How long before I can walk, park and drive on new concrete?

This is the schedule we hand a customer when we strip the forms. 24 hours: you can walk on it, and keep pets off it until then, because a paw print at hour six is permanent. 3 days: patio furniture, a grill, planters, a bicycle. 7 days: a passenger car or pickup, driven straight on and straight off, no tight turns with the wheel cranked. 14 days: delivery trucks, trailers, an RV, a roll-off dumpster or a loaded work truck. 28 days: full design strength, and the point where a sealer can go on.

Those are the numbers for a standard 4 inch residential pour. A thicker slab, 6 inches for a trailer pad or a shop floor, carries more mass and more water, so we push the car back to 10 days and hold the full 28 for anything heavy. The driveway and slab pages explain where each thickness belongs.

If you are not sure which bucket something falls in, go by its weight, not its wheels. A 7,000 pound pickup is a car for this purpose. A loaded moving van or a concrete truck is not, and a slab that has had one week to build strength will crack under one even if it looks finished.

Concrete does not harden by drying. It hardens by hydration: the water in the mix reacts with the cement and grows crystals that lock the sand and rock together. That reaction keeps going as long as there is water inside the slab. Let the surface dry out in the sun in the first days and the reaction stalls, and the slab never reaches the strength it was designed for.

Strength comes on a curve. A normal mix reaches roughly 70 percent of its design strength at 7 days and its full design strength at 28 days, which is the benchmark the whole industry tests against. It keeps gaining slowly for months after that. The 7, 14 and 28 day marks above are not arbitrary; they are the points on that curve where the slab can take a car, then a truck, then anything, without the load outrunning the strength.

So a slab can be dry to the touch in a few hours and still be nowhere near cured. The reverse is also true: a slab we kept wet for a week is curing better than one that baked dry, even though it looks wetter and later. Dry is what the surface looks like. Cured is what the concrete can carry.

Once the air gets above about 90°F, most crews treat the pour as hot-weather concreting, and a DFW afternoon from June through September clears that with room to spare. Heat speeds up the set, so the surface stiffens fast, but it also pulls water out of the slab faster than hydration can use it. That is how you get plastic shrinkage cracks, a web of fine cracks across the surface inside the first day, and it is the main reason a summer pour needs more attention, not less.

A fast set fools people. The slab feels hard by evening, so they assume the clock can be shortened. It cannot. The early set is the heat talking, not the strength. A slab poured at 100°F and left to dry out can end up weaker at 28 days than one poured at 70°F, because it lost its water before the cement could use it.

On a hot day we pour early in the morning, wet down the base so it does not pull water out of the bottom of the slab, and get curing compound on the surface as soon as the finish will take it. Hot weather moves the curing work earlier; it does not move the 7, 14 and 28 day marks earlier. On a brutal week we tell customers to add a day or two. Why concrete cracks covers what goes wrong when that care is skipped.

Cold runs the other way. Below about 40°F the hydration reaction slows to a crawl, and if fresh concrete freezes before it has gained any strength, the ice breaks apart the structure the cement was building and the slab is ruined. DFW does not get much of that, but an ice week in January or February is enough, and a slab poured the day before one is in trouble.

On a winter pour we watch the forecast harder than the calendar. We order a warmer mix, sometimes with an accelerator so the concrete gains its early strength before the night temperature drops, and we cover the slab with blankets if a freeze is coming. The trade is time: a slab poured at 45°F needs more days before it can take a car than one poured at 75°F, so when it is cold we add days to the schedule and say so up front.

One more winter rule. Do not put salt or de-icer on a new slab in its first winter. The surface is still maturing and the salt will scale it. Sand is fine for traction.

Our part is the first 7 days. The slab either gets a spray-on curing compound, which holds the water in under a thin membrane, or it gets kept wet with a hose or a soaker line for the first several days. Both do the same thing: keep water in the slab so hydration can finish. Curing compound is the normal choice on a driveway or patio because it does not depend on anyone remembering to water.

Your part is to keep the load off and the surface quiet. No tires for 7 days, no heavy vehicles for 14 on a 4 inch slab or 28 on a 6 inch one, no dragging furniture, no hot-tire turns in the first weeks, and no pressure washer for the first month. If we cured with water instead of compound, keep it damp in the heat for the first 3 days; a light mist morning and evening is enough. Do not flood it and do not let a sprinkler beat on one spot. Sealing comes at 28 days: a sealer applied earlier traps water the slab still needs and turns cloudy white.

The same schedule governs anything that bolts to the slab. When we pour a pad for a carport or a shop, CJM Metals builds the steel on it, and the columns wait on the concrete's clock, not the other way around. For a pour on your own schedule, or a question about one already curing, call (214) 603-9142.