COMPARATIVE MECHANICAL PROPERTY EVALUATION OF ULTRA-HIGH-PERFORMANCE CONCRETE REINFORCED WITH HIGH-STRENGTH STEEL STRAIGHT AND HOOKED FIBERS

Abstract

This study investigates the tensile properties and ductility of ultra-high-performance concrete (UHPC) reinforced with high-strength steel hooked fibers with a tensile strength of 445 ksi (3,070 MPa) and compares its tensile performance to that of UHPC reinforced with commonly used high-strength smooth steel microfibers with a tensile strength of 2,160 MPa (313 ksi). The research is based on a proprietary mix developed at the University of Texas at Arlington and investigates the mechanical properties of UHPC with varying fiber volume fractions of 1.5%, 2%, 2.25%, and 2.4%. The experimental results indicate that UHPC reinforced with high strength hooked fibers generally exhibits superior tensile strength and ductility compared to UHPC reinforced with straight, smooth steel fibers, except for the 1.5% fiber volume fraction. Moreover, this study examines the improvement of UHPC flowability through the inclusion of specific constituents such as spherical-shaped silica fume, sands, or fly ash. Enhancing flowability is crucial for facilitating the production of UHPC-based SIFCON (UHP-SIFCON). Additionally, the study investigates the influence of varying curing regimes on UHPC mechanical properties by examining factors like heat treatment temperature (60°C and 90°C) and delay time for heat treatment (immediately after casting or after a 24-hour delay). To determine the tensile strength of UHPC, direct tensile test (DTT) is performed, complemented by the four-point loading flexure test and double punch test (DPT) for comparison purposes. Furthermore, analytical investigations are performed to evaluate the accuracy of the assumptions in the ASTM 1609 beam test standard, while the DPT results are confirmed with the DTT results to ascertain their reliability in measuring tensile response. The outcomes of this study will contribute to the understanding of the mechanical bonding effect of hooked fibers on the tensile performance of UHPC and UHP-SIFCON, as well as contribute to the development of a more reliable testing method for evaluating UHPC properties.