Contribution of fiber orientation to enhancing dynamic properties of UHPC under impact loading

This paper investigates the contribution of fiber orientation to enhancing dynamic properties of ultra-high performance concrete (UHPC) under impact loading. Dynamic properties of 27 cylindrical core samples taken from prisms that have predominantly perpendicular, random, and parallel fiber orientations relative to the loading direction were tested. The Spilt Hopkinson Pressure Bar test method was used with strain rates of 160–290 s −1. Fiber orientation of samples after impact was evaluated using computed tomography scan. Test results indicated that the dynamic compressive strength, peak strain, and energy absorption capacity increased by up to 65%, 105%, and 295%, respectively, with the increase in strain rate. For a given strain rate, samples with predominant fiber orientations perpendicular and parallel to the loading direction showed the highest and lowest dynamic properties, respectively. Such spread in dynamic compressive strength, peak strain, and energy absorption capacity due to fiber orientation was up to 40%, 90%, and 135%, respectively. Fiber orientation coefficients along the direction of tensile stress were around 0.7 and less than 0.3 for samples with predominantly perpendicular and parallel fiber orientations, respectively. Good relationships were established to estimate the dynamic properties of UHPC given the quasi-static compressive strength, strain rate, and fiber orientation. A constitutive model to estimate the stress-strain curves of UHPC subjected to impact loading was developed. The model considers the influence of fiber orientation and strain rate. The model was successfully validated using the experimental data and was found to provide accurate prediction of the ascending portions of the stress-strain curves of UHPC under impact loading. The maximum spread between predicted and experimental dynamic compressive strength was limited to 10%.