Towards understanding corrosion initiation in concrete – influence of local concrete properties in the steel-concrete interfacial zone

Abstract

Chloride-induced corrosion is the most common deterioration process for reinforced infrastructure objects. Improving the understanding of the conditions for initiation of localized corrosion is urgently needed. Research is focused on the influence of “defects” at the steel-concrete interface (SCI), as these weak points might be responsible for corrosion initiation. In contrast to numerous studies with “lab concrete”, this study reports results from reinforced concrete cores drilled from old infrastructure objects containing a non-corroding rebar. In contrast to laboratory studies, this guarantees real conditions at the SCI comprising also irregularities such as air voids, plastic settlement voids, cracks, etc. This allows to study chloride-induced corrosion in real conditions and to determine the so-called “critical chloride content” Ccrit. Visual inspection of the SCI enables to establish (or not) influences of the local conditions at the SCI and Ccrit. It was found that Ccrit strongly decreased with the carbonation depth, even if the carbonation front had not reached the steel. Moreover, coarse air voids and cracks were in this study not particularly susceptible sites for corrosion initiation.

The influence of defects at the steel/concrete interface for chloride-induced pitting corrosion of naturally-deteriorated 20-years-old specimens studied through X-ray Computed Tomography

“…This statement is also confirmed by Glass et al [10], who observed that the chloride threshold value for corrosion initiation of steel increases if the percentage of air voids at the SCI decreases. However, recent studies reported that defects at the SCI did not have any influence on corrosion initiation under certain environmental conditions [11,12]. Angst et al [4] observed by visual inspection that the location of corrosion onset did not coincide with the location of interfacial air voids.…”

“…So, the lack of electrolyte at these locations could prevent corrosion to initiate. Boschmann et al [12] found that corrosion initiation in the majority of cases did not occur at air voids in cores taken from existing structures, that were exposed in the laboratory to chloride solution. Although several studies have been conducted on the influence of interfacial defects for corrosion initiation [4][5][6][7][8][9][10][11][12][13], on-going research on the topic highlights the importance to clarify if, and how, interfacial voids may affect the chloride critical value and so initiation of corrosion.…”