New concrete
How long does new concrete have to cure before it can be coated?
New concrete has to dry as well as harden, and how long that takes comes down to what the slab reads on a moisture test.
The short version
- New concrete has to dry, not only harden. Those are two different conditions and the second one arrives later.
- The wait ends when the slab is dry, and how fast it dries depends on the air over it more than on the date.
- A slab poured in a cold month in an unheated space dries much slower than the same pour in a warm one.
- Somebody tests bare concrete for moisture and schedules the install off what it reads.
- A slab that has been rained on since the pour has taken water back on.
What is happening in a new slab
Concrete gets hard early and keeps changing for a long time after that. The cement is still reacting with water, and the slab is still full of the water that was mixed in to make it pourable in the first place. Hard enough to walk on and dry enough to coat are two different conditions, and the second one arrives well after the first.
What has to happen is that the extra water leaves. It moves up through the slab and out the top into the air of the room, and how fast it does that depends on how thick the pour is, how much water was in the mix, whether there is a vapor barrier under it, and what the air above it is doing.
What the wait depends on
The air is most of it. A slab drying in a heated space with the air moving loses water steadily. The same slab in a closed, cold garage barely loses any, because cold air holds very little and still air keeps whatever it already took up. So a floor poured in a warm month and one poured in a cold month are not on the same clock, whatever the calendar says.
The pour matters too. A thicker slab has farther to move its water. A slab poured over plastic can only dry from the top, which is slower than one that gives water off in both directions and is still the right way to build it. And a slab that has been rained on since it was poured has taken water back on.
How it gets confirmed
The same way an older slab does. A moisture test goes on bare concrete and gets left to sit, and you find out where your own floor actually is rather than where a rule of thumb says it ought to be. A new pour in a heated shop can be ready before an older floor in a cold basement, which is why nobody schedules this off a date alone.
So the sequence on a new pour is that somebody comes out, tests the slab, and schedules from the reading. If it is not there yet, it gets tested again later. That is a longer answer than a number would have been, and it is the one that keeps a new floor from blistering off a slab that was still giving up its water.