Abstract: Abstract: In order to study the change law of the interfacial bonding properties of recycled concrete for round steel tubes after high temperature water spray cooling, 27 circular steel tubes after high temperature water spray cooling were carried out with the maximum constant temperature, the replacement rate of recycled coarse aggregate, and the interface anchoring length as three parameters. The static push-out test of the steel tube recycled concrete specimen and 2 natural cooling specimens (control group) is mainly based on the displacement-controlled loading method, so that the steel pipe at the free end is stressed alone (the core concrete is not stressed), and the steel pipe in the round steel pipe is stressed. Concrete is pushed out from the bottom up. The whole process and shape of the failure of the recycled concrete specimen of the circular steel tube were observed through experiments, the load-slip curves of the loaded end and the free end were obtained, and the influence of various parameters on the bonding performance was analyzed. The test results show that after high-temperature water spray cooling, the load-slip curves of the loading end and the free end of the circular steel tube recycled concrete specimen are similar in shape, and similar to the curve of the natural cooling specimen, and are also similar to that of the ordinary concrete of the steel tube made in the previous stage. The curves are similar; the load-slip curves of the loaded and free ends are roughly divided into ascending, slowly descending, and gentle segments; and the initial slip of the free end is later than that of the loaded end. The ultimate bond strength τu and residual bond strength τr are defined. The strain on the surface of the circular steel tube recycled concrete specimen is approximately exponentially distributed along the height direction of the specimen. After being cooled by fire-fighting water, the bond strength first increases and then decreases with the increase of the maximum temperature. The average residual bond strength of the specimens after high temperature is 1.25 times, 1.75 times, 1.38 times and 1.50 times that of the specimens at room temperature, respectively. The change of the replacement rate of recycled coarse aggregate has no obvious effect on the bond strength. When γ=50% at room temperature, the bond strength reaches the maximum value; when the temperature is 200 ℃ and γ=75%, the bond strength also reaches the maximum. This is similar to the conclusion of the steel tube recycled concrete under natural cooling. The increase of anchorage length also increases the average ultimate bond strength and residual bond strength of recycled concrete specimens with circular steel tubes to varying degrees. The calculation formulas for the ultimate bond strength and residual bond strength of recycled concrete of circular steel tubes after high temperature water spray cooling are proposed, and the calculation results obtained by this formula are satisfactory. In the future, the research on the interface bonding performance can be discussed from the perspective of increasing the roughness of the inner wall of the steel pipe, adding stiffeners inside the steel pipe and adding bolts of different lengths in the steel pipe, and further enriching the theoretical research on the interface bonding performance, in order to provide fire water spraying. The other mechanical properties of post-building structural components provide a theoretical basis, and at the same time provide a certain reference for post-disaster assessment and reinforcement of steel-concrete composite structures subjected to fire in real life.

Key words: Key words: round steel tube, recycled concrete, high temperature spray cooling, ultimate bond strength, residual bond strength, bonding slip constitutive equation