Recent advances in the use of computed tomography in concrete technology and other engineering fields

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Recent advances in the use of computed tomography in concrete technology and other engineering fields

T

⁎

Miguel A. Vicentea,b, , Dorys C. Gonzáleza,b, Jesús Míngueza a b

Department of Civil Engineering, University of Burgos, c/Villadiego, s/n. 09001, Burgos. Spain Parks College of Engineering, Aviation & Technology, Saint Louis University, 3450 Lindell Blvd, 63103, Saint. Louis, MO, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Computed tomography scan Paleontology Heritage Ancient relics Metals Alloys Composites Pavements Asphalt Rock Soil Concrete

Over the past two decades, immense research efforts at a global level have extended CT-Scan technology across several engineering fields. The state-of-the-art of the most relevant research related to the use of CT-Scanning is presented in this paper, which explores microstructural studies of materials used in various fields of engineering, with especial emphasis on concrete technology. Its main aim is to present the range of new applications, in addition to the conventional uses of CT-Scan technology. Based on X-ray absorption, CT generates a visual display of the internal microstructure of a material at micro-range resolutions. In addition to its well-known usage in medicine, the current fields of application of this technology are very extensive. For example, CT is now an essential tool in paleontology that can reveal the internal structure of ancient relics without damaging (in many cases) unique specimens. It is extremely useful in material engineering, when analyzing the internal microstructure of the new and/or improved materials, because the images it generates can then be used to modify the material and further improve its macroscopic behavior. Mechanical engineers use it both in the analysis of internal flaws (i.e. voids, cracks, joints, and planes of weakness) in metals and in the study of composite materials. Likewise, its use among civil engineers extends to microstructural studies of rock and minerals (crack patterns, joints, voids, etc.). The advantages of this powerful tool are similar in concrete technology, because the macroscopic response of concrete components, as with so many other materials, is strongly related to the internal microstructure of the matrix and its internal flaws.

1. Introduction to the CT-Scan technology CT-Scan technology is based on the properties of X-rays and their interaction with matter. As X-rays pass through matter, their energy loss is governed by the law of Beer (Eq. 1).

I = I0·exp ⎡− ⎣

∫ μ (s) ds⎤⎦

(1)

where I0 is the initial intensity of the X-ray, I is the final intensity, and μ (s ) is the linear attenuation coefficient along its path. The last parameters mainly depends on the density, ρ , of the matter at each point through which the X-ray beam passes. The ratio μ/ ρ is approximately proportional to Z3, where Z is the atomic number of the element. Consequently, the direct relation between the loss of X-ray energy and the density of the matter is the underlying principle of CT-Scan technology. CT-Scan machines contain an intensity-controlled X-ray source and a detector, which measures the loss of X-ray intensity. During the scanning process, X-rays are emitted in all the directions and the initial and final X-ray intensity is measured and recorded for all the ⁎

X-rays, in such a way that every point of the specimen is traversed by different X-rays, from different directions. Different technical solutions for multiple-beam X-ray emissions can be found. The conventional solution is for the X-ray source to emit a line beam that is received by a single line detector. In this case, the CTscan machine emits a single X-ray beam along a fixed direction. During the scanning process, the specimen is rotated, elevated, and lowered in front of the emitter-detector device until the required areas are scanned. However, the scanning process is slow and the resulting scan is not highly accurate, because of the limitations of the mechanical components that are used to move the specimen up and down, which reduce the Z-axis resolution. Modern industrial facilities are equipped with an X-ray source that emits a cone ray received by an array of detectors (Feldkamp et al., 1984). Cone-beam devices became available in Europe in 1996 and US in 2001 only. During the scanning process, the relative movements between the CT machine and the specimen are much less than in the preceding example of the conventional solution. In this case, the

Corresponding author at: Department of Civil Engineering, University of Burgos, c/Villadiego, s/n. 09001, Burgos. Spain. E-mail address: [email protected] (M.A. Vicente).

https://doi.org/10.1016/j.micron.2018.12.003 Received 28 August 2018; Received in revised form 8 December 2018; Accepted 8 December 2018 Available online 12 December 2018 0968-4328/ © 2018 Elsevier Ltd. All rights reserved.

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done with the naked eye. Other researchers exploit the CT-Scan data through numerical evaluations of the data, using commercial or customized digital image processing software. In other situations, depending on the research, the information provided by the CT-Scan is exported to a finite element software, in order to develop numerical models of the real specimen. These models are useful when the specimen is then subjected to mechanical, thermal or environmental actions and where the aim of the research is to calibrate the numerical model for deeper comprehension of the behaviour of the material. This paper therefore reviews and summarizes the current state-ofthe-art of the most relevant research related to the use of CT-Scan technology that explores the microstructure of materials in various fields of engineering. The rest of the paper is structured as follows: the use of CT-Scan technology in paleontological and heritage-related contexts is examined in sections 2 and 3, respectively. Its use in metals and composites is then discussed in sections 4 and 5, respectively. Subsequently, the use of CT-Scan technology in civil engineering fields, such as pavements, rocks, and concrete is approached in sections 6,7, and 8, respectively. Finally, the conclusions are drawn in section 9.

scanning process is fast and highly accurate scans are produced of equal resolution on the X, Y, and Z axes. Post-scan data processing determines the density at each point of the specimen in accordance with the measured loss of X-ray intensity for all the X-rays. Several numerical strategies have been developed with the aim of increasing the quality and the sharpness of the images, while reducing the computational time; a problem that continues to attract scientific interest (Zhang et al., 2018a; Lee et al., 2017; Sidky et al., 2009; Wang et al., 2008). The final post-process result is a spatial image composed of voxels in grey scale (ranging from 0 to 255), where the grey value corresponds to the average density of the voxel. Clear grey tones correspond to high densities while dark grey tones correspond to low densities. This technology was first applied in medicine, in the 1970s, (Cormack, 1964; Hounsfield, 1973) as a non-invasive technique to visualize the internal body parts (organs, tissue, bones) of patients and to detect pathological abnormalities. Over recent years, CT techniques have been adopted in many other scientific fields, especially science and engineering. However, there are substantial differences between a CT-Scan for medical treatment and a CT-Scan for research and industrial purposes, the most relevant of which is radiation intensity. Low-intensity X-rays are used in medicine, so as not to damage human tissue; on the contrary, high-intensity X-rays can be used with inert objects to generate images of higher resolution. Over the past few years, the use of CT-Scan technology has become widespread and the most relevant research works regarding material properties, microstructure and macroscopic behaviour only use this technology or combine it with other conventional ones. Fig. 1 shows the number of journal papers published between 1990 and 2017 in different science and technology-related fields in which computed tomography scan technology is mentioned. The data were taken from the Web of Science that lists the most relevant international journals in all fields of science and technology. As can be observed in Fig. 1, there is a marked increase in the number of research papers that refer to CT-Scan technology. The strong increase of publications begins shortly after the availability of conebeam CT devices. Among the research fields studied in this work, CT technology is used most in the study of metals, composites, alloys, rocks, soils, and concrete. Its usage is less relevant in the other research fields referred to in this study. Many researchers hardly exploit the full potential of this technology in their work and use CT-Scan technology as a tool to generate images of the internal microstructure, while their analysis and interpretation is

2. Use of CT-Scan technology in paleontology The use of CT-Scan technology began early in this field, although it is less commonly used nowadays. The initial research carried out by Wong and published in 1981, applied CT-Scan technology in paleontology, using more intense X-ray sources on inert specimens that generated better quality images. A significant research effort using CT-Scan as the core research technology is found in this field (Pepinelli and Currie, 2017; Selden and Penney, 2017; Quam et al., 2016; Lautenschlager, 2013, 2016; Santos et al., 2013; Tafforeau et al., 2006; López-Polín et al., 2008). In all cases, fossil specimens were studied, with significant differences to the organic specimens commonly analyzed in medicine. The fossilization process of an organism occurs over thousands of years, during which time the soft tissues decompose, and bone loss and fragmentation continues until the formation of the complete fossil. In addition, breakage may occur during the manipulation and study, which can imply an enormous loss. In this context, the CT-Scan is a very useful tool that produces exact three-dimensional images that makes it possible, by means of digital image processing software, to undertake virtual reconstruction of skeletons, etc., without any need to manipulate the pieces. In addition, the

Fig. 1. Rise in journal references to CT-Scan technology (1990–2017). 23

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Fig. 2. Virtual cleaning of a fossil (Santos et al., 2013).

processing software, a virtual reconstruction can eliminate the layer of rocky sediment. Later on, an exact replica may be reproduced with a 3D printer with which scientist can then work, while preserving the original from deterioration. Likewise, museums can store ancient relics for their safety, exhibiting only the replica. The use of CT-Scan technology started relatively late and one of the first papers on the subject was published in 1986 by Notman et al. Additionally, this technology is not very popular in this research field, although some research works have had a great media impact, like the Antikythera Mechanism (Freeth et al., 2016). Over the past few years, some research that uses CT-Scan technology has been developed in this field (Ludwig et al., 2018; Van den Bulcke et al., 2017; Hoshino et al., 2017; Zhang et al., 2012; Abel et al., 2011; Morigi et al., 2010; Lehmann et al., 2005; Zhang and Bao, 2009) (Fig. 3). Sometimes, the main purpose is virtual reconstruction, when the relic is broken and its different parts have been found, or the study of the internal structure or materials. In other cases, the scanning is used to understand the fabrication process or internal mechanisms, in case of artefacts, (Greene et al., 2017; Freeth et al., 2016; Stelzner et al., 2016; Kahl and Ramminger, 2012; Krug et al., 2007). In both cases, an analysis by means of CT-Scan technology preserves the integrity of the piece.

information collected by the CT-Scan can serve as the basis for the regeneration of exact replicas using modern 3D printers. In other cases, it may be physically impossible to remove the rocky sediment that hardens around the fossil without breaking it. In this case, the CT-Scan can “virtually” eliminate the sediment, revealing the “clean” piece (Fig. 2): CT-Scan technology has immense potential in research. The main purpose of a CT-Scan is to determine the exact geometry of a piece, for greater comprehension of its behaviour, and its potential role within a larger body, mainly through comparisons with other specimens. In other cases, CT-Scan technology offers the possibility of detecting skeletal and spinal malformations, dental history, and even disease etc. Several research projects in that regard have been developed over the past few years (Lurino and Sardella, 2014; Brauer et al., 2003). A relevant amount of research in this field is being focussed on the study of the Egyptian mummies (Cramer et al., 2018; Longo et al., 2018; Schmidt et al., 2013; Gerloni et al., 2009; Melcher et al., 1997), and on the study of the bog bodies (Sydler et al., 2015; Villa and Lynnerup, 2012; Pestka et al., 2010). 3. Use of CT-Scan technology in heritage and ancient relics Ancient artefacts and relics share a few similarities with fossils. First, these specimens are unique and of singular value, so they have to be handled with care. Second, most of them have been extracted from their archaeological site covered with a hard layer of rock, the removal of which is a high-risk task, because of the risk of breaking the piece. In these situations, CT-Scan technology is a proven and powerful tool. First, the whole pieces are scanned and then, using digital image

4. Use of CT-Scan technology in metals Metals are without exception widely used materials in all fields of industrial engineering. Large amount of metals and alloys are consumed by industry, each with specific properties, depending on the use: low or high weight, strength, ductility, tenacity, energy absorption, electrical 24

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Fig. 3. Example of rendering a prayer nut (Ludwig et al., 2018).

Fig. 4. Example of analysis of mechanical behavior of metal under compression and CT-Scan analysis (Hangai et al., 2012). (a) Deformation recorded with a video camera (b) Deformation “recorded” with a CT-Scan.

and bioengineering sectors, among others). One very common research line, in which CT-Scan technology plays a core role, is the study of defects during the manufacturing process of a product for quality control and innovative improvements (Lu and Chan, 2018; Szkodo et al., 2016; Wicke et al., 2016; Yand et al., 2015a,b). In other cases, CT-Scan technology is combined with mechanical testing, in order to establish correlations between the internal microstructure and the macroscopic response of the material (Awd et al., 2018; Ali et al., 2018; Chen et al., 2018; Alsalla et al., 2016; Dahdah et al., 2016; Nemcko and Wilkinson, 2016; Chan et al., 2015; Hangai et al., 2012). The final purpose of this research line is to replace expensive mechanical quality control tests during the manufacturing process with scanning technology that is both cheaper and easier to implement in the manufacturing production line. Additionally, in some

conductivity, high or low thermal transmissivity, abrasion resistance, hardness, corrosion resistance, and porosity, among others. A wide range of manufacturing processes are currently used in industry, from the most conventional ones of smelting and casting to more modern systems of stamping, injection, and additive manufacturing. The use of CT-Scan technology in the study of metals and alloys started very early, drawing from earlier developments in medical applications. The first study specifically developed for industry was published in 1981 by Hildebrand and Harrington, and subsequent research has ensured that CT-Scan technology is now widely used in metallurgy. In general, the use of this technology is of increasing interest in the industrial sector, especially in industries that develop components and elements of high added value (aeronautical, aerospatial, automotive,

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joint. In consequence, the defects produced in the weld can be evaluated during welding with CT-Scan technology, helping to improve the process.

5. Use of CT-Scan technology in composites Composite materials are currently widely used in many engineering fields. They are generally composed of a matrix (plastic, metallic, etc.), and reinforcement (usually particles and/or fibres). The aim of the reinforcement is to modify the properties of the matrix, with the objective of achieving a material of the desired characteristics. In general, 3 phases can be identified in a composite material: matrix, reinforcement, and pores or cracks. As with metallurgy, the use of CT-Scan technology in the study of composites was developed from medical research at an early stage. The first research works specifically developed for industry were published in 1983 (More et al., 1983), since when copious research has been published in the field of CT-Scan technology in relation to composites. The distribution of the reinforcement and alignment (in the case of fibres) as well as the location of pores and cracks strongly affects the behaviour of composite materials. In this situation, CT-Scan technology becomes a powerful tool to generate images for microstructural observation and analysis of the composite material (Hayashi et al., 2017; Melenka et al., 2015; Grammatikos et al., 2014; Nikishkov et al., 2013; Mccombe et al., 2012; Shen et al., 2004). In many cases, the scanning is combined with mechanical and thermal characterization tests of the composite material, in order to establish empirical relations between the composite microstructure and its macroscopic response (Kahl et al., 2018; Song et al., 2018; Wang et al., 2017; Grammatikos et al., 2016; Jespersen et al., 2016; Stamopoulos et al., 2016; Yu et al., 2015) (Fig. 5). Likewise, there is evident interest in the analysis of the internal microstructure and crack patterns of composite specimens under mechanical or environmental stress and strain that may damage them (Li et al., 2018a; Elamin et al., 2018; Saoudi et al., 2017). In a similar way to metals, the data provided by the CT-Scan can be exported to a FEM software package that will perform numerical simulations of a workpiece for subsequent comparison with the experimental results (Yan et al., 2018; Sencu et al., 2016; Czabaj et al., 2014).

Fig. 5. Example of analysis of the microstructure of a composite (Melenka et al., 2015). (a) 3D braid geometry (b) 3D braid geometry with imperfections (c) 3D distribution imperfections (d) Detailed view.

cases, the information provided by the CT-Scan is used for the generation of FE models for the numerical simulation of the expected results and their subsequent comparison with the test results. Here, the advantage of CT-Scanning is that it permits the construction of exact numerical models, which not only include the different phases that constitute the specimen, but also the voids, defects, cracks, etc., in their exact position (Fig. 4). In relation to the same line of research, welded joints and their analysis may be mentioned (Kar et al., 2018; Myrach et al., 2017; Dinda et al., 2016; Kuryntsev and Gilmutdinov, 2015). Welding is the most commonly used process for joining two metallic parts. The way in which the welding is done is fundamental to the final quality of the

Fig. 6. Extraction of the area of interest using CT-Scan technology (Hu et al., 2015). 26

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Fig. 7. Example of the possibilities of CT-Scan technology in rocks (Bultreys et al., 2016a). This time sequence shows a pore filling event. Fig. 8. Example of rendering volumes of the changing pores in limestone (Dewanckele et al., 2014). The pores are color coded from red (large) to blue (small). Drawing A belongs to unweathered state, drawing B belongs to 6 days of weathering process and drawing C belongs to 21 days of weathering process. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

6. Use of CT-Scan technology in pavements

such as the shape and the distribution of the aggregate, the asphalt content, the pore content, shape and distribution, etc. Additionally, the degree of homogeneity of the mixture plays a relevant role in the mechanical behaviour of the pavement and its durability. The use of CT-Scan technology in the study of composites is relatively recent. The first research results in this field were published in 1999 (Masad et al., 1999). Moreover, the use of this technology for the study of pavements is not widespread and there are few research works on CT-Scan technology and pavement. The most recent research lines in this field have focused on

Pavement or asphalt concrete is a composite material widely used in the construction of roads, parking lots, and airports, because of the advantages that it contributes, among which high strength, easy manufacture, and maintenance, and low noise emission. In structural terms, pavements are made from heterogeneous materials, consisting of coarse and fine aggregates, asphalt, and porous networks. The mechanical properties of these materials show high levels of dispersion, given that those properties depend on many factors, 27

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macroscopic behaviour of the pavement (Hu et al., 2018; Liu et al., 2017; Hu et al., 2015; Yin et al., 2015; Wang et al., 2015; Kayhanina et al., 2012; Chen and Wang, 2011) (Fig. 6). In the case of special asphalts, such as porous and fibre-reinforced pavements, knowledge of the exact distribution of certain components is of great interest, with a view to understanding their effectiveness. This situation applies to both additives for pavement restoration (Dinh et al., 2018a; Zhang et al., 2015) and to fibre-reinforced asphalts (Dinh et al., 2018b; Norambuena-Contreras et al., 2016). In many other cases, CT-Scan technology may help to establish correlations between the macroscopic behaviour (under loads or environmental actions) of the pavement and its internal microstructure (Lu et al., 2018a; Zheng et al., 2017; Jing et al., 2016; Wang et al., 2016; Rinaldini et al., 2014).

7. Use of CT-Scan technology in rocks The application of CT-Scan technology to rock and mineralogy research that started around the 1980s has converted it into one of the first fields of civil engineering to use this technology. As with composites and asphalt concrete, rocks are also heterogeneous materials that contain pores and cracks. In addition, most rocks consist of different materials, each with different mechanical properties and/or densities. In most cases, the real structural behaviour of a rock is strongly conditioned by its internal (pores, voids, weakness planes) flaws. In terms of structures, rock is the most important structural material in a wide range of civil engineering works, especially in tunnels and dams. The mechanical properties of rocks, their porosity and their degree of internal cracking strongly condition tunnel stability and convergence, as well as the excavation procedure, and the tunnel support methods. Something similar occurs in the case of dams, especially arch dams. The structural elements of the arch dam are bonded with rock and their mechanical behaviour strongly conditions the structural safety of the

Fig. 9. Steel-Fiber Reinforced Concrete specimen. Steel fibers (in white), cement matrix (in grey) and cracks and porous (in black) can be identified. (Vicente et al., 2018a).

microstructural analysis as the basis of understanding the macroscopic behaviour. In this situation, and similarly to the previous cases, CT-Scan is a very useful technology that generates an image of the internal microstructure and the distribution of the components (especially voids and pores) inside the specimen. Moreover, the data provided by the CTScan can be used to perform FEA, in order to calibrate the numerical models with the results provided by mechanical tests, and to establish empirical correlations between the internal microstructure and the

Fig. 10. Voids’ distribution in concrete specimen with different ratio Air Entraining Agent / Cement: (a) 0%, (b) 0.1%, (c), 0.2%, (d), 0.3% and (e) 0.4% (Vicente et al., 2018b). 28

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Fig. 11. Interface paste-aggregate and porosity between matrices with recycled aggregate and natural aggregate (Leite and Monteiro, 2016).

developed in this field was published in 1982 by Petrovic et al. Currently, the use of this technology is very intense and copious research works on CT-Scan technology have been published in this field. Again, CT-Scan technology is a proven and powerful tool for exploring the microstructure of rock, leading to an understanding of the macroscopic behaviour of rock and hence its prediction. In these cases, small specimens are usually extracted from the rock for scanning and the results have to be extrapolated to the entire rock. (Smal et al., 2018; Zhou et al., 2018; Zhang et al., 2018b; Bultreys et al., 2016a,b; De Kock et al., 2015) (Fig. 7). One of the most interesting research fields is the study of capillarity and its influence on degradation of rock. Capillarity, voids and cracks in rocks are linked to the filtration of water, which coupled with freeze-thaw cycles will result in progressive degradation. Several studies on rock capillarity that use CT-Scan technology can be found (Menke et al., 2018; Bultreys et al., 2016c; Lin et al., 2014; Charalampidou et al., 2010). A further line of research, of great interest, is the study of petrous elements for use in construction. One particularly interesting line of research examines the porosity of limestone and other types of masonry such as façades and pedestrian pavements, as well as masonry for the rehabilitation of historic buildings. In all of these cases, the determination of porosity is very important, in order to determine whether it is convenient for use in a particular climate (Kalasova et al., 2018; Gibeaux et al., 2018; Dewanckele et al., 2014; Boone et al., 2016) (Fig. 8). Finally, mining may be mentioned in relation CT-Scanning, an area where the research has mainly been focused on understanding the formation of induced cracking and its propagation in rock, especially where size reduction of the material represents a major cost. Some works have recently been performed in this field (Nicco et al., 2018; Hartlieb et al., 2017; Weerakone et al., 2012). Regarding this issue, a significant amount of research has been conducted over the past few

Fig. 12. 3D view of the internal damage in a concrete specimen subjected to fatigue compression loading (Vicente et al., 2018a).

dam. In this case, flaws and weak planes in rocks, as well as excessive internal cracking can undermine the stability of the dam over time. The use of CT-Scan technology in the study of rocks and mineralogy started really early. One of the first research works specifically 29

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Fig. 13. Fiber distribution and orientation inside a concrete wall (Mínguez et al., 2018).

Fig. 14. Results of the rheology simulation. The glyphs represent the alignment of the fibres with each other, where a sphere represents isotropy and cigar-shaped fibres that are well aligned with each other. (a) and (d) No slip at t = 5 s; (b) and (e) slip length 0:08 m at t = 5 s, (c) and (f) slip at t = 5 s; (a), (b) and (c) top view, (d), (e), and (f) bottom view. (Herrmann and Lees, 2016).

and, eventually, fibres. In this situation, CT-Scan technology provides useful information on the internal microstructure of concrete (Fig. 9). The use of CT-Scan technology in the study of concrete started at a very early stage with the first research work published in 1980 by Morgan and coworkers. It is now very widespread and copious research based on CT-Scan images has been published. There are several lines of research into concrete and CT-Scan technology. In the case of ordinary concrete, the study of its internal porosity is of great interest, because it directly affects many macroscopic responses, among which durability, fracture behaviour, fatigue, and freeze-thaw cycles. In this case, CT-Scan technology can measure the exact geometry of the voids and several shape factors can be extracted to correlate porosity with the macroscopic response (Vicente et al., 2018b; Yuan et al., 2018; Lu et al., 2018b; Tian and Han, 2018; Balázs et al., 2018; Lu et al., 2017) (Fig. 10). This subject is of great interest in the case of two singular concretes: pervious concrete (Rifai et al., 2018; Chandrappa and Biligiri, 2018; Zhang et al., 2018c) and recycled concrete (Qi et al., 2018; Leite and Monteiro, 2016; Erdem et al., 2011) (Fig. 11).

years in relation to hydraulic fracturing (or fracking) for oil and gas (Li et al., 2018b; Mohammadmoradi and Kantzas, 2018; Álvarez et al., 2017). In all these cases, CT-Scan technology is used to explore the internal microstructure of the phases (soil, liquid, gas, voids and cracks) and their evolution over time. 8. Use of CT-scan technology in concrete Concrete is the most widely used construction material in the world. Its many important advantages include low cost, globally accessible production technology and durability. However, it also has various equally well-known disadvantages, among which the high scatter of its mechanical properties, because it is in fact a composite material made of coarse aggregate, fine aggregate, cement and water. In addition, other additives may be included to improve its behaviour: fly ash, silica fume, fibres, among others. The macroscopic behaviour of concrete depends, in most of the cases, on its internal microstructure. Different phases can be observed inside concrete: cement matrix, aggregates, water, empty pores, cracks 30

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Fig. 15. Stress-strain curve and damage propagation under tension. ∗DSF is displacement scale factor (Ren et al., 2018).

different external actions, among which compression, tension, fatigue, and creep (Mínguez et al., 2017, 2018; Skarzynski and Suchorzewski, 2018; Oesch et al., 2018; Balázs et al., 2017; Ponikiewski and Katzer, 2016; Herrmann et al., 2016; Vicente et al., 2014) (Fig. 13). Microstructural studies are not only of interest because they help to understand the macroscopic behaviour of hardened concrete, but also of fresh concrete. In this case, some interesting research projects have been conducted using CT-Scan technology to correlate the rheology of

Similarly, the crack distribution patterns inside concrete specimens are of particular interest, given their strong correlation with concrete ductility and several research works have been developed on this field (Vicente et al., 2018a; Gu et al., 2018; Suzuki et al., 2017; Zuguan et al., 2016, 2015) (Fig. 12). One of the most active research lines at present examines fibre-reinforced concrete. In this case, fibre alignment and distribution strongly affect the mechanical behaviour of this type of concrete under many 31

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fresh concrete and its microstructure (Jasiuniene et al., 2018; Herrmann and Lees, 2016) (Fig. 14). The data provided by the CT-Scan can be processed with finite element software applications as the basis for developing FE models. These models can be used to calculate the mechanical behavior of a concrete specimen under certain mechanical and thermal conditions and the results may be compared with the results of experimental tests. This research line contributes both to a deeper understanding of the failure mechanisms and to the development of new manufacturing methods of concrete components. It is a very promising line of research for fibre-reinforced concrete, as one of its technological challenges is the management of fibre alignment. To date, this is a completely new research line with very few works published on the subject (Ren et al., 2018; Zirgulis et al., 2016a,b; Svec et al., 2014) (Fig. 15). 9. Conclusions CT-Scan technology is a powerful research tool for researchers of many engineering fields and a huge amount of research has been conducted during the last two decades where this technology plays the core roll of the work. The enormous potential of the CT-Scan technology to generate images of the internal microstructure of materials for their analysis is currently improving our understanding of their macroscopic behaviour under mechanical, thermal, and environmental conditions, among others. The research fields to which this technology can be applied are almost unlimited, far beyond its well-known medical applications. In this paper, some of the most relevant CT-Scan-related research fields have been discussed where it has been widely used: paleontology, heritage and ancient relics, metals, composites, pavements, rocks, and concrete. Moreover, there are certainly some other research fields where this technology is used and others where this technology could be helpful. CT-Scan technology can be used “alone” to generate an image of the internal microstructure, leaving the interpretation of the results to the naked eye, although such methods underexploit the full potential of the technology that provides many other possibilities. The data provided by the CT-Scan can be post-processed using digital image correlation software, for in-depth analysis of the information, thereby distancing it from the subjective criteria of an observer. Another possibility is to export the data to a finite element software, in order to generate “exact” rather than (the commonly employed) idealized models, including the defects in their real positions with their real shape. In this way, it can help the researchers to understand the real behaviour of the material under external actions. Future generations of CT-scan machines will be faster, less expensive, more powerful and more accurate, generating sharper images of the internal microstructure of matter, which will help researchers to understand their behaviour and to improve them, yielding solutions adapted to each particular material. Acknowledgements The authors are grateful for financial support from the Ministerio de Economía y Competitividad, BIA2015-68678-C2-R, Spain. References Abel, R.L., Parfitt, S., Ashton, N., Lewis, S.G., Beccy-Scott, C., 2011. Digital preservation and dissemination of ancient lithic technology with modern micro-CT. Comput. Graphics 35, 878–884. Ali, U., Mahmoodkhani, Y., Shahabad, S.I., Esmaeilizadeh, R., Liravi, F., Sheydaeian, E., Huang, K.Y., Marzbanrad, E., Vlasea, M., Toyserkani, E., 2018. On the measurement of relative powder-bed compaction density in powder-bed additive manufacturing processes. Mater. Des. 155, 495–501. Alsalla, H., Hao, L., Smith, C., 2016. Fracture toughness and tensile strength of 316L stainless steel cellular lattice structures manufactured using the selective laser melting technique. Mater. Sci. Eng. A 669, 1–6. Álvarez, J.O., Rakananda Saputra, I.W., Schechter, D.S., 2017. Potential of improving oil

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