Cover to reinforcement is a key factor taken into account in the design and construction. It is directly related to the durability of the structure.
Providing the required reinforcement area, having use correct concrete grade and designing section correctly does not always constitute that the design is complete.
The selection of correct concrete cover based on the exposure conditions and fire requirements is a must for a successful structural design and to have a durable structure that can last at least its design life.
The durability of a structure and the design life of a structure is directly related to the cover to the reinforcements especially for the structures built in corrosive environments. BS 8110 does not provide much information for designs when it compared with the Eurocode 2.
Eurocode stipulates comprehensive information and selection method for cover to the reinforcements.
According to BS 8110, it provides a guideline to select the cover to reinforcements based on the fire rating and as per the environment(exposure condition) that the structure is to be exposed.
Cover to reinforcement in no case shall be less than nominal cover minus 5mm. The nominal cover is specified considering the following factors.
- Base on the recommendation for bar sizes and aggregate sizes
- Protection of steel against corrosion
- Protection of steel against fire
- Allowing surface treatments such as bush hammering
There are six exposure conditions that cover the different conditions a structure can be exposed to. Following table extracted from BS 8110 indicates the information under each category.
Based on the location where the structure to be built, the exposure condition is selected. Cover to the reinforcement can be selected from different tables shown in the same guideline as per the grade of the concrete. There is no provision for selecting the cover for concretes that grade is less than C30.
Cover can be selected for each exposure class from the above table and cover can be selected from the following table based on the fire requirements.
In general, when designing a structure, the above procedure is followed when selecting the cover to the reinforcements. However, there is more to be looked into when the structures are to be built in special locations like the sea. In those situations, more attention shall be made on the durability as the structure is exposed to severe environmental conditions.
BS 8500-1 provides more information on durability aspects. Specifying higher grade of concrete could be done with the change of the exposure conditions to severe. the grade of concrete based on the durability aspects. Thus, the selection of the grade of the concrete could also be done based on the exposure class and based on the selected cover, obtained considering other aspects.