Petrography and Geochemical Analysis for the Forensic Assessment of Concrete Damage

Concrete deterioration was recognised in the early 1900s and at the time was considered a natural consequence of aging. Since then, a number of different damage mechanisms have been identified, compromising performance and reducing service life. Proper identification of primary and secondary causes of deterioration is essential to determine correct rehabilitation strategies, and to prevent future damage. Results from such assessments have been used to decide disputes and warranty claims especially, in recent structures, from a forensic perspective. In older structures, such data are typically used to plan maintenance and rehabilitation and, in general, to provide workers with the required expertise and know-how to prevent the use of materials known to be deleterious, or mixes proven to have a poor performance in new structures. Reduced concrete performance can be assessed by a number of standard methods to produce data on, amongst others, compressive/tensile strength, water infiltration depth, total porosity, permeability, and chloride content. However, besides the characterisation of the actual performance of the material, it is necessary to identify the cause of deterioration, for which analytical methods, based in geological techniques, have proven to be powerful and versatile, notably petrographic microscopy and geochemistry. Petrography can be applied on plane sections from extracted drill cores, as well as on thin sections under an optical microscope using polarised light. Polished sections can be used for analysis by microprobe (EMPA), including element mapping. Appropriately prepared thin sections enable identification and assessment of the spatial distribution of micro-structural features, including capillary porosity. Petrographic data on modal content of coarse and fine constituents, and rock types and minerals present in concrete, are essential for correct interpretation of geochemi-cal assessment of bulk ‘whole rock’ concrete using methods such as XRF or ICP.

Issues which can prove challenging using conventional bulk testing methods can be more easily resolved using geological methods, especially when forensic issues are involved. This chapter presents the applications of these techniques in case studies, illustrating the potential of petrography, combined with geochemical analysis where applicable, to resolve the cause of deterioration where traditional methods failed.