Energy-Based Method for Evaluating Cracks and Resistance of Fiber Reinforced 1 Ultra-High Strength Concrete under Impact Loads 2 Mehran Aziminezhad 1 , Sahand Mardi 2 , Abolfazl R. Shourestani 2 , Pouria Hajikarimi 2,* , Fereidoon 3 Moghadas Nejad 3 4 1 Department of Civil Engineering, Imam Khomeini International University, Qazvin, Iran 5 2 Department of Civil Engineering, Qazvin Branch, Islamic Azad University, Qazvin, Iran 6 3 Department of Civil & Environmental Engineering, Amirkabir University of Technology, Tehran, Iran M X Abstract: c In order to adjust the lack of sufficient ductility of ultra-high strength concrete (UHSC), different types of 10 fiber were used in this study. This research investigates the effect of glass, polypropylene and steel fibers 11 on the impact resistance and crack propagation of fiber reinforced UHSCs by implementing slab 12 specimens with a dimension of 300×300×30 mm. The experimental program includes 18 specimens with 13 1%, 1.5% and 2% of concrete volume for each type of fiber which was made with two different mixing 14 methods (Ordinary fiber reinforced concrete (FRC) and high performance fiber reinforced concrete 15 (HPFRC)). In this study, specimens were placed under a low-velocity impact loading (5.42 m/s) within a 16 fixed rigid constrained setup. The health index and the crack propagation correlation are two criteria for 1M determining the trend of degradation and impact resistance reduction. Results demonstrate that the FRCs 1X show higher impact resistance in comparison with the HPFRC because the HPFRC method doesn’t 1c provide enough cohesion between concrete and fibers. The obtained results also show that FRC 20 specimens include polypropylene, endure higher impact resistance with a greater amount of health index 21 rather than other specimens. By increasing the fiber’s volume in the specimens fabricated with glass and 22 polypropylene, a more homogenous composite was formed and energy spread more uniform over all 23 faces of FRC specimen. 24 Keywords: 25 UHSC; fiber reinforced concrete; impact resistance; crack propagation 26 2M ACCEPTED MANUSCRIPT