The literature demonstrates that concrete produced at high temperatures [35 versus 18°C (95 versus 65°F)] or mixed and agitated for 90 versus 20 min, tends to have increased mixing water requirements and reduced strength. The research reported here demonstrates that the additional cement required to compensate for this strength loss can be very modest.
Concrete was mixed at temperatures of 18 and 35°C (65 and 95°F). Two series of mixtures were proportioned to produce strengths of 28 to 31 MPa (4000 to 4500 psi) and 34 to 38 MPa (5000 to 5500 psi), utilizing three different material combinations: cement without admixture, cement plus water reducer, and cement plus fly ash. Two cement sources were used and concrete delivery times of 20 or 90 min were simulated. The concrete was retempered to maintain slump. To simulate nonstandard how weather initial curing, extra cylinders were cured at 38°C (100°F) for the first 24 h.
The main considerations were the effect of the research variables on the required cement content to produce a given strength level, on the required mixing water content to maintain a target slump, and on drying shrinkage. The results show that, with one exception, the higher mixing temperatures and extended delivery time had little effect on the concrete properties. One of the two cements used tended to have more strength loss in hot weather when it was used without admixtures. However, when that cement was used with fly ash or a water-reducing admixture, rapid loss of slump and strength was avoided.
An increase in the concrete mixing temperature from 18 to 35°C (65 to 95°F) required an average increase of 4.7 kg/m3 (8 lb/yd3) of cement to maintain the strength level. An increase in the delivery time from 20 to 90 min required an additional 13.6 kg/m3 (23 lb/yd3) of cement, on the average. The water required to produce the target slump at the higher temperature increased by 2 to 6%. The extended delivery period affected the water requirements by 3 to 13%, and when the extended period was coupled with the higher temperature, the increase in mixing water ranged from 7 to 18%. The hot first-day curing period had a significant effect on the strength, causing a strength loss of about 10%. The variables appeared to have little effect on the drying shrinkage of the concrete.