Experimental Study On Preparation And Properties Of High Ductility Fiber Reinforced Engineering Cementitious Composites

Engineered Cementitious Composites(ECC)have two major characteristics of multi-joint cracking and strain hardening,which largely compensate for the defects of brittle and cracked ordinary concrete,so it has a wide range of engineering applications to repair and strengthen existing concrete structures or directly build engineering structures with special requirements.Therefore,ECC has received a lot of attention and favor from researchers.However,as the traditional ECC is prepared by quartz sand and polyvinyl alcohol(PVA)fiber,the material cost is expensive,which restricts its application in practical engineering,so reducing the cost of ECC has become an urgent problem to be solved.In this paper,we use river sand and stone powder instead of quartz sand for ECC preparation tests to study its mechanical and microstructural properties,which is beneficial to the promotion of ECC in engineering structures.Firstly,the mechanical properties of ECC prepared with different sand-cement ratios,different types of fine aggregates,different particle sizes of river sand and different types of fibers were investigated by designing and optimizing the parameters of the ratio,and the effects of each factor on the mechanical properties of ECC were analyzed.The test results showed that the ultimate tensile strain of ECC prepared with river sand of 0～0.3 mm particle size reached 5.17%,the peak stress reached 5.23 MPa,the cubic compressive strength was42.5 MPa,and the flexural strength was 16.4 MPa at a water-cement ratio of 0.4 and a sand-cement ratio of 0.65;the ECC prepared with three kinds of fine aggregates,namely river sand,mechanism sand and quartz sand,all had excellent multi-seam cracking and flexural strength.The ultimate tensile strain reached 5.35%,peak stress reached 6.78 MPa,compressive and flexural strengths were 50.4 MPa and 18.7 MPa,respectively;the experimental results of river sand with different grain sizes showed that the ultimate tensile strain and peak stress decreased with the increase of river sand grain size,and the ultimate tensile strain and peak stress decreased when the maximum grain size exceeded 1.18 mm.When the maximum particle size exceeds 1.18 mm,the decrease is more obvious,and when the particle size is in the range of 0～0.6 mm,the specimen has the maximum compressive strength,while the flexural strength also decreases with the increase of the particle size of river sand;among the four kinds of fibers,PVA fiber has good coordination with river sand,while polyethylene(PE)fiber and polypropylene(PP)fiber are less effective,and the tensile specimen prepared by basalt fiber(BF)only produces A crack followed by damage,that is,the use of BF fibers can not prepare ECC.Secondly,the mechanical properties,microstructural properties and self-shrinkage properties of ECC prepared by replacing river sand with granite porphyry stone powder,limestone powder and quartzite powder at different replacement rates were investigated to reduce the environmental burden caused by the lack of full utilization of stone powder,since the particle size of stone powder is generally below 75μm,which meets the particle size requirement of fine aggregate for ECC materials.The results of the mechanical properties tests showed that the peak stress and ultimate tensile strain of ECC tensile specimens decreased and then increased with the increase of stone powder substitution rate,and the compressive strength and flexural strength increased with the increase of stone powder substitution rate,and the mechanical properties were optimal at 100%substitution rate.The mechanical properties of ECC prepared by three kinds of stone powder all reach peak tensile stress more than 4.4 MPa,ultimate tensile strain more than 4.2%,compressive strength more than 50 MPa,flexural strength more than 18 MPa,which have a greater degree of improvement than the mechanical properties of the reference group specimens;The results of self-shrinkage test show that when granite porphyry stone powder,limestone powder and quartzite powder replace river sand,the self-shrinkage of each ECC specimen becomes larger with the increase of stone powder replacement rate;The microstructure performance test found that the incorporation of stone powder can be used as the nucleation matrix of hydration products and Promote its reaction,improve and optimize the pore structure of matrix,when the amount of stone powder is larger,the distribution of polyvinyl alcohol fiber in the matrix is more uniform,the matrix and fiber can jointly withstand the load and coordinated deformation,making full use of the strength of the matrix and fiber itself.Finally,combining the tensile stress-strain curve of ECC in this test,the steady-state cracking stage is optimized on the basis of the bilinear model,and a tensile principal structure model of ECC considering the range of stress changes in the steady-state cracking stage is established,which can accurately characterize the steady-state cracking stage.In addition,in this paper,the non-linear curve fitting was carried out by the logistic model through the method of fitting the self-shrinkage test results,and the self-shrinkage calculation model of ECC with a high degree of agreement(R~2>0.9)was obtained.