A major environmental challenge facing metropolitan areas is recycling and removing waste plastic materials including PET from the environment. On one hand, using waste materials as a substitution for a portion of natural aggregate in concrete has been an efficient solution for dealing with the environmental problems associated with these materials. On the other hand, despite the degradation of concrete properties due to the presence of waste materials, the incorporation of fibers into concrete can improve its mechanical performance. In this regard, the compressive performance of fiber-reinforced concrete containing recycled PET chips being exposed to high temperatures, was investigated in this experimental effort. A total of 108 specimens, divided into 9 mix designs, were then fabricated in order to perform the compression test. The variables under consideration were the volume percentage of PET chips replacing natural sand (0, 5, and 10%), the volume ratio of steel fibers (0, 0.5, and 1%), and the temperature (25, 200, 400, and 600 °C). Further, a comparison was made between the compressive strength values obtained from tests and the associated predictions of ACI 216 and EN 1994-1-2. The findings demonstrated that the presence of PET chips replacing a percentage of sand by volume and steel fibers in the concrete mix lowered the compressive strength of the specimens with and without the thermal loading. Also, the codes overestimated the compressive strength values of all thermally treated concrete specimens. Moreover, the response surface method (RSM) was utilized to optimize the design parameters so as to obtain the maximum compressive strength of concrete with PET chips and steel fibers subjected to high temperatures. Finally, a closed-form formula was developed to predict the compressive strength using the gene expression programming (GEP) approach. The optimization results demonstrated that by replacing 10% of natural sand with recycled PET chips and adding 0%
