Effect of inclination angle on hooked end steel fiber pullout behavior in ultra-high performance concrete

The bond relationship between the concrete matrix and steel fiber is a significant factor that affects the performance of ultra-high performance fiber reinforced concrete (UHPFRC). In the present research, pullout performances of hooked end fibers embedded in ultra-high performance concrete matrix under various inclination angles are systematically investigated, with special attention on fiber dimension and embedded length. Pullout load-slip curves are obtained and experimental observations including complete fiber pull-out, fiber rupture and matrix failure are analyzed in detail. The effects of the pullout angle are then studied quantitatively by parameter calculations and mechanism analysis. A new analytical model for evaluating the snubbing and spalling effects of the hooked end steel fiber is proposed and validated. It is shown that the influences of the inclination angle on the peak pullout load vary with different fiber types, embedded lengths and fiber diameters, which are also associated with the occurrences of the fiber rupture and the matrix failure. In addition, optical microscope and scanning electron microscopy observations at mesoscale are performed to further analyze the effects of orientation angle.