Research On Single Steel Fiber Pullout Behavior In UHPC Mixed With Granite Stone Powder

Ultra-high performance concrete(UHPC)is a new type of cement-based composites with ultra-high strength,high durability and high toughness.Improving the ITZ behavior between steel fiber and the UHPC is a key to improve the toughness of UHPC,which has the important significance in making full use of UHPC material performance,reducing the project cost,and popularizing the application of UHPC.A large number of abandoned granite stone powder in Fujian province cannot be used efficiently.Using the granite stone powder as a material for the preparation of UHPC benefits the environment.The variable parameters are the type of steel fiber,surface modification of steel fiber,the curing system and the curing age.A self-made single fiber pullout device was designed to obtain the pullout load and displacement(P-s)curve of eight kinds of steel fiber,which were embedded in UHPC mixed with granite stone powder.The microscopic methods such as SEM,EDS and XRD were used to investigate the surface morphology of steel fiber,hydration products of the UHPC and the pore structure of the UHPC.The research elaborates the bonding mechanism of steel fiber and UHPC,achieving the following results.1.The analysis of particle surface morphology,particle size distribution,water demand and other properties demonstrated the feasibility of using granite stone powder instead of quartz powder.A mix proportion of UHPC was obtained by an orthogonal experiment,namely the water-binder ratio of 0.18,the ratio of silica fume to cement of 0.3,the ratio of fine to total silica sand of 0.2,and volume content of straight steel fiber(S)of 2%,the replacement ratio of quartz powder by granite stone powder of 100%.When S steel fiber-was pulled out from the UHPC,the average bond strength and the pullout energy of S steel fiber in UHPC reach the maximum values,which are 14.0 MPa and 160.0 N mm,respectively.2.Hooked-end fiber(G3),small crimped steel fiber(L3)and end-paddles fiber(B)should not be used for the preparation of the UHPC due to the low tensile strength.Under the standard curing for seven days,among eight kinds of steel fibers,the average bond stress and the pullout energy of copper plated small crimped steel fiber(L2)reach maximum values of 33.3 MPa and 632.2 N mm,respectively.3.Under the autoclaved curing for seven days,among eight kinds of steel fiber,the average bond stress and the pullout energy of copper plated hooked-end steel fiber(G1)reach maximum values of 31.5 MPa and 540.9 N mm,respectively.G1,copper plated bi-line hooked-end steel fiber(G2)and copper plated crimped steel fiber(L1)were gradually straightened since there is a larger mechanical interlocking between G1 and UHPC compared to G2 and L1.More hydration products are attached to the surface of zinc phosphate-deposited steel fiber,which improves the interfacial adhesion strength between steel fiber and matrix.The average bond strength and the pullout energy of zinc phosphate-deposited S are markedly increased by 13.37%and 39.19%,respectively.4.Under the 90 degrees celsius water curing,the average pore size and the cumulative pore volume of UHPC decrease,and the interface performance between steel fiber and UHPC becomes better with the growth of curing age(3 d,7 d and 28 d).5.In differcnt curing systems(standard curing,90 degrees celsius water curing and autoclaved curing),the crystallinity of the hydration products increase.In terms of the steel fibers pulled out from the UHPC,the order of the average bond stress and the pullout energy from large to small is G1>G2>L1>S.The ranking of methods in improving fiber material utilization ratio(namely the maximum tensile stress divided by the fiber tensile strength)is curing scheme>curing age>modified treatment method.