n When the temperature of the concrete itself exceeds 20°C, the rate of reaction between the cement and the water is increased and this in turn leads to an increased rate of stiffening and loss of consistence. There is also an increased risk of early-age thermal cracking because the peak temperature will be increased
n High air temperatures, especially when accompanied by a low humidity, can increase the rate at which water evaporates from the concrete. Evaporation of water due to delays between mixing and placing will cause loss of consistence. Evaporation will also be increased from exposed horizontal surfaces after the concrete has been placed.
Loss of consistence
Stiffening due to high temperatures and/or water loss can cause problems by:
n Making it difficult to place and compact the concrete.
n Increasing the risk of 'cold' joints in large pours
n Creating surface finishing problems with floors and paved areas.
The existence of drying conditions makes it more important to ensure that exposed surfaces of concrete after compaction and finishing are protected against loss of moisture by efficient curing methods.
Accelerated stiffening and loss of consistence can best be minimized by placing the concrete as soon after mixing as possible. It is essential that concrete should be of the required consistence at the point of placing, and any delays due to prolonged transportation should be allowed for by designing the concrete so that the consistence of the concrete at the mixer is higher than required at placing to allow for consistence loss.
It should be noted that this higher consistence may require an increased cement content or the use of a plasticizing admixture in order to maintain the correct free water/cement ratio.
Over a delay period of 30 minutes the loss in slump can be 20 - 50 mm and will become progressively worse with cement contents higher than 300 kg/m3. Rapid stiffening can be minimized by using a retarding admixture and/or cements containing pfa or ggbs, which reduce temperature rise and minimize the risk of early-age thermal cracking.
Another factor which should be taken into account is that the higher the temperature of the batch ingredients (and hence the concrete temperature) the greater will be the quantity of water needed to produce any given consistence class. For example, concrete with S2 consistence class at 20°C is likely to have a consistence class of only S1 at a temperature of 30°C when made with the same free water content.
Moisture loss
The rapid loss of moisture from the surface of exposed concrete increases the risk of plastic cracking, and the concrete should be cured thoroughly as soon as possible after finishing. As soon as the surface has hardened sufficiently, polythene sheeting can be used, or a sprayed-on curing membrane applied, (see Figure 24) preferably using a pigmented type that reflects solar radiation (see Curing on page 46).
Concrete that has lost workability due to early stiffening should not be retempered by additional water.
NOTE: Low water content makes concrete more susceptible to the adverse effects of the moisture loss.
It is generally considered inadvisable for concrete to be placed when its temperature exceeds about 30°C unless special procedures are followed, such as those that apply in very hot climates.