Implication of Fiber Distance on Pullout Behaviors of Steel Fibers in UHPC Under Impact Loads

Abstract

Fiber pullout tests provide information for a deeper understanding of the fiber behavior in fiber reinforced cementitious materials. This study examines the effects of the distance between fibers and geometry on the dynamic pullout behavior of steel fibers embedded in ultra-high-performance concrete (UHPC). To achieve this, three different geometries of steel fibers (straight, hooked, and twisted), four different distances between fibers (corresponding to fiber volume fractions of 1%, 2%, and 7% and a fiber bundle), and three different loading rates (static, 4, and 8 kN load) ranging from 0.018mm/s to 740.3 mm/s were considered. The test results indicate that the single straight fibers exhibit the most sensitive according to loading rate and get a largest load capacity dynamic increase factor (DIF), the DIFs are generally less for hooked, and twisted fiber. In multiple fibers specimen test, the straight and hooked fibers showed that the DIFs generally increased according to loading rate. However, as the distance between hooked fibers decreases, the matrix tends to be broken during the fiber pullout test and the DIF decreases. Multiple twisted fibers specimen exhibited the less sensitive response to loading rate compared to single twisted fiber test, and this is because the increased matrix spalling which deteriorate the load capacity of fiber pullout. However, the fiber bundle has an increased stiffness due to an increase of cross-sectional area, DIFs for deformed fibers bundle specimens increased notably even more than that of single fiber specimens.