Steel fibers are a critical component of ultrahigh performance concrete (UHPC). They are added to give the material its characteristic strength, toughness, and ductility. Fibers are the most expensive component of UHPC and account for up to 72% of the raw material cost. The steel fibers in current use have a smooth surface that reduces their ability to effectively bond with the UHPC matrix and therefore requires them to be used in high dosage at commensurately high cost. This project developed a novel type of steel fiber with micron-scale striations on their surface to dramatically enhance their bond with UHPC. The initial research effort focused on identifying the optimal striation pattern and developing a viable technique to manufacture the fibers in commercial volume. An experimental testing program was then conducted to assess the effectiveness of the new surface modification technology. It was shown that the surface striations delivered a 130% increase in fiber pull out capacity and 235% increase in pull out energy over smooth fibers. This translated to a 21% increase in flexural strength, 62% increase in flexural ductility, and 62% increase in the direct tensile ductility of UHPC (the so-called localization strain). When used at a reduced dosage of 1% by volume, i.e., half the regular dosage, the new fibers resulted in a performance that was on par with non-modified fibers at full 2% dosage. Once the new fiber technology was perfected, a demonstration effort was undertaken to showcase its capabilities. The demonstration effort focused on the Bricker Road bridge over the Quackenbush Drain, which is owned by the St. Clair County Road Commission (a research partner). The bridge had a deteriorated reinforced concrete slab deck that was due for replacement. A set of replacement UHPC deck panels reinforced with the new fibers were designed and constructed. The UHPC panels were only about one third of the weight of the original reinforced concrete panels. Data collected during the project demonstrated that mixing the new fibers into the UHPC went smoothly and that the resulting field mix’s spread was comparable to laboratory-mixed UHPC. Tension coupons and compression cube samples collected during the field trial showed that the field-mixed UHPC achieved mechanical properties that were just below the properties of lab-mixed UHPC, but still well above the design values. The demonstration bridge is the first bridge in the US in which the entire deck was made with open-recipe (non-proprietary) UHPC mixed in a traditional ready-mix truck. It decisively demonstrates the potential for using common concrete equipment for mixing UHPC. This paves the way for broader utilization of UHPC across the United State. The results of this project also indicate that by using the new steel fibers, it is feasible to reduce the dosage of steel fibers to 1% to 1.5% by volume from the traditional 2% by volume. Since steel fibers are the most expensive component of UHPC, reducing fiber dosage this way will profoundly reduce the upfront cost of UHPC and hence broaden its appeal and usage.
The final report is available.