A drop in temperature below the normal temperature during concrete casting can cause significant inconveniences. Mainly the slowdown in the maturation of concrete. In fact, in the case of temperatures of 5°C the resistance of the concrete after 3 days is barely half that of that with a temperature of 20°C. At the end of the 28 days the resistance obtained will be no more than 80% of that in the case of a normal situation.
Indeed, if the temperature reaches 0°, truly serious deteriorations can occur, especially if the drop in temperature occurs during the initial phase of the setting of the concrete and its hardening (in this phase the concrete has no sufficient compressive strength has yet been achieved). Frost can cause the disintegration of the mass of the material, which is all the more harmful the lower the thicknesses of the structural elements involved. For this reason, the reference regulations require that concrete not be poured at temperatures lower than 0°C, unless appropriate precautions are taken.
The precautions are as follows:
- use of rapidly hardening cements, i.e. which have high initial resistance, higher heat of hydration, and increase their dosage;
- use heated water and aggregates stored in sheltered environments;
- use aerating additives that protect against freezing, or alternatively use anti-freezing additives that lower the freezing point of the water;
- carry out the castings during the hottest hours and cover them with heat-insulating materials, so as to reduce the dispersion of the hydration heat of the concrete mass;
However, these measures are not effective at temperatures below -3°C. In these cases it is advisable to suspend processing.
The elements most exposed to risk are those with a limited thickness and a large surface exposed to the air (such as attics). Elements enclosed on all sides, such as pillars, in formwork are the most protected, especially if the formwork is made of wood.
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