Size Dependent Axial and Lateral Stress Strain Relationships for Actively Confined Concrete

The axial and lateral stress/strain behaviour of concrete under compression, both in the ascending and descending branches, is crucial in determining both the strength and ductility of reinforced concrete members, particularly when confined. Both the axial and lateral stress/strain relationships for concrete under any degree of confinement are generally considered as material properties which are usually determined directly from compression tests of cylinders or prisms. It is also widely recognised that these material properties are both dependent on the size and shape of the specimen from which they are extracted and on the method of measurement of strains. In this paper, the global deformation of concrete subjected to compressive forces is considered to comprise of two separate components; the local deformation of the material which is size independent; and the local deformation due to the shear-friction mechanism associated with the formation of wedges which is size dependent. It is shown that by separating the size independent and size dependent components of the deformation, it is possible to derive size dependent stress-strain relations for confined concrete from tests using one specimen size and to derive the dilatory deformation directly from the axial deformation. This should considerably reduce the amount of testing required for new concretes as only one size of specimen need be tested to obtain stress-strain relationships for all sizes. The proposed approach has been used to reanalyse 692 published test results on confined concrete to provide size dependent stress/strain relationships for both axial and dilatory strains in both the ascending and falling branches and for a wide range of confinements. It is also shown that the allowance for the size dependent component of deformation can substantially reduce the scatter.