Effects of test conditions and mixture proportions on behavior of high-strength concrete exposed to high temperatures

Mechanical properties of high-strength concrete exposed to elevated temperatures were measured by heating 100 x 200 nun cylinders at 5 C/min to temperatures of up to 600 C Heating was carried out with and without a sustained stress, and properties were measured at elevated temperatures as well as after cooling to room temperature. Four mixtures with water-cementitious materials ratios (w/cms) ranging from 0.22 to 0.57 and room-temperature strengths ranging from 51 to 98 MPa were used. Two of the mixtures contained silica fume. Measured compressive strengths and elastic moduli were normalized with respect to room temperature values, and analysis of variance was used to determine whether the test condition, the value of w/cm, or the presence of silica fume affected the results. The influence of these variables on the tendency for explosive spalling was also examined. Results indicate that losses in relative strength due to high-temperature exposure were affected by the test condition and w/cm, but there were significant interactions among the main factors that resulted in complex behaviors. The presence of silica fume does not appear to have a significant effect. Measurements of temperature histories in the cylinders revealed complex behaviors that are believed to be linked to heat-induced transformations and transport of free and chemically combined water.