Effect of fiber length and placement method on flexural behavior, tension-softening curve, and fiber distribution characteristics of UHPFRC

• Enhancement of flexural performance was achieved through increase of fiber length.
• Shorter fiber length exhibited better fiber orientation and dispersion and higher number of fibers in unit area.
• Improved flexural strength and fiber dispersion were obtained for the specimen with concrete placed at maximum moment region.
• Generalized tension-softening model of UHPFRC was suggested considering fiber length and placement method.

This study investigates the effect of fiber length and placement method on the flexural behavior, tension-softening curve, and fiber distribution characteristics of ultra-high-performance fiber-reinforced concrete (UHPFRC). Four different fiber lengths (Lf = 13, 16.3, 19.5, and 30 mm) were considered for two different placement methods. The ultimate flexural strength increased with increasing fiber length up to 19.5 mm, despite no noticeable difference in the first crack strength. Conversely, fiber length of 30 mm showed deterioration of flexural performance due to the decrease of fiber number existed across the crack surface. Both of first crack and ultimate flexural strengths were affected by the placement method; the specimen with concrete placed in the center (at maximum moment region) exhibited higher strength than that with concrete placed in the corner. The reasons were confirmed by image analysis that poorer fiber dispersion and fewer fibers across the crack surface were obtained for the specimen with concrete placed in the center than its counterpart. Finally, a tri-linear softening curve for UHPFRC was suggested based on inverse analysis and verified through comparison between the finite element analyses and the test data.