STRESS-STRAIN RELATIONSHIP OF CONFINED AND UNCONFINED CONCRETE

Experimentally determined full stress-strain curves from standard tria xial tests are presented. Confining pressures of between 1 to 20 MPa ( 145 to 2900 psi) were applied. Five mixes were studied using three typ es of aggregates, two of the mixes without silica fume and three with silica fume. The uniaxial compressive strengths varied between 60 to 1 30 MPa (8700 to 18,855 psi). An analytical model for the full stress-s train relationship for confined and uniaxially loaded concrete is empi rically developed and is shown to be applicable to a broad range of co ncrete strengths between 20 and 130 MPa (2900 to 18,855 psi). The main parameters required to establish the full stress strain relationship are peak stress, elastic modulus, strain at peak stress, stress and st rain at the inflection point of the descending curve, and, for confine d concrete, the stress and strain at one further point on the descendi ng curve. Empirical expressions for these parameters are developed tha t are functions of uniaxial compressive strength and level of confinem ent. Data on the form of the descending curve from the triaxial result s is used to extrapolate the descending curve for the uniaxial case. I t is argued that the uniaxial descending curve obtained in this manner is a better representation of the softening behavior under uniaxial c onditions than that obtained from standard cylinder tests.