Investigation Of Static And Dynamic Viscoelastic Behaviour Of Concrete

Concrete is one of the most widely used materials in civil engineering.Similar to many solid materials,the viscoelasticity is one of the fundanmental properties of concrete,which has consideable influence on creep,damping and mechanical properties of concrete.Although the viscoelasticity of concrete has been continuously studied and considered in structural analysis,due to the complexity of load changes in actual engineering,it is still lack of systematic research on static and dynamic viscoelastic properties of concrete under different load conditions.In this thesis,the static and dynamic viscoelastic properties of concrete are analyzed by short-term sustained load/recovery test under different loading/unloading rates and axial compression hysteresis tests under different frequencies.Based on the experimental data,the applicability of the fractional derivative theory(the derivative order between 0 and 1)in the description of viscoelastic of concrete materials is analyzed.One modified model and a revised parameter identification method are proposed in the thesis.The conversion relationship between the static and dynamic viscoelasticity of concrete is also discussed.The main contents of this thesis are as follows:(1)Based on the research methods of concrete creep/creep-recovery,the experimental study on the static viscoelastic properties of concrete is carried out.In the test,the influence of loading rates,unloading rates,and holding time on the short-term creep,the creep-recovery and residual deformation after unloading,and the elastic modulus were analyzed.In order to better exclude the influence of environmental factors and reflect the viscoelastic properties of the concrete material itself,the load duration time is controlled at minute-level during the test process.(2)The static viscoelastic of concrete were described by the viscoelastic models.Firstly,the prediction results of the existing creep models for the deformation of the concrete under different loading rates conditions are compared.The causes of the random errors are analyzed.From the perspectives of creep recovery and residual deformation,the existing creep recovery calculation methods are also compared.A more accurate combination of creep and creep recovery models is recommended in this thesis.Then,the applicability of the fractional derivative theory in the static viscoelastic analysis of concrete is analyzed from the perspective of short-term creep and creep-recovery.A modified fractional model is proposed by considering the deformation characteristics of concrete,and the model parementers of different conditions are given in combination with the experimental data.(3)In order to analyze the dynamic viscoelastic property of concrete,the axial hysteresis test under different load frequencies was carried out according to the dynamic modulus test method of asphalt mixture.During the experiment process,the influence of the load frequency on the hysteretic energy dissipation and dynamic modulus of the concrete was considered,which provided data support for further understanding the dynamic viscoelastic properties of concrete.(4)Based on the experimental results,the dynamic viscoelastic property of concrete were further calculated.Based on the fractional derivative theory,the theoretical formulas of the dynamic viscoelastic are derived.From the perspectives of hysteretic energy-frequency correlation and hysteresis curve,the applicability of fractional derivative theory in dynamic viscoelastic analysis of concrete is analyzed.Combined with the experimental data,the values of parameters of different models are given.A simplified identification method is proposed for the identification of fractional derivatives in fractional order models.The effectiveness of the method is verified by comparison with experimental data.(5)Based on the viscoelastic theory,the conversion of the static and dynamic viscoelastic characteristics of concrete materials were carried out.Firstly,based on the fractional order model,the problems of direct conversion calculation between static and dynamic viscoelastic characteristics were analyzed.Then the generalized viscoelastic model is used to calculate the conversion between static and dynamic viscoelastic properties.Combined with the experimental data,the accuracy of the generalized viscoelastic model for the viscoelasticity description of concrete is analyzed.Considering the computational complexity of Laplace transform and Laplace inverse transform,the approximate transformation formulas are derived based on the Volterra operator.By comparing the conversion calculation results with the experimental data,the feasibility of the method is verified,which provides a new reference for the conversion of static and dynamic viscoelastic properties of concrete.