Multi-scale behaviour of hybrid fiber reinforced recycledaggregate concrete: Mechanical and microstructural analysis

Abstract

The use of recycled aggregate concrete (RAC) offers a sustainable solution for construction; however, its structural performance is often limited by weak interfacial transition zones (ITZ), increased porosity, and brittle failure behaviour. To address these limitations, the present study investigates the effectiveness of a multi-scale hybrid fiber reinforcement system incorporating steel fibers (SF) and polypropylene fibers (PP) in RAC. Concrete mixes with 25% and 50% recycled aggregate replacement were developed and reinforced with mono (SF0.5) and hybrid fiber combinations (SF0.75–PP0.25 and SF1–PP0.5). The mechanical behaviour was evaluated through compressive, splitting tensile, and flexural strength tests, along with detailed stress–strain analysis under uniaxial loading. The results demonstrated that hybrid fiber reinforcement significantly enhanced compressive strength (up to ~45 MPa), tensile capacity, and post-peak ductility compared to plain RAC. Digital image processing revealed a transition from localized cracking in control and RAC mixes to distributed microcracking in hybrid fiber systems, indicating improved crack control and stress redistribution. Microstructural analysis using scanning electron microscopy (SEM) confirmed the refinement of the ITZ and effective fiber–matrix interaction, where polypropylene fibers controlled microcrack initiation and steel fibers bridged macro cracks. Durability performance was assessed through water absorption and acid attack. While recycled aggregates increased permeability, hybrid fiber systems improved resistance to crack propagation and maintained structural integrity under aggressive conditions. However, excessive fiber content led to increased porosity, highlighting the need for optimal fiber dosage. Overall, the study demonstrates that the synergistic interaction between steel and polypropylene fibers can effectively mitigate the drawbacks of RAC and produce a durable, ductile, and high-performance sustainable concrete suitable for structural applications.