Influence of cold weather during casting and curing on the stiffness and strength of concrete

The supposition that casting and curing at near freezing temperatures leads to the loss of stiffness of the hardened concrete and causes microstructure disruption and abnormalities was investigated. The research programme reported here involved 25 concrete slabs, 750 × 750 × 300 mm, which were subjected to various curing regimes in the range of 20 °C to −5 °C. The extent of loss of stiffness and the degree of microcracking were quantified using the Stiffness Damage Test (SDT). This test method has previously been shown by one of the authors to be very effective in quantifying the extent of disruption to the microstructure of concrete caused by various damage mechanisms. In addition to the SDT, the cold-cured concrete was evaluated by the full stress–strain test as well as ultrasonic, dynamic, and petrographic methods. Concreting at temperatures near freezing resulted in approximately 20% reduction of the 28 day stiffness and a Damage Index of 5 as determined by the SDT. Cold weather curing resulted in the development of microcracks in the paste and adversely affected the interfacial zone. Storage of the constituents of concrete at near freezing conditions prior to mixing adversely affects the long term stiffness and strength of concrete. The water absorption of hardened concrete increased as a result of cold-curing. The loss of stiffness because of low temperature curing is not remedied by using stiff aggregate such as granite nor by PFA cement replacement.