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This paper presents the pull-out characteristics of inclined hooked steel fiber from cementitious matrix. The effect of fiber embedded length and angle of inclination are evaluated together with the interaction of these parameters. The experimental program involved single fiber pull-out test of five inclination angle and four embedded length. The studied inclination angles were 0, 15, 30, 45 and 60 degrees. The embedded lengths were 10, 15, 20 and 25 mm. Compressive strength of matrix was 40 Mpa. The length and diameter of hooked steel fibers were 50 mm and 1mm, respectively and their tensile strength was 800 Mpa. At least five specimens were prepared and tested for each combination of inclination angle and embedded length. A special mold supplemented by a cross shaped device was designed to hold the fiber in desired angle and embedded length. X-ray radiography was used to verify the inclination angle and embedded length of fiber. All the specimens were tested at 28-day age. Pull-out test performed under displacement control condition in order to record descending branch of pull-out curves. A load cell and a displacement transducer were used to acquire pull-out load and slip during pull-out test. Pull-out load versus slip were recorded and parameters such as maximum pull-out force and its associated slip, pull-out energy, fiber efficiency and matrix spalling were drawn for comparison purpose. Based on the experimental results, the pull-out response of hooked steel fibers is predominately influenced by fiber embedded length and inclination angle. The results indicate that the peak pull-out load is maximized at approximately 30 degrees, although at greater inclination angle, the peak pull-out load decreases. The fracture load also decreases as fiber inclination angle increases. The additional shear stress imposed on inclined fibers; provide mechanisms favoring slip between the crystals in the steel. This causes a reduction in both yield and ultimate strength of the finer, resulting in a reduced fracture load. The results indicate that providing the hook is fully mobilized, the peak pull-out load is almost independent of embedded length of fiber. The results indicate that fracture of fiber is more presumable at greater inclination angle. Slip associated with peak pull-out load increases as the inclination angle increases. This can be attributed to matrix spalling. Matrix spalling also causes the drop of pull-out load in pull-out curves. The load drop is directly related to the size of crushed matrix. Matrix starts to spall at 30 degrees inclination angle. The results indicate that increase in embedded length and inclination angle result in increase of pull-out energy. An inclined fiber with respect to the loading direction absorbs a greater amount of energy at a given slip than an aligned one, with maximum pull-out energy occurring around 30 degrees. Fiber efficiency increases as the embedded length of fiber increases. Maximum fiber efficiency occurs at 30 to 45 degrees and decreases at greater inclination angle. The effect of elastic deformation of fiber during pull-out test was taken into account by calculation of elastic deformation and subtracting from slip, although, its effect was negligible.

Keywords: Hooked Steel Fiber, Pull-out Behavior, Cement-based Matrix

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