Concrete is a most important geomaterial as it is used for a large part in our buildings and infrastructures. According to Planetoscope (2012) its production is about 6 billion m3 per year (190 m3 each second) which makes it the most used manufactured material in the world. The alteration of concrete, tightly associated with the durability and security of our infrastructures, depends on its primary composition but also on a wide range of environmental factors (mechanical solicitations, freezing-thawing cycles, alkali-aggregate reactions, etc.). Hence, the contribution of geological or geological-related analysis to understand concrete alteration processes is required because this material is mainly composed of aggregates made of different minerals and rocks, the type and quality of which are important due to their influence on concrete behaviour. In this study, high resolution X-ray Computed tomography (X-ray micro-CT) was used to image 3D different types of concrete cores in order to characterize their respective state of alteration. The global alteration index (GAI) developed by Christe et al. (2010) for natural cataclastic rocks was applied to the segmented X-ray CT images of these concrete cores and compared to the results of microstructural analysis on thin slices and of compression tests. Also, an internal attack of concrete by hydrochloric acid was carried out in laboratory to simulate artificial alteration through carbonate dissolution and its process was monitored along time by X-ray micro-CT imaging (4D monitoring). Our first results show that X-ray CT imagery enables, without any destruction of the specimens, to characterize concrete internal features in terms of macroporosity, highly microporous cement paste, standard cement paste or aggregates. In particular, initial entrapped porosity as well as cracks are easily detected, characterised and quantified before and after mechanical testing. Petrographic analyses on thin sections enabled to verify the physical meaning of the X-ray CT-based detected features, such as the highly microporous cement paste, the opening of the microcracks and the detachment halos around aggregates which all act as weakening parameters. Despite the limited number of samples, a coherent relation between the GAI and the compressive strength of the concrete specimens is observed as the concrete compressive strength clearly decreases when the GAI increases. In this case, it logically means that the concrete with a higher porosity is less resistant. Moreover, the 4D monitoring of the acidic attack test led to dynamically show how the carbonated structure of the concrete was progressively altered. These preliminary results demonstrate that GAI, based on XRCT imagery analysis, can be used to evaluate the degree of alteration of concrete and could thus lead to estimate its strength. In more general terms, X-ray CT analysis opens new perspectives to relate the quality of concrete with its mechanical properties. This method could probably be further applied to a wide range of problems related to the inspection and maintenance of concrete infrastructures.
Summary Bulk density was determined indirectly in peat samples by X ‐ray computed tomography ( X ‐ray CT ) and compared with density values obtained by standard laboratory methods. Five Histosols were collected in the same cut‐over peatland, representing various degrees of disturbance related to the process of peat extraction. Soil cores were fully imaged by X ‐ray CT with a voxel size of about 0.25 mm. Each one of these five attenuation profiles was analysed and compared with direct density measurements. A linear relationship, to convert attenuation values into density values, is proposed to determine the variation in bulk density with a spatial resolution clearly greater than standard laboratory determinations. It is also shown that X ‐ray‐based density values can be effectively used to characterize the structure of peat soils and the possible consequences of disturbances after drainage and peat mining. Under the accepted limitations of the method, X ‐ray CT opens up new opportunities to determine the structural quality of peat and to monitor its modifications with time. This indirect diagnostic could be particularly useful to study peatlands' hydraulic systems or evaluate the effectiveness of restoration measures.
To “Geoelectric mapping of near-surface karstic fractures by using null arrays” (S. Szalai, L. Szarka, E. Pracser, F. Bosch, I. Muller, and P. Turberg, GEOPHYSICS, 67, 1769–1778).
Abstract Soil structure is closely linked to biological activities. However, identifying, describing and quantifying soil aggregates remain challenging. X‐ray micro computed tomography (X‐ray μCT) provides a detailed view of the physical structure at a spatial resolution of a few microns. It could be a useful tool to discriminate soil aggregates, their origin and their formation processes for a better comprehension of soil structure properties and genesis. Our study aims to (a) determine different X‐ray μCT‐based aggregate parameters for differentiating earthworm casts belowground (earthworm aggregates) from aggregates that are not formed by earthworms (non‐earthworm aggregates), and (b) to evaluate if these parameters can also serve as specific “tomographic signatures” for the studied earthworm species. For this purpose, we set up a microcosm experiment under controlled conditions during 8 weeks, including three species of earthworms tested separately: the epigeic Lumbricus rubellus , the anecic Lumbricus terrestris and the endogeic Allolobophora chlorotica . Our results show that X‐ray μCT analysis helps distinguish earthworm aggregates from non‐earthworm ones using (a) the relative volume of the components within aggregates and (b) the volumetric mass of aggregates and their global volume. In particular, the volume ratio of mineral grains within the aggregates is significantly different according to earthworm species. So, X‐ray μCT is a powerful and promising tool for studying the composition of earthworm casts and their formation. However, future research is needed to take into account the shapes and spatial distribution of the aggregates' components, in particular the different states of organic matter decomposition. Highlights Can earthworm belowground casts be differentiated from other soil aggregates using X‐ray μCT? The use of X‐ray μCT for characterizing soil aggregates at a spatial resolution of a few microns. A combination of X‐ray μCT variables discriminates earthworm casts from non‐earthworm aggregates. X‐ray μCT, used alone, is relevant for defining species‐specific signatures of earthworm casts.
