Cement) is preferred, which has a lower amount of free lime and can therefore offer resistance to mild acid attacks. The attacking acid can usually be identified by the salt of the acid deposited in concrete, for example when H2therefore4 Ca(OH) reacts with2,
h2therefore4 + Ca (OH)2 → Caso42 h2hey
Due to acid attack, all the cement gradually disintegrates and flows away. Examples of some acids are:
Mineral Acids: Sulfuric Acid, Hydrochloric Acid, Nitric Acid, Phosphoric Acid
Organic chemicals: acetic, lactic, tannic and formic
It is only because of acid attack that the walls which people use to urinate start getting damaged and deteriorate because urine is acidic in nature.
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