As the most widely used material in protective structures, a better understanding of the resistance of reinforced concrete (RC) to high-speed projectile striking is needed. It has been accepted that concrete material possesses limited impact resistance in virtue of its poor energy absorption capacity. To improve the performance of RC structures under impulsive loading, affixing externally-bonded Fiber Reinforced Polymer (FRP) sheets on structural elements is a practical method. Due to the emphasis of most existing literature on the penetration behavior of projectiles, the studies of impact-induced spalling damage on normal and FRP strengthened RC plates are not sufficient and in-depth. In this paper, the local response of un-strengthened and FRP strengthened RC plates under projectile impact is systematically investigated with experimental approaches. In consideration of the complexity of transient behavior as well as multiply parameters involved, empirical approaches based on dimensionless analysis are proposed to predict the diameter and depth of spalling craters on normal RC plates and strengthened ones with frontal CFRP layers, which fill current research gap and are potential tools for damage assessment and repair. The validity of the proposed approaches is verified by test results from the present and published studies.
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