It is important that steel reinforcement, whether bar or fabric is properly embedded in the concrete, in order to perform its strengthening role. The steel must also be protected from fire and against corrosion. This is achieved by what is known as concrete cover. Cover Steel bars and fabric must be placed accurately to achieve durability, structural performance, fire resistance. Correct cover enables the two materials to bond and work together.
Insufficient cover can result in corrosion of the reinforcement.
This can lead to expansion of the steel bars as they rust, which in turn can lead to cracking and spalling of the concrete surfaces, which again accelerates corrosion and reduces fire protection. However, too much cover on reinforcement is not a good idea, as this may mean that the reinforcement is not in the position in which it was designed, and that it is not in the tension or compression zone. 150 mm would be considered too much cover. Hence spalling and poor knitting of the concrete may occur . The amount of cover does however vary with the diameter of the bar and BS 8110 gives permissible deviations.