Issue 50

P. Qiu, Frattura ed Integrità Strutturale, 50 (2019) 300-309; DOI: 10.3221/IGF-ESIS.50.25 301 As the intrinsic characteristics of materials, scholars from all over the world have studied the fracture properties of concrete at room temperature extensively and have formulated standard test methods for mode I fracture related fracture parameters [7,8]. However, there are few studies on fracture properties after high temperature in China and abroad, and the research results of scholars are slightly different. A study [9] shows that the chemical composition, physical properties and mechanical properties of concrete have changed greatly at high temperature or after high temperature. In real society, besides buildings damaged by fire, there are many concrete structures affected by high temperature, such as cooling towers of power plants, prestressing nuclear reactor containers, etc. Therefore, it is of far-reaching significance to understand the high temperature fracture properties of concrete for the design, identification and strengthening of such structures. Based on the double K fracture toughness criterion, Yu et al. [10] carried out the experimental study on mode I fracture behavior of concrete after high temperature. The results showed that the initial fracture toughness Q IC K decreased gradually with the increase of temperature, while the instability toughness S IC K , fracture energy G F and characteristic length l CH all showed the trend of “unchanged-rising- falling” with the increase of temperature, which indicated that the three parameters could be used as ductility indicators of concrete after high temperature. Baker [11] studied the fracture energy of concrete under different cooling regimes. The results showed that the fracture energy of concrete increases first and then decreased with the increase of test temperature, the maximum value of fracture energy appeared near 300 °C, which showed that the toughness of concrete was the best at about 300 °C and the influence of different cooling regimes on fracture energy was very small. Menou et al. [12] pointed out that the value of fracture energy of concrete after high temperature was higher than that at normal temperature before reaching a certain temperature and the value of fracture energy decreased rapidly when the temperature exceeded that value. In this study, the fracture behavior of high strength concrete after high temperature processing was studied through the test of three-point bending beams with initial cracks. Comparative experiment was carried out using C70 concrete and five temperature gradients. Based on the double K fracture theory, the law of fracture toughness and fracture energy of high strength concrete varying with temperature was analyzed to obtain the relationship between relevant fracture parameters and temperature, so as to provide a reliable basis for damage assessment of building structures after disaster. O VERVIEW OF TEST he standard three-point bending beam specimens with a span-height ratio (s/h) of 4 proposed by International Union of Laboratories and Experts in Construction Materials, Systems and Structures were used in this test. A notch which was 30 mm deep were reserved at the middle and lower part. The basic shape of the three-point bending beam is shown in Fig. 1. Figure 1 : Schematic diagram of the three-point bending beam specimen (mm). The highest temperature endured was 25, 200, 400, 600 and 800 °C respectively. Considering the discreteness of the fracture test, five specimens were set up at each temperature, and another one was prepared for standby application. The specimen number was expressed as plain concrete strength - the maximum temperature endured, for example, 70-25 indicates that strength of the specimen was C70 and the maximum temperature that could endure was 25 °C. The mix proportion of the concrete specimen is shown in Tab. 1. Conch PO52.5 Portland cement was used. The maximum diameter of coarse aggregate was 10 mm, and medium sand with the diameter of 0.25-0.5 mm was used as the fine aggregate. After 28 days of sprinkling water curing, the compressive strength of the concrete cube reached 88.2 MPa, respectively. T