The alkali-silica reaction damage in concrete at the mesoscale: characterization by X-ray tomography

Abstract

The alkali-silica reaction (ASR) is the source of one of the most deleterious durability issues of the worldwide concrete infrastructure. It consists of chemical reactions between aggregates and the alkaline pore solution. The ASR products lead to deformations and cracking and, consequently, extensive struc tural damage, causing major safety, economic and environmental issues. Since the first investigations (eight decades ago), some basic ASR (cracking) mechanisms have been discovered. However, a com prehensive understanding has not been achieved yet, due to its complex multiscale nature. One of the major knowledge gaps stems from lack of characterization of the deformations and crack propagation at the mesoscale (few - few ). Results from systematic non-destructive studies have been lacking, although they are necessary to track the ASR cracking. That is because the typical petro graphic characterization techniques rely upon 2D microscopy (electron and optical), which require de structive specimen preparations. X-ray tomography (XT), as a non-destructive technique, has been re cently adopted, although, so far only in limited cases, mainly for simplified model systems and exploited only for qualitative analysis. During this PhD project, embedded within the framework of the Swiss National Science Foundation Sinergia project Nr. 171018 ("Alkali-silica reaction in concrete"), a comprehensive study of ASR crack ing at the mesoscale was carried out using time-lapse XT and various image analysis approaches. The study involved mainly specimens undergoing ASR acceleration in the laboratory. Its key goal was to characterize, for the first time, the spatial-temporal evolution of both ASR products and cracks. To achieve such goal, several challenges in the application of XT needed to be addressed. The result was the development of a new, integrated experimental and image analysis framework. One challenge consisted of enhancing the X-ray attenuation contrast between ASR products and the other material phases of concrete at the mesoscale, the former being normally undistinguishable from the latter in standard X-ray tomograms. Overcoming this challenge allowed analyzing both qualitatively and quan titatively their spatial-temporal distributions. Another challenge dealt with the contrast enhancement between aggregates and the cement paste, typically very low but needed to distinguish between prod ucts/cracks located either inside or outside aggregates. Finally, the qualitative and quantitative analysis of the tomographic time series required the implementation of distinct 3D image registration steps, to enable the time-lapse qualitative comparisons as well as the quantitative analysis. 3D image registration was additionally exploited both for estimating from the time series the global and local ASR-induced deformations and for increasing the robustness of the cracks and products detection (segmentation), compared with what achievable by conventional segmentation approaches. The overall, new XT-based methodology was extensively validated, concerning any eventual spurious effect on the natural ASR (cracking) course. It was possible to show its applicability and it delivered both new information and respective quantitative data about the ASR crack networks, about the extent of ASR products transport along them and the associated cracking itself as well as about the localized deformations, which accompany the cracking. The obtained, 3D datasets about products, cracks and deformations provided a uniquely new integrated knowledge and data base for advancing the basic un derstanding of ASR (cracking) and for supporting the formulation and validation of respective compu tational models at the concrete mesoscale. In addition, the developed XT-based methodology could be immediately exploited by the ASR research community for investigations also at other space-time scales not addressed in this project, e.g., the early stages of products formation and cracking initiation.