Pullout Behavior of High-Strength Steel Fibers Embedded in Ultra-High-Performance Concrete

Research on the pullout behavior of single steel fibers embedded in ultra-high-performance concretes (UHPCs) was conducted to investigate the bond properties of straight and deformed steel fibers. The main research objective was to compare the physicochemical interfacial bond properties between brass-coated straight steel fibers and the ultra-high-performance cementitious matrix with the mechanical bond properties of hooked-end and twisted steel fibers embedded in the same matrix. The results show that the enhanced bond properties provided by the ultra-high-performance cementitious matrix led to the failure of fibers having a high mechanical bond component. Tailoring of the fiber strength and mechanical bond to the matrix strength is needed for optimal pullout behavior. It is observed that the equivalent bond strength of deformed fibers embedded in UHPC reaches up to 47 MPa (6.8 ksi)-that is, almost five times the equivalent bond strength of straight fibers (10 MPa [1.4 ksi]) embedded in the same matrix. Furthermore, the equivalent bond strength of straight steel fibers, which are commonly used in ultra-high-performance fiber-reinforced concrete (UHP-FRC), can be doubled to a value exceeding 20 MPa (2.9 ksi) by optimizing the UHPC matrix through composition and particle size distribution, leading to an atypical pullout load-slip-hardening behavior. Such behavior is desirable for high tensile strength, high-energy-absorbing, strain-hardening UHP-FRC.