For many contractors, pouring concrete in cold weather is a nerve-wracking experience -— so much so, that they shorten their construction season to avoid problems associated with cold pouring.
But it doesn’t have to be that way. With today’s improved concrete designs and pouring methods any contractor can be confident of producing good results even in cold weather with a little planning.
Concrete’s strength is in part a function of the temperature at which it cures. Normal compressive strength is based on curing concrete at its ideal curing temperature, 72 degrees F for a period of 28 days. Warmer temperatures reduce the cure time. On the other hand, cooler temperatures increase the length of the cure period.
Cold weather is technically defined as a period when, for more than 3 consecutive days, the average daily air temperature is less than 40 degrees F, and the air temperature is not greater than 50 degrees F for more than half of any 24-hour period.
Pouring concrete in these conditions actually can be advantageous for contractors. As long as the differential between ambient air temperature and concrete temperature is minimal, concrete poured under cold conditions will produce higher than ordinary ultimate strengths and better long-term durability.
The secret to a good cold weather pour begins with good site preparation. In cold weather this means making sure you have all your insulating and heating equipment on-site and working properly before the pour begins. Straw, tarpaulins and insulating blankets should be all dry and in good condition. If foam sheeting is being used for insulation, be sure to figure out how it will be held in place in case of high winds. Check all heaters for performance and check that they have sufficient fuel for the entire period of the cure.
Concrete should not be placed on frozen subgrade. If freezing is anticipated, the subgrade must be protected and its temperature brought up to above freezing. Usually this means using about 6 in. of straw cover to insulate the pour area. The straw should be held in place with tarpaulins or plastic sheeting. Be careful not to compact the straw too much or you will reduce its insulating abilities.
These precautions extend to include forms at grade and above grade as well as reinforcing steel brought up to above-freezing temperature prior to the pour. Forming components and reinforcing steel must be free of ice, snow and frost. The forms themselves should be warmer than 32 degrees F but not warmer than the concrete that will be placed against them. Insulated blankets and plastic foam panels can be used to insulate the forms. In the case of post holes, make sure there is no ice accumulation at the bottom of the holes that can cause movement of the posts after thawing.
In some instances you will want to construct enclosures to protect curing concrete from wind and too rapid evaporation.
In the mix
Good communications between your concrete supplier and your pour supervisor are another key ingredient in successful cold weather concreting. The plant needs to know the temperature at your site, especially if the batch plant is some distance away.
Your batch plant has three methods of ensuring that the concrete arriving at the work site is suitable for the weather conditions.
First of all, the plant will simply raise the temperature of the water used in the mix. As long as the aggregates are free of ice, this method works quite well for moderately cold days. Water temperatures ranging between 140 and 180 degrees F can be used, but temperatures above 180 degrees F will compromise the mix. To prevent the possibility of concrete flash setting, at no point should the cement be allowed to contact the hot water; instead, the aggregate should be mixed into the water prior to the addition of cement. Alternatively, aggregates and cement could be added to hot water by ribbon loading the batch.