Research On The Influence Of The Amount Of Expansion Agent On The Bearing Capacity Of Self-compacting Concrete-filled Steel Tube Under Axial Compression

During the setting process of concrete columns in concrete-filled steel tubes,the shrinkage of the volume or the failure of compaction will lead to the phenomenon of "vacuum" between the steel tube and the concrete.The use of self-compacting concrete and the addition of expansion agents can prevent such phenomena,The reasonable amount of expansion agent is not conclusive.This article combines the existing theories and research results,experiments,and finite element simulation to carry out research on the subject.Through the test of 18 specimens with 0%,5%,7%,and 9% expansion agent content and the simulation analysis of 279 specimens,the effects of the expansion agent content and the hoop coefficient on the bearing capacity of the specimens were discussed.Research indicates:(1)After the expansion agent is added,the initial tightening force p will increase,so that the steel pipe and concrete enter a cooperative working state in advance,and the expansion agent can effectively improve the bearing capacity of the concrete filled steel pipe.However,if the amount of expansion agent is too small,the axial bearing capacity of concrete-filled steel tube will be slightly improved;if the amount of expansion agent is too much,it will not only destroy the original structure of the concrete,but also increase the initial hoop stress of the steel tube and reduce the longitudinal compressive stiffness,The bearing capacity is reduced.(2)When the expansion agent content is 5%,the yield bearing capacity and ultimate bearing capacity increase the most,which are 11.9% and 49.85%,respectively;when the expansion agent content is 7%,the ultimate bearing capacity is increased compared with ordinary steel tube concrete.It is about 41.47%,and the yield bearing capacity is increased by about 8.53%.Therefore,it is recommended that the content of the expansion agent is about 5%.(3)The ferrule coefficient can reflect the combined effect between steel tube and concrete,which has a great influence on the axial bearing capacity of concrete filled steel tube.With the increase of the hoop coefficient,the load-displacement curve appears in the elastic phase,the elastoplastic phase,the plastic phase or the strengthening phase;as the hoop coefficient increases,the displacement interval of the load-displacement curve gradually increases.(4)The "calculation formula for bearing capacity" was deduced based on research and analysis;its feasibility was verified through comparative analysis.Through a series of studies,it can be obtained that the bearing capacity is the best when the amount of expansion agent is about 5%;the formula for calculating the axial bearing capacity of self-compacting steel tube concrete with the amount of expansion agent is derived.