This study first presents an extensive experimental research program on the true uniaxial and triaxial compression behavior for both high strength concrete (HSC) and steel fiber reinforced high strength concrete (SFHSC). The experimental study mainly focuses on the octahedral shear stress strain relationship of those two types of concrete, which is adopted as the basis to develop a new incremental constitutive model. Emphasis is also put on the investigation of the variation of tangent Poisson's ratio under not only uniaxial but also triaxial stress conditions. The effect of cyclic loading on this parameter is also addressed.

According to this research, under triaxial compression, there is no apparent advantage of steel fiber reinforced high strength concrete (SFHSC) over high strength concrete (HSC) in terms of triaxial strength, ductility and stress ~ strain behavior. The compressive meridians and the peak octahedral shear stress (ζ_octp) versus peak octahedral shear strain (ϒ_octp) relationships for the two types of concrete can be virtually expressed by a single expression respectively.

Unlike most of the previous incremental constitutive models, the proposed new model utilizes the experimentally acquired octahedral shear stress (ζ_otcp) octahedral shear strain (ϒ_oct) relationship instead of the fictitious concept of "equivalent uniaxial strain" to locate the peak point of the triaxial stress ~ strain curve, which ensures its capability of simulating the whole load ~ deformation process for both HSC and SFHSC, including the descending branch in the stress ~ strain curve. The results from the model analysis comply with the experimental data fairly well under moderate confining pressures.