Issue 50

P. Qiu, Frattura ed Integrità Strutturale, 50 (2019) 300-309; DOI: 10.3221/IGF-ESIS.50.25 305 Figure 6 : The curve of P-δ of the test specimens under different temperatures. Fig. 5 and 6 are the typical P-CMOD curve and P-δ curve of C70 concrete specimen at different temperatures. It was found that both types of curves showed a trend of shortening and fattening with the increase of temperature. The peak load decreased with the increase of temperature, and the corresponding critical CMOD and deflection increased. Linear regression was made on a part of the ascending section of P-CMOD curve that was approximately straight line, and the coincidence degree between them was observed. The point where the curve began to deviate from the fitting straight line was found, and the corresponding load was the initial cracking load P ini, T . Details are shown in Tab. 2. Temperature/°C P ini, T /kN P max, T /kN COMD corresponding to P max, T /mm C70 C70 C70 25 3.80 5.78 46.88 200 3.86 5.65 50.31 400 2.33 3.66 79.13 600 1.11 2.40 141.62 800 0.25 0.73 368.36 Table 2 : Calculated results of fracture parameters (average values). Calculation of Fracture Toughness Fracture toughness of concrete is the ability of material to resist crack propagation. Initial fracture toughness Q IC K represents the resistance to crack propagation when the crack is about to crack and the instability toughness S IC K represents the resistance to external forces when the material is in critical instability state. Crack initial fracture toughness Q IC K and instability toughness S IC K can be calculated by the formula given in the Norm for Fracture Test of Hydraulic Concrete [16]: 2 3 1/2 int, 0 0 2 mg 1.5( 10 ) 10 2 ( ) T Q IC P Sa K f th        (1) where 0 ( ) f  stands for a function related to the value of 0 / a h , and its expression is:      0 2 0 0 0 0 0 0 3/2 0 0 1.99 (1 ) 2.15 3.93 2.7 ( ) , 1 2 1- a f h                (2)