  The steel-concrete interface (SCI) is known to play a major role in corrosion
of steel in concrete, but a fundamental understanding is still lacking. One
reason is that concrete's opacity complicates the study of internal processes.
Here, we report on the application of bimodal X-ray and neutron microtomography
as in-situ imaging techniques to elucidate the mechanism of steel corrosion in
concrete. The study demonstrates that the segmentation of the specimen
components of relevance - steel, cementitious matrix, aggregates, voids,
corrosion products - obtained through bimodal X-ray and neutron imaging is more
reliable than that based on the results of each of the two techniques
separately. Further, we suggest the combination of tomographic in-situ imaging
with ex-situ SEM analysis of targeted sections, selected on the basis of the
segmented tomograms. These in-situ and ex-situ characterization techniques were
applied to study localized corrosion in a very early stage, on reinforced
concrete cores retrieved from a concrete bridge. A number of interesting
observations were made. First, the acquired images revealed the formation of
several corrosion sites close to each other. Second, the morphology of the
corrosion pits was relatively shallow. Finally, only about half of the total 31
corrosion initiation spots were in close proximity to interfacial macroscopic
air voids, and above 90 percent of the more than 160 interfacial macroscopic
air voids were free from corrosion. The findings have implications for the
mechanistic understanding of corrosion of steel in concrete and suggest that
multimodal in-situ imaging is a valuable technique for further related studies.
