Effects of heat and mixing time on self-compacting concrete

The coupled effects of ambient temperature and mixing time on the slump loss of self-compacting concrete (SCC) are critical for hot weather concreting. Ordinary Portland cement concrete mixtures were made with a water/cement ratio of 0·38 and incorporating polycarboxylate-, melamine sulfonate-, or naphthalene sulfonate-based superplasticisers. The concrete mixtures were continuously agitated for up to 110 min using a low-shear rate mixer under controlled temperature ranging from 22 to 45°C. The effects of such a prolonged mixing scheme under various temperatures on the slump loss and compressive strength of concrete at ages of 12 h, 1, 3, 7 and 28 days were investigated. The results show that concrete can undergo substantial slump loss when subjected to prolonged mixing at high temperature. Knowledge of the superplasticiser effects at high temperature and prolonged mixing time is critical to achieve adequate rheological properties of concrete in hot weather. The results also indicate that the compressive strength of concrete mixtures is dependent on temperature, mixing time and superplasticiser type. The early-age strength of concrete at high temperature may be about four-fold that at moderate temperature. On the other hand, the long-term compressive strength of concrete subjected to early-age high temperature can decrease by more than 10%.