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.
The local bond stress slip behavior is a fundamental property required for the analysis and design of concrete structures at both serviceability and at ultimate limit. The addition of fibers has been shown to significantly improve the bond between normal strength concrete and steel reinforcement but little work has investigated the bond between reinforcing and ultra high performance fibre reinforced concrete (UHPFRC). In this paper a series of 69 pull tests are carried out on UHPFRC with either short straight or long hooked steel fibers including mixes where the two fiber types have been blended. The results of this study were combined with the results from existing literature to regress a material model for the bond slip behavior between UHPFRC and ribbed steel reinforcing bars. Importantly, it is shown that models for normal strength fiber reinforced concrete cannot be extrapolated to UHPFRC.
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