Winter Concrete 1

[wdc]

How long must a concrete pile cap be covered when temps range from 10 - 25 degrees. Size is approx 3-6 x 4-0 x 3' deep. from above grade. Also when pouring concrete wall in temps of about 40 degrees and temps drop to 20 degrees at night is heat needed or can the wall be covered with blankets if so how long for each

 

[JohnGn]

It all depends on the mix you are using, on the temperature of the concrete on delivery, and on the efficiency of your insulating blankets. I've done jobs where the concrete itself generated enough heat to cure itself even in very cold (below freezing) temps were experienced. On the other hand, if concrete is delivered cold (say, 60 or below) and is covered only with tarps and not insulating blankets, it's likely to be too cold to cure properly.

In short, without monitoring the in-place temps as the concrete cures there is no way of knowing how long you need to leave protection on it--or how long you must wait before stripping formwork, for that matter.

--John Gnaedinger

 

[concrete2]

I am using an 800lb cement factor placed at a maximux 4" slump in a 20" thick wall. The wall is covered with 1" thick insulating blankets. The temp is expected to go down to 10 degrees F. I believe that I am more than safe, even at zero degrees. What are your opinions.

 

[JohnGn]

To Concrete2:

You should be safe with a 9-sack mix in a 20" thick wall covered with an insulating blanket and a good w/c. Air temp will mean very little in this case. In fact I'd say that your concrete will likely cure better in a cooler environment than a hot one.

Four thoughts:
1. you do not specify what the temperature of the concrete must be on delivery. This is a very important factor in how quickly the concrete begins it's hydration reaction, and it is the single biggest factor in determining the early-age curing rate. I would recommend at least 60F, warmer if possible. Even a few degrees makes a remarkable difference.
2. You do not mention if you will be pre-heating your forms. I would strongly urge you to preheat your forms using a hydronic heater since then you will not be shocking the concrete as it is placed in the cold forms.
3.How soon after placement are you expecting to remove the forms? I ask because many times contractors want to strip forms very soon but that takes away a lot of insulation that keeps the concrete warm, especially at the surface, where the risk of freezing is greatest.
4. I would recommend also that you place a recording thermometer inside the wall. Then you'll know exactly how the wall cured. You could also do maturity testing to determine the in-place strength which would help you decide when you can safely pull the forms off.

I hope this is helpful.

John
Con-Cure Corp.