Issue 50

P. Qiu, Frattura ed Integrità Strutturale, 50 (2019) 300-309; DOI: 10.3221/IGF-ESIS.50.25 307 The relationship between the average values of Q IC K and S IC K of the concrete with different strength grades at different temperatures and temperature could be expressed by the following regression equations (25 °C≤ m T ≤800 °C). -0.001 0.8392, 0.9532 Q IC m K T r    (6) -0.0022 2.2086, 0.9901 S IC m K T r    (7) As shown in Fig. 7(a), Q IC K of the concrete of both strength grades decreased with the increase of temperature (C70 is abnormal at 200 °C). The slope of the regression fitting curve was 0.001. The variation law of S IC K with temperature was similar with Q IC K , as shown in Fig. 7(b). As shown in Fig. 7(c), with the increase of the maximum temperature, the Q IC K / S IC K of the concrete decreased (except 200 °C), indicating that Q IC K decreased faster than S IC K with the increase of the temperature, i.e., the sensitivity of Q IC K to temperature was higher than that of S IC K . Calculation of Fracture Energy Fracture energy is defined as the energy consumed to form a unit fracture surface, usually expressed by GF [17]. The fracture energy of concrete can be directly obtained from the load-midspan displacement curve (i.e. P-δ curve) obtained from the fracture test. In the fracture test, the work on concrete specimens consists of two parts, i.e., the work done by the loading axis applied on the beam and the work done by the gravity of the beam itself. In the calculation, according to the principle of equal bending moment acting on the notch section, the beam weight was equivalent transformed into the concentrated external force w P acting on the loading point, 2 w mg P  . The deadweight equivalent force of the beam w P was applied to the beam together with the applied load a P , so that the total load became w a P P  . The displacement at the time of 0 a P  was denoted as 0  , the area under the total load-displacement curve could be divided into three parts, 0 W , 1 W and 2 W , as shown in Fig. 8. 0 W is the area under the curve of 0 a P   , 1 0 w W P   , and 2 W represents the area of the curve after point 0  . Reference [18] analyzed the value of 2 W and approximately considered 1 2 W W  . Therefore, the total work done by the external force was: 0 1 2 0 0 0 0 2 w W W W W W P W mg          (8) Mid-span displacement P  P O W W P 1 W 0  2 W   Figure 8 : The diagram for the analysis of fracture energy calculated based on the load-midspan displacement. It was assumed that all the work done by external forces flew into the fracture zone for fracture development, then the expression of fracture energy per unit area was: