Mechanical Performance And Mechanism Of Wollastonite Modified Ultra-high Performance Concrete (WFMUHPC)

Wollastonite Microfiber(WF),a kind of inorganic mineral microfiber with high quality and low price,has been proven to have microscopic strengthening effects on cement-based materials.Based on the concept of multi-scale reinforcement,this paper applies it to ultra high performance concrete(UHPC)to make up for the deficiency of steel fiber enhancing UHPC at micro scale and improve its cost performance.The strength and toughness of WF modified UHPC with steel fiber(WFMUHPC)under different external loads were systematically studied.The effects of WF dosage,curing system and steel fiber type on the micromechanical behavior of the interface between steel fiber and matrix of WFMUHPC were analyzed.The modification mechanism of UHPC by WF at the micro level and the synergistic enhancement mechanism of WF and steel fiber were revealed by means of characterization methods such as microstructure and composition.The main research work and results of this paper are summarized as follows:(1)The strength of WFMUHPC,including compressive strength,flexural strength,splitting tensile strength.The UHPC matrix was modified by WF to obtain WF-UHPC.Taking traditional steel fiber UHPC(SF-UHPC)as reference,the effects of WF on UHPC strength were analyzed and discussed.The experimental results show that WF can improve the compressive and tensile strength of UHPC.Compared with the compressive strength,WF improved the splitting tensile strength more significantly.(2)The toughness of WFMUHPC,including compression toughness,flexural toughness and fracture toughness.The experimental results show that WF-UHPC exhibits brittle failure characteristics in both compression and bending tests,but WF can effectively delay the occurrence of brittle failure behavior and improve the stiffness of matrix under compressive stress.WF further improved the energy dissipation capacity of SF-UHPC under different external loads,which was mainly related to the improvement of WFMUHPC’s post-peak behavior.In addition,accelerated curing is beneficial to the peak toughness of beam specimens,but not to the increase of flexural toughness.The flexural behaviors of WFMUHPC plate specimens are affected by the content of steel fiber,which is mainly related to group effect and wall effect.Compared with WF-UHPC or SF-UHPC,WFMUHPC has higher fracture toughness.(3)The effect of steel fiber types and curing systems on the micromechanical behavior of the interface between steel fiber and WF-UHPC was studied by single fiber pull-out test..The experimental results show that WF can significantly increase the average peak bond stress and residual pull-out stress of straight steel fiber in UHPC matrix,which improves of the strength,toughness and post-peak behavior of WFMUHPC.Accelerated curing can further improve the interface bonding property between straight steel fiber and WF-UHPC.The pull-out of straight steel fiber and hooked-end steel fiber includes three stages: fully bonded,debonding and frictional sliding.The anti-cracking mechanism of WF on the matrix around hook improves the interface bonding behavior between hooked-end steel fiber and WF-UHPC at different pull-out stages.(4)The synergistic effect of WF and steel fiber on the strength and toughness of WFMUHPC was evaluated by synergistic effect index,and the fiber combination with the best gain for different mechanical properties was given.The results show that S1.5W3(UHPC specimens with 1.5 vol.% steel fiber and 3 vol.%WF were added)has the most significant positive hybrid effect in both compressive strength,flexural strength and splitting tensile strength.Both S1.5W3 and S1.5W6(UHPC specimens with 1.5 vol.% steel fiber and 6vol.%WF were added)fibers are cost-effective to improve the toughness of UHPC.(5)Optical microscope(OM),Scanning electron microscope(SEM),X-ray diffraction(XRD),Nanoindentation(NI)and Backscattered electron imaging(BSE)microscopic tests were carried out to study the strengthening mechanism of WF on UHPC matrix and the pullout behavior of steel fiber embedded in WF-UHPC.The results show that WF acts as both microfiber and microparticle.WF can bridge micro-cracks and filling matrix,and limit the formation and development of microcracks at the interface between steel fiber and matrix,so as to improve the properties of matrix and interface,and enhance the strength and toughness of WFMUHPC.In addition,WF is beneficial to the hydration of cement and active minerals,and promotes the formation of different C-S-H phases in UHPC,thus improving the microstructure of UHPC.