A new elasto-plasticity constitutive model for concrete under multiaxial compression based on experimental observations.

This thesis comprises of two different kinds of work. The first part is focussed on existing experimental data. Investigations and observations of the behaviour of plain concrete under triaxial and multiaxial compression following cyclic loading and a variety of stress paths has been presented. The behaviour of concrete with different constituents was also investigated. The directions of the plastic strain vectors were identified. Two loading surface were also identified: (i) the Peak Nominal Stress surface (PNS) which was identified from the peak stresses recorded from stress control tests and (ii) the Volume Transition Stress surface (VTS) which determines the onset of the volumetric dilation. The plastic VTS is the surface which was identified from plastic strain components only. At this surface, the directions of the plastic strain vectors are purely deviatoric. A proposal for the shapes of the yield surface for concrete is given. These shapes were identified by the plastic work contours and also from the directions of the plastic strain vectors assuming the associated flow rule. This assumption has been verified by examining the normality of the plastic strain vectors to the PNS surface. Following the investigation of the experimental data, an examination of various advanced plasticity models for concrete revealed the need to develop a new constitutive model with a suitable shape of the loading surfaces and with a better prediction for the stress-strain response. A new constitutive model for plain concrete has been developed using the previous work in this field at the University of Sheffield. The new yield surface was developed as a combination of a reflection of part of the peak nominal stress surface (PNS) and a quartic function. The continuity, the convexity and the normality of the yield surfaces were ensured. The model was calibrated and the optimum values of the thirteen material constants are presented. This is followed by a sensitivity study with simulations of a wide range of existing experimental data. Simulations of concrete with different constituents are also presented. The formulation of the model was simplified and verified by using numerical derivatives. A comparative study between the analytical and numerical derivatives of the constitutive model is presented. The sensitivity study and the simulations of experimental tests showed that the new constitutive model is: (i) easy to calibrate using only data from uniaxial compression tests and one triaxial compression test, and (ii) gives very good predictions of stressstrain response of different types of concrete under triaxial compression stresses and at different levels of confinement all the way to the peak stress state.