Evaluation of fracture energy, toughness, brittleness, and fracture process zone properties for lightweight concrete exposed to high temperatures

Abstract

Proper understanding of the fracture mechanism of lightweight concrete (LWC) after exposure to high temperatures has a significant role in LWC proportioning design which provides a better comprehension of the structural behavior during a fire event. In this study, the effect of high temperatures (250, 500, and 750 °C) on the fracture parameters of LWC and its brittleness compared to normal-weight concrete (NWC) was experimented using a total of 120 notched beams subjected to the three-point bending test. For this purpose, the size effect method (SEM) and the work of fracture method (WFM) were employed to analyze and explain the fracture parameters. The results demonstrate a reduction of 85 and 77% in the size-independent total fracture energy (GF) and a decrease of 37 and 28% in the characteristic length (Lch) based on WFM, respectively for NWC and LWC with increasing temperature up to 750 °C. As the temperature reaches 750 °C, the size-dependent initial fracture energy (Gf), fracture toughness (KIC), and fracture process zone length (Cf) based on SEM, are reduced by 73, 64 and 76% for NWC. The corresponding reduction values for LWC are 68, 47 and 69% compared to specimen at ambient temperature. However, the brittleness number (β) based on SEM is increased. The results of the parameters of effective size of the process zone (Cf) in SEM and characteristic length (Lch) in WFM indicated that normal-weight concrete is more brittle and its ductility is reduced compared to lightweight concrete exposed to high temperatures. Based on the size effect phenomenon, the behavior of normal and lightweight concrete became closer to the linear elastic fracture mechanics (LEFM) criterion with increasing temperature. Finally, the average ratio of GF obtained from WFM to Gf obtained from SEM for NWC and LWC at elevated temperature exposure show 1.82 and 1.7, respectively.