Influence of hydrogen evolution and corrosion on the bond strength of galvanized reinforcing steel in concrete with different binder types

The service life of reinforced concrete structures is often compromised by corrosion of the reinforcing steel, primarily due to carbonation and chloride ion diffusion, especially in marine environments. Zinc coating, particularly through hot-dip galvanizing, offers a cost-effective solution by enhancing resistance to chloride-induced corrosion. Galvanized steel requires higher chloride content to start corrosion and produces less expansive corrosion products than normal steel. Yet, zinc may corrode early in fresh concrete, releasing hydrogen and increasing porosity at the steel–concrete interface, potentially weakening bond strength. This study investigates the effects of corrosion products on the bond strength of galvanized steel in three concrete types: Portland composite cement (PCC), fly ash (FA), and calcined clay (CC) concrete. Electrochemical tests (potentiodynamic and linear polarization resistance), pull-out tests, and chemical analyses (XRF and SEM) were conducted. PCC was used as the reference binder, with FA and CC replacing 30% by weight of the total binder. Water-to-binder ratios of 0.4 and 0.6 were tested. Results showed that galvanized steel initially exhibited low open circuit potential (OCP) and hydrogen evolution, later stabilizing with a protective zinc compound layer. FA concrete demonstrated the highest bond strength due to superior compressive strength. Surprisingly, CC concrete, despite lower pH, exhibited higher corrosion resistance than PCC and FA due to limited oxygen diffusion from its dense structure and early strength development. However, hydrogen evolution in CC reduced the bond strength. Adding 2% chloride by weight led to high corrosion rates but increased bond strength due to corrosion product filling at the steel–concrete interface, benefiting both galvanized and bare steels.