Post-fire residual fracture characteristics and brittleness of self-compacting concrete containing waste PET flakes: Experimental and theoretical investigation

Abstract

Correct understanding of the fracture mechanism of self-compacting concrete (SCC) after tolerating elevated temperatures plays a big role in the design of SCC mixes and helps better understand the behavior of SCC structures during a fire incident. Besides, to obtain eco-friendly concretes incorporating wastes and be able to employ them in concrete structures, knowing their mechanical characteristics is essential. Therefore, in this research, the three-point bending test was conducted on 240 notched beams to determine the effect of the partial substitution of polyethylene terephthalate (PET) flakes for fine aggregates at volume percentages of 0, 5, 10, and 15% on the fracture characteristics and ductility of SCC in two states: without heat treatment and after the application of 200, 400, and 600 °C. For this purpose, the two methods of size effect method (SEM) and work of fracture method (WFM) were used to analyze and interpret fracture parameters. The analysis of the obtained results demonstrated in the specimens without PET, after the heat treatment, fracture energy values obtained from WFM and SEM, as well as the fracture toughness (KIC) obtained from SEM, declined. Moreover, the results obtained for the effective size of the process zone (cf) parameter in SEM and characteristic length (lch) parameter in WFM demonstrated that the concrete without PET became more brittle and less ductile. On the contrary, in concretes incorporating PET, as the temperature increased, the fracture energy and toughness levels of the SCC declined, but its ductility increased. Moreover, considering the size effect design, the behavior of the SCC approached the linear elastic fracture mechanics (LEFM) criterion as the temperature increased and the substitution percentage of PET for fine aggregate decreased. On average, in all the specimens, the fracture energy values obtained from WFM were 2.35 times those obtained from SEM. Finally, based on the obtained mechanical characteristics and test variables, multivariate models were presented to predict the fracture parameters of SCC incorporating PET after tolerating elevated temperatures and compared with the experimental results of the present research and literature. A proper accuracy was observed for the models.