Curing is not drying
Concrete does not harden by drying out. It hardens because cement and water react chemically, a process called hydration, and that reaction needs water to continue. Concrete left to dry quickly stops gaining strength — permanently. The strength it did not reach in the first week is not recovered later.
This is the single most misunderstood thing about concrete on site. A slab that looks dry on top is not a slab that has finished working.
The 28-day figure, and what happens before it
Twenty-eight days is the age at which concrete strength is specified and tested. It is a convention, not the moment hardening stops. A rough picture of an ordinary mix kept properly moist:
- 1 day: About 15 per cent of its 28-day strength. Enough to walk on carefully.
- 3 days: Around 40 per cent. Formwork to vertical faces can usually come off.
- 7 days: Around 65 per cent. The critical curing window has passed.
- 14 days: Around 90 per cent.
- 28 days: The specified strength. Loading can begin.
Strength continues to climb slowly for months and years afterwards, but nobody designs around that.
What to actually do in the first week
The aim is to keep water in the concrete, not to add water to it. In rough order of effectiveness:
- Leave the formwork on: The cheapest curing there is. Formwork holds moisture against the concrete.
- Cover with polythene: Lapped and weighted at the edges. Traps the concrete's own moisture.
- Wet hessian under polythene: Better still for slabs, and the standard on any job that cares.
- Curing compound: A sprayed membrane. Useful on large pours where sheeting is impractical.
Seven days of curing is the usual minimum for ordinary Portland cement. Cold weather extends it, because the reaction slows down as the temperature drops.
Weather changes everything
Temperature is the biggest external variable:
- Below about 5°C: Hydration nearly stops. Concrete placed and left in a frost can be permanently damaged, because the water expands as it freezes and tears the structure apart before it has any strength to resist.
- Hot, dry, windy: The worst combination. Surface water evaporates faster than it can rise from below, and the top plastic-shrinkage cracks while the slab is still soft. Wind matters more than temperature here — a hot still day is kinder than a warm windy one.
The practical rule is that the first 24 hours decide the surface and the first 7 days decide the strength.
Why a richer mix is not a shortcut
There is a temptation to specify a stronger mix so that the slab can be loaded sooner. It works up to a point, but it brings its own problems: more cement means more heat, and more heat in a thick pour means a temperature difference between the core and the surface, which cracks it.
The reliable way to load early is to cure properly, not to enrich the mix.
What this means for your programme
Curing time is not dead time — but it is time in which the slab cannot take load, cannot take traffic, and should not be covered with a screed or a finish that traps or draws moisture unevenly.
Plan the pour so the seven days fall where you can afford them. Pouring on a Friday buys you a weekend of curing that costs you nothing in labour, which is a small piece of programming that experienced site managers do without thinking about it.