Issue 50

P. Qiu, Frattura ed Integrità Strutturale, 50 (2019) 300-309; DOI: 10.3221/IGF-ESIS.50.25 303 The concrete fracture test was carried out on a 2000 kN electro-hydraulic servo universal testing machine. The whole test process was controlled by displacement, and the loading velocity was 0.02 mm/min to achieve stable splitting test. The average loading time of specimens which were processed with temperature lower than 400 °C was 20 minutes, while that of specimens which were processed with temperature higher than 400 °C was 40 minutes due to the long time spent in the descending section of the load-crack opening displacement curve. As shown in Fig. 2, the crack opening displacement (CMOD) and the crack tip opening displacement (CTOD) of the specimens were measured with a clamp extensometer, and the deflection of the middle position of the specimens was measured with two displacement meters. Experimental Observations The experimental phenomena of the three-point bending beams after high temperature processing are shown in Fig. 3. When the temperature that the concrete specimen ensured did not exceed 400 °C, the crack was vertical and downward, and there was no split crack. The splitting surface was relatively flat, and there was aggregate breaking on the fracture surface. When the load has not reached its peak value, visible cracks did not appear at the tip of the specimens; but once there were visible cracks, the load of the specimens suddenly dropped and then steadily decreased. When the temperature of the concrete specimen was 600 or 800 °C, the cracks bent downward; at the beginning, several small cracks appeared on the surface of specimens, and with the increase of the load, a main crack and several split cracks appeared. With the increase of temperature, the splitting surface became more tortuous, and the phenomenon of cracks passing through aggregates decreased obviously, instead, it went around between aggregates. When the load did not reach the peak value, visible cracks appeared at the tip of the prefabricated crack; when the load reached the peak value, the crack opening became larger and the length expanded, but then they slowly decreased, and the descending section lasted longer with the increase of the temperature of the specimen. For the specimens which were processed by 400 °C, 600 °C and 800 °C, obvious cracks detoured between the aggregate. Figure 3 : The pictures of test phenomenon. T EST RESULTS AND ANALYSIS Measurement of ignition loss of specimens after high temperature he loss of free water and bound water occurred in the concrete specimens at high temperature, and the weight of the concrete specimens decreased significantly compared with that before heating. In order to prevent water vapor from entering after heating, the specimen was wrapped by plastic bags. By measuring the mass change of the T