Projectile impact and drop weight resistance of functionally graded fiber-reinforced magnetite aggregate concrete

Many researchers have focused on the behavior of fiber-reinforced concrete (FRC) in the construction of various defensive structures to resist against impact forces resulting from explosions and projectiles. However, the lack of sufficient research regarding the resistance of functionally graded fiber-reinforced concrete against projectile impacts has resulted in a limited understanding of the performance of this concrete type, which is necessary for the design and construction of structures requiring great resistance against external threats. Here, the performance of functionally graded fiber-reinforced concrete against projectile impacts was investigated experimentally using a (two-stage light) gas gun and a drop weight testing machine. For this objective, 12 mix designs, with which 35 cylindrical specimens and 30 slab specimens were made, were prepared, and the main variables were the magnetite aggregate vol% (55%) replacing natural coarse aggregate, steel fiber vol%, and steel fiber type (3D and 5D). The fibers were added at six vol% of 0%, 0.5%, 0.75%, 1%, 1.25%, and 1.5% in 10 specimen series (three identical specimens per each series) with dimensions of 40 × 40 × 7.5 cm and functional grading (three layers), and the manufactured specimens were subjected to the drop weight impact and projectile penetration tests by the drop weight testing machine and gas gun, respectively, to assess their performance. Parameters under study included the compressive strength, destruction level, and penetration depth. The experimental results demonstrate that using the magnetite aggregate instead of the natural coarse aggregate elevated the compressive strength of the concrete by 61%. In the tests by the drop weight machine, it was observed that by increasing the total vol% of the fibers, especially by increasing the fiber content in the outer layers (impact surface), the cracking resistance and energy absorption increased by around 100%. Note that the fiber geometry had little effect on the energy absorption in the drop weight test. Investigating the optimum specimens showed that using 3D steel fibers at a total fiber content of 1 vol%, consisting of a layered grading of 1.5 vol%, 0 vol%, and 1.5 vol%, improved the penetration depth by 76% and lowered the destruction level by 85%. In addition, incorporating the 5D steel fibers at a total fiber content of 1 vol%, consisting of the layered fiber contents of 1.5%, 0%, and 1.5%, improved the projectile penetration depth by 50% and lowered the damage level by 61% compared with the case of using the 3D fibers.