Behavior of over-reinforced concrete beams including longitudinal voids in the compression zone reinforced with aluminum tubes

This study investigates the structural performance of over-reinforced concrete (RC) beams incorporating longitudinal compression-zone voids, a solution proposed to improve spatial efficiency for utility integration. However, such voids may significantly reduce strength and promote brittle failure. To address this issue, aluminum tubes (AT) are introduced as internal reinforcement within the voids. An experimental program comprising eleven beams (one solid, five hollow without reinforcement, and five hollow with AT) was conducted. Key parameters included void ratio (3.1–8%), number (single/double), orientation (vertical/horizontal), and AT ratio (0.75–1.8%). Results show that unreinforced voids reduce ultimate load capacity ( Pᵤ ) by up to 36.5% and energy absorption (EA) by 44.8%. In contrast, AT reinforcement significantly enhances performance, increasing Pᵤ by 57–135% and EA by 126–264%, while improving ductility and delaying brittle crushing. Horizontally oriented ATs achieved superior performance, with up to 10.8% higher Pᵤ compared to vertical configurations. Increasing AT ratio (≥ 0.75%) further enhanced post-cracking behavior with minimal effect on initial stiffness. The findings demonstrate that aluminum tubes effectively restore and enhance the structural efficiency of voided over-reinforced beams. For practical applications, void ratios up to 8% are recommended with internal AT reinforcement as a lightweight, corrosion-resistant alternative to conventional external strengthening methods.