Size-Dependent Stress-Strain Model for Unconfined Concrete

The stress-strain behavior of concrete under compression, both in the ascending and descending branches, is crucial in determining both the strength and ductility of reinforced concrete members. This material property is generally determined directly from compression tests of cylinders or prisms. However, it is widely recognized that this material property depends on both the size and shape of the test specimen and the method of measurement. This paper shows that concrete deformation because of compression is both a material property and a shear-friction mechanism and that by taking both of these deformations into account it is possible to derive a stress-strain relationship that is size-dependent. This paper also shows how the stress-strain from cylinder tests of one specific length can be modified to determine the stress-strain relationship for any size of a cylinder. With this new procedure, the authors reanalyzed the results from 380 published tests on unconfined concrete to extract size-dependent strains at the peak stress and then used these results in existing curve-fitting models to produce size-dependent stress-strain models for unconfined concrete. This paper shows how these size-dependent stress-strain models can be used in a size-dependent deformation-based approach to quantify both the strength and ductility of reinforced concrete members.