Residual mechanical properties of compound concrete containing demolished concrete lumps after exposure to high temperatures

The structural use of compound concrete (CC) made of demolished concrete lumps (DCLs) and fresh concrete (FC) has been demonstrated to be a feasible alternative for waste concrete recycling. Previous studies have put focus on the properties of CC at ambient temperature, but little is known about the fire related performance of such material. To address this gap, a total of 159 CC specimens were tested after exposure to high temperatures up to 600 degrees C. The samples' failure modes, residual mechanical properties, and deformation characteristics were presented. The findings suggest that despite the thermal exposure, the CC specimens possessed similar relative compressive strength and elastic modulus to their FC counterparts; however, above 200 degrees C both CC and FC showed a slightly decreased capacity of strength retention with temperature compared to recycled aggregate concrete. The CC specimens' shape effect was prominent, since as the exposed temperature and the replacement ratio increased, the cube-to-cylinder strength ratio rose dramatically from 1.23 to 1.72. Moreover, it was found that incorporating DCLs had a non-negligible impact on the specimens' ambient and post-fire dilation behaviors. The ratio of the critical load (i.e. the load at which the dilations initiated) to the peak load exhibited a decreasing trend with the DCLs content; but such effect almost diminished at 600 degrees C. Lastly, a stress-strain relationship of CC after exposure to high temperatures was proposed.