curing hearth slab in lower temps

[jimkramer]

I'm going to frame my hearth slab this weekend, and pour it next weekend, I hope. How long will it take to cure? We should get just about a week of nights above 40 or so (just north of NYC). Is this enough time? If it dips a bit below that, will a worklight under the slab at night keep it warm enough?

[dmun]

Re: curing hearth slab in lower temps

40f seems to be the magic number. You may cover it with a tarp and leave a light bulb burning underneath at night. You absolutely don't want anything near freezing.

[Tscarborough]

Re: curing hearth slab in lower temps

Once there is no free water in the concrete (it has "set"), then freezing can't hurt it. It WILL retard the cure, though not stop it. A tarp and shop light will speed up the cure.

[Ilfornaio]

Re: curing hearth slab in lower temps

If the temperature goes below 40 F the concrete will stop curing. It is advisable to cover the concrete with a thick layer of straws or some kind of insulating blanket for at least a week, if ambient temperature are forecasted to drop below 40F.

[SteveP]

Re: curing hearth slab in lower temps

As a general rule, the concrete should not be allowed to get below 50F. As T stated, when concrete gets close to freezing, hydration will practically stop and the slab will not gain any strength. You will get some heat in your mix due to the exothermic reaction, but I don't think it will be enough to overcome the cold temperatures. You could end up with a weak "green" slab if it gets too cold. If you could put a tarp around it with a heater or a heavy duty shop light, this will probably work. On a side note, I poured a 3' x 3' x 3' foundation when it was <50F outside and the concrete was still green and brittle after 24 hours.