Enhancing structural protection of reinforced concrete structures against high dynamic loads using post-installed UHPFRC/UHPFRSC

Abstract

Recently there has been an observed change in security policy across the globe which implies that humanity must prepare for hybrid and asymmetric acts of war. This security situation will result in spontaneous attacks from non-military groups. The threats include ballistic attacks from commercially available weapons, detonation of explosives, and impact of vehicles. These attacks are focused on critical infrastructures. Reinforced concrete (RC) elements with ballistic and explosion protection are of huge relevance. This requires new concepts and innovative solutions for the protection of new buildings and/or for the strengthening of existing structures. In this paper a method for the increase of structural safety is shown using Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) or sprayed UHPFRC, so called UHPFRSC, as a surface layer for reinforced concrete structures. The UHPFRC/UHPFRSC is applied in thin layers of 30 - 80 mm on the protection side. The interaction of the two different cementitious materials results in increased resistance to high dynamic loads. The traditional reinforced concrete on the impact side absorbs a major fraction of the energy of impact. On the back side (protection side), tensile forces are generated by the impact, which can excellently be handled by the UHPFRC/UHPFRSC. For proof of concept of the hybrid strengthening method, tests were carried out on UHPFRC - RC composite elements. In the test campaign, ballistic tests were performed using test specimens with dimensions of 500 × 500 mm. The layer thicknesses were 120 mm reinforced concrete and 40 mm UHPFRC. Additional steel rebars in the UHPFRC layer were applied for half of the specimens. The investigation demonstrated the effectiveness of the strengthening method, showing high resistance to high-dynamic loads. This behavior is attributed to the absorption and transfer of tensile stresses by the UHPFRC, that reduces penetration depth and damage, ensuring remaining load-bearing capacity after gunfire.