Concrete Heritage Conservation and the Viability of Migrating Corrosion Inhibitors

Steel is embedded within concrete to create a material that combines the compressive strength of the concrete with the tensile strength of the steel. This composite has a wide range of applications, but corrosion of the steel in reinforced concrete creates a serious problem with no single solution. Complex conservation treatments can be evaluated through the lens of preservation theory in order to choose a path that best fits the values of the structure and the elements in which those values are instilled. This type of values-based assessment of options illuminates the fact that each treatment poses its own upsides and downsides for the specific needs of a structure. These pros and cons may then be considered in conjunction with factors such as efficacy of the treatment, parameters set by the site's programming, and cost, to decide on the best path forward. One such treatment, migrating corrosion inhibitors (MCIs), despite having been on the market since the 1980's, still has a number of questions surrounding it. MCIs are products that are designed to diffuse through concrete and slow the corrosion of embedded reinforcement by forming a protective layer around it. Because of their ability to be applied easily to the surface of concrete, and the claim that they can prevent or retard corrosion without change to appearance or texture, MCIs present an appealing option for corrosion treatment in concrete heritage, offering an "invisible hand" to the conservator. This is a particular advantage for structures in which significance lies in the original appearance of the concrete. Unfortunately, the study of MCIs, and of their risks and their benefits, seems incomplete today due to unreliable evaluation methodologies, leading many practitioners to question their risks and efficacy. This thesis begins with a discussion of the composition of concrete and the implications of the differences between historic and modern concrete for conservators. It then offers an explanation of the process of corrosion in reinforced concrete, its causes and effects, to inform an understanding of conservation options. Next, the care of concrete heritage will be contextualized in a values-based preservation theory, to aid in the subsequent discussion of treatment options. This leads to a discussion of one treatment in particular, MCIs, in which a description of their classifications and compositions guides an evaluation of the theoretical benefits for concrete heritage. This is followed by a discussion of the limitations of existing evaluation methods for MCIs, and an exploration of how to improve these.