Research On Dynamic Mechanical Characteristics Under High Strain Rate And Damage Constitutive Model Of Roller-Compacted Concrete

Currently,roller-compacted concrete（RCC）is widely used in hydraulic dam structures.Concrete dam may experience high-strain-rate impact and blast loads,meanwhile stress wave propagation under dynamic loads and the complexity and the heterogeneity of RCC’s composition make the material is in an unusually complex stress state during service.The dynamic analysis of RCC structure is inseparable from the deep understanding of mechanical properties and constitutive relations of dam material,especially for the understanding of the dynamic constitutive relation of RCC at high strain rate and complex stress state is very limited so far.Based on the forming method and the mix proportion of actual hydraulic dam,experimental research on the dynamic mechanical properties of RCC under high-strain-rate loads has been carried out,and different constitutive models have been selected to be modified for RCC and numerical verification has been made to prove the validity.Research and summary of the relevant methods and technical problems in the study of the dynamic mechanical properties and constitutive relations of RCC have been achieved,main contents and results are as follows:（1）Begin with the structural characteristics and the forming mechanism of RCC,the influence of stratified structure on the mechanical properties of RCC is analyzed and summarized.Based on the construction practice of one hydraulic concrete dam and the scale effect of coarse aggregate,the mix proportion of RCC has been designed and a set of forming method of specimen which undergoes mixing,spreading,rolling,curing,drilling,cutting and polishing has been established,making the characteristic of RCC is consistent with engineering construction material.Specimen prepared through this method owes high engineering reduction degree,and the results obtained from this experiment are convenient for popularization and application.（2）The mechanical properties of RCC under different stress conditions have been systematically studied and revealed.Ottosen failure criterion has been established based on the test data of mechanical properties of RCC.Split Hopkinson Pressure Bar（SHPB）test technology improved for quasi-brittle material can obtain reasonable and reliable dynamic mechanical properties of RCC under high-strain-rate impact loads(strain rate from 20s^-1 to 150s^-1).Test data above provide the basis for further study of the constitutive relationship of RCC.（3）Results of SHPB test show that the trends of the curves of RCC specimens with and without bedding surface are similar.As expected,the specimen without bedding surface has slightly higher strength than the specimen with bedding surface at the similar strain rate,the specimen strength is therefore negligible.Failure mechanism of RCC can be concluded from the energy absorption,the failure mode and the screening result of the specimen during the loading process of SHPB test,which means cracks spread along the nearest path instead of the interfacial transition zones（ITZ）around the aggregates and specimens,even pass through the coarse aggregate directly affected by viscous effect and inertia effect.It can be concluded that the sensitivity of failure mode of RCC material to strain rate is similar to that of the normal concrete.The quadratic polynomial expression relationships between the strain rate effect and the size effect have been given qualitatively,and a full-scale understanding of the mechanical behavior of RCC under dynamic impact load is achieved.The dynamic mechanical properties of RCC shows an obvious strain rate strengthening effect.（4）The classical concrete constitutive theory has been systematically summarized.Considering the feasibility of finite element calculations,the original HJC model and K&C model have been modified according to the triaxial compression test result and SHPB test data which contains strength plane,strain rate effect,damage criteria and damage equations.Improved models for describing the dynamic mechanical behavior of RCC with lower strength have been established.（5）Numerical model of SHPB test that satisfies the test hypothesis has been proposed.The constitutive relation of RCC has been simulated using the parameters of the modified HJC model and the modified K&C model developed on the FORTRAN Workspace platform.The stress-strain relationship reconstructed from simulation result is consistent with the SHPB experimental one.Specimen failure mode obtained via the modified HJC model shows a nuclear retention phenomenon which is the same as the experimental result while failure mode acquired by the modified K&C model shows a nuclear elimination phenomenon.The effectiveness of describing the mechanical behavior of RCC under high-strain-rate impact loads through the modified model has been proven.