Optimizing ultra-high performance cementitious functionally graded composite slab towards bullet impact resistance.

• The functionally graded composite slabs (FGCS) exhibit enhanced penetration and crack resistance. • The FGCS target is fabricated by one-time grouting cast to achieve improved interfacial bond. • A reduced perforation limit is observed for the FGCS target under in-service bullet impact. This study develops functionally graded composite slab (FGCS) by a one-time grouting approach based on the design concepts of ultra-high performance fibre reinforced concrete (UHPFRC) and two-stage concrete (TSC). The anti-penetration performance of the FGCS targets is investigated against 7.62 mm in-service bullets with shooting velocity of 843 m/s. The results show that while the TSC target presents superior bullet penetration resistance, it concurrently poses a significant threat of widespread cracking damage. Upon weighing the factors of bullet penetration and risk of cracking, the FGCS emerges as a more viable candidate for protective structures. The FGCS target is cast through a one-time grouting method, and there is no interface delamination under the projectile impact. The optimized FGCS exhibits enhanced in-service bullet impact resistance and a reduced safety thickness of 80 mm compared to the normal UHPFRC. [ABSTRACT FROM AUTHOR]