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Malamute
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# Posted: 7 Nov 2011 12:39am
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I would like to know how long I should worry about new concrete freezing, and when I should strip forms. I poured concrete Friday morning and it was in the 50's, about 30 that night. It's been 40's in day, 20's at night since. I had them add hot water to it when they batched it, as there was a couple places where there were small frozen places in the ground when I set forms. I covered it on the top and one side with straw the first night and since. I'd guess it past time to be concerned about it freezing, but I don't know for sure.
So, good to go without straw now? Fine to strip forms?
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Rob_O
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# Posted: 7 Nov 2011 09:50am
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I'd say if it hasn't set by now it never will. Might as well strip the forms and see what you have
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Retired
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# Posted: 30 Nov 2011 02:47pm
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Generally speaking it takes 28 days for concrete to reach its full strength. Putting hot water in the mix the day you poured it would help it set up quicker that day so you could put a finish on it. It would not in cold weather make it cure out quicker. Accelerators seem to help a slab cook the first day as well. They seem to give you a jump in curing but protection is still needed from the first night and through the first week is critical.
I would have to know more about what it is your have poured, foundation, slab on grade? Did you have compacted gravel base?
I would say the first week would be the most risk in it freezing, after a couple weeks your pretty safe. The straw on top assuming it is 6 inches or more should help.
Any freeze damage to the surface of a slab on grade will likely appear in the first week. It would appear as a round discoloration above the stone below it. When it happens the top 1/16th of the surface pops off over the rock sometimes as much as 1/8"
I have only seen a couple slabs freeze; we poured them in extremely cold conditions where it dropped to about 25 degrees. The slab had a maximum amount of accelerators and hot water in it. Still got to around 9pm before we got a finish on it. We had insulating blankets on it, but the edges froze, we had to saw cut off about 6" inches of the edge of the slabs.
If it's a foundation and you can leave the forms on for 30 days I would do it. If it were a slab I would leave the straw on a minimum of two weeks, preferably 30 days. Insulating blankets can be rented at various contractors concrete supply businesses. They allow you to work during the day and cover at night. No mess.
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