Fracture toughness of high performance concrete subjected to elevated temperatures. Part 2 : the effects of heating rate, exposure time and cooling rate

In this study, the fracture toughness K_IC of high performance concrete (HPC) was investigated by conducting three-point bending tests on a total of 240 notched beams of 500 mm×100 mm×100 mm subjected to heating temperatures up to 450ºC with exposure times up to 16 hours and various heating and cooling rates. For a heating rate of 3ºC/min, K_IC for the hot concrete sustained a monotonic decrease trend with the increasing heating temperature and exposure time, from 1.389 MN/m^1.5 at room temperature to 0.942 MN/m^1.5 at 450ºC for 4-hour exposure time, 0.906 MN/m^1.5 for 8-hour exposure time and 0.866 MN/m^1.5 for 16-hour exposure time. For the cold concrete, K_IC sustained a two-stage decrease trend, dropping slowly with the heating temperature up to 150ºC and then rapidly down to 0.869 MN/m^1.5 at 450ºC for 4-hour exposure time, 0.812 MN/m^1.5 for 8-hour exposure time and 0.771 MN/m^1.5 for 16-hour exposure time. In general, the K_IC values for the hot concrete up to 200ºC were larger than those for the cold concrete, and an inverse trend was observed thereafter. The increase in heating rate slightly decreased K_IC, and at 450ºC K_IC decreased from 0.893 MN/m^1.5 for 1ºC/min to 0.839 MN/m^1.5 for 10ºC/min for the hot concrete and from 0.792 MN/m^1.5 for 1ºC/min to 0.743 MN/m^1.5 for 10ºC/min for the cold concrete after an exposure time of 16 hours. The increase in cooling rate also slightly decreased K_IC, and at 450ºC K_IC decreased from 0.771 MN/m^1.5 for slow cooling to 0.739 MN/m^1.5 for fast cooling after an exposure time of 16 hours. The fracture energy-based fracture toughness K_IC' was also assessed, and similar decrease trends with the heating temperature and exposure time existed for both hot and cold concretes. The relationships of two fracture toughness parameters with the weight loss and the modulus of rapture were also evaluated.