Combined neutron and X-ray computed microtomography to study the transport and precipitation of corrosion products in the cementitious matrix

Background

Steel reinforcement corrosion leads to the premature degradation of reinforced concrete structures. Corrosion products have a larger volume than pristine steel, and their build-up causes pressure inside the concrete matrix. Given the relatively low tensile strength of concrete, the formation of corrosion products may cause cracking and spalling of the matrix, dramatically compromising the serviceability and durability of infrastructures. The resistance of concrete to the build-up pressure caused by corrosion product formation depends on many interrelated parameters, such as the characteristics of the mixture, the presence of interfacial defects, and how the corrosion products migrate within the concrete matrix surrounding the steel reinforcement. Understanding how the transport and precipitation mechanisms of corrosion products are influenced by some characteristics of concrete (e.g., composition, porosity, etc.) would help to enhance the reliability of service life models of concrete structures. However, monitoring of the steel corrosion progress over time is hampered by the concrete matrix surrounding the reinforcement, making this mechanism not easily accessible and observable.

Through a novel methodology based on the simultaneous application of neutron and X-ray computed microtomography (CT), this project attempts to study the transport and precipitation mechanisms of corrosion products within the surrounding matrix, also in relation to the characteristics of the concrete.

Aim and objectives

This project aims to study over time the mechanisms of transport and precipitation of corrosion products in the surrounding concrete matrix. To this aim, neutron and X-ray computed microtomography are employed to observe the formation of corrosion products at the steel-concrete interface (SCI) due to the exposure to chlorides and carbon dioxide as well as the consequences that corrosion product build-ups have on the corrosion propagation resistance of the surrounding matrix.

Methodology

The application of neutron and X-ray computed microtomography gives a complete overview of the internal state of reinforced mortar and concrete specimens over time. By inducing corrosion through the exposure of reinforced mortar and concrete specimens to chlorides and carbon dioxide, the corrosion initiation mechanism can be observed over time and precious information about how the corrosion develops can be acquired. The bimodal imaging approach allows for enhanced segmentation of features of interest for the corrosion propagation phenomenon, namely the steel, the corrosion products, the characteristics of the surrounding concrete matrix, and its moisture conditions. In this project, specimens cast under laboratory conditions as well as samples taken from engineering structures will be studied simultaneously. The formation, transport, and precipitation of corrosion products will be studied in mortar and concrete specimens subjected to both chloride- and carbonation-induced corrosion.