Investigation Of Material Damping And Nonlinear Characteristics Of Recycled Aggregate Concrete

As one of the three dynamic characteristics of structures,damping has a significant influence on the dynamic response of structures.It is an important indicator to identify the energy dissipation behavior of concrete materials or structures under dynamic load.However,as a kind of weak viscoelastic material,concrete damping has complex damping mechanism and it is influenced by different factors compared with damping materials such as polymer and asphalt concrete mixtures.On the other hand,with the consumption of natural resources such as natural sand and gravel,companied with the generation of a large number of construction and demolishment waste,recycled aggregate concrete(RAC)has received extensive attention.The incorporation of recycled coarse aggregate(RCA)further improves the complexity of concrete material components.Considering the complexity of concrete damping,the influence of the incorporation of RCA on material damping is still lack of systematic investigation.A combination of experimental research and theoretical analysis is used in this research.The variability of material damping of RAC was firstly studied.The main problems existing in the damping of RAC was systematically analyzed,such as: RCA material properties and the mechanism of the influence of load conditions on the material damping of RAC.The main contents of this research are as follows:(1)The variability of dynamic characteristics of natural aggregate concrete(NAC)and RAC,such as fundamental frequency and damping ratio,was studied by forced resonance test.Classical and Bayesian statistical methods were used to study the dynamic characteristics of NAC and RAC.The probabilistic prediction model of generalized stiffness,fundamental frequency and damping ratio of RAC were established based on Bayesian generalized linear model,which can well consider the variation of damping.The results show that the incorporation of RCA will change the optimal distribution of fundamental frequency and damping ratio of concrete.The coefficient of variation of RAC damping follows a normal distribution,and the confidence interval of 95%confidence level is(0.107,0.119),which is higher than that of NAC.(2)The nonlinear material damping of RAC was studied by carrying out cyclic uniaxial compression test and strength test.The nonlinear relationships between compressive strength,elastic modulus and RCA properties were analyzed.Meanwhile,the nonlinear material damping models were established.Finally,the optimum RCA particle size and replacement percentage with the consideration of both RAC's compressive strength and damping property under different application scenarios are proposed.The results show that there is an obvious nonlinear relationship among the material damping,RCA properties and cyclic load.The static mechanical properties and damping properties of concrete materials are mutually restricted.Different load indexes will lead to different damping properties and optimization results of RAC.(3)Consideing the viscoelastic properties of creep and damping,the rate-dependent material damping properties of RAC were studied based on creep test with RAC beams,free vibration test and literature data investigation.The relationship between damping and creep is explored based on the interconversion method between viscoelastic material functions in time domain,Laplace domain and frequency domain.Based on the viscoelastic theory,the rate-dependent material damping models of RAC were established.The effect of the residual elasticity on damping under different load levels of concrete was analyzed.The results show that the damping characteristics of RAC have obvious correlation with load rate and load levels.The residual elasticity of concrete is related to the stored strain energy of the material,which is necessary to estimate the residual elasticity when calculating the concrete damping.The RAC damping is a kind of timedependent properties.(4)By carrying out creep and creep recovery tests under three load levels and four loading times with RAC beams,the influence of the load history on material damping performance of RAC was analyzed.Firstly,the viscoelastic parameters of creep and creep recovery under different load conditions were identified based on FTPM.Then the influence of load holding history on viscoelastic parameters of NAC and RAC was analyzed.The influence of load level,loading time and unloading on the variation of damping of NAC and RAC was studied based on interconversion method between creep and damping.The results show that the load history has a great influence on the damping of concrete.For the load level,increasing the load level not only means the change of the components of elasticity and viscosity,but also the plasticity and the unrecoverable deformation caused by damage will incease with the increase of load level.The damping performance of concrete will be changed by changing the residual elasticity of concrete.As for the loading time,it is mainly due to the hydration reaction inside the concrete,which changes the viscosity of the concrete,and its unrecoverable deformation will increase at the same time.(5)Based on the principle of compromise in competition in the mesoscience and the principle of minimum energy dissipation,the plastic damage energy dissipation mechanism and viscoelastic energy dissipation mechanism of NAC and RAC were studied.The energy dissipation rate of plastic damage energy dissipation was derived based on the plastic damage constitutive model considering the coupling of damageplastic deformation.At the same time,the energy dissipation rate of NAC and RAC based on viscoelastic energy dissipation was derived based on FTPM.Then the transition point of the two energy dissipation mechanisms was identified based on the EMMS(Energyminimization multi-scale)principle of compromise in competition.The energy dissipation composition of the two energy dissipation mechanisms under cyclic load was calculated and analyzed.The results show that the viscoelastic energy dissipation component of RAC is higher than that of NAC,but have little difference is found between NAC and RAC.Although the incorporation of RCA improves the energy dissipation capacity of concrete,the proportion of energy dissipation mechanism does not change significantly.NAC and RAC have similar energy dissipation mechanism,but there are still obvious differences in total energy dissipation.