How Long Does Concrete Take to Cure?

Reviewed by Bryan Stanley, Creative Concrete Inc.

New concrete looks finished long before it is finished. The surface goes hard in a few hours, the crew packs up, and the slab sitting in your yard looks ready for anything. It is not. Concrete gains strength on a schedule, and that schedule runs a lot longer than most homeowners expect.

Here is the short answer, then the detail behind it.

Concrete reaches roughly 70 percent of its design strength in the first 7 days and hits full design strength at 28 days. It keeps gaining strength slowly for years after that, but 28 days is the number the industry builds around.

Those are general ranges. Mix design, slab thickness, weather and what the slab has to carry all move the numbers, so follow whatever your contractor told you for your specific pour.

This is the part that trips people up, and it matters for how you treat a new slab.

Drying is water evaporating off the surface. Curing is a chemical reaction called hydration, where cement and water bond and form the crystals that give concrete its strength. Curing needs water. If a fresh slab dries out too fast, hydration stalls and the concrete never reaches the strength it was mixed for.

That is why you may see a crew wetting down a slab, covering it with plastic or blankets, or spraying on a curing compound. They are not keeping it clean. They are keeping the reaction going.

It also means a slab that feels bone dry on top can still be early in its curing. Hard is not the same as strong.

The crew floats, trowels and edges the surface, then cuts or tools the control joints. Those joints are the straight lines you see across a driveway. They are deliberate. Concrete shrinks slightly as it cures, and control joints give it a planned place to crack instead of a random one.

You can usually walk on a new slab after 24 hours. Walk, not drag. Keep shoes on, keep pets off, and do not drag anything across the surface. The top layer is still soft enough to scuff and scar.

Forms typically come off in this window too.

Around 7 days, the slab has about 70 percent of its strength, which is enough for a passenger car or a light pickup. Park straight in, avoid hard turning on the slab, and keep the heavy stuff off.

If the pour happened in cold weather, give it longer. More on that below.

At 28 days the concrete has reached full design strength. This is when RVs, loaded trailers, dumpsters and heavy trucks are safe. It is also when you can seal it.

Temperature drives the chemistry, and the Twin Cities give concrete a workout at both ends of the calendar.

Cold slows hydration down. Below about 50 degrees, curing slows noticeably. Below about 40 degrees it nearly stops, and concrete that freezes before it has gained strength can be permanently damaged. That is why late fall and winter pours involve insulating blankets, heated enclosures, or a mix adjusted to set faster. It is also why a late October pour may need more than 7 days before you park on it.

Heat and wind speed evaporation up. A hot, dry, windy July day pulls moisture out of the surface faster than hydration can use it, which causes shallow surface cracking. The fix is the same idea in reverse: keep water in the slab with wet curing or a curing compound.

Either way, the 28 day target does not change. What changes is how much help the slab needs to get there.

A few things protect the investment while the slab is young.

Keep de-icing salt off it the first winter. This is the big one in Minnesota. New concrete is more porous than mature concrete, and chlorides cause surface scaling and spalling. Use sand for traction instead, and skip the salt until the slab has been through a full season. Even after that, go easy.

Do not seal it too early. Sealing traps moisture that still needs to escape. Wait the 28 days.

Expect hairline cracks. Thin surface cracks are normal in concrete and are not a structural problem. Control joints exist to manage where the real movement happens. If you see a crack that is wide, offset, or growing, that is worth a call.

Keep heavy loads off the edges. Slab edges are the weakest part of any pour. A truck tire riding over the edge of a driveway does more damage than the same truck parked in the middle.

Curing time is part of planning a job, not an afterthought. If you need a driveway usable by a certain date, that date works backward 7 days from the pour at minimum, and longer in cold weather. A patio you want furniture on for a party needs the same kind of math.

Creative Concrete has been pouring in the Twin Cities since 1998, which means a lot of fall pours, a lot of cold mornings, and a lot of slabs that had to be driveable on a deadline. We will tell you up front what the schedule looks like for your project and what the slab needs from you while it cures.

Looking at a project? See our work on concrete driveways, concrete patios and concrete walkways, or request a free estimate.