Evaluation of the degradation of concrete composites using digital microscopy

Evaluation of the degradation of concrete composites using digital microscopy Radoslav Gandel1 * , Premysl Pavka2, Andrea Peknikova1, and Jan Jerabek1 1VSB – Technical University of Ostrava, Faculty of Civil Engineering, Department of Building Materials and Diagnostics of Structures, Ludvíka Podéště 1875/17, 708 00 Ostrava-Poruba, Czech Republic 2STAVBY VANTO, s.r.o., nam. Svobody 362, 6804 Kunovice, Czech Republic Abstract. The paper deals with an alternative possibility to evaluate the degradation of concrete composites by means of surface changes of surfaces exposed to the relevant degradation agents using digital microscopy. A uniform, rapid and efficient determination of the nature and level of degradation for the various methods dealing with the durability and service life of portland cement-based systems is crucial for eliminating the timeeconomic costs in the case of diagnostic, verification or type tests. The experimental section specifically addresses the issue for specimens subjected to the test of the resistance to water and de-icing agents and acid attacks. 1 Introduction Concrete is one of the most widely used building materials due to its availability and favourable physical-mechanical and chemical properties. However, these properties are inevitably compromised by the external influences of the environment in which the concrete element is placed. The issue of degradation of concrete composites is a significant problem in the construction industry in relation to the long-term durability and resistance of structures exposed to adverse environmental conditions. Concrete degradation is a natural but undesirable process that threatens not only the durability but also the reliability and safety of structures. Degradation agents can be divided into several groups depending on the nature and intensity of specifics, such as physical-mechanical (sub-zero temperatures [1, 2], carbonation [3, 4], mechanical wear [5]), chemical (sulphate attack [6, 7], chloride attack [8, 9]) or biological (biocorrosion caused by microorganisms [10]). Their effect is the same - reduction of the strength properties of concrete and shortening of its service life. These negative factors often occur simultaneously, or one precedes the other and therefore should be seen as a complex subset. Understanding these changes and quantifying them are key to increasing the service life of concrete composites or materials similar to concrete and reducing the cost of diagnosis and rehabilitation. In this context, digital microscopy plays an important role as an advanced and accurate method to assess changes on surfaces subjected to different types of degradation. Unlike traditional approaches, which often require time- * Corresponding author: ra[email protected] E3S Web of Conferences 641, 01027 (2025) https://doi.org/10.1051/e3sconf/202564101027 Young Scientist 2025 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/). consuming processes, digital microscopy enables fast, uniform and efficient analysis. In spite of its considerable advantages, the use of digital microscopy as such in the study of the properties of concrete composites is not very extensive. However, similar efforts could be seen e.g. in [11], where the authors combine optical microscopy with DIC methods for quantitative measurement of deformation and thickness of the ITZ transition zone in different types of concrete formulations and subsequent comparison of the results with SEM microscopy with DIC methods. The comparison of optical and SEM microscopy is also the focus of [12], who investigated the interface between the concrete surface and the geopolymer coating layers at the microscopic level. The aim was to analyze how different compositions of geopolymer materials and their application conditions (on dry or wet concrete surface) affect their adhesion to concrete and the migration of ions between the layers. The purpose of this article is to show to reader the possibility of an alternative assessment of degraded concrete surfaces using digital optical microscopy, specifically samples exposed to water and chemical de-icing agents and sulphuric acid. This evaluation consists of a comparison of the surface area before and after exposure to a given degrading agents. 2 Experimental part 2.1 The principle of measurement The schematic in Fig. 1 illustrates a simple principle for comparing the surfaces of degraded concrete samples. For the samples shown on the left, the area of the ideal surface will be S = x2, where x is the side of the square of the observed surface. When a certain type of degrading agent is applied and the surface of the concrete element is severely damaged, the cementitious sealant leaches out and cavities and cracks form, increasing the ruggedness of the degraded surface and with it the surface area, as shown graphically on the right side of the sample in Fig. 1. Fig. 1. Illustration of the scanned surface before (left) and after (right) exposure to the degrading agent.The ratio of surfaces is determined by relation (1): 𝑆R=𝑆𝐷 𝑆= (𝑥2+𝑦 𝑥2− 1) ∙ 100 (1) E3S Web of Conferences 641, 01027 (2025) https://doi.org/10.1051/e3sconf/202564101027 Young Scientist 2025 2 Where S (x2) is the scanned surface area of the sample before exposure to the degrading agent, SD (x2+y) is the scanned surface area of the sample after exposure to the degrading agent, in which y is the surface area of cavities, cracks formed after exposure to the degrading agent and SR is the percentage increase in surface area after exposure to the degrading agent. Fig. 2 shows realistic images of the surface of the undegraded (left) and degraded (right) sample taken with a Keyence VHX-7000 digital microscope, showing the difference in the change in surface ruggedness. The Keyence VHX-7000 is an advanced digital 4K microscope designed for detailed surface analysis with magnification up to 6000×. It works on the principle of combining high quality optics, automated focusing and high dynamic range imaging. It enables both 2D and 3D measurements, displays fine contours and produces fully focused images without manual intervention. Fig. 2. Demonstration of the surface of undegraded (left) and degraded (right) sample. 2.2 Practical application The test samples were 40x40x160 mm prisms made of high-performance concrete (HPC) with partial replacement of 0-4 mm aggregate with 0-4 mm concrete recyclate, on which scanned areas with dimensions 10x10 mm were marked. After the initial scanning, the tested areas were subjected to a given degradation trigger: in the first case, the HPC was exposed to 0.5% H2SO4 for 30 days (the actual photodocumentation is shown in Fig. 3), in the second case, the HPC was exposed to water and chemical de-icing agents (3% NaCl solution) for 200 freeze-thawing cycles (test surfaces see Fig. 4). Tests of resistance to water and chemical de-icing agents have been carried out in accordance with the standard ČSN EN 73 1326 [13]. Fig. 3. Surfaces of HPC samples before and after exposure to 0.5 % H2SO4. E3S Web of Conferences 641, 01027 (2025) https://doi.org/10.1051/e3sconf/202564101027 Young Scientist 2025 3 Fig. 4. Surfaces of HPC samples before and after exposure to water and chemical de-icing agents (3 % NaCl solution). In Figures 3 and 4, the visual difference between the original and degraded samples is visible. Further details are given in the following Table 1: Table 1. Surface areas of individual samples and their percentage increase after exposure to degrading agents. Type of degrading agent Sample Surface area [µm2] Percentage increase in surface area Before exposure to the degrading agent After exposure to the degrading agent 0.5 % sulphuric acid solution 1A 1.13E+08 1.19E+08 5.31% 1B 1.08E+08 1.11E+08 2.78% 1C 1.09E+08 1.17E+08 7.34% 1D 1.00E+08 1.03E+08 3.00% 3 % sodium chloride solution + freeze-thawing cycles 2A 9.57E+07 9.62E+07 0.52% 2B 9.59E+07 9.69E+07 1.04% 2C 9.03E+07 9.34E+07 3.43% 2D 9.65E+07 1.03E+08 6.74% Table 1 shows the surface area values of each sample before and after exposure to the degrading agents and their percentage increase. As can be seen, the values vary considerably for both types of degradation. In the case of exposure to a 0.5 % sulphuric acid solution, the average increase in HPC surface area was approximately 4.6 %, whereas in the case of an environment in which the sample was exposed to water and chemical de-icing agents, the average increase in surface area was approximately 2.9 %. 3 Conclusion The method of assessing the degree of degradation by surface area change using digital microscopy appears to be an effective and alternative approach to traditional methods. Nevertheless, there are several factors that limit the use of this method in practice: E3S Web of Conferences 641, 01027 (2025) https://doi.org/10.1051/e3sconf/202564101027 Young Scientist 2025 4 • The accuracy of the measurement depends not only on the quality of the instrument, but also on the human factor, where the user's manual marking of the test area introduces an error into the accuracy of the measurement - it is necessary to replace it with another type of marking (e.g. engraving) or to prepare samples of the corresponding size (here, however, the grain size of the filler must be taken into account). • Lenses with higher resolution are unsuitable due to the small coverage of the measured area. • To support the introduction of modern microscopic methods for the assessment of surface degradation of concrete samples, it is essential to establish clear correlations between the microscopic findings and the results obtained by conventional assessment methods. In current engineering practice, degradation caused by chemical attack - for example, by exposure to sulphuric acid [14] - is usually assessed by a combination of visual inspection and weight loss measurements, which provide a quantifiable measure of the extent of chemical degradation over time. In addition, a number of analytical methods such as SEM, FTIR, XRD, TGA, etc. are used to better understand the chemical processes and microstructural changes. In contrast, the resistance to physical and chemical stresses caused by freezing and thawing cycles in the presence of water and chemical de-icing agents (3 % NaCl solution) [13] is evaluated differently. In this case, degradation is assessed primarily on the basis of the amount of peeled material obtained after the number of cycles. The cumulative weight of this waste material is then used to determine the degree of deterioration or classification, which reflects the resistance of the sample to the action of a given degradation agent. These traditional approaches provide tangible, standardized parameters that are critical for evaluating the durability of concrete. Therefore, if microscopic methods are to be adopted as valid alternatives or complementary tools, their outputs - e.g. changes in surface morphology, microcrack development, etc. - need to be compared. • If significant degradation and large mass losses are expected, it seems more efficient to compare and scan the change in volume of the body rather than its surface area. This paper aims to demonstrate the applicability and effectiveness of modern microscopic methods for the alternative assessment of surface degradation levels in concrete elements. The data presented in this study are available on [15]. This paper was supported by project No. CZ.02.01.01/00/22_008/0004631 Materials and technologies for sustainable development within the Jan Amos Komensky Operational Program financed by the European Union and from the state budget of the Czech Republic. 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