Control of cracking in reinforced concrete

Cracking is normal in reinforced concrete. Since shrinkage is restrained, tension cracks can form, and if uncontrolled these can create visual defects that are particularly obvious after rain. Cracking occurs when the tensile strain exceeds the cracking strain. Tensile strains occur due to loads, shrinkage and temperature changes. If concrete is insufficiently reinforced, then the rebar will yield when the first crack forms as the element shrinks due to heat of hydration movement. The heat of hydration expansion does not induce compression stresses, because the concrete is not hardened. Problems occur when the concrete gains strength and cools. Instead, the main loading is from shrinkage movements which induce tensile stresses. These stresses are transferred to the steel when the concrete cracks. The roof of a shopping mall car park comprises 450 mm deep precast concrete floor slabs, which span 16 m between supports.