Concreting In Cold Weather
At low temperatures, the hydration process, setting and gaining strength within concrete are delayed, so protecting concrete from the cold for extended time periods is necessary. Curing periods are generally doubled during winter months but, even if the concrete can be kept above freezing temperatures, the strength of concrete laid at low temperatures can be as much as 25 per cent lower than that of concrete placed at warmer times of the year.
One of the key problems is that water expands when it freezes, which causes massive problems if freezing occurs within partly set concrete. At best, the concrete strength can be significantly reduced as a result of the freezing water inducing stresses that weaken the bond between the aggregate and the cement paste. This can also result in increased porosity, spalling of the concrete and loss of durability. At worst, the damage can be so severe that the concrete is rendered useless and must be taken up and replaced.
Possible courses of action include raising the temperature of the mixing water, ensuring that all aggregates are free from ice and not frost-bound, and protecting the finished concrete with polythene sheeting with hessian quilting on top, held down firmly at the edges. However, the best course of action is not to carry out concreting at temperatures below 5 degrees centigrade or where there is a risk of the temperature dropping below 5 degrees centigrade overnight.
The use of Rapid-Hardening Portland cement instead of Ordinary Portland has benefits for working in cold weather. Rapid-Hardening Portland cement and Ordinary Portland cement are similar in composition but the former is more finely ground. The increased surface area of the cement powder does not make concrete set more quickly, but it does increase the rate of hydration at early stages and leads to an increased rate of early hardening and strength gain. This is important when trying to combat the effects of cold weather on new concrete. The increased rate of hydration, being an exothermic reaction, also causes an increase in the evolution of heat, which helps to reduce the possibility of water freezing within the concrete.