# Pullout Response of Ultra-High-Performance Concrete with Twisted Steel Fibers
## 1 Introduction  
Fibers with different types, shapes, aspect ratios have been used to enhance the mechanical properties of concrete regarding its fracture toughness, flexural strength, tensile strength and energy absorbing capacity under impact [1; 2; 3; 4; 5; 6; 7; 8]. This improvement of mechanical properties is achieved by producing the strain-hardening behavior and by increasing the strain capacity of the fiber-mixed composite with supplementary cementitious materials such as silica fume and ground granulated blast-furnace slag. It was reported that the maximum compressive strength and tensile strength of the steel fiber reinforced concrete (FRC) could be as high as 292 MPa and 37 MPa, respectively and that the strain at peak stress up to 1.1% was obtained [2]. These values are at least 5-10 times higher than those of the conventional concrete without steel fibers. As a result, this ultra-high-performance fiber reinforced concrete (UHPFRC) has been widely used in different structural members such as slabs, beams, columns and beam-column joints for various construction applications [9]. Due to the superior performance and the increased applications of UHPFRC, extensive studies have been performed to understand the mechanical properties of UHPFRC and the corresponding reinforcement mechanism. The current studies focus on the optimization of the factors that influencethe fiber-matrix bond strength, cementitious matrix packing density and the tensile response of UHPFRC through either the direct tensile tests of dog bone specimens or the flexural bending tests. These factors mainly include fiber type [10; 11], fiber volume fraction [11], fiber embedment length [12], fiber morphology [1; 12; 13; 14], fiber orientation [15; 16], loading rate [3; 13], cementitious material property [17], fiber-matrix interface property [12] and curing of cementitious material [12]. For example, [PERSON] et al. [3] showed that energy absorption capacity significantly increased with the fiber volume fraction of up to 3%. It should be noted that most of the above studies adopt material-level experimental studies which however are often expensive and difficult to carry out, particularly at meso/micro scales [18]. For instance, it is challenging to carry out strain measurements on short discontinuous fibers, causing the difficulty in developing the bond-shear stress-slip relationship between fibers and cementitious matrix. This could limit the detailed understanding of material's damage mechanisms and thus considerably limit the wider application of fibers. Compared to the smooth or hooked steel fibers used as reinforcement in high-performance cementitious composites, a relatively new type of steel fibers with twisted morphology has been proposed to further enhance the mechanical properties of HPFRC [1; 11; 13]. This twisted steel fiber with optimized section geometry (triangular and square section) was experimentally explored regarding its superior performance in contributing the pull-out load and strain hardening capacity. Note that the pull-out test is commonly used to evaluate the bond strength, pull-out response and strain hardening behavior. The twisted fibers could theoretically provide a larger lateral surface area than smooth or hooked steel fibers and thus increase frictional and adhesive bond forces at the fiber-matrix interface. Consequently, the twisted fibers generated peak pullout forces on the order of 3-5 times those of straight, smooth fibers [1]. Note that their high-performance-concrete strength values are generally less than 100 MPa. Due to the complexity of fiber morphology, the fabrication of the fiber materials becomes challenging. For example, only the rectangular-section twisted steel fibers are available in Chinese material market, leading to the difficulty in investigating a variety of twisted steel fiber geometries in experimental studies. Also, the pullout test with detailed measurements on fiber strain, bond shear force, untwisting torque at meso/micro scales is still in an early stage.