Long-term Static Behaviour Of Concrete-filled Steel Tubes Influenced By Temperatures

Concrete-filled steel tubular arch bridges have been widely used in China in rencent years because of their high bearing capacities, long-spanning abilities, convinient and economical construction and attractive appearance. The increase of span length makes time effects of the concrete core have more pronounced influence on the static responses of CEST arches. Arch bridge is located in the open environment, the static responses of which is considerably affected by the change of environmental temeratures. Temperature is the second largest environmental factors which affects creep of concrete. Therefore, it is necessary to study how the long-term static behaviour of concrete-filled steel tubes influenced by temperatures.Creep is an important long-term performance of concrete, which has been widely studied around the world. Though research on temperature effects on concrete creep has been conducted for decades, at present there is no unified recognition on the prediction model. In particular, the research on the effects of temperature on creep of the concrete-filled steel tubes is still in its starting stage.This study collected and investigated experimental data on the temperature effects of concrete creep to analyse how the trend of creep development and the value of final creep deformation are affected by temperatures. Four existing creep models,(i.e. CEB, EC2, ACI, and B3 model), which involve temperature effects were compared with the collected test data to verify the reliability of these models in predicting the creep deformation of concrete under different temperature levels. It was found that under the normal temperature, the calculation results of EC2 model could best fit the test data, but the EC2 model could not well predict the influence of temperature on the creep. On the other hand, B3 model could not fit the test data very well at room temperature, but could well describe how concrete creep varies with different temperatures. In this context, calculation method to conside the influences of temperature on creep in B3 model was introduced to EC2 model to make the model feasible for the prediction of concrete creep at both room temperature and elevated temperatures.Based on the modified model, the creep of concrete under constant temperatures, variable temperatures, and cycle temperatures were investigated. Creep of concrete at elevated temperatures is higher. Variable temperatures, and cycle temperatures both induce higher creep than constant temperature. Modified EC2 model was combined with step-by-step method to study the creep behaviour of CFST. Considered parameter includes water-cement ratio, loading age and steel ratio. It was found that the creep behaviour of CFST were more sensitive to the variation of temperatures for specimens with lower water-cement ratio, lower the steel ratio of CFST, or subjected to long-term loading at older concrete ages. Temperature has significant influence on creep of concrete-filled steel tube. Temperature effect is suggested to be considered in predicting the creep of CFST structures in open air.