cold wether concreting

[kuntay]

I am looking for strength gain curves for cold weather concreting. In fact we are pouring concrete in cold weather conditions and protect it by heater fans and cover it by tent canvas nearly for 24 hours. After that we leave it open. The weather conditions range from +5C to -15C. In normal conditions the concrete gains 65% strength in one week but what is the situation if it is kept at 30-35C for the first 24 hours and then left uncovered. What are the effects of freeze thaw on this concrete. If you have any recommendations I will be glad to know. Thanks.

 

[Ron]

Don't allow the concrete to freeze until at least 7 days and preferably 28 days. Placing the concrete at this temperature will make it more susceptible to expansion issues during high temperatures. Further, allowing the concrete temperature to drop immediately after placement will subject it to higher shrinkage stresses and potentially more cracking.

 

[jheidt2543]

Here are a few thoughts to consider:

1. The American Concrete Institute (ACI) and the Portland Cement Association (PCI) both have much information on cold weather concreting.

2. You should not let concrete freeze until it has reached it's design strength. Depending on mix design, cement content, cement type, admixtures and curing that can be any where from 3-4 days up to 28 days as Ron has said.

3. When using heaters and fans, it is very important to check CO2 and CO levels, both for worker safety and protection of the concrete. Both gases can damage the surface of fresh concrete causing "dusting" (carbonization). Don't direct the warm air directly onto the concrete.

4. When pouring cold weather concrete we would get an indoor/outdoor thermometer and place the "tail" into the wet concrete, cover the fresh concrete and forms with insulated tarps and leave the thermometer outside the cover. We could then read both the concrete temperture and the air temerture during the curing period. You will find the records very helpful and it is amazing how much heat curing concrete can generate! We've poured heated concrete (the materials were heated in the ready-mix truck) in large spot footings, in +15F weather, on UNFROZEN ground (we had the forms protected) and had the night time temperture drop to -15F after the pour. The concrete didn't drop below +70F for more than 7 days!

Cold weather concreting can be done successfully, BUT it takes planning and attention to details.

 

[dicksewerrat]

The concrete will continue to cure at low temps but should not be allowed to freeze for at least 3-4 days. It may not reach design strength for 6-8 weeks if it is kept cold though. I would heat with an air-exchange type heater. One that does not blow the combustion gases on the concrete but air heated through a heat exchanger. As Jheidt2543 said CO and CO2 will damage the surface of the fresh concrete. Get rid of the canvas and buy insulated blankets. Leave the insulated blankets on the concrete as long as possible. You will also want to redesign your mix and eliminate the use of Fly Ash to increase the heat generated by the chemical reaction. Maybe even up the cement content and theoretical strength to compensate for the cold curing temps.

 

[KRSServices]

There are several additives that will ensure a quick cure or promote heating during the cure process. These additives have to be incorporated into the design to ensure the correct 7 day strengths. Here in Canada, we preform cold pours on a regular basis and I can forward some excellent references if required. ACI also has some excellent materials on this subject.