Pullout Response of Ultra-High-Performance Concrete with Twisted Steel Fibers

This paper aims to develop a pullout force formula and increase the understanding of the damage mechanisms of ultra-high-performance fiber reinforced concrete (UHPFRC) with twisted steel fibers (TSFs) through a pull-out test and finite element analysis (FEA). The formula was first obtained through a theoretical force analysis with model assumptions that are based on the experimental data in the literature. A microscale in-situ X-ray computed tomography (µXCT) was used to prepare 3D images of the cross-section of concrete before and after TSFs with three embedment lengths were pulled out. The tested pullout force values were used for comparison with the developed formula values. The µXCT images show the concrete matrix was preserved after the TSF was pulled out, indicating the stable pullout force values at the strain hardening stage was mainly caused by the fiber untwisting. FEA results show this untwisting behavior occurs on the effective untwisting length of TSF close to the exterior concrete surface. The theoretical formula values were found match well with the testing data. The developed formula is potentially used to analyze the pullout behavior of TSF with different geometries; thus, the design of the UHPFRC with TSFs can be optimized in the field.