Residual fracture toughness of normal- and high-strength gravel concrete after heating to 600°C.

Fracture toughness represents the resistance of concrete to cracking and fracture, and is significantly influenced by heating scenarios such as heating temperature and exposure time. In this paper, two parameters—the classic fracture toughness and the fracture toughness related to fracture energy—were used for assessing the residual fracture toughness of heated concrete. The former is an instantaneous parameter that represents the crack resistance of concrete at peak load, while the latter is a synthetic-process parameter that represents the crack resistance over the entire failure process. The effects of the heating temperature, exposure time, and curing age on the fracture toughness were experimentally investigated and analyzed by conducting three-point bending tests on 87 notched normal- and high-strength concrete beams that had been heated between 100 and 600 ×C over varied exposure times up to 168 h and cooled down to room temperature. Four testing ages from 7 to 90 days were adopted. A higher heating temperature of over 200 °C generally decreased fracture toughness but, below 200 °C, some strengthening and toughening effects were observed. A similar phenomenon was also found for longer exposure times, but this effect was more significant at the early exposure stage, under 12 h. A longer curing age led only to slightly greater toughness in the first 28 days, and had very little influence thereafter. Weight loss was also measured to distinguish different stages of the fracture toughness of the concrete with the heating temperatures. The quick evaporation of the capillary water hardly affected the fracture toughness, but the evaporation of gel water and chemically bound water and the decomposition significantly decreased the fracture toughness