Report Title: Scale Effect Investigation on RC and HRC Beams: Ductile-to-Brittle Transitions and New Design Standards Presenter: Postdoctoral Researcher BINEET KUMAR (Shantou University) Time: 10:00, October 19, 2026 Venue: Room 301, East Engineering Building

Abstract

Traditional design generally adopts reinforcement ratio independent of structural dimensions as the primary controlling index. For quasi-brittle concrete structures, however, mechanisms including crack propagation, reinforcement bridging, shear cracking and compressive zone crushing exhibit distinct size dependence. This study systematically investigates the size effect, fracture behaviour and ductile-to-brittle failure transition of reinforced concrete (RC) and hybrid-reinforced concrete (HRC) beams. Large-scale experiments, nonlinear fracture mechanics modelling, back analysis and dimensional analysis are adopted to explore the influences of structural size, reinforcement level, steel area, slenderness ratio and fibre volume fraction on structural bearing capacity, deformability and fracture stability.

First, four-point bending tests are carried out on RC beams without stirrups to systematically study the effects of longitudinal reinforcement ratio, beam depth, constant reinforcement ratio, constant steel area and slenderness ratio on load-deflection responses, crack evolution and final failure modes. The results reveal that RC beams may experience evident transitions among flexural cracking, steel yielding, shear-dominated failure and concrete crushing in the compressive zone. A Cohesive/Overlapping Crack Model (COCM) is employed to interpret such behaviours, and a staged back-analysis method based on different dominant failure mechanisms is established to identify parameters for tensile fracture, steel yielding and compressive zone crushing respectively. On this basis, the minimum and maximum longitudinal reinforcement requirements of RC beams are further examined. It demonstrates that both reinforcement limits vary with structural size, and a fixed reinforcement ratio cannot fully reflect the fracture stability of structures.

Second, the research is extended to HRC beams reinforced with both longitudinal steel bars and discrete steel fibres. An Updated Bridged Crack Model (UBCM) is used to describe matrix cracking, steel fibre bridging and pullout, as well as the contribution of longitudinal reinforcement. Mechanical parameters of each component are identified via back analysis. Combined with systematic four-point bending tests, the coupled effects of reinforcement ratio, fibre volume fraction and beam size are investigated. The results show that the stable cracking capacity of HRC beams relies on the combined action of two bridging mechanisms from steel rebars and fibres, with prominent size effect. Fracture stability boundaries and size-dependent minimum hybrid reinforcement requirements are established based on dimensionless brittleness parameters, providing mechanical foundations for the collaborative design of steel rebars and fibres.

In addition, this study examines the influence of surface geometry of GFRP bars on bond-slip behaviour and structural pseudo-ductility. From the perspective of fundamental fracture mechanics, edge effect, notch sensitivity and the competition between plastic collapse and brittle fracture are analysed. The research indicates that ductility and brittleness are not solely determined by material properties; instead, they are structural characteristics governed jointly by material strength, fracture resistance, structural dimension, reinforcement layout and geometric conditions. Overall, this work develops a unified fracture-mechanics understanding of size effect and failure mode transition for RC and HRC beams. Size-dependent minimum/maximum reinforcement criteria for RC beams and minimum hybrid reinforcement principles for HRC beams are proposed, offering theoretical and experimental support for structural design targeting fracture stability and ductility control.

All faculty and students are welcome to attend!

Postdoctoral Research Station in Civil Engineering, Shantou University October 16, 2026