Strength impairment of concrete mixed in hot weather: relation to porosity of bulk fresh concrete paste and maturity

Abstract

Normal-strength concrete mixed in summer can be less strong than the annual average. One cause of these variations is the effect on concrete strength of the temperature of the constituents. Concretes were cast at different temperatures because of different temperatures of the constituents. The slump value, which is the criterion for acceptance on the building site, was kept constant whatever the initial temperature may be, either by the addition of water or by the use of a plasticiser at the end of mixing. Concretes were cured for 24 h under either the conditions specified in the French standard for manufacturing control or simulated hot weather conditions, and then stored in water at 20°C up to the time of test. Compressive strength was determined at 1, 7 and 28 day ages on 11 × 22 cm cylinders. The results show that the decrease in strength is dependent on the combined effective water and air content of the bulk paste of fresh concrete, and on the maturity of the concrete, which is influenced by its temperature history. By combining two classical relations: strength and design factors (Feret's relation) and strength and maturity (Arrhenius Law), decreases in strength could be analysed for any mixing and curing conditions.