Study On Compressive Properties Of Steel And Macro-polypropylene Hybrid Fibers Reinforced Recycled Coarse Aggregate Concrete

The proper mixing ratio of two and more fibers with different sizes and performances can provide synergy effects to enhance the properties of recycled aggregate concrete(RAC),which makes up the shortfall single fibers and gives play to respective advantages.The optimization of hybrid fibers and compressive properties under different loading conditions for hooked-end steel(HES)and macropolypropylene(MPP)hybrid fibers reinforced recycled coarse aggregate concrete(HFRRCAC)were studied using the experimental research,mechanistic analysis and theoretical modeling.The following conclusions can be summarized from the experimental results:(1)The optimum combination of parameters and the prediction models for four property indexes of hook-end steel and macro-polypropylene hybrid fiber reinforced recycled coarse aggregate concrete through an orthogonal experimental design is presented.The replacement ratio of recycled coarse aggregate(RCA)is the prominent influence factor of PIs.Yet,the interactions between recycled coarse aggregate and fibers and the hybrid effect between fibers have little impact.The parameter combination with the highest score was obtained through the verified evaluation method.The mixed regression models are more accurate prediction and smaller residual standard deviation,and the reduced mixed regression models can optimize the predictive ability and well agree with experimental results.The evaluation method and prediction models can be extensively applied to the parameter optimization and property index prediction of multi-index system.(2)The fresh properties and uniaxial compressive behavior of HFRRCAC with HES and MPP fibers were studied.For this purpose,three series(A,B and C)of HFRRCAC were designed with different RCA replacement ratios and hybrid fibers with different combinations at the total volume content of 1.5%.The results indicated that the effect of HES fibers on the workability of fresh concrete was much greater than that of MPP fibers.For natural aggregate concrete and recycled aggregate concrete with partial RCA,the hybrid combination of 1.0% HES fibers and 0.5% MPP fibers were observed as an optimum synergy effect in compression characteristics and damage evolution.However,the positive effect of single hook-end steel fibers on toughness and damage evolution was superior to that of hybrid fibers when RCA entirely replaced the natural coarse aggregate.The empirical formulas for predicting the characteristic indices and shape parameter of the piecewise constitutive model were proposed,taking the reinforcing index of hybrid fiber and RCA replacement ratio into account.Besides,the redefined damage parameter well described the damage evolution process of HFRRCAC.The cohesive zone model of HFRRCAC can describe the fracture failure process and predict the damage evolution process under compression loading.(3)The dynamic compressive behavior of HFRRCAC was studied,and four different strain rates were conducted.The results show that with the strain rate increasing,the peak stress,elastic modulus and compressive toughness increase,while the peak strain reduces.In the strain rate range,the strain rate sensitivity of peak stress,elastic modulus and compressive toughness increase with the RCA,and the RCA replace-ment ratio does not affect the strain rate sensitivity of peak strain.The strain rate sensitivity of peak strain and elastics modulus decrease with the addition of fiber.The addition of fibers enhances the strain rate sensitivity of peak stress and compressive toughness for natural aggregate concrete.While the strain rate sensitivity of peak stress and com-pressive toughness decrease for recycled aggregate concrete(RAC).The dynamic constitutive damage model was proposed considering the reinforcing index of fibers,RCA replacement ratio and strain rates.(4)Experimental investigation was conducted on the mechanical behavior and micro-structure analysis of HFRRCAC under uniaxial cyclic compression.The results show that the micro-structure of matrix and interface transition zone(ITZ)of RCA is loose for RAC.The fibers change the orientation of hydation products,improve the matrix and ITZ of RCA,inhibit the self-shrinkage of RAC.However,the incorporation of fibers introduces the many weak interfaces.The stiffness degradation rate of RAC is slightly higher,and the incorporation of fibers slows down the stiffness degradation.The influence of fibers and RCA replacement ratio on plastic strain is not significant.The hysteresis loop area reaches the maximum in several cycles after peak stress.The hybrid fibers can improve the energy dissipation capacity of concrete,and the synergy effect is obvious.Considering the RCA replacement ratio and fibers,the HFRRCAC uniaxial cyclic compression constitutive model was proposed.