Key reasons for concrete decay
Concrete cancer or alkali-silica reaction (ASR)
ASR is caused by the reaction between unreacted calcium oxide in the cement mixture dissolving in water and reacting with the silica used as an aggregate. This produces a gel that swells as it draws water from the surrounding cement, inducing pressure and subsequent cracking of the aggregate and surrounding paste.
Expansion of metal reinforcement
Corrosion of metal reinforcements causes them to expand. Corrosion is speeded up if the reinforcement is too near to the surface of the concrete.
This is because carbon dioxide in the air dissolves in any moisture (usually rainwater or condensation) on or under the surface of the concrete. It forms carbonic acid that migrates into the concrete, reducing its alkalinity and hence its ability to protect the reinforcement against corrosion.
It is possible to measure the ‘carbonation front’ (the depth to which the concrete is affected in this way), and to judge if, or how soon, the reinforcement will be reached.
There may also be some other destructive chemical agent present, either in the environment or as an impurity or misguided additive in the mix. Specific concrete types can be more vulnerable.
Examples include concrete formed with experimental additives to make it set faster, make it set in colder conditions, or allow it to flow more freely. The use of poorly washed aggregates can also be a problem.
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