Aging infrastructure is increasingly costing taxpayers due to increased repair and replacement costs. Ultra-high performance concrete (UHPC) has recently been recognized as a viable material for both the repair of concrete and steel infrastructure as well as a replacement material for new structures due to its enhanced mechanical and durability properties. Such uses require a much better understanding of the multiaxial tensile properties of UHPC to utilize the material more efficiently. This study focused on developing a novel apparatus capable of subjecting specimens to tensile forces in each of the three principal directions simultaneously. Such an apparatus could collect data for a portion of the failure surface that currently only has a small dataset to establish trends. The “Looney Bin” was designed to test 50-mm cube specimens in triaxial tension, biaxial tension, tension-compression, and tension-tension-compression stress states. Once the apparatus and fixtures were designed and fabricated, trial tests were conducted on a non-proprietary UHPC without steel fibers to establish a test method for each of the stress states evaluated. Data were then collected for different stress states using the established procedures and plotted against previously published failure models for UHPC to verify that the collected data were reasonable.

The tensile strength of conventional concrete is often ignored in structural design due to its relatively low value when compared to compressive strength. However, when designing ultra-high performance concrete structures, ignoring tensile strength is overly conservative and could lead to inefficient designs. Uniaxial tensile data is available for UHPC mix designs, but very little data exists examining the multiaxial tensile strength of the material. To help fill this data gap, an apparatus was designed at the University of Oklahoma capable of applying multiaxial tensile stress states to a cube specimen. A UHPC mixture developed at the University of Oklahoma with fiber contents of 0%, 1%, 2%, 4%, 5%, and 6% by volume was characterized in this apparatus to assess the effect varying the fiber content had on its multiaxial tensile strength. The data collected was compiled and compared to previously published multiaxial compressive UHPC data. This dataset was then used to fit two previously published models with different meridian shapes (parabolic and cubic) to ascertain which shape best describes the failure criteria of UHPC in the tensile region. It was found that triaxial tensile strengths decreased as the fiber dose increased over 4%. Also, the combined dataset fit a cubic meridian shape more closely than parabolic.