Question

Why do BS8007 specify permissible crack width of water retaining structure as 0.2mm for severe, extremely severe exposure?

Answer

For break width less than 0.2mm, it is thought that mechanism of autogenous healing will take place in which crack will routinely seal up and this would not cause problem of leakage and reinforcement corrosion in water retaining organization.

When cracks are in inactive state anywhere no movement takes spaces, autogenous healing occurs in presence of water. Still, when there is a continuous flow of water from side to side from these cracks, autogenous healing would not acquire place since flow removes the lime. One of mechanisms of autogenous healing is that calcium hydroxide in concrete cement reacts with carbon dioxide in atmosphere, resulting in formation of calcium carbonate crystals. Slowly these crystals build up and grow in these tiny breaks and form bonding so that cracks are sealed. Because first documented discovery of autogenous healing by French Academy of Science in 1836, there have been numerous preceding proofs that crack be sealed up naturally by autogenous healing. Since of its self-sealing property, designers normally bound crack width to 0.2mm for water retain arrangement.