DOI: 10.14457/tu.the.2024.519
title: Effects of corrosion on bond strength of galvanized reinforcing steel in concrete

description: The service life of reinforced concrete structures is typically limited by the corrosion of reinforcing steel caused by carbonation (CO₂ penetration) and/or chloride ion diffusion from the external environment, particularly in marine exposure. Reinforcing steel remains passivated in concrete due to its high alkalinity (pH 12.6–13.5). However, CO₂ diffusion lowers the pH of carbonated concrete, leading to steel de-passivation and increased corrosion. Similarly, corrosion can occur without pH reduction if the chloride concentration around the steel reaches a critical level, triggering de-passivation and corrosion initiation. Once the steel becomes active, corrosion reduces the rebar’s diameter, compromising the structure’s load-bearing capacity. Additionally, corrosion generates expansive products that induce tensile stress in the concrete cover, leading to cracking.To extend structural service life, researchers and engineers focus on corrosion protection methods. A common strategy involves providing adequate concrete cover. Cathodic protection using sacrificial anodes or direct current is also effective but often expensive and not universally applicable. Corrosion inhibitors offer another method, though they may degrade concrete’s mechanical properties. Alternatively, using non-steel reinforcement materials can help but may affect tensile stress transfer and other mechanical characteristics.Zinc coating on reinforcing steel presents a cost-effective option that preserves steel's mechanical integrity. Hot-dip galvanizing is a widely used method to apply zinc coatings, significantly enhancing resistance to chloride-induced corrosion—typically offering 2–4 times more resistance than uncoated steel. A higher chloride concentration is needed to initiate corrosion on galvanized steel. However, direct exposure of zinc to concrete’s internal environment may cause early corrosion and hydrogen evolution, potentially increasing porosity at the interfacial transition zone (ITZ) and weakening the bond strength. Despite this, zinc corrosion products are less expanded than those of uncoated steel and thus cause less damage to surrounding concrete.This study explores the impact of corrosion on the bond strength of galvanized reinforcing steel embedded in different concrete types: Portland composite, fly ash, and calcined clay concrete. Three key laboratory tests are performed: electrochemical (corrosion rate, potentiodynamic and linear polarization resistance), bonding (pull-out test), and chemical (XRF for oxide content and SEM for microstructure and corrosion morphology). Portland composite cement (PCC) serves as the primary binder, with a reference mix containing 2.0 wt.% initial chloride. Fly ash and calcined clay replace 30% of PCC by weight of binder. Two water-to-binder ratios (0.4 and 0.6) are tested, and a superplasticizer is used to maintain workability.As the result, galvanized steel has lower open circuit potential than bare steel. However, those negative OCP is lower than -1.07 V versus CSE which show the evident of hydrogen evolution. The OCP value then shifts to a positive value after a reaction that forms CHZ layer to passivate the galvanized layer. Fly ash results in the highest bond strength than the PCC and CC, this is because of compressive strength. Despite having lower pH when compared with PCC and FA which theoretically has the weakest passive film, CC turned out to have higher linear polarization resistance than PCC and FA. This is due to the limited oxygen supplied from the dense structure of CC by observing from the electrical resistivity measurement. CC also results in lower corrosion rate when compared to PCC and FA. Also, by having low pH and rapid setting time of CC concrete, there exists a hydrogen evolution that can’t escape the concrete, leading CC to have lower bond strength when compared to PCC and FA. By added 2% by weight of chloride, an unacceptable corrosion rate occurs, leading to received high value of corrosion rate. However, those corrosion rate filled pore at steel concrete interface that increased the bond strength of chloride concrete to have higher bond strength when compared with non-chloride concrete for both galvanized and bare steel.