Types
Several forms of concrete defect are possible: water penetration causes the concrete reinforcement to rust and expand which in turn creates stresses on the surrounding concrete which can then spall (break away). The use or presence of Chloride based compounds, together with a reduction in alkalinity, can cause corrosion of the reinforcing steel bars then expansion and spalling. Alternatively, if the cement component is too alkaline, it reacts with atmospheric carbon dioxide, and the structure will begin to deteriorate as star-shaped cracks appear which allow rainwater to penetrate. This deterioration is then accelerated by freeze-thaw cycling of water in the cracks, which, again, causes the surface to spall.
The initial cause of concrete cancer is usually water penetration. When calcium oxide reacts with water that penetrates the concrete it forms a solution of calcium hydroxide. The chemical formula for this is:
CaO(s) + H2O(l) → Ca(OH)2(aq)
Over time this calcium hydroxide solution will reach the edge of the concrete slab. When this happens the solution reacts with carbon dioxide in the air and transforms into calcium carbonate. On the top of the slab calcium carbonate causes cracks above the slab (allowing more water penetration, and below the slab stalactites form:
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Read more about this topic: Concrete Cancer
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