Concrete does not dry hard; it hardens by a chemical reaction between cement and water, and that reaction goes on for weeks. “Cured” therefore means different things at different times: hard enough to walk on after a day or two, strong enough for cars after about a week, and at its design strength after 28 days. This guide shows how strength builds with time, what slows it down, and when you can walk on, drive on, drill into or seal new concrete. For quantities, use the concrete slab calculator.
Concrete strength by age
| Age | Type I cement, moist cured | Type III (high early strength), moist cured | Type I, steam cured | A 3,000 psi mix (Type I, moist cured) |
|---|---|---|---|---|
| 1 day | 21% | 31% | 51% | 619 psi |
| 3 days | 46% | 59% | 78% | 1,374 psi |
| 7 days | 70% | 80% | 92% | 2,111 psi |
| 14 days | 88% | 92% | 98% | 2,642 psi |
| 21 days | 96% | 97% | 100% | 2,883 psi |
| 28 days | 101% | 100% | 101% | 3,022 psi |
| 56 days | 109% | 104% | 103% | 3,256 psi |
| 90 days | 112% | 106% | 104% | 3,354 psi |
For ordinary concrete made with Type I cement and kept moist, the relation gives about 21% of the 28-day strength after one day, 46% after three days and 70% after seven. That is the origin of the familiar rule that concrete reaches about 70% of its strength in a week. High early strength (Type III) cement gets there in roughly half the time, which is why it is used for cold weather and fast schedules.
Typical timeline for a slab, patio or driveway
| When | What is usually OK | Notes |
|---|---|---|
| Same day | Finishing, then start curing (keep the surface wet or covered) | Do not add water to the surface to make finishing easier; it weakens the top layer. |
| 24 to 48 hours | Light foot traffic; strip side forms on flatwork | Keep curing. Avoid scraping, dragging or point loads. |
| About 7 days | Passenger cars; normal use of patios and walks | Around 70% of design strength for moist-cured Type I concrete. |
| About 28 days | Heavy vehicles, full design loads, most coatings and sealers | Design strength; many paints, epoxies and sealers ask for 28 days so moisture can escape. Follow the product label. |
Curing: keep it wet, keep it warm
- Moisture. The reaction needs water. Keep new concrete continuously moist for at least the first 7 days for Type I cement (about 3 days for Type III) with wet burlap, plastic sheeting, ponding or a curing compound sprayed on after finishing. Concrete that dries out early can lose a large part of its potential surface strength and is more likely to crack and dust.
- Temperature. Cold slows the reaction and heat speeds it up. In cold weather (the industry guide, ACI 306, uses air temperatures at or below 40°F as its threshold) protect the concrete with insulated blankets or heated enclosures, and do not let it freeze before it reaches about 500 psi, roughly a day or two for normal mixes in mild conditions and longer in the cold. Never pour on frozen ground.
- Hot, dry and windy days pull water out of the surface before it can react. Pour early, keep materials shaded, and start curing as soon as finishing allows (ACI 305 covers hot-weather practice).
- Rain. Rain on freshly placed concrete can wash out the surface; cover it until it has set. Once it has hardened, rain does no harm and actually helps curing.
Before drilling, anchoring, painting or loading
- Drilling and anchors: anchor makers publish their load ratings for concrete of a stated strength and minimum age; most assume fully cured concrete. Check the anchor instructions before loading anchors in young concrete.
- Fence and mailbox posts: fast-setting post mixes state on the bag how long to wait before attaching rails or applying load; standard concrete needs about a day before light work and longer before heavy loads. See the post hole concrete calculator and our fence post guide.
- Paint, epoxy and sealers: most need the concrete to dry, not just cure. Follow the product’s waiting time and moisture test (a taped plastic sheet test is a common quick check).
- Backfilling against walls and building on slabs: follow your plans and engineer; foundation walls usually need floor framing in place, as well as strength, before backfill.
Worked example
A driveway is poured on a Monday with a 3,000 psi mix and kept covered and moist. By Thursday (3 days) the ACI relation puts it at about 1,374 psi, by the next Monday (7 days) about 2,111 psi, and it reaches its design strength around four weeks after the pour. Parking the car on it after a week is the usual advice; a loaded delivery truck should wait the full 28 days. If the nights drop near freezing that first week, add insulating blankets and expect the schedule to stretch.
Frequently asked questions
How long does concrete take to cure?
It reaches about 70% of its design strength in 7 days and its design strength at 28 days when kept moist at moderate temperatures (Type I cement, ACI 209R relation). It keeps gaining strength slowly after that.
When can I drive on new concrete?
Usually after about 7 days for cars and 28 days for heavy trucks, in moderate weather. Cold weather lengthens both.
How long does concrete take to cure at 40 degrees?
Much longer than at 70°F, and below about 40°F it needs protection. Keep it covered and insulated and do not let it freeze before it reaches about 500 psi; plan for extra days before loading.
Does concrete need to be kept wet?
Yes. Keeping it moist for at least the first 7 days (or using a curing compound) lets the cement keep reacting and gives a stronger, more durable surface.
Sources
- American Concrete Institute, ACI 209R-92, Prediction of Creep, Shrinkage, and Temperature Effects in Concrete Structures: time-strength relation f(t) = t / (a + b t) × f28 and its constants for moist- and steam-cured Type I and Type III cement (equation and constants as published; values computed by us).
- ACI 306 (cold weather concreting) and ACI 305 (hot weather concreting) guides, and ACI 308 (curing): thresholds and practices summarised, not reproduced.
- Timeline milestones: common industry guidance for normal-weight concrete, summarised; follow your mix supplier, contractor and product labels.
Last reviewed: 24 October 2026. Numbers in the tables are our arithmetic from the sources listed; code sections are summarised, not quoted. How we check our work: methodology and corrections. Spotted an error? Please let us know.