In service cracks
Reinforced concrete is designed on the assumption that the concrete has no significant strength in tension and that all tensile stresses will be carried by the steel reinforcement. Thus all reinforced concrete members subjected to significant loads may show some evidence of cracking. However, the width of the cracks is controlled by the reinforcement. In addition to carrying applied loading, structures are designed to satisfy serviceability criteria (such as deflections and crack widths) and to be durable for the intended life.
Maximum allowable widths of cracks under normal service loads are set to ensure that the structure is: Aesthetically acceptable; Durable; Watertight, where appropriate. The most common types of cracks are Flexural (bending) cracks, Shear cracks, Torsion cracks, Punching shear cracks, Cracks due to anchorage or lap failure, Cracks induced by temperature changes, Cracks due to freeze-thaw, Cracks due to reinforcement corrosion and Cracks due to alkali-silica reaction.
For reinforced concrete structures, crack widths are limited to 0.3 mm. For prestressed concrete structures and for bridges, the maximum crack width is related to the environment. Lower maximum crack widths a re specified for water-retaining structures. In all structures, a few cracks will be wider than specified, but this should not necessarily cause concern unless the wider cracks are at critical locations.
Some engineering judgement is required to determine their significance and this will depend on the location in the structure, the type of structure and the environment.