Placement of cover blocks for columns

Correct Concrete Cover to Reinforcement

Correct cover to reinforcing steel not only affects the durability of the concrete but also the extent and severity of cracking under working loads. In conditions of fire the amount of cover also governs the time scale before damage to the reinforced concrete elements occur. Attention to correct detailing practices, which include making allowances for permissible tolerances of both reinforcing and form-work, the maintenance of good construction practices and the specification of and use of the correct number and type of reinforcing cover devices will ensure that the rebar can be placed and maintained in the position intended.

Normal cover required should be clearly shown on all relevant drawings. The amount of nominal cover is dependant upon the size and type of the structural member, the environmental conditions to which it will be exposed (including fire), the class of concrete and the type and size of reinforcing bar which will be used.

Actual cover obtained on site should never be less than the minimum nominal cover specified by the relevant authorities, nor more than the minimum, plus variations as specified.


Positive tolerances on cover are as important as negative. To ignore them would invalidate design assumptions relating to effective depth and would also influence the width of surface cracks under design loads.

Correct Cover

1. Ensures protection of the Rebar from the environment by providing a     physical barrier.
2. Ensures that concrete completely encircles Rebar allowing passivation.
3. Ensures that concrete completely encircles Rebar allowing complete bond to     develop.
4. Allows the Rebar to assume position and act as designed.
5. Prevents staining.
6. Privides a time lag in cases of fire.

The engineer and detailer should specify and schedule the correct number and type of cover devices required.