At higher temperature (about 100 degree Fahrenheit or 38 degree Fahrenheit) hydration of cement is faster so the produced concrete has high strength at first but it is reduced at the last. Due to the faster chemical activity rapid setting and rapid evaporation of water causes difficulty in mixing, placing and curing. Evaporation of water causes shrinkage and subsequent cooling would introduce tensile stresses and cracking.
Hence, for hot weather concreting, the temperature of concrete make at the site should be less than 60 degree Fahrenheit and should not be greater than 40 degree Fahrenheit (i.e. 16 degree Celsius - 29 degree Celsius).
- Shading to the aggregates, cement and the mixture should be given to keep the temperature low.
- Cold water sprinkling over aggregate can be done to reduce the temperature of aggregate.
- Mixing water tanks and pipes should be thermally insulated.
- Use of crushed ice is useful to lower the temperature of mix. But ice should be changed to liquid at mixing stage.
- Working at night and protection of the work from direct sun rays can control rapid hardening and frying of concrete.
- For curing, moisture-retaining materials, sprinklers may be used to keep the concrete cool.
Concreting in cold weather:
At freezing temperature the following effects may be seen;
- After placing, if freezing occurred, setting of cement is interrupted.
- If freezing is before gaining of sufficient strength, the formation of ice causes disruption in strength gaining.
- If freezing takes place after full hydration, no water is left in the gel pores due to full utilization of water in reaction then the concrete will be able to withstand freezing action without damage.
Mixing water may be heated to increase the temperature of the fresh concrete. It is heated to no > 60 80 degree Celsius because of flash set.
- Cement shouldnt be in contact with hot water.
- If heating of water doesnt raise the temperature, the aggregates mat be heated by passing steam through coils for uniform heating.
- Aggregates should not be heated above 52 degree Celsius.
- To control the temperature of concrete in early stages after placing the following methods are employed;
- Use of cements containing more C3S and C3A which produces more heat and creates low initial setting time.
- Use of accelerators such as calcium chloride, which makes the initial and final setting time lower and helps to achieve greater strength (than that of concrete without it) in cold weather concreting.
- Concreting shouldnt be done in frozen surfaces.