Mechanical Behavior Of Fiber Continuous Joints In UHPC Decks

Fiber reinforced concrete contains a lot of steel fibers.The “bridging effect” of steel fibers has a good action on improving the tensile strength and toughness of fiber reinforced concrete.However,for the joints produced in the construction of step-bystep pouring or prefabricated installation structures,the joints make the steel fibers which should be continuously distributed in disorder be "truncated" artificially at the joints section,and there is no steel fibers which effectively penetrate the section at the joints,resulting in the problem of fiber discontinuity.Fiber discontinuity will greatly reduce the tensile strength of fiber reinforced concrete at joints.At present,there is no better joint structure to solve the problem of "fiber discontinuity".A method of continuous fiber passage at joints is proposed,and the joint form is applied to UHPC materials to solve the problem of discontinuity of fiber in UHPC joints.Accordingly,the mechanical properties of continuous joint between UHPC board and fiber are studied experimentally and theoretically in this paper.In the UHPC plate specimens poured separately,the fiber parameters of directional insertion length(10mm,15 mm,20mm and 25mm),diameter(0.3mm and0.5mm),dosage(0.2%,0.3%,0.4% and 0.5%),tensile strength(1100MPa and2500MPa)and shape(round-straight and hook-ended)at the interface of joints were studied.Axial tension tests of 48 UHPC specimens in 6 groups were carried out.The results show that the tensile strength of the joint increases with the increase of the length,diameter and content of the fibers embedded in the joint,and the strength of the joint increases with the increase of the fibers themselves and the alienation of the fibers shape.When the directional fibers at the joints are 30 mm in length,0.5 mm in diameter and the volume fraction of the directional fibers is less than 0.5%,which can ensure that the joint strength is not lower than the ultimate tensile strength of UHPC matrix.Bending tests were carried out on 24 prefabricated UHPC plates in 8 groups on the basis of axial tension test.The joints were overlapped joints of steel bars,continuous joints of fibers,continuous joints of fibers and steel bars,and stiffener and steel bars.The steel fibers with 0.25% and 0.5% interfacial content of fibers are taken as the research parameters in the two forms of continuous fiber connection and continuous fiber + steel bar overlapped connection.Through static test,the mechanical properties of each joint form are preliminarily discussed,and the deflection,crack,strain of steel bar and concrete of each plate are analyzed.The results show that the joint reduces the initial crack load and ultimate flexural capacity of UHPC specimens;the overlap joint form of reinforcing bars can’t guarantee the overall mechanical performance of the structure,while the combination joint form of continuous fiber and overlap of reinforcing bars can ensure that the mechanical properties of the joint structure are basically equivalent to the whole structure;the stiffener joint can also improve the ultimate flexural capacity of the structure,but the initial crack load is not guaranteed.2D plane strain models of concrete matrix,steel fibers and their bonding layers of axially tensioned specimens are established by ABAQUS software.3D models of flexural specimens are established according to different joint forms.The theoretical derivation and calculation of fiber pull-out load and flexural capacity of slabs are carried out.The results show that the experimental,simulated and theoretical values are in good agreement;the 2D plane strain model of fiber drawing test based on cohesion model can better simulate the actual test results;the UHPC joint prefabricated plate model with different joint forms can simulate the actual stress state of the plate;the theoretical formulas of fiber drawing load and flexural capacity of the UHPC plate can be applied to design calculation.