Optimization Of Interfacial And Tensile Performances Of Steel Fiber Reinforced Cement-based Composite

Adding steel fibers is a common method to improve the tensile and flexural performance of concrete and increase its toughness.Interfacial property is the key factor for the fiber to fully exert the bridging effect and the fiber and matrix to work together.However,the interfacial microstructure between steel fiber and cement matrix is poor,such that the fiber is easily pulled out when the fiber stress is significantly lower than the failure strength.Consequently,the strength utilization rate of steel fibers is low,and the strengthening and toughening effect is poor.Furthermore,due to the requirements of construction performance,the range of commonly used fiber reinforced toughening regulation and control methods such as increasing the fiber aspect ratio and using deformation fibers are limited,resulting in serious constraints on the overall performance improvement space and design theory.Therefore,this paper aiming at the steel fiber–matrix interfacial property,develops and optimizes nano surface modification method,proposes a multi-parameter interface matching method,studies the mechanism of interface modification to achieve efficient synergistic modification,and forms a new method for the optimization and design of tensile performance of steel fiber reinforced concrete（SFRC）.The main contents are as follows.Firstly,a modification method for growth of nano-SiO₂ hydration active layer on steel fiber surface is studied.Nano-SiO₂ modified steel fiber is prepared by sol-gel method and the morphology and composition of modified layer are comprehensively characterized by scanning electron microscopy（SEM）,atomic force microscopy（AFM）,energy dispersive spectroscopy（EDS）and Fourier transform infrared spectroscopy（FTIR）.The effects of reaction environment and steel fiber types on the modification effect are evaluated and the modification scheme is optimized.A simulatied hydration environment test is carried out to verify the reactivity of the nano-SiO₂ modified layer with the cement hydration product Ca（OH）₂.The fiber pull-out test is designed,and it is found that nano-SiO₂ on the fiber surface could significantly improve the interfacial bond properties by more than 70%by reacting with Ca（OH）₂.The effect and mechanism of fiber surface nano-SiO₂ modification and matrix nano-SiO₂ modification on interfacial bond properties are compared and analyzed.The effect of fiber surface nano-SiO₂ modification on the flexural performance of SFRC,especially the initial cracking behavior,was verified by macro mechanical test.Secondly,the enhancement law of interfacial bond properties under the single and coupled effects of various modification methods is systematically studied.A single fiber pull-out test was designed to evaluate the effects of water to binder ratio（W/B）and silica fume on the pull-out resistance of steel fibers.The W/B ratio threshold（0.25）for the sudden change of interfacial bond properties and the optimal silica fume content（30%）of the interfacial bond properties were found,and the interaction rule was revealed.The whole process characteristics of interfacial debonding and slip curves were studied when fiber surface nano-SiO₂ modification was used in conjunction with traditional modification methods such as W/B ratio and silica fume.It is found that the optimal range of W/B ratio and silica fume content in which fiber surface nano-SiO₂ modification can significantly enhance the interfacial strength and pull-out energy.The regression analysis of the interfacial bond properties under the action of multi-parameter coupling is carried out,and a multi-parameter design method for the interfacial bond properties is formed.Then,the relationship between the interfacial microstructure and the interfacial bond properties and the interface modification mechanism are studied.The interfacial microstructure,product distribution and micro-mechanical property under various modification methods are systematically characterized and analyzed quantitatively based on the backscattered election image analysis（BSE-IA）,EDS and micro-hardness test.Different interfacial failure modes are divided by observing the composition,thickness and distribution of the adhered matrix on the surface of the pulled-out fibers.The influence characteristics of the interface structure and its matching relationship with the matrix on the interface failure mode are analyzed.The mechanism of different interface structures,interface failure modes and interfacial properties induced by different interface modification methods is revealed.A relational model of interface partition porosity and interfacial bond properties is constructed,the physical mechanism of parameters such as the W/B ratio threshold of 0.25 is revealed,and a synergistic modification method based on interfacial property is proposed.Finally,the relationship between interfacial bond properties and macro tensile performance is investigated.The effects of W/B ratio and silica fume on the tensile performance of SFRC are evaluated by direct tensile test,and it is found that the tensile strength increases significantly when the W/B ratio decreased from 0.30 to 0.25,and reached a peak at 20%silica fume content.The influences of W/B ratio and silica fume on the effect of fiber surface nano-SiO₂ modification to improve the tensile properties are studied.It is found that the enhancement range increases with the increase of the curing age,and the enhancement is more significant in the early stage of curing.The bond-slip relationship between the fiber and the matrix is simulated by the method of constraint coupling,and the composite material constraint coupling finite element model established by it can predict the tensile properties according to the material property of each phase and interface behavior.An optimal design method for tensile performance of SFRC based on interfacial bond properties is developed.