ADVANCED MATERIALS & PROCESSES | MAY 2026 35 parameter tends to vary for the steel supplied on the site, due to microstructural changes and the completeness of the Tempcore treatment. The corrosion susceptibility at an initial stage after 24 h exposure to aggressive solutions may vary depending on the parameters used during the treatment and the changes in the microstructure. At the beginning, the steel rebars (Fig. 3, left) seem to be more sensitive to the water (Fig. 3, center) rather than to the 3.5 wt% chloride solutions (Fig. 3, right). At a later stage, starting from 30 days of exposure, the corrosion difference between the rebars is largely reduced. The outer shell of steel rebars, which exhibits a tempered martensitic microstructure, indicates a higher susceptibility to corrosion after 24 h of exposure in a 3.5 wt% NaCl solution as compared to the ferritic-perlitic micro- structure of the core (Fig. 4, top). Intergranular corrosion is seen along the rebar’s border (Fig. 4, center) and pitting at MnS inclusions are par- ticularly present along the borders (Fig. 4, bottom)[7]. CONCLUSIONS The Tempcore treatment increases mechanical properties and allows steel rebars to reach the required tensile strength even in steels containing a large quantity of inclusions. On the other hand, the corrosion resistance of the tempered martensitic outer shell surface’s layer is reduced as compared to the ferrite-perlite microstructure of the rebar’s core. This fact may endanger the high service life requirements of 100 years for structures. To address this concern, a satisfactory balance between mechanical properties and long-term corrosion resistance needs to be targeted to achieve long-lasting, reinforced concrete infrastructures. ~AM&P Acknowledgments The author would like to thank S. Antonietti, C. Mosca, and the technician of the Institute of Materials and Construction, SUPSI, for sampling and testing. Note: Tempcore is a registered trademark of Centre for Research Metallurgy (CRM Group). For more information: Christian Paglia, Institute of Materials and Construction, SUPSI, Via Flora Ruchat 15, CH-6850 Mendrisio, Switzerland, christian.paglia@ supsi.ch. References 1. Standard SIA 262, Concrete Structures, 2013. 2. M. Economopoulos, et al., Application of the Tempcore Process to the Fabrication of High Yield Strength Concrete-reinforcing Bars, CRM Rep., 45, p 1-17, 1975. 3. Standard EN 10080, Steel for the Reinforcement of Concrete — Weldable Reinforcing Steel — General, 2006. 4. Standard EN ISO 15630-1, Steel for the Reinforcement and Prestressing of Concrete — Test Methods, Part 1: Reinforcing Bars, Rods and Wire, 2019. 5. R. Rodríguez and I. Gutiérrez, Mechanical Behavior of Steels with Mixed Microstructures, 2nd International Conference of Thermomechanical Pro- cessing of Steels, Liege, Belgium, p 356363, 2004. 6. Standard EN 1998-1:2005, Euro- code 8: Design of Structures for Earthquake Resistance — Part 1: General Rules, Seismic Actions and Rules for Buildings, CEN — European Committee for Standardization, 2005. 7. U.M. Angst and B. Elsener, Forecasting Chloride-induced Reinforcement Corrosion in Concrete — Effect of Realistic Reinforcement Steel Surface Conditions, Concrete Repair, Rehabilitation and Retrofitting IV — Dehn et al. (Eds), Taylor & Francis Group, London, ISBN 978-1-138-0284, 2016. high number of inclusions, especially MnS, notoriously brittle in the mechanical behavior (Fig. 2, top), can reach a satisfactory strength and an adequate balance with toughness and ductility. The strength is mainly supported by the tempered martensite of the outer shell (Fig. 2, center) and the ferritic- perlitic microstructure of the core contributes to the ductility (Fig. 2, bottom). The difference in the rebar’s elongation to rupture may reach 8% in the same steel bar batch. This latter mechanical Fig. 3 — Steel rebar (left) immersed in tap water (center) and in a 3.5 wt% NaCl solution (right). Fig. 4 — Cross-section of a B550B steel rebar showing an increased corrosion of the outer shell (top), intergranular corrosion (center), and pitting (bottom)[1].
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