Study On The Mechanical Properties Of Steel-concrete Components Before Cracking At Ultra-low Temperature

With the continuous development of the economy and society,the application conditions of building materials are evolving from normal temperature to extreme temperature,among which reinforced concrete including special materials of LNG storage tanks,is widely used at ultra-low temperature.The project of "extreme temperature test research" for providing theoretical guidance for the design of reinforced steel at ultra-low temperature based on collaboration with the Engineering Company of China and Wuhan University was carried out,which explores linear expansion coefficients of steel bar,cement mortar,limestone and elastic modulus of mortar,limestone at ultra-low temperature,as well as the bending and axial tensile properties of reinforced concrete components at ultra-low temperature.The detailed researches are as follows:（1）An ultra-low temperature mechanics testing system is independently developed and designed by our group,which is used to investigate the mechanical properties of axial compression specimens and bending components at ultra-low temperature.Temperature and load can be applied independently and simultaneously at a range from room temperature to-165 ℃.Under certain temperatures and loads,the strain data can be collected independently,respectively.（2）Establishing in-situ compensation method.An in-situ compensation method named self-compensation of the strain gauge temperature was proposed by our group through accurately monitoring and controlling the temperature around the specimen,which could obtain the real-time strain output at every measuring point.The strains caused by load and temperature are obtained,respectively,based on the output mathematical models of temperature and strain at the measured points through fitting relative data.（3）The expansion coefficient and elastic modulus at ultra-low temperature were tested.The linear expansion coefficient of steel bar,mortar,and limestone and elastic modulus of mortar and limestone at ultra-low temperature are measured.The thermal output data of specimens at different ultra-low temperatures were separated and extracted.The variation of linear expansion coefficient of steel bar,mortar and limestone at different testing temperatures as well as the relationships between elastic modulus of mortar and limestone and temperatures were studied by analyzing the thermal output data at different ultra-low temperatures.（4）Mechanical properties of reinforced concrete beams at ultra-low temperatures.The relationships between temperatures and load strain of axial tension beam and bending beam under ultra-low temperature were investigated by the separation-extraction method.Through analysis and research,this paper draws the following conclusions:（1）With the decrease of temperatures,the linear expansion coefficient of reinforcement and the linear expansion coefficient of mortar and limestone decrease linearly.The linear expansion coefficient of mortar and limestone under loading is higher than that under no loading,which means the material loading can affect the variation of the linear expansion coefficient.After testing different volumes of mortar and limestone,It can be concluded that the linear expansion coefficient for smaller volume is smaller,which indicates that material volume also exebits an effect on the linear expansion coefficient.（2）When temperature decreasing from 0 to-165 ℃,the elastic modulus of mortar increases and the deformation of mortar under resistance stress decreases,demonstrating its large elasticity and large stiffness,while the elastic modulus of limestone decreases and the deformation under the resistance of stress increases,which demonstrates its large elasticity and the small stiffness.（3）When temperature decreasing from 0 to-165 ℃,the axial tension stiffness and bending stiffness of reinforced concrete beams behave similar variation trend.When temperature decreasing from 0 to-100 ℃,the tensile stiffness and bending stiffness increase non-linearly;When the temperature is below-100 ℃,the tensile stiffness and flexural stiffness of concrete beams stay unchanged.（4）When the temperature decreases from 0 ℃ to-165 ℃,the constrained strain between the steel bar and the concrete in the concrete beam caused by the temperature change can not be ignored and should be considered in the design.