Concrete does not dry out like paint. It hardens through a chemical reaction between cement and water called hydration, and that reaction carries on for weeks. Understanding the difference between setting, curing and drying is what stops a slab being ruined in its first 48 hours.
Three different things
- Setting is the concrete stiffening from a fluid to a solid. Initial set is typically a couple of hours after batching, final set within about six, though both move with temperature, cement type and any admixtures.
- Curing is keeping enough moisture in the concrete for hydration to continue. This is the part that builds strength, and it is the part most often skipped.
- Drying is moisture leaving the concrete afterwards. It matters when you are laying a floor finish on top, and it is far slower than people expect.
Concrete can be set and still be nowhere near strong. It can also be strong and still be too wet to floor over. Which mix you started with changes all of it — see what concrete mix do I need.
A working timeline
As a guide for a general-purpose mix in ordinary British weather. Cold slows every figure down, sometimes dramatically.
| Stage | Typical timing | What it means |
|---|---|---|
| Initial set | 1–3 hours | The surface will no longer take an impression easily. Finishing work is done around this point. |
| Final set | About 6 hours | Stiff throughout. Too late to work the surface. |
| Foot traffic | 24–48 hours | Careful walking. Keep barrows, trestles and point loads off it. |
| Strike vertical formwork | 24–48 hours | Sides of a base or wall. Soffits and anything propped stay far longer — take that from the engineer. |
| Two thirds of strength | About 7 days | A useful rule of thumb rather than a guarantee. |
| Light vehicles | About 7 days | A car on a domestic driveway, if it has been cured properly. |
| Characteristic strength | 28 days | The figure the mix is specified against. Heavy plant and full design loads wait for this. |
| Dry enough to floor over | Far longer | Governed by drying, not strength — see below. |
Why 28 days
Hydration is quick at first and then slows down, without ever quite stopping. The industry settled on 28 days as the age at which characteristic strength is measured because by then the great majority of the strength has developed and the remainder is slow enough not to matter for design. When we test cubes from a load, that is the age they are crushed at, alongside an earlier 7-day result as a check.
Weather changes everything
Cold
Below about 5°C hydration slows sharply, and near freezing it effectively stops. Fresh concrete that freezes before it has gained strength can be permanently damaged, because the water expands inside it. In cold weather, protect the pour: insulating blankets, covers, and leaving formwork on longer all help hold the heat the mix generates itself. Expect every timing in the table above to stretch, and tell the plant when you order — our ready mixed concrete can be batched to suit the conditions you are pouring in.
Hot and windy
Heat, low humidity and wind pull moisture out of the surface faster than it can rise from below. That causes plastic shrinkage cracking — short, random cracks appearing within hours of the pour — and a weak, dusty surface. On a hot or breezy day, pour early, keep the surface covered between operations, and start curing as soon as you can.
How to cure properly
Curing is not complicated. It is keeping the water in for the first few days:
- Cover with polythene, lapped and weighted, as soon as the surface will take it without marking.
- Or spray a proprietary curing compound, if nothing is being bonded to the surface afterwards.
- Or keep it damp under hessian. Do not let it wet and dry repeatedly, which is worse than leaving it alone.
- Three days is a sensible minimum in warm weather, seven in cold, and longer for anything structural.
The mistakes that cost slabs
- Adding water on site. It buys workability and gives back strength, surface durability and dimensional stability. Ask for the right consistence when you order instead.
- No curing at all. An uncured slab can lose a substantial share of its potential surface strength, and it is the surface that wears.
- Loading too early. Seven days is not 28 days. A skip lorry on a week-old driveway is a repair bill.
- Working the surface too soon. Power floating before the bleed water has gone seals it and traps moisture underneath.
- Assuming dry means ready. A slab can look bone dry on top and still be far too wet for a floor covering.
Concrete curing is not screed drying
If you are laying a floor finish, the number you need is not strength but residual moisture. Screeds and slabs both have to dry to a level the floor covering will tolerate, and that takes far longer than curing. Our guide to traditional and liquid screeds covers the drying rules and where they differ.
Planning a pour around the weather or a tight programme? Ring the plant covering your postcode and we will talk it through.