3D imaging and micro-structural characterization of reinforced concrete structures naturally-deteriorated due to corrosion

Abstract

In this study, a three-scale characterization of eight 20-years-old reinforced concrete specimens naturally deteriorated due to chloride-induced corrosion was carried out. The main aim of this research was to investigate some relevant parameters to accurately model corrosion of reinforced concrete structures, such as which are the locations where corrosion pits are more likely to initiate as well as if the Elastic modulus of corrosion product depends on the concrete mix that the reinforcement was embedded in. Each scale of characterization had a specific goal: the macro-scale characterization aimed to analyze the state of the specimens after 20 years of outside (unsheltered) exposure through Non-destructive techniques (i.e. potential measurements, corrosion rate, resistivity of concrete), visual inspections and analysis of historical records. Also, since specimens were mis-labelled during the years, the cement type that each specimen was cast with was identified through polished sections of concrete portions analyzed under the Scanning Electron Microscope (SEM). The meso-scale characterization of the reinforcement aimed to quantify the volume loss of the steel as well as to investigate which were the locations where corrosion pits occurred the most. Out of each reinforced concrete specimen, a core of 20 mm diameter and 100 mm height has been drilled, embedding a steel bar of 8 mm diameter. Each core has been then analyzed through X-ray Computed Tomography (CT-scan) and image analysis was performed. Apart from revealing different volume loss for bars embedded in different mixes (i.e. CEM I was the most corroded, CEM III/B the least), this characterization suggested that the most influencing factor for corrosion pits to form is the presence of defects (i.e. air voids) at the steel/concrete interface. Finally, the micro-scale characterization of corrosion product aimed to investigate the relation between Elastic modulus of corrosion product (퐸cp), its micro-structure and its chemical composition, with regard to the different concrete mix that steel was embedded into. 퐸cp was measured through Nano-indentation, while chemical composition was measured through SEM/EDS (Energy Dispersive Spectrometry) elemental mapping and spot analysis. This phase of the research revealed that differences between Elastic modulus of corrosion product generated in different mixes are not significant, and that the relation between 퐸cp and Fe/O seems to be (linearly) proportional. In the last chapter, the findings of the research are summarized and some recommendations are made regarding future endeavors.