Two temperatures must be considered during cold-weather concreting: the ambient air temperature and the concrete temperature. If freshly placed concrete falls below 0°C, water in the mix will freeze and expand, causing structural damage. However, if the concrete reaches a strength of approximately 2 N/mm², it can resist this expansion. For most mixes, this strength is achieved within 48 hours if the concrete is maintained at or above 5°C. After this point, low temperatures will still slow strength gain, so the primary objective during cold weather is to keep the concrete warm for the first 48 hours and then allow strength to develop gradually.
When the ambient temperature is below 5°C but there is no frost, the risk of permanent damage is low, but strength gain will be slower. Formwork should not be removed too early, as premature removal can lead to broken edges or even structural failure in beams and slabs. The time required before removal depends on factors such as temperature, cement type and content. If necessary, strength development can be accelerated by increasing the cement content and reducing or eliminating supplementary materials such as GGBS or PFA.
If there is slight frost at night, it is critical to prevent freezing. Concrete should be delivered at a temperature of at least 5°C. If this cannot be guaranteed, you must assess whether adequate protection can be provided before proceeding. Protection measures include frost blankets, insulated formwork, and covering exposed surfaces with insulating material. Timber formwork often provides sufficient insulation, but additional measures such as temporary covers and space heaters may be required. Before placing, ensure that sub-bases, formwork, reinforcement, and equipment are free from ice and snow.
In severe frost conditions, additional precautions are necessary. Heated water should be used to ensure the concrete temperature remains above 5°C. Heated water is available from selected Procon Readymix plants, so check availability in advance. If heated water is not available, it is advisable to delay concreting until the ambient temperature rises.
In summary, maintaining concrete temperature above 5°C for the first 48 hours is essential, along with immediate protection against frost and adjustments to mix design where necessary. For guidance on heated water and mix modifications, contact Procon Readymix before starting work.