Research On Constitutive Models For Engineered Cementitious Composites

Engineered Cementitious Composites(ECC)is one kind of high performance fiber reinforced cementitious composites,which shows excellent quasi strain hardening and multiple cracking behaviour in tension and higher ductility than concrete in compression.ECC has been widely applied in practical engineering such as bridge deck slab,bridge link slab,structural earthquake prevention component and concrete protective cover.However,there is no relatively mature constitutive model for the numerical modeling of ECC structural components,and there is no ECC constitutive model in commercial finite element procedures such as Abaqus and Ansys.Existing ECC constitutive models do not take into account the variations of compressive strengths under biaxial compression and the fiber bridging mechanism when describing the quasi strain-hardening behaviour of ECC under uniaxial tension.Developing constitutive models which correctly describe the mechanical behaviour of ECC is significant for the engineering applications of ECC.The work of this paper can be divided into two parts.First,based on the Darwin-Pecknold method which considers biaxial behavior of concrete,a two-dimensional constitutive model for engineered cementitious composites(ECC)taking into account the nonlinear mechanical behavior and the variations of compressive strengths under biaxial compression is developed.A nonlinear stress-equivalent uniaxial strain relationship is established after introducing equivalent uniaxial strain in two orthotropic directions caused by biaxial loading to account for the biaxial nonlinear behavior of ECC.A biaxial strength envelope is used to determine the compressive strengths of the two directions.An explicit numerical algorithm of the model is derived,and user-defined material subroutine UMAT containing the numerical algorithm is coded and implemented in finite element procedure Abaqus v6.14.The efficiency of the proposed model is validated through numerical simulations of two sets of ECC cube specimens with different mix proportions under biaxial compression loading at various biaxial stress ratios.The stress-strain curves and the compressive strengths in the major compressive stress direction obtained by numerical simulations are in good agreement with the experimental results.It is shown that the proposed ECC constitutive model under biaxial compression can effectively predict the biaxial nonlinear mechanical behavior and the failure strengths of ECC under biaxial compression.The proposed ECC constitutive model is used in numerical simulation of a ECC column under uniaxial compression and equal biaxial compression.The predicted ultimate load of ECC column under equal biaxial compression is higher than uniaxial compression,showing that the bearing capacity of ECC compression member can be underestimated without considering the strength increasement of ECC under biaxial compression.Second,based on the Nguyen’s constitutive framework which contains a localised band to account for localised fracture behaviour,a constitutive model for ECC which considers fiber bridging mechanism of ECC after cracking under uniaxial tension is developed.An implicit numerical algorithm of the model is derived,and user-defined material subroutine UMAT containing the numerical algorithm is coded and implemented in finite element procedure Abaqus v6.14.The efficiency of the proposed model is validated through numerical simulations of ECC specimens under uniaxial tension.The results obtained from numerical simulations show that the proposed ECC constitutive model under uniaxial tension does not exhibit mesh size sensitivity and element integration scheme sensitivity.The simulation result of a dumbbell-shaped specimen of ECC under uniaxial tension are in good agreement with experiment result.It shows that the proposed ECC constitutive model can effectively predict the mechanical behaviour of ECC under uniaxial tension.