Issue 50

P. Qiu, Frattura ed Integrità Strutturale, 50 (2019) 300-309; DOI: 10.3221/IGF-ESIS.50.25 308 0 0 0 ( ) F lig W mg W G A t h a      (9) where 0 W stands for external work, 0  stands for the CMPD when the specimen is damaged, lig A stands for the ligament area of the specimen, and 0 a stands for the height of the prefabricated crack of the specimen. Relationship between fracture energy and temperature The relationship between the average fracture energy of concrete with different strength grades at different temperatures and temperature can be expressed by the following regression equations. 0.095 470.32 , 0.8766 F G T r    (10) It was noticed from Fig. 9 that for C70 concrete specimens, the fracture energy decreased sharply with temperature when the temperature was lower than 200 °C. When the temperature was higher than 200 °C, temperature had little effect on fracture energy, the fracture energy showed a trend of slow decreasing, and the fracture energy was basically maintained at 297 N/m. The small variation of the fracture energy was because that the water-cement ratio of high-strength concrete was small, the proportion of cementitious material was large, and the crack development was relatively uniform. Overall, the fracture energy showed a law of decreasing rapidly first and then decreasing slowly with the development of temperature. The development law of fracture energy of concrete specimens with different strength grades with temperature was comprehensively analyzed. When the concrete strength was C50, the fracture energy increased first and then decreased. When the concrete strength was C70, the fracture energy decreased rapidly first and then slowly with the development of temperature. Figure 9 : Relationship between fracture energy of concrete with different strength grades and elevated temperature C ONCLUSIONS he fracture behavior of high strength concrete after high temperature was studied by the experiment of three-point bending beam with initial cracks. Comparative experiment was carried out by using C70 concrete. The load-midspan displacement curve and load-CMOD curve of concrete of different strength grades before and after high temperature were obtained. The fracture parameters such as initial cracking load, maximum load, critical value of CMOD, initial fracture toughness and instability toughness were calculated based on the load-CMOD. The fracture energy of the concrete was calculated based on the load-midspan displacement curve. The relationship of fracture parameters and fracture energy of concrete with temperature was analyzed and studied. The main contents and conclusions are as follows. 1. The ignition loss of the high-strength concrete decreased first and then increased with the increase of strength at high temperatures. 2. The initial fracture toughness Q IC K and instability toughness S IC K decreased with the increase of temperature, and the sensitivity of Q IC K to temperature was higher than that of S IC K . T