| Hydraulic concrete structures,such as concrete arch dams and gravity dams,often consider the influence of dynamic loads such as earthquake action during their service,so that most of them will be in biaxial or triaxial stress state under high strain rate.However,the experimental research on the multi-axial dynamic characteristics of large aggregate concrete is still rare,and the design indexes based on the multi-axial dynamic strength are still to be further studied.In this paper,a triaxial hydraulic servo test system developed by the laboratory was used to carry out a multi-axial dynamic loading test for three graded concrete used for dam surface and its two graded wet-screened concrete.The loading conditions included uniaxial dynamic compression,uniaxial dynamic tension,biaxial constant proportional dynamic compression,biaxial constant proportional dynamic tension-compression,and triaxial constant proportional dynamic tension-compression-compression,considering the strain rate of 10-5s-1?10-4s-1?10-3s-1?10-2s-1 corresponding to earthquake action.Finally,a design index based on multi-axial dynamic strength for the seismic design of hydraulic structures was put forward.The results of the test and the main conclusions were as follows:(1)Through the test of uniaxial dynamic compressive and tensile loading for three graded concrete and its two graded wet-screened concrete,the dynamic uniaxial strength,peak strain,modulus of elasticity and stress-strain relationship of two kinds of concrete were obtained.The test results show that the dynamic uniaxial strength of three graded concrete and its two graded wet-screened concrete is much greater than that of the normal concrete.The ratio of the dynamic strength for the two concrete is not obvious with the strain rate,and the dynamic peak strain increases slightly with the strain rate.Through regression analysis,a series of empirical formulas were obtained to reflect the relationship between the dynamic mechanical properties and its influencing factors.(2)The dynamic biaxial compressive strength of two kinds of concrete were obtained by biaxial constant proportional dynamic compressive loading test for three graded concrete and its two graded wet-screened concrete.The combined effects of strain rate,stress ratio and wet-screened concrete on the dynamic compressive strength of large aggregate concrete were analyzed.The test results show that the dynamic biaxial compressive strength increases with the strain rate,the greater the stress ratio is,and the slower the growth speed is.The dynamic strength growth rate under biaxial compression is less than that under uniaxial compression.The influence of the stress ratio on the two concrete strength ratios is greater than that of the strain rate.Two kinds of strength criterions for hydraulic concrete in principal stress space and octahedron stress space were given respectively.(3)The test data of dynamic biaxial tension-compression strength and peak strain for two kinds of concrete were obtained by the dynamic biaxial constant proportional tensile-compressive loading test for three graded concrete and its two graded wet-screened concrete.The influence of strain rate,stress ratio and wet-screened concrete on the dynamic tensile-compressive strength of large aggregate concrete was analyzed.The experimental results show that the increase rate of dynamic biaxial tensile-compressive strength is greater than that of uniaxial dynamic tension,uniaxial dynamic compression and biaxial dynamic compression,and decreases with the increase of the absolute value of stress ratio.The effect of stress ratio on peak strain is greater than that of strain rate.The dynamic biaxial tensile-compressive strength criterion of hydraulic concrete,which considers the interaction of strain rate and stress ratio,was given.(4)The dynamic strength test data of two kinds of concrete under triaxial tension-compression-compression were obtained by the dynamic triaxial constant proportional tensile-compressive-compressive loading test for three graded concrete and its two graded wet-screened concrete.The influence factors on triaxial dynamic tension-compression-compression characteristics and their differences with biaxial dynamic tension-compression characteristics for two kinds of concrete were analyzed.The test results show that the dynamic strength growth rate under triaxial tension-compression-compression is equal to uniaxial dynamic tension.The test results also show that the strength ratios of the two concrete are the smallest under triaxial tension-compression-compression,second under biaxial tension-compression,and biggest under uniaxial tension.The triaxial dynamic tensile-compressive-compressive strength criterions of three graded concrete and its two graded wet-screened concrete under octahedron stress space were given.(5)Based on the analysis of the existing design indexes such as safety factor and failure distance coefficient,a design index based on the dynamic multi-axial strength,named dynamic failure distance coefficient,was proposed.The index is based on the shortest distance of the stress point to the dynamic multi-axial failure surface.It can directly and conveniently reflect the safety of the different parts of structures,and can be used as a checking index of seismic design and calculation for hydraulic structures. |
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