Abstract:Compared with conventional concrete structures, cracking of underwater concrete structures have a worse effect on structural safety. Accurate and timely monitoring of cracks in the process of occurrence and development is of great significance to ensure the safety of wading engineering and prevent accidents. Based on the coupling effects among fluid, heat source, and crack, a temperature tracer method for crack monitoring in underwater concrete structures was proposed and three crack monitoring schemes were designed. These three monitoring schemes realize the conversion between cracked information and thermodynamic information of cracked position using assemblies with monitoring tube and microporous casing, monitoring tube and hollow casing, and monitoring tube and irrigation tube, respectively. Among them, the first two monitoring schemes mainly use the principle of heat conduction to change the thermodynamic parameters of the medium around the cracks after cracking, so as to change the heat transfer law of the heat source. The third scheme mainly uses the principle of convective heat transfer and makes the crack section produce a convective heat transfer effect through irrigation, so as to improve the heat transfer speed. Concrete beam specimens were made according to the above three monitoring schemes, respectively. The transient heat transfer model tests were carried out by using the sensing heating element composed of a fiber bragg grating temperature sensor and a ceramic heating tube. A discriminant index reflecting the cooling rate was defined to identify crack information according to the subsection characteristics of the heat source cooling curve, and the identification effects were analyzed and compared. The results showed that these three monitoring schemes could well judge whether there were cracks. In terms of crack location, the third scheme had a better effect. In terms of quantitative identification of crack width, the first scheme could not identify the crack width quantitatively, the second scheme could only identify the crack width in the flowing water environment, and the third scheme could identify the crack width according to the leakage flow.