The Influence of Coating Thickness and Composition on the Corrosion Propagation Rates of Galvanized Rebar in Cracked Concrete

Abstract

A study has been conducted of the chloride-induced corrosion behavior of four different batches of galvanized steel reinforcement embedded in sound and in cracked concrete. One batch of bars was of conventionally produced hot-dipped galvanized (HDG) steel, two were prototypes of continuously galvanized rebar, and the fourth was a hot-dipped bar with an experimental Zn-Al alloy coating. Carbon (black) steel bars were also tested for comparison purposes. The continuously galvanized process is aimed at producing a thinner, but more ductile coating than that formed by conventionally hot-dipped galvanizing process. Metallographic examination of the as-received galvanized bars showed a wide variation of the coating thickness around and along the bars, and the continuously galvanized coatings were consistently thinner than specified. All bars were cast in concrete which was subsequently cracked either parallel to or perpendicular to the embedded bars. Additional specimens were tested in the sound (non-cracked) concrete. All specimens were constantly exposed to a chloride brine for 64 weeks, and were electrochemically assessed bi-weekly during the exposure period. The electrochemical results and visual examination after autopsy showed that no active corrosion was initiated in either the galvanized or black rebar reinforced non-cracked concrete specimens. Therefore, the data in this project give no indication of initiation time or chloride threshold concentration for corrosion of these bars. On the other hand, in all cracked concrete specimens, corrosion initiated at the base of the crack and extended along or around the bars. In the cracked specimens, all galvanized bars exhibited lower current densities than the black bars, with the HDG being the lowest. Recommendations are given for appropriate interpretation of half-cell potentials of the galvanized bars investigated in terms of high or low probability of active corrosion.