The reinforcement must be fixed with appropriate accuracy in the position specified by the designer to ensure proper structural performance, durability and fire resistance. Insufficient cover can result in corrosion of the reinforcement, leading to cracking and spalling of the concrete and eventually, if left unchecked, to a reduction in the cross-sectional area of the reinforcement and in the strength of the structure. The cover must also be sufficient to provide the required degree of fire resistance. Too little cover reduces the fire resistance; too much, unless precautionary measures are taken, can make the structure vulnerable to spalling, which again reduces the fi re resistance. Incorrect positioning of the reinforcement can seriously reduce the load-carrying capacity of the structural member.
Unless otherwise specified, the concrete cover to the reinforcement should be within the following permissible deviations from the dimensions specified in BS 8110:
Bars up to 12 mm diameter: +5mm –5mm
Bars over 12 mm up to 25 mm: +10 mm –5mm
Bars over 25 mm: +15 mm –5mm
- At each end of reinforcing bar, net less that 25 mm or less than twice the diameter of the bar.
- For a longitudinal reinforcing bar in a column, not less than 40 mm not less than the diameter of such bar. In case of columns of minimum dimension of 20 cm or under, whose reinforcing bards do no not exceed 12 mm a cover of 25 mm to be used.
- For longitudinal reinforcing bars in a beam, not less than 30 mm or less than the diameter of the bar.
- For tensile, compressive shear or other reinforcements in a slab or wall not less than 15 mm, not less that the diameter of such bar.
- For any other reinforcement not less than 15 mm, not less than the diameter of such bar.
- For footings and other principal structural members in which the concrete is deposited directly against the ground, cover to the bottom reinforcement shall be 75 mm. If concrete is poured on a layer of lean concrete, the bottom cover maybe reduced to 50 mm.
- For concrete surfaces exposed to the weather or the ground after removal of forms, such as retaining walls, grade beams, footing sides and top etc. not less than 50 mm.
- Increased cover thickness shall be provided as indicated on the drawings, for surfaces exposed to the action of harmful chemicals (or exposed to earth contaminated by such chemicals), acid, alkali, saline atmosphere, sulphorone, smoke etc.
- For liquid retaining structures, the minimum cover to all steel shall be 40 mm or the diameter of the main bar, whichever is greater. In the presence of sea water and oils and waters of a corrosive character the covers, shall be increased by 10 mm.
- Protection to reinforcement in case of concrete exposed to harmful surroundings may also be given by providing a dense impermeable concrete with approved protective coatings. In such a case the extra cover mentioned in (b) & (i) above may be reduced.
- The correct cover shall be maintained by cement mortar cubes (blocks) or other approved means. Reinforcements for footings, grade beams and slabs on a sub-grade shall be supported on re-cast concrete blocks as approved by EIC. The use of pebbles or stones shall not be permitted.