Issue 50

P. Qiu, Frattura ed Integrità Strutturale, 50 (2019) 300-309; DOI: 10.3221/IGF-ESIS.50.25 304 concrete specimens before and after high temperature, the burning loss curves of concrete with different strengths at different temperatures were obtained (Fig. 4). It was seen that the burning loss of general concrete basically presented three stages [14, 15]. The first stage was a physical change process. When the temperature did not exceed 200 °C, the main reason for the increase of the burning loss was the rapid evaporation of free water in the specimens. The second stage was a physical-chemical change process. When the temperature that the test specimen ensured was between 200 and 400 °C, the growth rate of the loss of ignition was slower than that in the first stage. In this stage, the free water has been exhausted and the bound water was difficult to separate. The third stage was a chemical change process, in which the temperature of the specimen was between 400 and 800 °C, and the loss of ignition was mainly caused by the decomposition of chemical components of concrete mortar and aggregate at high temperature, which was manifested as the dehydration of C-S-H gel crystal water and the dehydration of Ca(OH) 2 . Overall, the loss of ignition showed a law of increasing with the improvement of temperature. Figure 4 : Relationship between ignition loss and elevated temperature. For concrete of different strength grades, the physical and chemical effects were dominant respectively due to the large difference of water-cement ratio. For C50 concrete, the water-cement ratio was large, and the fast increasing stage of ignition loss was 0 °C ~ 400 °C, in which free water loss was the main phenomenon. The loss caused by the physical effect was more obvious than that caused by the chemical effect. For C70 concrete, the water-cement ratio was moderate, the physical and chemical effects were equal, and the ignition loss remained stable as a whole. Overall, the loss of ignition presented a law of decreasing first and then increasing with the increase of the strength. Load-displacement Curve Of Specimens after High Temperature Complete load-displacement curves including load-crack mouth opening displacement (P-CMOD) curve and load- deflection (P-δ) curve can represent a series of important results, such as initial stiffness, ultimate bearing capacity, opening displacement corresponding to cracking load and instability load and hardening and softening characteristics, and the geometric shape of the curve can also reflect the toughness and brittleness of the material to a certain extent. Therefore, the full load-displacement curve plays a key role in the calculation of concrete fracture parameters. Figure 5 : The curve of P-CMOD of the test specimens under different temperatures.