Direct tension testing of SFRC - Some peculiar effects of the end restraints

In principle, a direct tension test is the ideal test that should be used in experimentally determining the softening, or residual, parameters of steel fibre reinforced concrete (SFRC). In the context of design, these material properties are then used to feed into models for shear, flexure etc. This is the approach that has been adopted in the recently released standard for the design of concrete structures in Australia (AS3600-2018). However, there are many parameters which may influence the results of the uniaxial tension test, and the choice of boundary conditions for the test is one of the most relevant ones. Three boundary or end conditions are possible: fixed-fixed, fixed-rotating, and rotating-rotating. In this paper, results of uniaxial tensile tests on twelve identical SFRC direct tension specimens tested with the end conditions listed above are presented. Each condition exhibits behaviours not present in the theoretically ideal tensile softening curve and investigating this is the focus of the present study. It is concluded that the fixed-rotating end conditions serves as a compromise to the issues associated with the other test setups and seems to be more suited for uniaxial tension testing of softening SFRC