This paper investigates a potential application of hard-to-recycle plastic waste as polymeric addition in high strength concrete, with a focus on the potential to mitigate heat-induced concrete spalling and the consequent effects on the mechanical properties. The waste corresponds to soft and hard plastic, including household polymers vastly disposed of in landfills, although technically recyclable. Mechanical and physical properties, cracking, mass loss, and the occurrence of spalling were assessed in high strength concrete samples produced with either plastic waste or polypropylene fibers after 2-h exposure to 600 °C. The analysis was supported by Scanning Electron Microscopy and X-Ray Computed Tomography images. The plastic waste is composed of different polymers with a thermal degradation between 250 to 500 °C. Polypropylene (PP) fibers and plastic waste dispersed in concrete have proved to play an essential role in mitigating heat-induced concrete spalling, contributing to the release of internal pressure after the polymer melting. The different morphology of plastic waste and polypropylene fibers leads to distinct mechanisms of action. While the vapor pressure dissipation network originated by polypropylene fibers is related to the formation of continuous channels, the plastic waste seems to cause discontinuous reservoirs and fewer damages into the concrete matrix. The incorporation of plastic waste improved heat-induced concrete spalling performance. While 6 kg/m3 of plastic increased the mechanical performance after exposure to high temperature, the incorporation of 3 kg/m3 resulted in mechanical properties comparable to the reference concrete.

The physical, chemical and mechanical properties of concrete incorporating rejected recyclable plastic waste (RRPW), the potential for the mitigation of heat-induced concrete spalling and the effects of high temperature on the residual properties of concrete were investigated. Concrete compressive and tensile strengths, Young's modulus, crack width, mass loss, water absorption by capillarity, chemical composition and evidence of heat-induced concrete spalling were monitored in concrete samples made with RRPW particles and compared with those of concrete samples made with commercial polypropylene (PP) fibres, after exposure to 200°C, 400°C and 600°C for 2 h. The use of 0·125% by volume of PP fibres and plastic waste particles was found to improve the heat-induced concrete spalling performance, due to the release of internal pressure after melting by different mechanisms. The positive effects on concrete properties show the technical potential of incorporating RRPW particles in construction materials.