Multi-objective Optimization Design Of Mix Ratio Of Hybrid Fiber Recycled Aggregate Concrete Of Steel-macro Polypropylene For Road Use

Adding hybrid fibers and active mineral admixtures to recycled aggregate concrete(RAC),through the optimal design of the mix ratio,can effectively exert synergistic effects,achieve complementary advantages,and improve the comprehensive performance of RAC.In this paper,through the response surface method(RSM)test,macroscopic mechanism analysis and multi-index optimization,the influence of multiple factors on the working performance,mechanical properties and wear resistance of hybrid fiber recycled aggregate concrete(HFRAC)was analyzed,and the optimal parameters of the mix ratio were determined to improve the mechanical and wear resistance properties of RAC,and the flexural toughness and fatigue properties of the optimized HFRAC were explored.The main research work and conclusions are as follows:(1)Through experiments and theoretical calculations,the particle gradation and basic physical and mechanical properties of recycled coarse aggregate(RCA)were explored;And based on the RSM test,the influence of the volume content of steel fiber(SF),macropolypropylene fiber(MPPF),sand rate and their interaction on the slump of HFRAC was quantitatively studied.The results show that the RCA used in this study has good particle gradation,and its physical and mechanical properties meet the requirements of grade III or above in the specification,and can be applied to roads with medium and light traffic loads;compared with natural coarse aggregate,the apparent density and bulk density are lower,but the water absorption rate and crushing value are higher;Among the single factors,the volume content of SF and MPPF have a significant impact on the slump,the effect of MPPF volume content on slump is weaker than that of SF volume content.In the interaction,the interaction between SF volume content and sand ratio has a very significant effect on slump,and the interaction between SF volume content and MPPF volume content has a significant impact on slump.(2)Using the Box-Behnken test design in RSM,and constructing the HFRAC mix ratio optimization model based on the NSGA-Ⅱ coupling entropy weight TOPSIS method,the factors of SF volume content,MPPF volume content and sand rate were studied,and the HFRAC flexural strength,compressive strength and wear amount as the evaluation index for the optimal design of the mix ratio.The results show that: The prediction model fitting effect of each evaluation index in the test range is good;among the single factors,the volume content of SF has a very significant effect on the flexural strength,compressive strength and wear volume;the volume content of MPPF has a great influence on the wear volume Significantly,the impact on the flexural and compressive strength is significant;the sand rate has a very significant impact on the amount of wear.In the interaction,the interaction between SF volume content and MPPF volume content has a very significant impact on the flexural and compressive strength,and has a significant impact on wear;the interaction between MPPF volume content and sand ratio has a significant impact on the flexural strength;when SF volume content The overall performance of HFRAC is optimal when the content of sand is 1.39%,the volume content of MPPF is 0.97%,and the sand rate is 36.10%.The feasibility of the method is proved by the confirmatory test.(3)Combine SF and MPPF hybrid fibers with solid waste resources(fly ash-microsilica fume).Using the CCF test design in the response surface method,combined with the analytic hierarchy Process(AHP)-entropy weight method(EWM)performance index comprehensive weighting method and the RSM-desirability function(DF)multi-index optimization algorithm,the hybrid fiber ratio(SF volume content: MPPF volume content),Fly ash substitution rate and micro-silica fume substitution rate are factors,and HFRAC flexural strength,compressive strength,splitting tensile strength and wear amount are used as evaluation indicators for the optimal design of the mix ratio.The results show that each response model has good fit and high prediction accuracy;each evaluation index is not only affected by a single factor,but also by the interaction between factors.When the four evaluation indicators are optimally coordinated,the corresponding combination is as follows: the proportion of hybrid fibers is1.14,the replacement rate of fly ash is 23.38%,and the replacement rate of microsilica fume is 11%.Through the verification test,it is in good agreement with the theoretical prediction results;the optimal group HFRAC’s flexural strength,compressive strength and splitting tensile strength are all higher than the reference group,and the wear amount is lower than the reference group,respectively increased by 65.8%,30.6%,44.8%,and the amount of wear is reduced by 38.7%.(4)The flexural toughness and fatigue properties of the optimal group HFRAC were studied.And the mechanism analysis of the damage characteristics of RAC and HFRAC was carried out at the macroscopic level.The results show that the incorporation of hybrid fibers makes RAC change from brittle failure to ductile failure,the number and size of cracks are significantly improved,the load-displacement curve is fuller,and the descending section is gentler than that of the control group.The optimal group HFRAC flexural toughness Compared with the control group,it increased by 4.02 times.The bending fatigue life of the optimal group HFRAC conforms to the two-parameter Weibull distribution,and a doublelogarithmic bending fatigue life equation considering the failure probability is established,which can be used as a reference for engineering applications.