Cracks in concrete vary in width from 0.1 to 1.0 mm. Concrete begins to crack, even in its plastic state and when hardened concrete is exposed to weathering, due to thermal and shrinkage stresses. Cracks in concrete are mainly due to the following reasons.
Temperature gradient including frost action
Moisture gradient (dry shrinkage)
Structural overloading, cyclic or impact loading
Rapid drying conditions (shrinkage of plastic)
Inadequate structural design and detailing
Chemical causes including corrosion of reinforcement
Shrinkage stress causes stress in the concrete, and when it exceeds the concrete’s tensile strength, cracks begin to form. However, due to the viscoelastic behavior of concrete (creep), some stress is relieved and it is the residual stress, after softening due to creep, that is responsible for cracking.
Cracks on concrete surfaces seriously affect the durability of concrete and 0.15 mm is often recommended as the maximum allowable crack width.
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