CONCRETE COVER AND COVER BLOCKS

NOMINAL COVER (or Clear Cover)
Nominal cover is the design depth of concrete cover to all steel reinforcements, ,including links. It is the dimension used in design and indicated in the drawings. It shall be not less than the diameter of the bar.

Nominal Cover to Meet Durability Requirement

Minimum values for the nominal cover of normal weight aggregate concrete which should be provided to all reinforcement, including links depending.on the condition of exposure described below:

NOTES

1. For main reinforcement up to 12 mm diameter bar for mild exposure the nominal cover may be
reduced by 5 mm.

2. Unless specified otherwise, actual concrete cover should not deviate from the required nominal cover by +I0 mm

3. For exposure condition ‘severe’ and ‘very severe’, reduction of 5 mm may be made, where concrete grade is M35 and above.

 * However, for a longitudinal reinforcing bar in a column nominal cover shall in any case not be less than 40 mm, or less than the diameter of such bar. In the case of columns of minimum dimension of 200 mm or under, whose reinforcing bars do not exceed 12 mm, a nominal cover of 25 mm may be used.

 * For footings minimum cover shall be 50 mm.

 For Minimum values of nominal cover of normal-weight aggregate concrete to be provided to all reinforcement including links to meet specified period of fire resistance, refer in code section - 26.4.3

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There is no maximum cover, but the combination of inherent shrinkage and other factors may make excessive cover break away from underlying concrete 

Minimum Concrete Cover for Reinforcement

Below are the specifications for reinforcement cover for different structural members in different conditions.

a) At each end of reinforcing bar, concrete cover not less that 25 mm or less than twice the diameter of the bar should be provided.

 b) For a longitudinal reinforcing bar in a column, concrete cover not less than 40 mm not less than the diameter of such bar should be provided. In case of columns of minimum dimension of 20 cm or under, whose reinforcing bars do no not exceed 12 mm, concrete cover of 25 mm to be used for reinforcement. 

c) For longitudinal reinforcing bars in a beam, not less than 30 mm or less than the diameter of the bar.

 d) 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. 

e) For any other reinforcement not less than 15 mm, concrete cover not less than the diameter of such bar.

 f) 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. 

g) 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. cover should not be less than 50 mm.

 h) 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, sulphur, smoke etc.

 i) For liquid retaining structures, the minimum cover to all steel shall be 40mm 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.

 j) 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.

 k) 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 precast concrete blocks as approved by EIC. The use of pebbles or stones shall not be permitted. 

l) The minimum clear distance between reinforcing bars shall be in accordance with IS:456 – 2000 or as shown in drawing.

Minimum Clear Cover for Reinforcement

Effective cover 

Effective cover = nominal cover + half dia of reinforcement

CONCRETE COVER BLOCKS

Cover block is a precast concrete block to provide the necessary cover or spacing for the

reinforcement bars from the formwork. PVC or Plastic Cover blocks are also available.

By placing Cover blocks, rebar gets proper bonding with concrete and saved from corrosion.

The distance between the two cover blocks depends upon the diameter of the rebar, the type of RCC structure, and rebar directions (longitudinal or transverse). When the rebar diameter increases there will be less sagging and hence the number of cover blocks requirement becomes less as the bar dia. increases.

Average distance between Cover blocks

 Slabs - For 8mm - 60 cm and for 10mm - 70 cm. For 100 Sqm, approx. 280 nos. required for 8 mm bars and approx. 210 nos. required for 10 mm bars.

Columns - all sides, for 12mm - 20mm - 60 -90 Cm and above 20mm - 100 cm spacing required.

Beams - 12mm - 20 mm - 40 -70 Cm and above 20 mm - 80 Cm required.

Footings - 60 - 70 Cm required

Various types of Cover blocks

Single Cover spacers, Multiple Cover Spacers and Circular Spacers are in precast concrete blocks.

Clip Spacers, wheel spacers and chair spacers are PVC material blocks.