BEM investigation of fracture characteristics in polyethylene terephthalate-modified self-compacting concrete after thermal exposure

Evaluating the performance of self-compacting concrete (SCC) at elevated temperatures significantly contributes to understanding the behavior of structures during fire. The inclusion of polyethylene terephthalate (PET) as a partial replacement of fine aggregates in SCC addresses environmental challenges while modifying the failure behavior. However, the failure performance and thermal response of self-compacting concrete (SCC) containing PET at elevated temperatures are still not fully understood. Understanding the changes in toughness, ductility, and fracture energy of this type of concrete at elevated temperatures is essential for the safe design of concrete structures. In this study, to assess the effect of shredded PET sheets as a partial replacement for natural fine aggregates, self-compacting concrete mixtures incorporating 0, 5, 10, and 15% PET by volume were produced, and their fracture behavior and ductility were investigated under unheated conditions and following thermal exposure at 200, 400, and 600 °C through three-point flexural loading applied to notched beam specimens. The results showed that the characteristic crack length as a ductility index ( α ∞ ∗ ), the fracture toughness ( K IC ), the initial fracture energy ( G f ), and the size independent fracture energy ( G F ) decreased by approximately 7.5, 20, 22, and 27%, respectively, for the concrete specimens compared with the reference sample (without PET and without heating) as the PET replacement ratio increased to 15%. In addition, exposure of the specimens to 600 °C resulted in the greatest reduction in the investigated parameters compared with the other temperature levels. Analysis of the fracture parameters using the BEM method indicated that, in PET-free concrete and PET-containing concretes subjected to heating, the fracture energies and fracture toughness decreased by about 26 to 50% and 38 to 47%, respectively. Conversely, increasing the temperature within the range of 20 to 600 °C resulted in a reduction observed in both groups, with the difference that PET-containing concretes exhibited considerably more ductile behavior than PET-free concrete at high temperatures. Examination of the design criterion indicated that the fracture behavior of SCC becomes more consistent with linear elastic fracture mechanics (LEFM) with a rise in temperature, such that the design criterion at higher temperatures, with greater initial notch depth, conforms to LEFM. Finally, based on the achieved results and the experimental variables, multivariable equations were created to predict PET fracture parameters containing self-compacting concrete exposed to high temperatures. Evaluating these models against the present experimental findings, as well as with data available in academic writing, demonstrates that the equations reveal satisfactory precision.