Study On Pullout Resistance Of Hybrid Fiber UHPC Stud Connector

Ultra high performance concrete(UHPC)is a new type of cement-based composite material with high strength and high durability.Compared with the traditional steel-concrete composite structure,the new steel-UHPC composite structure composed of structural steel is lighter in weight,higher in strength,and has a longer service life.As the key component to ensure the cooperation between UHPC and steel,it is necessary to conduct a thorough study on the pull-out resistance of the stud connector.At the same time,mixing fibers of different sizes and properties can maximize the strength and crack resistance of concrete while reducing the amount of fibers,but there are few studies on hybrid fiber UHPC.Therefore,this paper firstly optimizes the UHPC benchmark mix ratio was optimized through the mechanical properties test of cement mortar.And then the impact of length-diameter ratio of steel fiber or polypropylene crude fiber,and the fiber content on the workability and basic mechanical properties of UHPC were studied.Under the selected length-diameter ratio of steel fiber and polypropylene crude fiber,the two fibers were mixed into the UHPC matrix,and the reinforced and toughening mechanism of the hybrid fibers was explored by compression,splitting,bending toughness and microscopic tests.Finally,the hybrid fiber UHPC material is considered to be used in new steel-UHPC composite structure.The pull-out behavior of the stud connector of new composite structure is analyzed and its force mechanism is clarified.A prediction model of ultimate tensile capacity of UHPC stud connector for concrete cone failure is proposed.It can provide reference for theoretical research and engineering application of UHPC materials.The main conclusions of the above study are as follows:(1)The change of raw material composition has a significant effect on the mechanical properties of UHPC material pure paste.With the increase of the water-binder ratio,the flexural and compressive strengths of UHPC materials decrease significantly;the flexural and compressive strength of UHPC increase with the increase of sand-binder ratio.(2)The length-diameter ratio and the content of fiber will affect the fluidity and mechanical properties of UHPC.The smaller the diameter and the longer the length of the fiber,the greater the negative effect on the fluidity of UHPC;when the fiber content continues to increase,the fluidity of UHPC will be greatly reduced;it is advantageous to increase the length-diameter ratio of steel fiber to increase the strength of UHPC in certain content range,but the length-diameter ratio should not be too large,and there is an optimal length-diameter ratio.(3)The interface transition between the UHPC matrix phase and the aggregate phase is very dense,and the bond between steel fiber or polypropylene crude fiber and the UHPC matrix is also very strong.And the two fibers can be mixed into UHPC matrix,play a positive synergistic role,and achieve the purpose of strengthening and toughening.The prediction model of UHPC splitting tensile strength and the conversion formula between compressive and splitting tensile strength were proposed considering fiber characteristic parameters.(4)The failure mode of UHPC stud connector is related to the fiber added into the UHPC matrix and the embedding depth and size of the stud.Concrete cone failure occurred in all the UHPC without fiber,and stud tensile failure occurred in the UHPC specimens mixed with fibers at the maximum embedding depth.The ultimate tensile capacity and initial stiffness of UHPC stud connectors are increased with the increase of stud size and embedding depth,but the ductility coefficient of specimens is decreased.Fiber incorporation can significantly improve the energy dissipation and crack resistance of specimens,and the stud connectors based on hybrid fiber UHPC have higher ultimate tensile capacity,initial stiffness and ductility coefficient.(5)Based on the pull-out test data of UHPC stud connectors,and on the basis of CCD model,a prediction model of ultimate tensile capacity of UHPC stud connectors with concrete cone failure was proposed by taking split tensile strength instead of compressive strength as strength factor and considering the influence of stud size and component size.