Monotonic and cyclic behavior of high-strength concrete with polypropylene fibers at high temperature

Experimental studies were conducted to examine the monotonic and cyclic response of high-strength concrete (HSC) with polypropylene (PP) fibers at 2 kg/m3(0.125 lb/ft3) at temperatures up to 700°C (1292°F). At temperatures below 500°C (932°F), HSC with PP fibers is a brittle material with a relatively steep descending branch in the stress-strain curve (that is, the postpeak part of the stress-strain curve). Compressive strength decreases rapidly when the temperature increases from room temperature to 100°C (212°F) but recovers when the temperature increases from 100 to 400°C (212 to 752°F). From 400 to 500°C (752 to 932°F), it drops sharply and there is a strength loss of 50% at 500°C (932°F). A further increase in temperature only leads to a slight reduction in the compressive strength. At 700°C (1292°F), the strength loss is approximately 55%. Peak strain increases considerably with increasing temperature. Based on the test data, a complete stressstrain relationship is proposed. Examination with a scanning electron microscope shows distinct changes in the morphology of concrete, caused by exposure to elevated temperatures. Cyclic stress-strain envelopes at 500 and 700°C (932 and 1292°F) match the stress-strain relationship of monotonic loading. In particular, the unloading plastic strain ratio varies linearly with the unloaded strain ratio and progresses at a faster rate as compared with that obtained at room temperature. This indicates that the variation of the unloading strain ratio is sensitive to the change in temperature, particularly at high temperature.