Study Of Static Mechanical Properties Of Ultra-high Performance Concrete Under Triaxial Stress States

Ultra-high performance concrete(UHPC)has been widely used in civil engineering structures.However,in practical applications,most of the concrete structures are in multiaxial stress state.At present,the research on the mechanical properties and constitutive relations of UHPC under multiaxial stress state is still very limited,and the corresponding design specifications are also very imperfect.To this end,this paper carried out experimental research and theoretical analysis on the multiaxial mechanical properties of UHPC.The main research work and innovations include the following aspects:(1)Based on the direct shear test of 16 UHPC specimens,the influence of steel fiber volume ratio and stirrup ratio on the shear performance of UHPC was studied.Based on the experimental data,a new empirical formula was derived to predict the shear strength of UHPC with different volume of steel fibers and stirrup ratio.(2)Through the conventional triaxial test and true triaxial test of UHPC materials,the effects of confining pressure and steel fiber volume fraction on the failure mode,triaxial compressive strength,peak strain and compressive toughness of UHPC materials were studied.An improved Mohr-Coulomb failure criterion is proposed to provide a reference for predicting the mechanical properties of UHPC materials under triaxial conditions.Based on the triaxial test of UHPC materials under true triaxial conditions,it is found that the anisotropy characteristics of UHPC materials are not obvious,but there are still slight damages inside the specimens;the intermediate principal stress and stress ratio are the damage forms of UHPC specimens.The effect of triaxial compression strength is obvious.Through the results of conventional triaxial and true triaxial tests,a simple expression for obtaining the UHPC material tensile and meridian is derived based on the unified strength theory,which has the advantages of being simple and easy to use.(3)Proposed a high temperature resistant UHPC material formulation.The UHPC material was subjected to conventional triaxial test and true triaxial test after high temperature.The results show that the UHPC specimens enter the compaction stage after high temperature,and the stress-strain curve shows that the slope of the starting curve is small.When the specimen is compacted,the slope gradually increases.The damage,stress-strain curve,triaxial compressive strength and peak strain of the specimen under different confining pressures and different high-temperature three-axis conditions were studied under the conventional triaxial test.Under the support of unified strength theor y,the triaxial compression failure criterion of UHPC after different high temperature is established.(4)Based on the unified strength theory,the UMAT subroutine was used to develop and verify the UHPC material triaxial constitutive in ABAQUS software.The developed UMAT material model can well simulate the UHPC triaxial mechanical properties.The development of this model lays a part of the theoretical foundation for the complex force analysis of UHPC structures.In summary,through a large number of experiments,numerical simulations and theoretical analysis,the mechanical properties of UHPC materials under normal temperature and high temperature are systematically studied.The unified strength theoretical formula of UHPC materials is proposed,and the UHPC material is successfully developed in ABAQUS software.Three-axis constitutive under and after high temperature.The research results are of great significance for promoting the wide application of UHPC materials,and can provide some theoretical bas is and technical support for the safe and reliable design of UHPC structures.