Curing of concrete means keeping freshly placed concrete moist and at a suitable temperature so the cement keeps reacting with water and gains strength. IS 456 asks for exposed surfaces to be kept continuously damp for at least 7 days when ordinary Portland cement (OPC) is used, and at least 10 days for blended cements such as PPC or PSC, or when the weather is dry and hot. For blended cements, the code recommends extending this to 14 days.
On many house sites in Nepal, curing is the step that gets cut short. The slab is cast, water is sprinkled for two or three days, and then everyone moves on to the next floor. The concrete looks fine, but it never reaches the strength the design assumed, and the surface cover over the steel stays weak and porous. This post explains why curing matters, how many days are needed, which methods work and how to handle Nepali winter and summer.
Why curing of concrete matters
Cement gains strength through hydration, a chemical reaction with water. If the water in fresh concrete evaporates too early, the reaction slows down or stops, especially near the surface. The results are:
- Lower strength. Poorly cured concrete can end up well below its intended grade, even if the mix was correct.
- Plastic and drying shrinkage cracks. Fine cracks on slabs that appear within hours of casting are a common sign of fast drying.
- Weak, dusty surface. Floors and roof slabs that dust off when swept usually did not get enough curing.
- Poor durability. The outer 20 to 50 mm of concrete is the cover that protects reinforcement from moisture and rust. If that layer is porous, steel corrodes sooner and roof slabs leak.
Strength gain of concrete with age
Concrete does not reach full strength in a few days. The commonly quoted figures for OPC concrete cured at normal temperature are shown below. They are approximate and vary with cement type, mix and temperature. PPC usually gains strength more slowly in the early days but continues to gain later.
Worked example: M20 concrete
M20 concrete has a characteristic 28-day strength of 20 N/mm². Using the approximate percentages above:
| Age | Approx. % of 28-day strength | Approx. strength for M20 (N/mm²) |
|---|---|---|
| 1 day | 16% | 3.2 |
| 3 days | 40% | 8.0 |
| 7 days | 65% | 13.0 |
| 14 days | 90% | 18.0 |
| 28 days | 99% | 19.8 |
This is why a 7-day cube result of about 13 N/mm² for M20 is usually read as "on track", and why heavy loading such as stacking bricks or casting the next floor on a 3-day-old slab is risky. These figures assume the concrete is actually kept moist. Without curing, the later gains may not happen at all.
How many days of curing are needed
| Situation | Minimum curing (IS 456) |
|---|---|
| OPC, normal weather | 7 days |
| OPC, dry and hot weather | 10 days |
| Blended cement (PPC, PSC) or mineral admixtures | 10 days, recommended to extend to 14 days |
Much of the cement sold in Nepal is PPC. If you do not know which cement your contractor used, check the bag. For PPC, plan for 10 to 14 days of curing as a normal practice, not an exception.
Methods of curing used on site
- Ponding: for slabs and floors. Small bunds of cement mortar or clay are made on the slab and filled with water. Start once the surface has set enough not to be damaged, usually the next morning.
- Wet hessian or jute bags: for columns, beams and walls. Wrap the member and keep the cloth wet throughout the day. Dry hessian does more harm than good because it draws moisture out.
- Sprinkling: works only if done often enough that the surface never dries. Two sprinkles a day in May is not curing.
- Plastic sheets: polythene laid over a wet surface and held down at the edges keeps moisture in. Useful where water is short.
- Membrane curing compound: a liquid sprayed on the fresh surface that forms a film and slows evaporation. Used on roads and large slabs. Apply as per the maker's instructions, and note it can affect bonding of later plaster or toppings.
Worked example: water for ponding a slab
A roof slab of 10 m x 8 m = 80 m² ponded to 25 mm depth needs 80 × 0.025 = 2 m³, which is 2000 litres, just to fill the ponds once. Evaporation and leakage mean you will top up daily. Plan your water tank and tanker supply before the casting day. The water tank capacity calculator helps check whether your site tank can hold enough.
Curing in Nepali winter
In Kathmandu and the hills, winter nights in Poush and Magh can drop close to freezing, and higher areas go well below. Cold slows hydration, so concrete gains strength more slowly than the chart above suggests.
- Cast in the morning so the concrete gets the day's warmth before night.
- Cover fresh concrete with plastic and dry hessian or straw at night to hold heat, then keep it moist during the day.
- Do not let fresh concrete freeze. Concrete that freezes in its first day or two can be permanently damaged.
- Extend the curing period and leave formwork and props longer than the summer schedule.
Curing in Nepali summer
In the Terai, Jestha and Asar temperatures can exceed 40 °C with hot dry winds before the monsoon. Water evaporates from the slab surface very fast.
- Start curing as soon as the surface can take it. Cover with wet hessian or plastic within hours of finishing.
- Cast early in the morning or late afternoon, and keep aggregates shaded.
- Treat 10 days as the minimum, since IS 456 itself raises the period for hot dry conditions.
Formwork removal times
Curing and formwork removal go together. IS 456 gives these minimum stripping times for OPC concrete at normal temperatures (about 15 °C and above). For PPC or cold weather, the times should be increased.
| Formwork | Minimum period |
|---|---|
| Vertical faces of columns, walls, beams | 16 to 24 hours |
| Slab soffit (props left under) | 3 days |
| Beam soffit (props left under) | 7 days |
| Props to slabs, span up to 4.5 m | 7 days |
| Props to slabs, span over 4.5 m | 14 days |
| Props to beams, span up to 6 m | 14 days |
| Props to beams, span over 6 m | 21 days |
Once formwork is removed from columns and beams, wrap them with wet hessian straight away. Removing shuttering does not end the curing period.
If you are planning materials for a pour, the concrete calculator gives cement, sand and aggregate quantities, and the house construction cost estimator shows how concrete work fits into the overall budget.
Frequently asked questions
Can curing be stopped after 3 days if the concrete looks hard?
No. Concrete feels hard within a day, but at 3 days it typically has only around 40% of its 28-day strength and the surface is still forming. Stopping early leaves the concrete weaker and more porous than designed.
Is curing for more than 14 days harmful?
No. Longer moist curing only helps. The limit is practical, since water and time on site cost money.
Should we cure brick walls and plaster too?
Yes. Cement mortar in brickwork and plaster also needs moisture. Plaster is usually kept damp for about 7 days to reduce cracking.
Does rain count as curing?
Steady rain keeps concrete wet, but heavy rain on concrete that has not yet set can wash out cement from the surface. Cover fresh concrete until it has set, then rain is helpful.
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