Short answer: 28 days for full design strength. Long answer: it depends on the weather, the mix, and what happened in the first two hours after the truck pulled away.
Under normal conditions — about 70°F, decent humidity, no wind problems — concrete follows a pretty predictable strength curve. These are the numbers that matter on a job site:
The slab stiffens up enough that you can't work it anymore. All your finishing has to be done before this point. Hot weather pulls this in fast — sometimes you've got 90 minutes instead of 4 hours. Cold weather stretches it out, which is why you might still be troweling at sundown in November.
Foot traffic is fine. Forms can usually come off vertical surfaces. Don't put a wheelbarrow on it. Don't let a kid ride a bike on it. The slab is still drinking up moisture and hardening — keep it wet or covered.
Most specs let you remove form supports for slabs and beams. Light equipment can roll across it. This is also when most curing compounds have done the bulk of their work.
Driveways are typically opened to vehicles at this point — passenger cars only, not concrete trucks. ACI considers 7 days the minimum curing period for most flatwork. If you're post-tensioning, this is usually when you stress.
The number on the spec sheet. The number on the cylinder breaks. This is when full structural loading is allowed, heavy trucks can run on the driveway, and the engineer signs off. Concrete keeps gaining strength past 28 days, but this is the design target.
Those numbers above? They assume you didn't lose moisture during placement. If the wind picked up to 15 mph and the slab was warm and the humidity was 25%, you might have hit 0.20 lb/ft²/hr evaporation in the first two hours. That means moisture left the surface faster than bleed water could replace it. The top inch of your slab is now under-hydrated. Permanently.
Curing — the wet burlap, the curing compound, the plastic sheets — protects the concrete after placement. It can't bring back moisture that was lost during finishing. That's the difference between a slab that hits its 28-day strength and a slab that breaks 15% low and nobody can explain why.
Cold weather. Below 50°F, hydration runs at maybe half-speed. Below 40°F it basically stops. If you pour at 45°F and it drops to 35°F overnight, your 7-day strength might come in like 3-day strength. Cure blankets and heaters fix this. Ignoring it doesn't.
Hot, dry, windy weather. The double whammy. The mix sets faster (less time to finish) AND moisture leaves faster (less water for hydration). This is exactly the condition ACI 305R was written for, and it's the one PourDay flags loudest.
Bad curing. No curing compound, no wet burlap, no plastic — concrete left to fend for itself. The surface dries out in 24 hours and hydration in the top inch quits early. This is why your 28-day cylinders crush at the spec value but the slab itself dusts and crazes.
If your customer asks "when can I drive on the new driveway?" — tell them seven days for a passenger car, twenty-eight for the heavy stuff. If your engineer asks "when's it at full strength?" — twenty-eight days, assuming nothing went sideways at placement. If anyone asks why the surface looks chalky after a year — that conversation starts with what happened on pour day.
Read pour conditions like a 20-year foreman. PourDay tells you what evaporation, temp, and wind will be the day of your pour — and the next 16 days. Free on iOS.
How cure time changes from 50°F to 90°F — the chart you wish you had on the truck dash.
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Read articleThe three weather conditions where the right call is to push the pour.
Read articleReal-time pour conditions. 16-day forecast. Pour logging. Free for contractors.