Guides
Why cover blocks matter in RCC
A cover block holds the steel off the shuttering by the distance IS 456 asks for, which is 20 mm in mild exposure, 30 mm in moderate, and 45, 50 or 75 mm as the exposure gets worse.
- Guides
- 1 table of figures
- Own delivery, 34 blocks
- Open all 7 days
Cover is the concrete between the steel in a member and the finished face of that member, and clause 26.4.1 measures it to all the steel, links included, so on a beam it is the ring and not the main bar that sets it. The block is the one item on a slab order whose entire job is to hold that distance while the concrete goes in. Jay Shree Infra keeps cover blocks at the Saharsa godown and sends them out across Saharsa, Supaul and Madhepura. Two Indian Standards decide what to buy: IS 456:2000 fixes how much cover a member needs, and IS 2502:1963 says what the block itself has to be. Which forms and materials reach this shelf is on the cover block page.
IS 456 has been amended six times since 2000, in 2001, 2005, 2007, 2013, 2019 and 2024, on BIS's own record, and the cover figures below are the ones the unamended 2000 text prints. Where a drawing or a tender names a different figure, that figure governs your job. IS 2502 has no amendment on record.
The table
Cover IS 456 asks for, exposure by exposure
The five exposure classes of IS 456 Table 16, the nominal cover each one needs, and the block height the trade sells against it.
Scroll the table sideways to see every column
| Exposure class | Nominal covermm, not less than | Trade block heightmm |
|---|---|---|
| Mild | 20 | 20 |
| Moderate | 30 | 30 |
| Severe | 45 | nothing at that height |
| Very severe | 50 | 50 |
| Extreme | 75 | 75 |
The first two columns are Table 16 as IS 456:2000 prints it. Three notes printed under that table each move a figure. Main reinforcement up to 12 mm in mild exposure may take 5 mm less. Severe and very severe may take 5 mm less where the concrete grade is M35 or above. And the actual cover may run up to 10 mm over the nominal figure, with nothing allowed under it. The third column is the cover heights the trade casts, 15, 20, 25, 30, 40, 50 and 75 mm. Not every height is standing on a given day; call and we confirm what is.
Severe is the row with a gap beside it. The trade list steps from 40 straight to 50 and the code asks 45, so a job specified as severe is the one where the number on the drawing and the number in a pack are least likely to meet. The deviation note settles what to do about that: cover may go over by 10 mm and may go under by nothing, so a 50 mm block answers a 45 mm requirement and a 40 mm block does not.
The other three heights come out of the same code. 15 mm is Table 16's own first note, mild exposure with main bars up to 12 mm taking 5 mm off the 20. 40 mm and 25 mm are the two column figures in the member rules below. So all seven heights the trade casts trace back to IS 456, and 45 is the one figure IS 456 names that the trade list leaves out.
Four members, four figures
One house does not have one cover
Table 16 is the floor for the environment. Three further rules in clause 26.4 sit over it, and each one can raise the block you need. Cover is never less than the diameter of the bar, so a 32 mm bar cannot sit on a 20 mm block whatever the exposure class says. A longitudinal bar in a column takes at least 40 mm, and a column of 200 mm minimum dimension or under, with bars no thicker than 12 mm, may come down to 25 mm. A footing takes 50 mm minimum, and that figure is flat: no exposure class lowers it.
So a footing, a column, a beam and a slab in one house can want four different heights, poured on four different days. One height ordered for the whole building answers one of them. The diameters those figures are being read against are on the sariya page, and the rest of this shelf is on the hardware and accessories page.
IS 2502
What the standard asks of the block itself
IS 2502:1963 is the code of practice for bending and fixing bars for concrete reinforcement, and its clause 7.3 is about cover blocks. Blocks are generally of cement mortar, generally square or rectangular in plan, cast with or without binding wire embedded in them so they can be tied to the steel as it is placed. Rings with a hole at the centre may be used instead.
Two lines in that clause decide orders. Clause 7.3.2 says the mortar or concrete in the block shall not be leaner than the mortar or concrete it is going to be embedded in, so the mix going in over the blocks is part of choosing them. Clause 7.3.3 says only single cover blocks shall be used to provide the cover at any section, which closes off the usual repair for a short block, stacking a second one under it.
At this counter
What the shop can settle, and what it cannot
We stock cover blocks by material, form and cover height, not by maker name or strength grade, so nothing here can promise on its own that a given pack satisfies clause 7.3.2 against your mix. What we can do is read out what a pack in the godown is made of and what is printed on it.
Not every height stands in every form on a given day. Ring with the cover figures off your drawing, member by member, with the count against each, and the blocks go on the vehicle the cement and the steel are already taking to your block.
Sources
The two standards behind these figures, and their amendment record
BIS's own records for both codes, checked directly with the Bureau.
Questions
Drawing ke baad ke sawal
Cover kam pad gaya to kya hota hai?
IS 456 allows nothing under the nominal figure: the note under Table 16 permits up to 10 mm over and zero under. So a block taller than the drawing asks for is inside the code and a shorter one is outside it, which is the whole reason the 45 mm row on the table above matters. Order the height the drawing carries, member by member.
Chhote block ke upar dusra block rakh dein to?
IS 2502 clause 7.3.3 says only single cover blocks shall be used to provide the cover at any section, so two stacked to make up a height is outside the code. Order the height the drawing asks for. Where a height is short on the day, say so on the call and we will read out what is standing against it.
Footing me cover kitna rakhna hota hai?
50 mm at least. Clause 26.4.2.2 of IS 456 sets that as the minimum for footings on its own, without reference to the exposure class, so it holds even where the rest of the building is on 20 or 30 mm. The exposure table can push it higher and nothing pushes it lower.
Column ke sariya par 40 mm cover kyun likha hota hai?
Clause 26.4.2.1 asks for at least 40 mm on a longitudinal bar in a column, and never less than the diameter of that bar. There is one relaxation in the same clause: a column whose minimum dimension is 200 mm or under, with bars no thicker than 12 mm, may use 25 mm.
Block ka masala slab ke mix se kamzor hua to?
IS 2502 clause 7.3.2 rules it out: the mortar or concrete in the block shall not be leaner than the concrete it is embedded in. We don't hold a strength grade against a cover block, so this is checked at the counter against the pack and against the mix you are pouring, and it is worth asking before a pack is opened.
Before you order
Have the drawing open for these
What cover is written against each member on the drawing?
Read them out member by member. The structural design sets the figure and it can differ between a footing, a column, a beam and a slab in one house.
Is any of this going into a footing?
A footing carries a 50 mm minimum of its own under clause 26.4.2.2, so it is usually the odd height on an order and the one left short.
What grade of concrete is going in over the blocks?
Clause 7.3.2 of IS 2502 asks that the block not be leaner than the concrete around it. That comparison needs your mix and the pack in front of you, so it is made on the call.
How many of each height, and which day is the pour?
Blocks follow the bar layout, so the count comes off your side. The pour date decides whether they travel with the rest of the load or on their own.
Contact
The member first, then the figure beside it
A short block cannot be topped up and a buried one cannot be swapped, so the heights come off the drawing member by member before anything is loaded. Blocks are light and they ride out on a vehicle already carrying the cement and the steel, which is why they belong on the first call and not the second. Ring +91 6202 967 218 with the cover against each member, the grade of concrete going over it, and the count against each height.
Shop hours: All 7 days, 9:30 AM to 6:00 PM