How do I cure concrete in hot or cold weather?

Why curing is about keeping water in rather than letting it dry, and the protection each temperature extreme needs.

Difficulty: intermediate
Time: 30 min
Read: 4 min
Safety: caution

Short answer

Concrete hardens by a chemical reaction with water, not by drying out. Keep it damp and covered for at least seven days. In heat, shade it, dampen it and cover it to slow evaporation; in cold, insulate it and protect it from frost, because concrete that freezes before it sets is permanently ruined.

Curing is the most skipped step in domestic concreting and the one with the largest effect on the result. A slab left to dry in the sun can end up substantially weaker than the same mix kept damp for a week.

Safety

Wet cement is strongly alkaline and causes serious chemical burns on prolonged skin contact, often without immediate pain. Wear waterproof gloves, long sleeves and eye protection, and wash any splashes off immediately. Kneeling in wet concrete without protection is a common cause of severe burns.

What you'll need

polythene sheeting
hessian or old blankets (optional)
frost blankets or insulation in winter (optional)
water spray

Step by step

Understand what curing does.Cement hydrates — it reacts chemically with water — and that reaction is what produces strength. If the water leaves before the reaction has run, the strength never develops.
Cover it as soon as the surface will take it.Once the surface is firm enough not to mark, cover with polythene sheeting taped or weighted down at the edges to trap moisture.
Keep it covered for at least seven days.Most of the strength gain happens in that first week. Longer is better, especially in cool conditions.
In hot weather, slow the evaporation.Shade the pour, dampen the sub-base and any formwork before placing, work in the cooler part of the day, and cover immediately. Wind dries concrete even faster than sun.
Damp-cure in heat.Wet hessian under polythene, kept damp, is the traditional and very effective method. A light mist of water on the surface under a cover works too.
In cold weather, keep it above freezing.Concrete that freezes before it has set is permanently damaged. Insulating blankets, frost sheets or straw under polythene hold the heat of hydration in.
Do not pour onto frozen ground.The base must be frost-free. Concrete placed on frozen ground loses heat straight downwards and the bottom of the slab never cures.
Expect much longer setting times in the cold.At five degrees a slab may take two or three times as long to become workable as at twenty. Do not force it by adding heat directly to the surface.
Avoid adding water at the surface to help finishing.Sprinkling water onto the surface while trowelling produces a weak, dusty top layer. Use the right mix and the right timing instead.
Delay loading it.Foot traffic after a day or two in warm conditions, but vehicles and real loads need considerably longer — 28 days is the conventional figure for full design strength.

Tips

Polythene taped down over new concrete for a week is the cheapest strength improvement available.
Wind is as damaging as sun in hot weather. A windbreak matters as much as shade.
Never pour concrete when frost is forecast within the next few days unless it can be properly protected.

Common mistakes

Leaving new concrete uncovered to dry — Drying is not curing — it is the opposite. Water leaving early stops the hydration reaction, and the concrete never reaches its intended strength.
Pouring with frost forecast — Water in the mix expands as it freezes, disrupting the structure before it has developed strength. The damage is permanent and the slab has to come out.

If it doesn't work

Surface dusts off under foot

Cause: Cured too fast, over-watered mix, or frost damage — Fix: Apply a concrete surface hardener or sealer if it is mild. Severe dusting means the top layer is weak and may need grinding back.

Scaling and flaking after the first winter

Cause: Frost damage, often to concrete that was not fully cured — Fix: Patch with a repair mortar, then seal. Prevention is curing properly and protecting from frost during the pour.

Crazing across the surface

Cause: Rapid surface drying — Fix: Usually cosmetic. Seal to reduce dusting, and cure under polythene next time.

Concrete still soft after two days in winter

Cause: Low temperature slowing hydration — Fix: Protect and be patient. Do not add heat directly or attempt to work the surface — it will develop strength, slowly.

Questions people ask

How long until I can walk on it?

Typically 24 to 48 hours in warm conditions, considerably longer when cold. Keep it covered while you wait — walking on it does not require the cover to come off permanently.

Should I water new concrete?

Keeping it damp is good; adding water to the surface while finishing is not. Wet hessian under polythene keeps moisture in without diluting the surface.

Do curing compounds work?

Yes. They form a film that reduces evaporation, and they are convenient over large areas. Check compatibility if the surface will later be tiled or coated, since some compounds interfere with adhesion.