Residual Compressive Stress–Strain Relationship for Hybrid Recycled PET–Crumb Rubber Aggregate Concrete after Exposure to Elevated Temperatures

One efficient way of recycling polymeric wastes such as crumb rubber and polyethylene terephthalate (PET) is to use them in a concrete mix. A prerequisite in accomplishing this application is to determine the general stress–strain relationship of this concrete type when subjected to fire in order to examine specific fire-performance criteria and better understand the actual behavior of structures made of it during the fire. In this research, the compressive stress–strain behavior of concrete containing polymeric recycled materials consisting of crumb rubber and PET as well as their combinations as natural sand replacements was investigated after exposure to elevated temperatures (200°C, 400°C, 600°C, and 800°C). For that purpose, the physicomechanical properties of the concrete specimens, namely, compressive strength, elastic modulus, strain at peak stress, ultimate strain, toughness, stress–strain curve, weight loss, and visual observation, were evaluated after exposure to elevated temperatures. Then a series of empirical equations were developed to predict the mechanical properties. Furthermore, a comparison was conducted between the experimental results and those predicted by international codes of practice, together with a comparison between the equations proposed here and the experimental results reported by other researchers. Finally, using the empirical equations obtained for the mechanical properties of the concrete containing recycled polymeric materials under elevated temperatures, a stress–strain model was proposed to predict the compressive behavior of this concrete, which demonstrated a good consistency with the experimental results. The results showed that as the temperature increased, a significant degradation occurred in the physical and mechanical properties of the concrete specimens. Moreover, the mentioned codes properly estimate the experimental results of compressive strength at higher temperatures and the tangential elastic modulus at all the temperatures, while they give a considerable overestimation of the strain at peak stress results.