Improvement of flexural and tensile strength of layered-casting UHPC with aligned steel fibers

Abstract

In order to further improve flexural and tensile strength as well as ductility of ultra-high performance concrete (UHPC), this research developed an improved device for casting layered UHPC with better alignment of the fiber. The effect of fiber content (2% or 3%) and fiber type (straight fiber, end hooked fiber and combining the two fibers) on the flexural strength was first discussed. The optimal fiber content and fiber type were used to study the effect of fiber alignment on the mechanical performance of UHPC under flexural and direct tension loading. Through image analysis, the specific influence of fiber aligning on fiber orientation and dispersion in UHPC was analyzed in detail. The results showed that the vibration effect of the improved device significantly improved the slurry fluidity, and the movable mold provided additional drag force during the casting process to ensure the effectiveness of fiber aligning. Fiber aligning not only improved the ultimate tensile and flexural strength of UHPC, but also significantly improved the cracking strength and ductility of UHPC due to the orientation of fibers parallel to the casting direction after fiber aligning. However, the UHPC after fiber aligning showed anisotropy, and had a higher probability to develop diagonal cracks under tension. In addition, fiber aligning made the overall fiber dispersion uniform, while it led to the uneven fiber distribution at the interlayer boundary and the weak areas formed there.