Comparison of properties of homogenous and heterogenous hardened cement pasts exposed to high temperature at different ages

Abstract

Fire and high temperatures are the most destructive effect on concrete structures. To better understand the behaviour of the concrete exposed to high-temperature series of tests were carried out on homogenous and heterogenous hardened cement pastes. At three ages (28, 90, and 180 days) with three water-cement ratios (0.300; 0.250; 0.222), at eight different heat steps (from 20 °C to 900 °C), pure Ordinary Portland cement was tested. A study at the same ages and heat steps on heterogenous cements with two dosages (20% and 35%) of cementitious material with constant water-fine ratio was also carried out. The examined fines were powdered quartz, magnetite, boron carbide, powdered chamotte, and powdered perlite. Our experiment showed that the local maximum value between 200-300 °C of the relative residual compressive strength of Ordinary Portland Cement specimens decreased 10-30% from the values of 28 days old specimens to the 180 days old samples. However, the lowest strength value was at least 85% of the initial strength. The disappearance of the local strength maximum points is also noticeable in the rates of the Integrated Temperature Endurance. Among the cementitious materials, boron carbide and perlite eventuated remarkable results. The boron carbide resulted residual strength increment between 600 °C to 900 °C and ended 43-56% of the initial strength in all ages and water-fines ratios. The perlite powder containing cement pastes produced nearly constant, at least 80-90%, residual compressive strength up to 600 °C.