MixRatio

Reference

Concrete cure time

When you can walk on it, drive on it and load it — and why curing is something you do, not something you wait for.

Cure milestones

AgeMilestoneNotes
4–8 hoursInitial setStiff enough to finish — floating, trowelling and edging happen in this window.
24–48 hoursForms can come offSide forms on slabs and walls, once the concrete holds its own shape. Leave load-bearing shoring in place.
24 hoursFoot trafficLight foot traffic is usually fine. Keep it protected and keep curing.
3 days≈ 40% of strengthLight use. Still vulnerable to point loads and impact.
7 days≈ 65–70% of strengthLight vehicles on a domestic driveway are commonly allowed from here.
28 days100% design strengthThe reference age all specified strengths are measured at.
Planning reference. Real timings shift with temperature, mix, cement type, admixtures and thickness — cold weather slows everything down substantially. Form striking and load times on structural work must follow your local building code and an engineer's instruction.

Setting is not curing

Concrete does not dry out to gain strength — it hydrates, a chemical reaction between cement and water that continues for as long as there is water available. Let the surface dry out early and that reaction stops where it stopped, permanently. The result is a weak, dusty, easily-scuffed surface over sound concrete underneath.

That is why curing is active work: keep the concrete damp and at a reasonable temperature for the first seven days, and ideally longer. Cover it with plastic sheet or wet hessian, mist it, pond it, or use a spray-applied curing compound.

The 28-day curve

Strength climbs quickly then flattens. Concrete typically reaches around 40% of its design strength in three days, roughly two-thirds by seven days, and its specified strength at 28 days — the age every quoted strength refers to. It keeps gaining slowly for months after that.

Weather

Below about 5 °C hydration slows dramatically and near freezing it effectively stops; fresh concrete that freezes can be permanently damaged. In hot, dry or windy conditions the opposite problem appears — the surface loses water faster than the concrete can bleed it back, and plastic shrinkage cracks open up within hours. Shade it, break the wind, and start curing as soon as the finish allows.

Planning a pour? Work out quantities with the volume calculators, and check the water-cement ratio — the other half of getting the strength you specified.