Non-invasive investigation of concrete deterioration in aggressive aqueous environments by means of neutron diffraction

Maximizing the service life of concrete structures is an important challenge in the context of a sustainable development in the construction sector. However, an improvement of the durability of cement-based materials requires a detailed knowledge of the mechanisms of relevant deterioration reactions. In particular, the related mechanisms due to an impact of aqueous solutions are still debatable. Here, an interplay of transport processes and reactions disturb the equilibria between solid phases and the solutions in the pore structure of the materials resulting in significant changes in the mineralogical composition. In the medium and long term, these processes lead to serious damages to construction materials. Investigations of the damaging reactions are, however, usually “invasive” and therefore only suitable to a limited extent to characterize the relevant in situ processes. In this context, neutron diffraction appears to be a promising tool for time-resolved monitoring of the reactions of crystalline compounds in mineral-bound materials. So, non-destructive measurements can be performed on defined volume elements. In this study, investigation concepts were developed for different scenarios of chemical attacks relevant to practice. ... mehrFor example, the corrosion of the steel in reinforced concrete induced by chlorides, which penetrate into the materials due to permanent contact with chloride-containing solutions (e.g. sea water and de-icing salt), represents a serious problem. Here, however, the influencing variables that regulate the reaction are not sufficiently well understood to perform reliable assessments as a basis for prevention concepts. For example, the literature contains a wide range of values for a “critical” chloride content, which is necessary to initiate corrosion. Nevertheless, non-destructive analysis could make a valuable contribution to a more detailed understanding of these mechanisms. The aim of this study was to investigate whether neutron diffraction can be used to monitor the reaction progress by time-resolved measurements on samples contaminated with chloride. Over a period of several months, it should be investigated whether the formation of corrosion products or a degradation of metallic components can be monitored. Another example is the sulphate attack on cementitious materials. In this case, sulphate uptake into the microstructure of the material leads to the crystallization of ettringite. The associated crystallization pressure causes mechanical stress and thus substantial damage. The study tried to follow the mineral reactions directly. In this contribution, first results of the research activity are presented and neutron diffraction is evaluated regarding its potential for the detailed study of material damaging reactions.