Strength impairment of concrete mixed in hot weather: analysis in relation to physical and chemical properties of hardened concrete

摘要

A previous paper reported that reductions in 28-day strengths were observed for manufacturing control specimens produced in summer conditions. It was shown that they were dependent on the porosity of the bulk paste of fresh concrete according to the initial temperature of the mix. Here, further investigations using physical and chemical approaches to seek explanations for these strength decreases in relation to the 28-day microstructure of concrete are presented. Scanning electron microscope observations revealed air bubbles and well-developed calcium hydroxide crystals which could explain in some cases the drops in strength. Porosity to water and absorption of water were increased whereas the degree of hydration was not affected. Secondary effects were highlighted when standard curing temperature (20℃) was not satisfied.