Abnormally cold or hot temperatures cause concrete materials to behave differently and affect the timing of the finishing operations. Protection may be required against the effects of such weather. Advanced planning for cold weather concreting and finishing operations will minimize the problems. Heaters and materials for enclosing and insulating should be available at the job site for ready use. Concrete mixes that are proportioned for cold weather and heated when necessary play an important role. Such mixes may require more cement, the use of Type III cement for high early strength, or the inclusion of one or two percent of calcium chloride to offset retardation caused by low temperatures. Other accelerating admixtures may be used but never in combination with calcium chloride. Remember, though, that no admixture will prevent concrete from freezing. At the time of placement, concrete should be no cooler than 50 degrees F. When the air temperature is between 30 and 40 degrees F it is advisable to heat the mixing water or aggregates to bring the concrete temperature within the desirable range. Maintaining good control of the temperature of the mixing water from one batch to another will avoid major variations in temperature which cause fluctuation in slump and disruptions in the timing of finishing operations. Curing temperatures should never be allowed to drop below 50 degrees F for a period of seven days with normal portland cement, or for a period of three days with Type III cement. Freezing must be prevented for at least another four days to avoid interruption of strength gain. Insulating blankets can help protect slabs on ground against cold weather during the curing period. In extreme cold, enclosures must be built for slabs exposed to the weather. One successful way of heating slabs above grade is to insulate the concrete surface, enclose the space below and heat it with salamanders. The forms below the concrete prevent the carbonation of the bottom surface while the top surface is warmed sufficiently through the slab. In planning for hot weather remember to cool the concrete using ice in the mixing water or sprinkling the aggregate. Finishing operations should go quickly and fog sprays may be used to replenish moisture loss.