Response of hybrid-fiber reinforced concrete slabs to hard projectile impact

In the present study, the effectiveness of hybrid-fibers (a combination of steel and plastic fibers) in improving the impact resistance of slabs was studied through a detailed experimental program. A total of 54 hybrid-fiber reinforced slabs were cast in the two groups; each group containing 27 slabs. The specimens of the first group were cast using normal strength concrete, whereas specimens of second group were cast using high strength concrete. All the slabs were 600 × 600 × 90 mm and contained different proportions of steel and plastic fibers. Out of a total number of 54 slabs, three slabs in each group were used as control specimens i.e. without fibers. The impact penetration tests were carried out using an air-gun system. The projectiles were made of hardened steel and were bi-conic in shape. Failures of the specimens were observed and size of the front and the rear face craters and the penetration depths were measured. The test results showed that the hybrid-fibers in the concrete lead to smaller crater volumes and reduce the spalling and scabbing damage. The hybrid-fibers arrest the crack development and thus minimize the size of the damaged area. The penetration depth and perforation thickness were predicted by modifying the impact function of NDRC equation to incorporate the effects of hybrid-fibers. The ballistic limit was also predicted. A simple formulation was then proposed for the prediction of the ejected concrete mass from the front and the rear faces of the specimens. Predictions matched well with the experimental observations.