The few-rupee part with a hundred-year job
Reinforced concrete works on a quiet bargain. Concrete is strong in compression but cracks in tension; steel carries the tension but rusts in air and water. The bargain holds because concrete is intensely alkaline, a pH of around 13, and at that alkalinity a thin passive film forms on the bar and corrosion effectively stops.
That film survives only as long as the alkaline concrete around the bar stays intact. The layer between the bar and the outside world, the cover, is therefore not a detail. It is the entire corrosion protection system of the structure. And on most sites it is entrusted to whatever the helper found lying near the mixer.
One correction first, because it causes half the cover failures I have seen in 25 years: cover is measured to the outermost steel, the stirrup or link, not the main bar. If the drawing says 40 mm and your stirrup is 8 mm, the main bar sits 48 mm in. Measure to the main bar instead and you have silently lost 8 mm before the pour even begins.
The two slow attacks: carbonation and chloride
Carbonation is the everyday killer. Carbon dioxide from the air seeps into the concrete's pores and reacts with the lime in the cement paste, converting it to calcium carbonate. The alkalinity collapses, pH falls from about 13 to below 9, and the passive film dissolves. From then on the bar rusts like any damp piece of exposed steel.
Carbonation advances as a slow front from the surface inward, roughly with the square root of time. As a rule of thumb, not a code figure, dense, well-cured concrete may carbonate 15 to 20 mm in 50 years, while porous, badly cured, watery concrete can do the same in 10 to 15. Whether that front reaches your steel in a decade or a century comes down to concrete quality and cover thickness. Nothing else.
Chloride attack is the coastal and bad-water killer. Salt from sea air, salty groundwater, unwashed sand or contaminated mixing water punches through the passive film locally even while the concrete is still alkaline. The result is pitting: deep, narrow corrosion that eats bar cross-section fast while showing almost nothing on the surface. This is why buildings a few kilometres from the sea spall at year fifteen while identical inland buildings look untouched.
Both attacks end the same way. Rust occupies several times the volume of the steel it consumes, so a corroding bar bursts its own cover from inside: the cracking and spalling you see under old balconies and chhajjas. Once the cover cracks, oxygen, water and salt reach the bar freely and the decline steepens.
What IS 456 actually requires, exposure by exposure
IS 456:2000 handles this by classifying exposure and prescribing a nominal cover for each condition (Table 16). Learn five numbers:
- Mild, interior surfaces protected from weather: 20 mm, relaxable to 15 mm for bars of 12 mm and under, which covers most residential slabs.
- Moderate, external members sheltered from severe rain, concrete in non-aggressive soil or continuously under water: 30 mm.
- Severe, alternate wetting and drying, severe rain, coastal environment: 45 mm.
- Very severe, sea-water spray, aggressive sub-soil: 50 mm.
- Extreme, tidal or splash zone, aggressive chemical exposure: 75 mm.
- On top of the table sit two overriding minimums: column main bars get at least 40 mm (or the bar diameter, whichever is more), and any footing cast against earth gets at least 50 mm.
- The code permits a 5 mm reduction in the severe and very severe classes when the concrete is M35 or better. Denser concrete buys back a little cover.
- The tolerance runs one way: slightly over the nominal figure is acceptable, under it is not. Plan to the number and let errors fall on the generous side.
Why brick bits and stone chips are a false economy
Walk most small sites and you will find the slab mesh propped on broken brick. It feels free. It is the most expensive shortcut on the job, for four reasons:
- Brick is porous. It soaks up water and holds it against the bar: a permanent moisture wick installed at exactly the point that needed a barrier.
- The sizes are random. One chip is 20 mm, the next 45 mm. The cover the code specifies to the millimetre becomes a lottery.
- They crush and shift. Labour walking the mesh crushes soft brick; the needle vibrator knocks loose chips over. The steel ends up on the shuttering with zero cover, invisible under wet concrete.
- They bond poorly. A foreign chip in the cover zone leaves a weak interface: a ready-made crack path for water straight to the steel.
- Stone chips fix the porosity but nothing else: wrong shapes, no way to tie them to the bar, and point contacts that pop off under vibration.
Placing cover blocks properly: type, density, supervision
Use cement mortar or concrete cover blocks cast to the exact specified cover, ideally of strength comparable to the structural concrete, with binding wire embedded so they cannot migrate during the pour. For the side faces of columns and beams, moulded PVC wheel or clip spacers that lock onto the bar give consistent cover where a loose block cannot sit.
How many? These are rules of thumb from practice, not code clauses:
Then supervise the pour itself. Blocks that were perfect at 8 am are knocked flat by the concrete pipeline and foot traffic by 10. Assign one person during concreting whose only job is to restore cover blocks and chairs just ahead of the pour front. It is the cheapest quality role on the entire site. A full house slab's worth of cover blocks costs less than a bag of cement, while breaking out and repairing spalled concrete later routinely runs into hundreds of rupees per square foot.
- Slabs: about 4 blocks per square metre, roughly one every 500 to 600 mm each way, with extras at edges, corners and along labour walking routes.
- Beams: blocks under the bottom bars every 600 mm to 1 metre, plus side spacers on both faces; a cage leaning against the shutter on one side is a classic hidden failure.
- Top steel: cover blocks do nothing for bars that need holding up. Use steel chairs for every top mat, above all in cantilevers, where top steel sinking during the pour is a collapse risk, not merely a durability one.
A one-minute check before your next pour
Whether you are the engineer, the contractor or the owner standing at the edge of the shuttering, five questions catch most cover problems before the concrete hides them:
Concrete forgives many small sins. Thin cover is not one of them. It is the difference between a structure your grandchildren use and one that starts shedding its balconies at year twelve. Samrit Bharat publishes Site Wisdom notes like this one as part of building in the open ahead of launch: nothing on sale today, just the practice that makes buildings last.
- What nominal cover does the drawing specify for each member? If nobody on site can answer, stop.
- Is cover being measured from shutter to stirrup, not to the main bar?
- Look at the cover block stock: uniform, cast to size, wire embedded, or a heap of brick bats?
- Are chairs in place for every top mat and every cantilever?
- Who is the named person guarding cover during the pour?
Quick questions
What is the minimum concrete cover for a normal house in India?
Per IS 456:2000: interior slabs and beams in mild exposure need 20 mm nominal cover (15 mm allowed for bars up to 12 mm), external members in moderate exposure need 30 mm, column main bars need at least 40 mm, and footings cast against earth need at least 50 mm. In coastal areas, exposed members move to the severe class at 45 mm or more.
Can I use broken bricks or stone pieces instead of cover blocks?
No. Brick is porous and wicks moisture straight to the bar, the random sizes make actual cover unpredictable, and both brick and stone crush or dislodge during the pour, leaving steel sitting on the shuttering with zero cover. Use cement cover blocks cast to the specified size with embedded tying wire, or moulded PVC spacers.
How many cover blocks are needed per square metre of slab?
A common rule of thumb, not a code requirement, is about 4 blocks per square metre, roughly one every 500 to 600 mm in each direction, with extra blocks at edges, corners and along the routes labour walk during the pour. Top steel needs steel chairs separately; cover blocks only handle the bottom cover.