Study On Mechanical Properties And Mechanism Of Hybrid Fiber Reinforced UHPC

Ultra-high performance concrete(UHPC)is considered to be the future building material due to its excellent mechanical properties and superior durability.In recent years,more and more engineering applications of UHPC have been applied in Europe,America,Japan and South Korea,and our country has gradually attached importance to the research and development and application of this material.However,due to the use of a large number of cementitious materials in UHPC,it will not only lead to high cost,but also cause a large amount of autogenous shrinkage,which is an urgent problem to hinder the large-scale engineering application of UHPC.In this study,starting from the mix proportion of UHPC,coarse aggregate was added into the matrix to optimize the particle packing system,and an advanced UHPC material with lower energy consumption(standard curing/water curing),low shrinkage and excellent comprehensive performance(workability and basic mechanical properties)was developed.In addition,in order to improve the toughness and strain hardening properties of UHPC,the effects of steel-steel fiber hybrid and steel-PE fiber hybrid on the basic mechanical properties of UHPC were investigated.The main research contents and conclusions are as follows:(1)Based on the modified MAA model,water and steel fibers were incorporated into the particle packing system,and different UHPC mix proportions were designed to compare their properties.The results show that the optimized design method is more suitable for the design of UHPC mix proportion.The effect of distributed modulus q on the workability and compressive performance of UHPC was investigated.The results show that in the range of 0.2-0.25,the higher the q value,the worse the flowability of UHPC,but the higher the compressive strength.According to the experimental results,the empirical equation of optimal coarse aggregate content,the comprehensive performance evaluation method of UHPC containing coarse aggregate(UHPC-CA)and the performance-based design method of UHPC-CA were proposed.(2)To reduce the autogenous shrinkage of UHPC and reduce the negative effects of commonly used shrinkage reducing materials on the mechanical properties or workability of UHPC,the combined incorporation of crushed granite(CG),superabsorbent polymer(SAP)and expansion agent(EA)into UHPC was proposed.The results show that CG,EA and SAP can all reduce the autogenous shrinkage of UHPC,and the reduction efficiency of double and triple admixing is higher than that of single admixing of UHPC.The addition of SAP can offset the negative effect of CG and EA on the workability of UHPC,while the incorporation of CG and EA can compensate the negative effect of SAP on the compressive strength of UHPC.This indicates that the combination of the three can bring out the advantages of each,thus giving UHPC excellent comprehensive performance.In this study,UHPC with the addition of CG,1%EA and 0.15%SAP showed the best comprehensive performance with no reduction in the workability,a slight increase in compressive strength,a 68%reduction in autogenous shrinkage and a 42.9%reduction in porosity compared with the reference group.(3)The effect of hybrid fibers on the basic mechanical properties of UHPC was investigated systematically by studying the hybridization of different fibers and different hybridization ratios,designing seven test groups,through compressive test,uniaxial tensile test,flexural test,combined with digital image correlation(DIC)technology.The results showed that the improvement of tensile strength,tensile ductility and flexural strength of UHPC in the steel-steel hybrid group was significantly higher than that in the steel-PE hybrid group.However,when 0.5%PE and 1.5%steel fibers were hybrid,the effect of hybrid fibers to enhance the flexural toughness of UHPC was the most obvious.In addition,the empirical equation proposed in this paper for predicting the elastic modulus of UHPC based on its cubic compressive strength has good prediction accuracy and is simple in form and easy to calculate.(4)To further investigate the effect of hybrid fibers on the properties of UHPC,the micromechanics principle was applied to systematically evaluate the pseudo-strain hardening properties of UHPC,and the reasons for the variation of UHPC properties at the microscopic level were explained by SEM images and mercury compression tests.The results show that,in general,it is feasible to apply micromechanical principles to evaluate the tensile properties of UHPC,but this paper suggests that the strength criterion be revised to PSH_s≥1.0.Generally,the denser the UHPC matrix is,the lower its total porosity will be,and the higher the compressive strength and tensile initial crack strength will be.However,the tensile initial crack strength of UHPC is also related to the fibers employed,and the PE fibers selected in this paper have a significant crack arresting effect at 0.5%volume doping.The SEM test results show that the PE fibers are pulled out of the matrix in various forms,including PE fibers completely pulled out,fibers being cut,fiber fracture and fiber pulling.However,the agglomeration of PE fibers led to a decrease in the compatibility of the fresh mixture,which increased the porosity and macropore percentage of the matrix.