Effects of fiber type and specimen thickness on flexural behavior of ultra-high-performance fiber-reinforced concrete subjected to uniaxial and biaxial stresses

In this study, we investigated the effects of steel fiber type and specimen thickness on the uniaxial and biaxial flexural behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC). For this purpose, three types of steel fibers (straight, three-times twisted, and six-times twisted) and three thicknesses of specimen (24, 48, and 72 mm) were used. The test results indicated that, owing to the larger perimeter of the triangular shape and mechanical anchorage effect, the twisted steel fibers exhibited better pullout resistance than the straight steel fiber with a circular shape, and its effectiveness increased with the number of ribs. In contrast, the best flexural behavior of UHPFRC was observed when the straight steel fiber was used under both uniaxial and biaxial stress states, and the six-times twisted steel fiber exhibited the worst flexural performance owing to the excessive bond strength of the composites. The uniaxial and biaxial flexural strengths of UHPFRC were insignificantly influenced by the sample thickness; however, the normalized toughness decreased with an increase in the thickness. A higher flexural strength, normalized toughness up to the peak, and deformability were observed under the biaxial flexural stress state than those under the uniaxial flexural stress state. The use of twisted steel fibers was more effective for slabs subjected to biaxial flexural stress than that for uniaxial beams.