Issue 50

P. Qiu, Frattura ed Integrità Strutturale, 50 (2019) 300-309; DOI: 10.3221/IGF-ESIS.50.25 302 Strength grade Water/ (kg·m -3 ) Cement /(kg·m -3 ) Water cement ratio River sand/ (kg·m -3 ) Gravel / (kg·m -3 ) Silica fume/ (kg·m -3 ) Water reducing agent/(kg·m -3 ) Unit weight/ (kg·m -3 ) C70 150.00 441.27 0.31 615.32 1194.44 39.21 6.74 2446.97 Table 1 : The mixing proportion of the concrete specimen An electric furnace with a furnace size of 500 mm × 500 mm × 1200 mm was used as the heating device, and an AI-518 (V7.1) programmed intelligent temperature regulator was used for temperature regulation and constant temperature control. The heating curve adopted the standard heating system, and the heating time was controlled according to the rate of 200 °C rising every half hour. When the heating temperature reached the predetermined temperature, it was switched to the constant temperature mode and kept for 3 hours. Finally, the samples were taken out after the power-off of the electric furnace and 24 h of natural cooling [13]. Test Procedures WAW-1000B microprocessor-controlled electro-hydraulic servo universal testing machine was used in this test. The computer was controlled manually and the testing machine starting testing automatically. The test device is shown in Fig. 2. The test steps are as follows. 1. The support of the universal testing machine was adjusted to keep a distance of 300 mm between the central lines of the two supports. The screw was tightened to keep the two supports fixed. Moreover, the loading shaft was adjusted parallel to the two supports. 2. After the support was adjusted, the specimen was slowly placed between the two supports, so that the two hand- drawn lines of the specimen were aligned with the center of the two supports, and the notch of the three-point bending beam was aligned with the loading axes on the testing machine. 3. The computer system was connected. The load sensor and displacement sensor of the computer were calibrated, and the bending parameters of the computer were set. Then the load, midspan displacement and notch displacement were adjusted to zero, and the initial position was adjusted. 4. The distance between the two load-bearing sheets of the electronic extensometer was adjusted to the shortest (10 mm) by holding them by hand. Because of its good elasticity, the two pieces of the electronic extensometer were stuck between the two steel sheets at the crack opening. The load-opening displacement curve of the specimen was measured by the extensometer, and the load-span displacement curve was also be measured by the testing machine to obtain fracture parameters. 5. Continuous loading method was adopted in this test. The loading speed was 0.08 KN/s. It was loaded directly until the specimen failed, and the control system automatically saved the data. 6. In the process of the experiment, the test machine was unloaded to prevent the oil pump from rising too high and damaging the test machine after the testing of five test specimens. The concrete blocks which needed to be processed by 200 °C and 400 °C were directly heated to the preset temperatures. The concrete blocks which needed to be processed by 600 °C and 800 °C were pre-heated to 200 °C, keeping for 24 h, and then heated to the preset temperature from 200 °C. Figure 2 : The testing device.