Cover to reinforcement
In nearly all instances, cracks develop at the surface of concrete and are restrained by the tensile reinforcement. If a good bond exists between the concrete and the steel, the width of the crack at the steel will be very small. The cracks therefore tend to be wedge-shaped: the further the surface is from the steel, the wider the crack will be at the surface.
It follows that if surface crack widths are to be controlled effectively, the reinforcement should be close to the surface of the concrete. The provision of adequate cover for durability by protecting the reinforcement from corrosion, and to provide fire resistance, should always take precedence over aesthetic considerations.
The designer has a significant role in determining whether or not the design minimum cover is achieved in practice. A realistic cover tolerance (Δc) must be added to the minimum design cover to give a practical nominal cover. Δc values of at least 10mm are required to ensure the minimum cover can be achieved. Research and experience has shown that Δc values of less than 10mm in in-situ construction will lead to a high risk of actual covers being less than the minimum cover, even with high standards of workmanship. A Δc value of 5mm maybe used for precast construction where quality control procedures can justify its use. Low nominal covers will increase the risk of plastic settlement cracking.