Study on the bilinear softening mode and fracture parameters of concrete in low temperature environments

The mode and fracture parameters of concrete at different temperature levels (20 degrees C, 0 degrees C, -20 degrees C, and -40 degrees C) were investigated using three-point bending fracture experiment in a low-temperature fracture test system. The three-point bending beams with the same sizes, 750 mm x 150 mm x 70 mm, were divided into two groups according to age: 28 days and 120 days. The bilinear softening parameters for each beam were determined using the inverse analysis; then, the fracture energy G(F) and characteristic length l(ch) were determined by the calculated softening curve. In addition, the fracture toughness including the initial fracture toughness K-Ic(ini), unstable fracture toughness K-Ic(un) and cohesive toughness K-Ic(c) were also analyzed. The experimental and calculation results show that the softening curve shape and fracture parameters of the concrete change significantly when the temperature decreases. Finally, the double-K fracture model was used to verify the validity of the bilinear softening curve calculated in this paper.