Penetration calculations of a given projectile into a reinforced concrete slab are commonly based on its penetration depth into a semi-infinite target, which is inversely proportional to the square root of the concrete compressive strength. A procedure of that kind is applied in the modified NDRC penetration equations, which are widely quoted in the literature as being able to predict well empirical results. However, simple extrapolation of these equations that were developed from normal strength concrete (NSC) tests to predict the impact resistance of High Strength Concrete (HSC) has no experimental support, and may be unsafe. This paper discusses some experimental results of HSC specimens and proposes a modified expression to represent the concrete strength effect that covers both NSC and HSC. Test results that were reported in the literature of striking and of post perforation projectile velocities with medium and high strength concrete barriers were analyzed. The analysis indicates a way to modify the concrete strength coefficients in present formulae (e.g., the coefficient 'K' in the NDRC formulae). The proposed modification reflects the different tensile-to-compressive strength relation of HSC as compared to NSC. Consequently, the discrepancy between experimental and theoretical results of HSC resistance to impact is reduced when the modified strength coefficient is used. This is shown by the relatively good agreement between measured penetration depths and their theoretical predictions, which has been obtained when this modification was applied. The proposed approach can be further confirmed, as additional experimental data of HSC resistance to non-deforming projectiles impact become available.
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