In wintertime, the primary concern of the concrete contractor is to be sure his concrete is placed and kept at a temperature high enough for normal strength development. Most existing mix design and strength development data are based on the assumption that concreting normally takes place at temperatures around 70 degrees F. Below 70 degrees F setting and hardening of concrete are retarded, and below 25 degrees F the reaction between water and cement practically ceases. The easiest and most economical way of raising the temperature of a concrete mix is by heating the mixing water. Hot water must near come into contact with cement alone. The temperature limit is 180 degrees F when the water is added to a mixture of cement and aggregates. This limit automatically restricts the use of heated mixing water alone when ambient temperatures are above 30 degrees F. For ambient temperatures below 30 degrees F, it is also necessary to heat the aggregate. Usually only the fine aggregate will need to be heated for temperatures down to zero. Below zero both fine and coarse aggregate should be heated. The condition of the aggregate has been stored dry and is free form ice and frozen lumps, heating is easier because excess thaw-out heat is not needed. Heating of the aggregate is accomplished most efficiently by means of a closed system of coils in the storage bin. Open steam jets are faster but should be avoided because they add varying amounts of moisture which will affect the consistency.. Concrete should be air-entrained. Research has proved that without air-entrainment, no additional protection during winter concreting, however efficient, can provide an ultimate durability equal to that obtained with entrained air. The air-entraining agent must be added to the concrete after the highest temperature of the heated ingredients has been lowered by mixing, if the tie's effect is not to be minimized.