Kumar IronWhat curing does
Cement reacts with water to form the compounds that give concrete its strength. If the surface dries out early, the reaction stops, the top layer stays weak and cracks form. Good curing gives strength, lower water entry and fewer shrinkage cracks.
IS 456 minimum curing period
IS 456:2000, the code for plain and reinforced concrete, asks for exposed surfaces to be kept continuously damp. The minimum period depends on the cement and the weather.
| Condition | Minimum days |
|---|---|
| OPC, normal weather | 7 |
| Mineral admixtures or blended cement (such as PPC) | 10 |
| Hot weather, OPC | 10 |
| Hot weather with mineral admixtures or blended cement | 14 |
Counting starts from the date of placing. PPC and other blended cements gain strength more slowly in the first days, which is why the code asks for more curing. Summers in western UP are hot and dry, so plan for 10 to 14 days on roofs and exposed slabs.
Curing methods
- Ponding: build a small mud or cement bund on the slab and keep 25 to 50 mm of water standing. Best for slabs and roofs.
- Wet gunny bags or hessian: cover columns, beams and walls and keep the cloth wet.
- Spraying water: fine for small jobs if done often enough that the surface never turns dry. Repeat several times a day in summer.
- Curing compound: a liquid membrane sprayed on after finishing, useful where water is scarce. Follow the maker's instructions and keep it on the surface.
- Cover with plastic sheet: reduces water loss, but it is not a replacement for adding water in hot weather.
Formwork and early curing
Start curing as soon as the surface is firm enough not to be damaged, usually within 12 to 24 hours of casting. Vertical members like columns are covered by the shuttering, so keep it wet or loosen it after the minimum stripping time and then wrap wet cloth. Shuttering removal times are set separately in IS 456, so do not strip early to start curing.
How strength grows
Concrete is rated at 28 days. As a rough guide, OPC concrete reaches about 65 percent of its 28 day strength in 7 days, and it continues to gain slowly after that when moisture is available. Dry concrete stops gaining strength. Cubes cast on site and cured in water give a better picture than a cube left in the sun.
Common curing mistakes
- Stopping curing after 2 or 3 days because the surface looks hard.
- Using the same days for PPC as for OPC.
- Curing only in the daytime in summer and leaving the roof dry overnight.
- Loading the slab with bricks or heavy material in the first week.
- Letting cold water hit hot concrete in a way that thermally shocks it. Use clean water and spread it gently.
Frequently asked questions
- How many days should a slab be cured?
- At least 7 days with OPC and 10 days with PPC or blended cement as per IS 456, and longer in hot weather. Ten to fourteen days is a safe plan for summer roofs.
- Can I stop curing after 3 days?
- No. The code asks for a minimum of 7 days. Stopping early leaves a weak top layer and more cracks.
- Is PPC curing different from OPC?
- Yes. IS 456 asks for at least 10 days when blended cement is used, against 7 days for OPC.
- Can I use a curing compound instead of water?
- Yes, where it meets the specification and is applied properly. It forms a film that holds moisture in. Water curing is still the cheapest and most dependable method on small jobs.