Probing the steel-concrete interface microstructure using FIB-SEM nanotomography

Abstract

While it is widely accepted that the steel-concrete interface (SCI) plays an important role in governing the long-term durability of reinforced concrete structures, the understanding about the primary features of the SCI that influence corrosion degradation mechanisms has remained elusive. This lack of knowledge can be attributed to, firstly, the complex heterogeneous nature of the SCI, and secondly, the absence of established experimental techniques suitable for studying the relevant SCI features. Here, we use focused ion beam—scanning electron microscopy (FIB-SEM) nanotomography to obtain high-resolution 3D tomograms of the SCI. Five tomograms, spanning volumes ranging from 8000 to \({200\,000}\,{\upmu \hbox {m}^{3}}\), of both non-corroded and corroded SCIs were acquired. The achieved voxel size falls within the range of 30–50 nm, which captures capillary pores highly relevant for moisture and ion transport. Potential pitfalls when applying the FIB-SEM technique to the SCI are highlighted, including aspects related to the electron detectors. We present an image processing pipeline that reduces artifacts and generates tomograms segmented into solid matrix and pore space. Furthermore, to characterize the SCI pore structure, diffusion tortuosity and porosity profiles. The analysis showed that there is a pronounced anisotropy in the pore structure. This work demonstrates that the FIB-SEM technique can be applied to acquire high resolution tomograms of the SCI pore structure, which can be digitally analyzed to inform transport models of the SCI.