Ask any experienced site engineer which step gets cut first when a project runs late, and the answer is usually curing. It looks like nothing is happening while water sits on a slab, so it feels safe to stop early. It is not. Curing decides whether your concrete reaches the strength it was designed for.
Why concrete needs water after it sets
Concrete does not harden by drying. It hardens through hydration, a chemical reaction between cement and water that slowly forms strong crystals binding the sand and aggregate together. If the surface dries out, hydration near the surface stops, leaving a weak, dusty, crack-prone layer exactly where you walk, plaster or waterproof.
How fast concrete gains strength
The figures below are the widely used rule of thumb for OPC concrete kept properly moist. They show why the first week matters most.
Blended cements such as PPC and PSC gain strength more slowly at first and keep gaining for longer, which is why they need longer curing.
Curing methods compared
| Method | Best for | Watch out for |
|---|---|---|
| Ponding (small bunds of mortar holding water) | Roof slabs and floors | Leaks at the edges; refill daily |
| Wet hessian or gunny bags | Columns, beams, vertical faces | They dry fast in summer; wet them several times a day |
| Plastic sheet covering | Slabs where water is scarce | Must be sealed at edges and weighed down |
| Curing compound (sprayed membrane) | Large areas, roads, high-rise work | Can affect bonding of later plaster or tiles |
| Water spraying | Small work | Easy to miss times and let the surface dry |
When to start
Start as soon as the surface is hard enough not to be damaged, usually once the shine of surface water disappears and the concrete resists a light finger press. On slabs, cover with wet hessian first and start ponding the next day.
What happens if curing is skipped
- Lower strength. Concrete left to dry from the first days can end up noticeably weaker than designed.
- Plastic shrinkage cracks. Fine cracks appear within hours on slabs exposed to sun and wind.
- Dusting surfaces that wear away and absorb water.
- Poorer durability. A porous surface lets in water and air, which speeds up steel corrosion.
Five curing mistakes to avoid
- Curing only the top of a slab and forgetting the edges.
- Letting columns dry because wrapping them is inconvenient.
- Using dirty or salty water.
- Stopping curing on Sundays and holidays. Concrete does not take weekends off.
- Removing slab props too early. Formwork removal times are a separate rule, and props stay longer than curing.
Frequently asked questions
How many days of curing does a roof slab need?
At least 7 days for OPC and 10 days for PPC, with 14 days strongly recommended in summer. Ponding gives the most reliable result.
Should plaster be cured too?
Yes. Cement plaster should be kept moist for about 7 days, starting a day after application, to prevent cracking.
Is curing needed in the rainy season?
Rain helps, but surfaces still dry between showers. Keep curing unless the concrete stays continuously wet.