Combined neutron and X-ray computed microtomography to study the influence of steel-concrete interfacial defects on chloride-induced corrosion initiation

Collaboration: external page PSI (Paul Scherrer Institut, CH), external page EPFL, external page HE-ARC CR

Responsible: Susanna Governo, Emanuele Rossi

Project: external page CORINT - Elucidating Corrosion of iron in porous media by new quantitative multimodal in-situ tomography

measurement process schematic — Applicability of X-ray/neutron microtomography to study steel corrosion in engineering concrete structures. a) Test procedure involving coring of the reinforced concrete specimens and the application of multimodal tomography; b) Example of the interfacial characteristics observable after image segmentation (blue=air voids, red=corrosion products).

Background

Chloride-induced reinforcing steel corrosion is the leading cause of premature degradation of reinforced concrete structures worldwide. Predicting chloride-induced corrosion initiation of steel is crucial for service life modeling of concrete structures, but reliable predicting tools are hampered by the lack of complete understanding of which are the most sensitive locations at the steel-concrete interface (SCI) for corrosion to initiate. Contradictory results have been reported in the literature about the influence that interfacial defects (e.g., air voids and bleeding zones) have on the corrosion initiation mechanism: while in some studies interfacial air voids were found to be the preferential sites for corrosion initiation, there are works in that report that corrosion initiation was completely independent of the presence of interfacial defects. The literature is generally vague about the exact location of corrosion initiation concerning interfacial defects and the moisture conditions at the SCI.

Through a novel methodology based on the simultaneous application of neutron and X-ray computed microtomography (CT), this project attempts to clarify the role that interfacial defects and the conditions at which they are found have on the corrosion initiation of steel reinforcement.

Aim and objectives

This project aims to elucidate the mechanism of corrosion initiation of steel reinforcement in concrete by detecting and understanding the most sensitive locations for the formation of corrosion sites, mainly in relation to the characteristics of the SCI. To this aim, neutron and X-ray computed microtomography are employed to observe the initiation of corrosion over time and in about the characteristics of the SCI.

Methodology

The application of neutron and X-ray computed microtomography gives a complete overview of the internal state of reinforced mortar and concrete specimens. By inducing corrosion through the exposure of reinforced mortar and concrete specimens to chlorides, the corrosion initiation mechanism can be observed over time and precious information can be acquired. The bimodal imaging approach allows for enhanced segmentation of features of interest for the corrosion phenomenon, namely the steel, the presence of macroscopic concrete voids, the location of pitting corrosion and the moisture characteristics of the surrounding matrix. This project implies the study of both specimens cast under laboratory conditions as well as samples taken from engineering structures: while the former ones are used to elucidate the fundamental mechanism of corrosion initiation, the latter ones will be used to assess how the mechanism occurred under more realistic conditions.