Impact of synthetic fiber use on the mechanical properties of ultra high performance concrete

Ultra High Performance Concrete (UHPC) is a new generation of fiber reinforced concrete with high compressive and tensile strength, high post-cracking deformation capacity and superior durability. The fiber content is one of the most important factors in achieving these superior mechanical properties of UHPC. Although steel fibers are the widely used fiber type in UHPC due to their advantages such as high strength and stiffness, they have disadvantages such as being exposed to corrosion, high specific gravity, and high cost. For this reason, studies on the use of different fiber types have increased. In this study, it is aimed to investigate the usability of synthetic fibers for UHPC in terms of mechanical properties. For this purpose, an experimental campaign was prepared on UHPC mixtures having macro polypropylene fibers with different volume ratios and the effects of synthetic fiber usage on the mechanical properties of UHPC were investigated. The compressive and splitting tensile strengths of the mixtures were determined by axial compression and splitting tensile tests on cube and cylinder specimens. In order to investigate the flexural behavior of the mixtures, notched beam specimens were produced and three-point flexural test were performed. The experimental load-deflection and load-crack mouth opening displacement relationships of the beams were obtained and the contribution of synthetic fibers to the flexural behavior was evaluated in terms of flexural tensile strength and fracture energy parameters. Two-dimensional digital image correlation was applied during the test to examine the crack behavior and fracture modes of the beams. As a result, it was observed that the use of polypropylene fibers had no significant impact on the compressive and splitting tensile strength of UHPC. However, significant increases in both flexural tensile strength and energy absorption capacity were obtained, especially when high volumetric fiber ratios were used. When the cracking behavior of the beams was examined, it was determined that polypropylene fibers could exhibit an effective cracking control in UHPC.