Constitutive model for confined ultra-high strength concrete in steel tube

- A constitutive model for confined ultra-high strength concrete is proposed.
- The dilation angle model is a function of lateral pressure and plastic deformation.
- The hardening/softening rule is dependent on pressure and plastic deformation.
- The interaction between the steel tube and the confined concrete can be predicted.

With the development of material science and production technology, ultra-high strength concrete (UHSC) with uniaxial compressive strength up to 200 MPa has been made available commercially and used for concrete filled steel tubular (CFST) columns in high-rise buildings. Finite element analysis is a necessary tool to analyze CFST columns, but its accuracy depends on the generic constitutive model for the confined UHSC in the steel tube. This paper proposes a new constitutive model for confined UHSC based on (1) a yield criterion which is a function of hydrostatic pressure and lode angle, (2) a non-associated flow rule with a dilation angle that is a function of the confining pressure and the equivalent plastic strain, and (3) a hardening/softening rule which is dependent on the confining pressure and the equivalent plastic strain. The parameters of the proposed constitutive model are calibrated by a series of uniaxial compression, biaxial compression and triaxial compression tests of UHSC specimens. The constitutive model is then implemented in ABAQUS and verified by the test results of short CFST columns. Comparison of the test and predicted results in terms of compression load versus axial strain and lateral strain curves demonstrates that the proposed model can predict accurately the maximum resistance of the stub CFST columns as well as the interactive behavior between the steel tube and the confined concrete core.