Tensile Behavior of Ultra-High Performance Concrete Under Direct Tension

Abstract

Ultra-High Performance Concrete (UHPC) is a product that has improved mechanistic properties as compared to traditional concrete and is defined by the Federal Highway Administrations (FHWA) as having a post-cracking tensile strength of 0.72 ksi with a minimum compressive strength of 21.7 ksi. UHPC has constituents that are used to improve the various properties of the concrete. One such constituent is steel fibers, which help to improve the flexural strength of the concrete and the post-cracking strength. There are a variety of different UHPC mix designs, but for this study, the non-proprietary J3 UHPC mix from the University of Oklahoma was used. This study also sought to determine the optimum fiber content in UHPC for strain hardening and to evaluate whether the cylindrical dogbone specimen offers the greatest potential for the Direct Tension Test (DTT). The cylindrical dogbone was chosen because it has reduced stress concentrations and uniform stress distribution compared to the typical prismatic FHWA specimen. This study examined the effects of different fiber contents (0% to 6%) on the tensile capacity and behavior of UHPC. Upon completion of the testing, it was found that 4% fiber content had the best overall tensile performance for the J3 UHPC mix. A study was also completed in order to show the stress distribution in three different specimen types – a cylindrical dogbone specimen, a 2 inch x 4 inch cylindrical specimen, and a 2 inch x 2 inch x 17 inch FHWA specimen. All three specimens were tested in direct tension and modelled in ANSYS. The results showed that the cylindrical dogbone specimen had lower stress concentrations as compared to the FHWA specimen and improved bonding with the end anchorages compared to the cylindrical specimen due to the enlarged end regions. The cylindrical dogbone specimen is recommended for tension testing of UHPC.