Jihane El Hamzaoui

The fatigue performance of reinforcing bars is essential to the durability and safety of reinforced concrete structures. BS 4449 assesses fatigue prequalification using a stress range of Δσ = 200 MPa, a stress ratio of R = 0.2, and a run-out criterion of 5 × 10⁶ cycles for reinforcing bars with a nominal diameter not exceeding 16 mm. However, the influence of stress ratio variations under these conditions remains insufficiently documented. This experimental study examines the effect of stress ratio on the BS4449 fatigue prequalification results for 10- and 16-mm steel reinforcing bars under constant-amplitude loading. The bars were characterized through microstructural and inclusion analyses to provide a complete assessment of their metallurgical condition. Most specimens reached 5 × 10⁶ cycles without failure. Two failures occurred: one at R = 0.3 for the 10 mm bars and one at R = 0.2 for the 16 mm bars. Most specimens maintained the required run-out across a broader range of loading conditions. SEM observations identified MnS inclusions as the predominant inclusion population. These inclusions may serve as crack initiation sites. These findings highlight the importance of steel cleanliness when interpreting fatigue qualification results under varying stress ratios.

Seismic-resistant reinforcing steels (rebars) are indispensable for improving the safety and resilience of reinforced concrete structures subjected to earthquake loading. To perform effectively under such severe conditions, these steels must combine high strength with adequate ductility, toughness, and fatigue resistance. Achieving this balance depends on the control of the steel’s microstructure. In this context, microalloying with vanadium (V), niobium (Nb), and titanium (Ti) has become an effective strategy for controlling microstructural evolution through grain refinement, precipitation strengthening, recrystallization control, and phase transformation, therefore enhancing both monotonic and cyclic mechanical performance. Although the effects of microalloying and strengthening mechanisms in reinforcing steels are well documented, studies examining their combined influence on seismic applications remain relatively limited. This review therefore provides an integrated perspective by examining how alloy composition, thermomechanical processing routes, microstructural evolution, and strengthening mechanisms interact to determine the mechanical and fatigue performance required for seismic-resistant reinforcing steels. Rather than considering these aspects separately, this review focuses on the interactions between processing, microstructure, and mechanical behavior that determine the overall performance of reinforcing steels. This review also highlights the main scientific and technological challenges in the field, discusses the remaining knowledge gaps, and suggests future research directions for the development of next-generation seismic-resistant reinforcing steels.

Seismic-resistant reinforcing steels play a key role in structures subjected to earthquake loading, requiring an optimal balance between strength, ductility, and weldability. Microalloying with vanadium (V), niobium (Nb), and titanium (Ti) is widely used to improve these properties through precipitation strengthening and grain refinement. This work aims to contribute to the development of seismic-resistant reinforcing steels for the Moroccan construction sector. A literature review identified key international requirements, including a tensile-to-yield strength ratio (Rm/Re) of 1.15–1.35 and a total elongation at maximum force (Agt ≥ 7%). In parallel, Moroccan reinforcing bars were mechanically and microstructurally characterized. A conventional steel containing 0.65 wt.% Mn and no vanadium was used as a reference. This steel exhibited limited strain-hardening capacity, with Rm/Re ratios between 1.12 and 1.15. To improve this behavior, steels containing 1.1 wt.% Mn with different vanadium additions were investigated. Preliminary results indicate that vanadium microalloying improves mechanical performance through combined precipitation strengthening and ferrite grain refinement. The increase in strength is likely associated with fine V(C,N) precipitates formed during cooling, while ferrite grain refinement appears to contribute to maintaining ductility. This synergistic effect results in a more favorable strength–ductility balance, supporting the development of seismic-resistant reinforcing steels for structural applications.