Effect Of Embedment Depth And Silica Fume Content On Interfacial Bonding Properties Of Steel Fiber-UHPC Matrix Under Double Fiber Pull-out

Ultra High Performance Concrete(UHPC),as a new composite material,can give full play to the advantages of fiber concrete and ultra-high strength concrete,so that it can be widely used in practical engineering fields.Incorporating steel fibers into the UHPC matrix can significantly improve the strength,crack resistance and durability of UHPC because of its ability to transfer loads in the fiber-matrix interface region.Therefore,the quality of interfacial bonding between steel fibers and UHPC matrix is related to the overall performance of UHPC.In this paper,in order to understand the bonding characteristics between the steel fiber and UHPC matrix and the pull-out behavior of steel fiber,a series of experimental studies were carried out using the double-fiber pull-out test with the content of silica fume,the embedment depth and type of steel fiber as variables.The obtained research results are as follows.(1)Starting from the raw materials of UHPC,the influence of different content of silica fume on the fluidity and mechanical strength of UHPC matrix was studied,and the enhancement mechanism of silica fume was analyzed from the microscopic level by Scanning Electron Microscope(SEM).The results show that with the increase of silica fume content,the expansion degree and compressive strength of the matrix increase first and then decrease.When the silica fume content is 25%,the expansion degree and compressive strength are the largest,respectively 63.0mm and 139.3MPa,under the content of 20%silica fume,the maximum flexural strength is 17.6MPa,so the content of silica fume in UHPC should be 20%～25%;20%and 25%silicon were observed by SEM The number of bubbles in the matrix with ash content is less,and the pore size of the bubbles is smaller than the other three,the distribution of quartz sand is more uniform,and the CSH gel increases significantly.(2)Through the single fiber and double fiber pull-out tests of steel fiber under different fiber embedment depths,the effect of double fiber embedment depth on the interfacial bonding properties between steel fiber and UHPC matrix is studied.The test shows that the bonding properties of single straight circular fiber with diameter of 0.2mm are better than those of double fibers with same diameter when the embedment depth is 5.0mm and 6.5mm,but worse than those of double fibers with the same diameter when the embedment depth is 8.0mm;For 0.3mm diameter straight circular fiber,the bonding performance of single fiber drawing is better than that of double fiber drawing when the embedment depth is 5.0mm and 8.0mm,but worse than that of double fiber drawing when the embedment depth is 6.5mm;The bonding performance of a single end hook fiber with a diameter of 0.3mm is worse than that of two fibers with the same diameter under three different embedment depths.This shows that compared with the embedment depth of single fiber,when the same fiber is used,the embedment depth of double fiber can have a significant impact on the bond strength of UHPC.It is also observed by SEM that the matrix tunnel of double fiber drawing specimen has matrix breakage,the pull-out outlet becomes wider.and there are scratches and large-area adhesion on the surface of steel fiber.(3)Through the double fiber pullout test with the amount of silica fume,fiber embedment depth and type as variables,the effects of fiber embedment depth and silica fume content on the interfacial bonding properties under different fiber types were discussed.The results show that for the copper plated straight circular fiber with diameter of 0.3mm,the content of 20%silica fume has the greatest influence on the fiber matrix bonding performance.At the content of 15%～25%silica fume,the embedment depth of double fiber has a great influence on the interfacial bonding performance;For the 0.2mm diameter straight circular fiber,compared with the other four kinds of silica fume content,the interfacial bonding strength is the smallest at 20%silica fume content.At the same time,it can be seen that when the fiber embedment depth is 8.0mm,the effect of double fiber drawing on the interfacial bonding performance is more obvious;For H0.3 steel fiber,with the increase of fiber embedment depth,its drawing work increases,and the two interfacial bond strengths decrease.When the fiber embedment depth is 5.0mm,its ultimate load presents different laws due to the end hook.