Ultra-high performance concrete (UHPC) is an extremely durable type of concrete that includes high volumes of cementitious material, fine aggregate, superplasticizer, and steel fibers with an overall low water to cement ratio and has mechanical properties that significantly exceed those of typical concrete. Researchers at the University of Oklahoma (OU) have developed a non-proprietary version of UHPC using materials available locally in Oklahoma under the name of J3 with properties similar to commercially available UHPC mixes. While all other ratios of materials in the mix have been established, the effect of different percentages of steel fibers on material properties were yet to be tested. The research described in this thesis includes varying the steel fiber concentration within the mix to identify how different amounts of fibers – more specifically 0, 1, 2, 4, and 6% – affect flow, compression, and tension properties of this non-proprietary UHPC mix. The goal of this research was to determine which percentage of fibers resulted in the best performance and identify the best concentration of steel fibers for a particular structural application. This was done by putting specimens with each percentage of fibers through rigorous testing including flow, compressive strength, modulus of elasticity, splitting tensile strength, flexural strength, and freeze-thaw.

Another objective of this study was to determine if it would be plausible to recreate the performance of the OU J3 mix even when using materials from other locations. The mix was recreated using sand, slag, and cement from sources available locally in Florida and provided by collaborators at Florida International University (FIU) who were partners for this phase of the research. Flow, compressive strength, and flexural strength were tested for specimens cast from this mix of J3.

The results of the research suggest that for both material locations increasing the percentage of steel fibers increases compressive and tensile strength with diminishing returns for higher fiber contents. Of those tested, the optimal mix for most applications would contain 2% steel fibers by volume, while for extreme cases where additional compressive or tension strength is required higher percentages could be used. The results also revealed that while there was an increase in strength between the material properties at 28 and 56 days of age, this difference decreased with increase in fibers. Additionally, both the OU J3 and FIU J3 mixes behaved similarly for all tests performed indicating that the mix design is repeatable using materials from other sources.