Research On Strengthening And Toughening Of Strain Hardening Cementitious Composites And Dynamic Tensile Properties

Strain Hardening Cementitious Composites(SHCC)is a kind of fiberreinforced cementitious composites with ultra-high toughness.This material will show a unique multi-crack failure mode after being loaded,so it has excellent deformation and energy absorption capacity.At the same time,the controllable crack width makes it obtain the durability far higher than that of ordinary concrete.Therefore,SHCC materials are widely used in building structural engineering,road engineering,water conservancy engineering,military protection,and many other fields.With the update and iteration of scientific and technological weapons,military protective buildings,anti-knock,and anti-shock engineering put forward higher requirements for building materials.How to further optimize the properties of SHCC materials through various methods has become the only way for its further promotion and application in national defense engineering.The mechanical properties of SHCC materials are determined by the interaction among fibers,matrix,and fiber/matrix interface properties,and the properties of the three must meet the requirements of the micromechanical design model.It is well known that there is a contradiction between the strength and toughness of cement-based materials.Therefore,the strength and toughness design of SHCC needs to start from the micromechanical model.Firstly,the influence mechanism of micromechanical parameters of fiber,matrix and interface on its macro properties should be clarified by numerical simulation.Then various modification methods are used to adjust its micromechanical parameters to realize the strengthening of SHCC.Based on this idea,this paper adopts the methods of fiber surface modification,matrix modification,and fiber mixing to control the micromechanical parameters of SHCC and optimize its mechanical properties of SHCC.Considering the huge application potential of high-strength and high-toughness SHCC in seismic and national defense engineering,the effects of the above modification methods on the dynamic impact properties of SHCC were studied.Firstly,starting from the micro-mechanical design theory of SHCC,considering the discrete type of matrix cracking strength caused by the inhomogeneity of flaw size and distribution in the matrix,the uniaxial tensile strength of SHCC material is realized by programming and the effectiveness of the model is verified.The influence of different micromechanical parameters on the uniaxial tensile stress-strain relationship of the material were studied by parameter analysis.Increasing the interface friction,reduce the fracture toughness of the matrix,and adjusting the size and distribution range of matrix flaws can significantly improve the tensile properties of SHCC,which lays the foundation for the strengthening and toughening design of SHCC.Then,based on the micromechanical model,three modification methods were adopted to achieve the strengthening and toughening of the SHCC material.(1)Silane Coupling Agent(SCA)was used to modify the surface of polyethylene(PE)fibers.First,SCA suitable for PE fiber modification was screened by surface energy analysis.After modification,the hydrophobicity of PE fibers was reduced,and the interfacial friction between the fibers and the matrix was significantly improved.After interfacial modification,the tensile strength and tensile strain of the SHCC material were increased by 48.1%and 112%,respectively.Micromechanical model analysis further revealed the important role of interfacial modification in improving material properties and saving fiber content.(2)The matrix is modified with re-dispersible powder ethylene vinyl acetate(EVA).With the increase of EVA content,the defect size in the matrix becomes smaller,the distribution becomes narrower,and the elastic modulus of the matrix becomes smaller.The amount and fracture toughness decreased,the interfacial friction between fiber and matrix increased,and the tensile strength and tensile strain of SHCC increased significantly,but excessive EVA would cause excessive loss of compressive strength.(3)Hybrid using steel fiber(ST)and PE fiber,the influence of the mixing ratio of steel fiber and PE fiber on the mechanical properties of SHCC was first studied.The compressive strength,tensile strength,and tensile strain can be further improved.By deriving the calculation model of Cracking Saturated Index(CSI),modifying the calculation model of cracking strength and bridge stress required to calculate the Pseudo Strain Hardening(PSH).A design method of hybrid fiber reinforced SHCC based on saturation cracking is proposed,and the determination method of fiber hybrid ratio is given.Finally,a test device for the tensile and impact performance of concrete with drop weight impact as the power source was independently designed and constructed,and the dynamic tensile properties of SHCC before and after modification under different strain rates were studied.The dynamic tensile strength and impact energy of the SHCC specimens modified by the silane coupling agent interface is higher than those of the unmodified specimens,but the dynamic intensity factor(DIF)of the modified SHCC is smaller than that before the modification.The addition of EVA has little effect on the tensile impact strength of SHCC material,but it will reduce the impact energy of SHCC,and the DIF of tensile strength decreases with the increase of EVA content;Steel fiber can significantly improve the dynamic tensile strength and first cracking strength of SHCC.At the same time,an appropriate amount of steel fiber mixing will not reduce the dynamic strain of the material.The first cracking strength DIF of the specimen decreased with the increase of steel fiber content,while the tensile strength and strain energy DIF increased with the increase of steel fiber content.Finally,the relationship between the experimentally measured DIF of tensile strength and first cracking strength as a function of strain rate is compared with the model used to predict the relationship between DIF and strain rate of cement-based materials.The results show that the law of cracking strength DIF is close to that of concrete,but the law of tensile strength DIF is greatly affected by the interface friction.