Curing a concrete slab: how many days, how to keep it wet, and how long the props stay
IS 456 asks for 7 days of wet curing with ordinary Portland cement and at least 10 with PPC or slag cement. Here is the rule, a curing calendar, the water a ponded slab takes, and the earliest prop dates to plan your shuttering hire around.
By Bulan Sarkar · Updated
IS 456:2000 clause 13.5.1 says the exposed concrete must be kept continuously wet, by ponding or by wet sacking, for at least 7 days from the day it was placed when the cement is ordinary Portland (OPC), and at least 10 days when it is a blended cement such as PPC or PSC, or contains fly ash or slag. In dry, hot weather the minimum is 10 days for any cement, and for blended cements the code recommends stretching it to 14 days. So a PPC slab cast on 2 November stays wet until at least 12 November, better 16 November.
Source: IS 456:2000 Plain and Reinforced Concrete, Code of Practice (with Amendments 1 and 2), clauses 11.3 and 13.5, checked Oct 2026.
How many days: read the cement bag first
The number of curing days depends on the cement, not on how strong the slab looks on day 3. IS 456:2000 clause 13.5.1 (as amended) gives three minimums for wet curing, counted from the date the concrete is placed:
| Cement or condition | Minimum | What the code adds |
|---|---|---|
| Ordinary Portland cement (OPC 43, OPC 53) | 7 days | Kept continuously damp or wet |
| Blended cement (PPC, PSC) or a mix with fly ash or slag | 10 days | Recommended: extend to 14 days |
| Any concrete exposed to dry and hot weather | 10 days | Not less than 10 days |
The cement type is printed on the bag: PPC carries IS 1489, PSC carries IS 455 and OPC carries IS 269. Check one bag from the lot before the pour and write the type in your slab-day notes. If the slab is ready-mixed concrete, the delivery challan or the mix design names the cement and any fly ash or slag; ask for it.
Hot weather has its own standard. IS 7861 (Part 1):1975 calls concreting hot weather work when the air is above 40 °C or the concrete goes in above 40 °C, and clause 8.2 again sets curing at not less than 10 days. It also changes the pour: when temperatures are critical, it suggests casting in the evening or at night.
No clause asks for 28 days of curing. Twenty-eight days is the age at which the test cubes are crushed, which is a separate job covered in our cube test guide.
How to keep the slab wet
Clause 13.5.1 allows two methods: ponding, or covering the concrete with sacking, canvas, hessian or something similar that is kept constantly wet. The aim is the same either way. The surface must never dry out between checks, because concrete that dries and gets wet again cracks at the surface.
- In hot weather, as soon as the surface is finished, lay wet (not dripping) gunny bags or hessian on it. IS 7861 cl. 8.2 asks for this straight after finishing, so the slab doesn't lose water while it hardens.
- Start the moist curing proper once the surface can take water without damage. IS 7861 puts that at roughly 12 hours after mixing in hot weather.
- For ponding, build low bunds of lean mortar on the slab to make bays, and keep water standing in them. This works well on a flat roof slab and is easy to check from the stairs.
- On beams, sloping slabs, sunshades and columns, wrap the concrete in hessian and wet it often enough that it stays soaked. IS 7861 cl. 8.2.1 says not to count on the shuttering to keep moisture in during hot weather.
- Clause 13.5.2 allows an approved curing compound in place of water only with the engineer's permission, sprayed on every exposed face once the concrete has set. Polythene sheet laid close on the surface is also allowed as a barrier.
- On the last day, leave the wet covers in place until they dry by themselves (IS 7861 cl. 8.2.3), so the slab doesn't dry fast in the wind.
Props and shuttering: the dates to plan your hire around
Most house owners hire shuttering plates and steel props by the day, so the number of days the props stay under the slab is a real cost. IS 456 clause 11.3 sets the rule: formwork comes off only when the concrete has reached at least twice the stress it will carry at that moment. Clause 11.3.1 then lists stripping periods that meet that rule in normal conditions, which means ordinary Portland cement, air not below 15 °C, and proper curing:
| Formwork | Minimum |
|---|---|
| Vertical faces of columns, walls and beams | 16 to 24 hours |
| Slab soffit shuttering (props refixed at once) | 3 days |
| Beam soffit shuttering (props refixed at once) | 7 days |
| Props under slabs spanning up to 4.5 m | 7 days |
| Props under slabs spanning over 4.5 m | 14 days |
| Props under beams and arches spanning up to 6 m | 14 days |
| Props under beams and arches spanning over 6 m | 21 days |
Two things in that clause matter for a house. First, the table is for OPC. For other cements and lower temperatures the code says the periods "may be suitably modified", so a PPC slab, or one poured on a cold winter day, needs the engineer's own dates. Second, "props refixed at once" means the plates can go back to the hirer early, but props stay. Clause 11.3.2 asks for enough props, of the right size and spacing, to carry the slab and any load on it while the next work goes on. Use these dates to plan the hire. The engineer decides on the day when anything comes out.
Keep a curing sheet
Curing is the one slab-day job that nobody can check later. A crack or a low cube result three weeks on sends everyone back to ask whether the slab was kept wet, and a sheet answers that. One line a day is enough:
| Column | Example |
|---|---|
| Day and date | Day 4, 6 Nov 2026 |
| Cement on the bag | PPC (IS 1489), so 10 days minimum |
| Morning check | 7:30, pond full, sacks wet |
| Afternoon check | 14:00, east bay dry, refilled |
| Evening check | 18:30, all bays full |
| Water used | About 900 litres from the sump |
| Checked by | Name |
Write the end dates on the sheet on day one: the minimum day, the recommended day, and the earliest prop dates from the table above. Then nobody has to work them out when the labour wants to move on.
Worked examples
1. Curing calendar for a PPC roof slab
- Slab cast on Monday 2 November 2026 with PPC from the bag (IS 1489)
- IS 456 cl. 13.5.1: blended cement, at least 10 days
- 2 Nov + 10 days = Thursday 12 November, the minimum last day of wet curing
- The code recommends 14 days for blended cement: Monday 16 November
- Had the bag said OPC: 7 days, so Monday 9 November
Result: Keep the slab wet until at least 12 November and plan for 16 November. Put both dates on the curing sheet on the day of the pour.
2. How long to hire the props
- OPC concrete, November, days above 15 °C, curing done properly
- Room spans up to 4.2 m, one room 4.8 m; longest beam 5.4 m
- Slab soffit plates: earliest day 3, with props refixed at once (cl. 11.3.1 b)
- Props under the 4.2 m rooms: 7 days; under the 4.8 m room: 14 days (over 4.5 m)
- Props under the beams (up to 6 m): 14 days
Result: Budget the props for at least 14 days and the plates for at least 3. With PPC or in a cold month, the table no longer applies as printed, so get the engineer's dates before you sign the hire.
3. Water for ponding a 1,000 sq ft slab
- Slab area: 1,000 sq ft × 0.0929 = 92.9 m²
- A pond 25 mm deep: 92.9 × 0.025 = 2.32 m³ = about 2,320 litres for the first fill
- Each 1 mm the water drops: 92.9 × 0.001 m³ = about 93 litres to top up
- If you measure a 10 mm drop overnight: about 930 litres to refill
- Measure the drop on day 2 with a steel rule in one bay, then plan the sump or tanker
Result: About 2,300 litres to fill a 25 mm pond on 1,000 sq ft, then roughly 93 litres for every millimetre lost. Check the sump has that much before the pour day.
Tips and tricks
- Count the days from the pour date, not from the day the ponding started. Clause 13.5.1 counts from the date the concrete was placed.
- Name one person for curing and agree their pay for the full 10 or 14 days before the pour.
- Build the mortar bunds for the ponds on the evening of the pour or the next morning, so the bays are ready when the surface can take standing water.
- Don't spray cold water straight onto a slab that has been sitting in the hot sun. IS 7861 cl. 8.2.3 warns that curing water much cooler than the concrete can crack it.
- Wrap the beam sides and the slab edges in hessian too. The pond only reaches the top surface, and the edges dry first.
- Write PPC or OPC, and the curing end date, in chalk on a column at the stair landing where everyone walks past.
What we would do
We'd check the cement on the bag before the pour and plan for 14 days of wet curing if it is PPC or PSC, which covers both the minimum and the code's recommendation. We'd pond the roof slab in bays, wrap beams and edges in hessian, and pay one person to check it morning, afternoon and evening, with a sheet they sign. For the props, we'd ask the engineer for the stripping dates before signing the hire, and budget at least 14 days for props under the beams and any room wider than 4.5 m. The slab casting day checklist puts this into the order of the whole day, with a sheet to print.
Frequently asked questions
How many days should a concrete slab be cured?
Does a slab need 28 days of curing?
How many days of curing for PPC cement?
When can slab shuttering be removed?
Can I use a curing compound instead of water?
Sources and date checked
- IS 456:2000 Plain and Reinforced Concrete, Code of Practice, incorporating Amendments 1 and 2: cl. 11.3, 11.3.1 and 11.3.2 (stripping time and props), 13.5, 13.5.1 and 13.5.2 (curing; cl. 13.5.3 deleted by Amendment 1). Later amendments exist; ask your engineer which edition the drawings follow.. Checked Oct 2026.
- IS 7861 (Part 1):1975 Code of Practice for Extreme Weather Concreting, Part 1 Hot Weather Concreting: cl. 2.1 (definition, above 40 °C), 8.1 (evening or night placing), 8.2 to 8.2.3 (protection and curing, not less than 10 days, wind, water temperature). Checked Oct 2026.
This guide covers curing days, curing methods, record keeping and planning the prop hire. When the shuttering and props come out, the slab's design, and what to do about a crack or a low cube result are for your structural engineer to decide.