Using X-ray computed tomography to study paving materials

Iowa State University

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Civil, Construction and Environmental Engineering

2-2007

Using X-ray computed tomography to study paving

materials

Kasthurirangan Gopalakrishnan

Iowa State University

, [email protected]

Halil Ceylan

Iowa State University

, [email protected]

F. Inanc

Iowa State University

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Using X-ray computed tomography to study paving materials

K. Gopalakrishnan

, H. Ceylan

and F. Inanc

X-ray computed tomography, a non-destructive three-dimensional imaging tool, has been very helpful in medical diagnosis since the first commercial tomographs were constructed in 1973. In recent years it has gained increasing applications in civil engineering materials research. Several investigators have illustrated the use of computed tomography scans for the non-destructive evaluation of soils and have shown promising applications in characterisation, modelling and computational simulation to optimise asphalt concrete mix design, predict performance and conduct investigative forensic studies. This paper provides a detailed review of X-ray computed tomography applications in characterising asphalt concrete and also describes preliminary studies conducted at the Center for Nondestructive Evaluation (CNDE) at Iowa State University for characterising asphalt and concrete materials using X-ray micro-computed tomography or micro-micro-computed tomography. Researchers are currently using the advanced imaging facilities available at the CNDE to develop a deeper understanding of the pavement internal structure, to develop and optimise the various parameters that describe the internal structure and to relate them to the performance of pavements in a scientific way. This will provide the foundations for building more durable and long-lasting transportation infrastructure systems.

1. INTRODUCTION

Recent research studies focusing on the fundamental behaviour of asphalt concrete (AC) have recognised the relationship between AC ‘internal structure’ and field performance. AC internal structure refers to the content and spatial distribution of asphalt, aggregates and air-voids, as well as the chemical and physical interactions among these

constituents.1 These individual materials and components

have different physical and mechanical properties and behaviours that have a significant effect on the performance of AC mixes.2 Realistic characterisation of the failure of asphalt mixes (in terms of cracking, etc.) necessitates the consideration of the internal structure of AC. It has been well recognised that the internal structure of asphalt mixes plays a significant role in the mechanical properties of AC and in the resistance of asphalt pavements to major distresses including rutting, fatigue, thermal cracking and low-temperature cracking.3,4

There is much key evidence that supports the significance of the aggregate structure or internal structure in AC. The better performance of stone matrix asphalt (SMA) is attributed to a better coarse aggregate skeleton in these mixes compared to the dense graded AC.5,6The Bailey method of gradation selection reportedly produces an aggregate blend that is packed together in a systematic manner, to form an aggregate skeleton with the required interlock and packing.7Thus, directly quantifying the AC internal structure and establishing its relationship to performance will be most beneficial.

The past mechanistic models of asphalt mixes have

concentrated mainly on the macroscopic behaviour of AC (e.g. indirect tensile strength, resilient modulus, etc.) and have been constructed on the general principles of continuum

mechanics.8,9 Sashidhar et al.,10 however, have demonstrated, using photoelastic techniques, that AC behaves more like a granular material in terms of load distribution characteristics. Load transmission in AC takes place in the form of force chains and particle-to-particle contact exists in the formation of these chains (at relatively high temperatures, the binder is soft and has little restraint on the particles). It was also shown using discrete element modelling (DEM) that different aggregate gradations have different aggregate structures and therefore produce different load distributions in the

pavements.10 Such computations require detailed information

regarding the size, spatial and directional distribution of the aggregates, voids and asphalt regions in the AC that can be obtained through three-dimensional (3D) particle packing simulations and X-ray computed tomography (CT) imaging techniques.11

The Center for Nondestructive Evaluation (CNDE) at Iowa State University has in-house X-ray micro-CT equipment with customised software for data acquisition, reconstruction, and visualisation. Preliminary studies were conducted at the CNDE using the micro-CT system to investigate the imaging capabilities and resolution of the system in studying asphalt materials and its potential to study air-void structure in Portland cement concrete (PCC). The advanced imaging facilities available at the CNDE can help in providing valuable insights towards an understanding of the relationship between pavement internal structure and performance, as well as serve as a powerful forensic tool in the hands of pavement engineers. This paper summarises the recent successful X-ray CT applications related to the characterisation of AC internal structure and highlights some of the proposed

Kasthurirangan Gopalakrishnan

Research Scientist, Department of Civil, Construction, and Environmental Engineering, Iowa State University, USA

Halil Ceylan

Assistant Professor, Department of Civil, Construction and Environmental Engineering, Iowa State University, USA

Feyzi Inanc

Scientist, Center for Nondestructive Evaluation, Iowa State University, currently with Baker Hughes In, Houston, TX, USA Proceedings of the Institution of

Civil Engineers Construction Materials 160 February 2007 Issue CM1 Pages 15–23 doi:10.1680/coma.2007.160.1.15 Paper 80007 Received 12/12/2006 Accepted 27/06/2007 Keywords: maintenance & inspection/pavement design/research & development

applications of X-ray micro-CT in the study of asphalt and concrete pavement materials.

2. IMAGING METHODOLOGY

Any imaging technique will involve three major steps: image acquisition, image processing and image analysis. The image-capturing device can range from a simple digital scanner to a sophisticated X-ray CT system. The quantitative information that can be extracted from the images will greatly depend on the quality of acquired images. Saadehet al.12used hydrofluoric acid to discolour different types of rocks in asphalt mixes, which facilitated the capture of quality photographic images that enabled the separation of the aggregates from the other phases on the basis of differences in colour.

The CT imaging technique has the significant advantage of producing cross-sectional [two-dimensional (2D)] images non-destructively and these images can be used in the reconstruction of a volumetric (3D) image of any specimen. Of the main imaging techniques available, CT has the advantage of imaging a 150 mm diameter core of hot-mix asphalt (HMA) with sufficient resolution and clarity for quantitative analysis.

In image-based characterisation methods, the acquired images are digitised as 8-bit, 12-bit or 16-bit images depending on the required contrast resolution. In an 8-bit greyscale image, the greyscales are divided into 28Z256 levels, and each pixel

(picture element) in the image has an intensity value ranging from 0 (black) to 255 (white). Since the accuracy of information extracted from an image depends greatly on the image quality (spatial and contrast resolution, clarity, etc.), great care is taken in enhancing the image and filtering out the noise. Readymade image processing and analysis software packages like Image-Pro Plusw

(by Media Cybernetics, Inc.) may be used for this purpose. Such software has several in-built features and it also facilitates writing macros to automate user-friendly image analysis procedures. Typically, during the image analysis phase, numerous operations are performed on the image such as assigning labels to objects of interest (e.g., aggregatesO1 mm), collecting their centroid positions, analysing their shape and orientation, etc. The major steps involved in an image analysis technique are illustrated inFig. 1.

3. X-RAY COMPUTED TOMOGRAPHY

After the breakthrough of medical CT in 1973, investigators worldwide began to look for possible applications of this new non-destructive, image-producing testing method in other fields. CT imaging has gained increasing applications in civil

engineering materials research in recent years.13–17A CT system typically consists of an X-ray source, a rotating turntable to hold the sample and a detector. The X-ray beam is usually modulated into a fan or cone beam by using a collimator. The schematic is shown inFig. 2.

Basically, an X-ray beam transmitted through a sample along several different paths in different directions is detected, manipulated electronically and stored in a computer. The intensity of the X-ray beam is measured before it enters the sample and after it passes through it. The transmitted X-ray beams have a modulated intensity dependent on the overall linear attenuation characteristics of the intervening material. This modulated or

varying intensity with respect to distance is referred to as a profile. This profile information is then manipulated to produce a reconstructed image of a slice of the sample.

The resulting CT image is a spatial distribution of the linear attenuation coefficients, where brighter regions correspond to higher values of the coefficient. Therefore, if two aggregates with different linear attenuation coefficients are present in an HMA specimen, they show up as having different brightness.18The linear attenuation coefficients vary as a function of the composition and density of the material. The CT is highly sensitive to small differences (!1%) between materials. In a typical slice from a HMA specimen, the aggregate is the brightest, followed by mortar (mixture of asphalt and very fine particles), followed by air-voids. The sample is then shifted vertically by a fixed amount (slice spacing) and the entire procedure is repeated to generate additional slices. These slices from a single sample can be put together and rendered to produce a volume image.

Figure 3shows a 3D rendered image from a series of 2D slices of an HMA specimen scanned in a CT system (ACTIS 600/420 CT

X-ray source Collimator Sample Turntable Detector

Fig. 2. Schematic of X-ray computed tomography system

Image acquisition Image processing Image analysis 23 22 21 14 10 21 24 15 13 12 6 7 2 9 11 4 1 5 8 3 19 16 17 18

system; Bio-Imaging Research, Lincolnshire, Illinois, USA) housed in the Federal Highway Administration’s

Turner-Fairbanks Highway Research Center at McLean, Virginia, USA.19

There are four parameters that affect the quality of a tomographic image: spatial resolution, contrast resolution, noise and artefacts. The spatial resolution of the image depends on the size of the sample and the system characteristics, namely detector pixel size and the X-ray source spot size. For example, using a 150 mm diameter AC core and a detector with a 512 pixel profile, a pixel size of 150/512Z0$29 mm/pixel can be obtained, which implies

that features as small as 1 mm could be imaged.20Finer resolution can be obtained with an X-ray high-resolution tomography system using a 160 kV microfocal X-ray source. Resolutions down to 10mm can be obtained with this technique. The contrast resolution is usually determined by the system specifications and the detector characteristics play a huge role in the contrast resolution. An increase in the contrast resolution leads to resolving materials that have much smaller differences in their linear attenuation coefficients.

Digital image analysis has been used to characterise qualitatively and quantitatively the internal structure distribution of HMA in conjunction with CT imaging. The next section reviews some of the recent X-ray CT applications in the study of AC.

4. LITERATURE REVIEW

Masadet al.21_{used image analysis techniques in studying the}

difference in internal structure of AC specimens compacted with the Superpave gyratory compactor (SGC) and the linear kneading compactor (LKC). The orientation and distribution of aggregates and the aggregate-to-aggregate contacts were used as

quantifying measures in studying the internal structure of AC. The results showed that aggregates have a preferred orientation toward the horizontal direction in SGC specimens and a relatively random distribution in the case of LKC specimens. In a related study, Masadet al.4measured aggregate orientations in AC specimens compacted to different numbers of gyrations and in field cores. They found that the anisotropy in gyratory specimens became more pronounced with increase in the compaction effort (more gyrations) up to a certain point. A further increase in the compaction effort, however, caused a reduction in the anisotropy level and produced a more random distribution of the

orientation. Tashmanet al.22reported a similar relationship between aggregate orientation and compaction effort. Aggregate anisotropic distribution was found to be higher in field cores than in SGC specimens.12,22_{Segregation analysis revealed a tendency}

for coarse aggregates to move toward the circumference in SGC specimens.1,23

Based on a sequence of 3D CT images, Synolakiset al.24

developed a new method for computing the microscopic internal displacement fields associated with permanent deformations of AC cores with complex internal structure. Brazet al.25analysed the CT images of AC specimens subjected to indirect tensile strength tests (IDT) and Marshall tests. Azariet al.26used CT images of AC specimens to study the effect of radial inhomogeneity on the shear properties of asphalt mixtures. Dessoukyet al.27related the aggregate structure stability measured during compaction in the SGC to the aggregate orientation.

Several cross-sectional images of a single AC specimen or core acquired using a CT system can be put together and rendered to produce a volume image. The volumetric images permit the study of various aspects of AC such as the structure of the aggregate skeleton, the orientation of particles, any lack of homogeneity (segregation) in aggregate sizes, distribution of air-voids, presence of cracks, the distribution of asphalt, etc. Information such as the inter-connectivity of air-voids, the number and the direction of aggregate-to-aggregate contacts and aggregate orientation cannot be accurately determined from 2D images. Masadet al.21found that the air-voids in AC gyratory specimens were non-uniformly distributed along the horizontal and vertical directions. A forensic analysis of Arizona’s US-93 Superpave sections using CT images revealed that the void structure contributed significantly to the distress of these sections.20 Shashidhar14studied field cores using CT and observed a relatively high void content present at the asphalt mixture interfaces. Tashmanet al.22observed a uniform air-voids distribution in the horizontal direction and a non-uniform distribution in the vertical direction from field cores studied using CT.

Ketcham and Shashidhar28developed a software program,

BLOB3Dw

, to analyse 3D images of HMA quantitatively. The

BLOB3Dw

program implements a 3D version of what is traditionally called a ‘blob analysis’ for extracting objects from 2D images. The 3D rendering of a sub-volume for HMA

quantitative analysis using BLOB3Dw

software is shown in

Fig. 4.19Ketcham and Shashidhar28demonstrated that it is possible to extract particle size, location, aggregate-to-aggregate contact vectors, and contact area using the BLOB3Dw

program. X-ray CT has also been used for crack detection in asphalt mixes.29,30Brazet al.31applied computerised tomography techniques to detect and follow the evolution of a crack, when an asphalt mixture is submitted to fatigue testing. Tashmanet al.32

used CT to capture the microstructure of HMA specimens before and after loading in triaxial compression tests at high

temperatures and used image analysis techniques to characterise the evolution of air-voids and cracks throughout the deformation

process. Wanget al.33presented methods to quantify damage

parameters from 3D reconstructed CT images of HMA specimens

Fig. 3. A three-dimensional rendered image from a series of X-ray CT sectional images

and also presented the relation between the quantified damage parameters and their applications in mechanical modelling.

5. PRELIMINARY STUDIES AT CNDE USING X-RAY MICRO-CT

X-ray micro-CT or high-resolution CT can resolve features to 10mm (or even lower) in size and detect density differences as small as 0$1%. Microtomography is similar to CT, except that it

uses a microfocus X-ray source and a high-resolution X-ray detector making it possible to measure the internal structure of materials in three dimensions at high resolution. Micro-CT has been successfully used to examine properties of cement concrete in recent years34–37as well as for characterising the physical properties of soil and particulate systems.38,39

5.1. Asphalt concrete

The application of micro-CT to the study of HMA has been very limited so far, at least as reported in the literature. Shashidhar14 used micro-CT to study a 32 mm diameter core from a pavement at the Federal Highway Administration’s (FHWA) accelerated loading facility. The micro-CT captured the mineralogical variation within the aggregate used in the mix. The aggregate

had three components: a dark siliceous phase, a denser aluminosilicate phase, and the brightest (densest) ferrous phase. It was also observed from the micro-CT images that although some asphalt mortar coated the large particles with a layer that was a few millimetres thick, the rest of the asphalt mortar occurred in pockets. This study indicated the potential of micro-CT for studying asphalt film thickness in AC.

The CNDE’s in-house X-ray micro-CT system used in this study (seeFig. 5) employs a Kevex 130 kV microfocus X-ray tube

capable of providing 2mm spatial resolution and 1400!

1400!500 voxel data volumes.40The detector is a Varian

amorphous Si panel using a Gd2O2S screen. The panel consists of

1936!1524 square pixels with a size of 127mm. The

source-to-detector distance is 1089 mm and the minimum source-to-object distance is 21$5 mm. The details of the X-ray micro-CT system at

CNDE are illustrated inFig. 6. With this system, CT images with 3mm can be obtained for a field view of 5 mm at the largest magnification levels. At a lower magnification, the field of view

reaches 200 mm with CT image pixels of 127mm. It should be

noted that there should be at least three pixels to resolve a feature in a given image. Depending on the needs, the system provides reconstruction software based on either a filtered back projection algorithm in two dimensions or a Feldkamp algorithm in three dimensions. The system is connected to a 64 processor Linux PC cluster and data acquired from the system is transferred to that system during acquisition to be reconstructed at a later stage.

Fig. 4. Quantitative analysis of three-dimensional images of AC using BLOB3Dw

software

Fig. 5. In-house X-ray micro-CT system used in this study

X-ray source generator Kevex Model P13006 Sample Varian PaxScan 2520 Amorphous-silicon detector Positioner

Results from two different studies related to asphalt materials are discussed in this paper. The objective of the first study was to demonstrate the usefulness of X-ray micro-CT in conducting forensic investigative studies on distressed/failed pavements. A random sample from an actual core obtained from a stripped AC pavement in Iowa was investigated using the micro-CT. The core exhibited signs of severe moisture damage. The front and back side of the investigated AC sample with varying dimensions are shown inFig. 7. Typical cross-sectional images captured using the micro-CT are displayed inFig. 8. This figure clearly shows the presence of large voids (in black) within the AC sample and the separation of aggregates from the surrounding matrix at several places due to stripping. A 3D reconstructed image of the stripped AC specimen is shown inFig. 9.

During the second study, three samples were prepared to assess the accuracy of micro-CT in capturing coarse and fine aggregates in the presence of asphalt. Aggregates retained on five different sieve sizes were considered: 12$7 mm (0$5 in.),

9$5 mm (0$375 in.), 4$75 mm (0$18 in.), 600mm, and 300mm.

The first sample (referred to as CA) was created by randomly placing the 12$7 mm, 9$5 mm, and 4$75 mm aggregates in a

cylindrical plastic container of 2$5 cm diameter and 5$5 cm

height and pouring asphalt into it. For all three samples, similar-sized cylindrical plastic containers were used. The second sample (referred to as FAC) consisted of 4$75 mm,

300mm, and 600mm particles mixed together in the cylindrical container prior to the pouring of asphalt. The third container (referred to as FAS) contained aggregates of same three sizes as

FAC, but were arranged in three different layers prior to the pouring of asphalt.

Figure 10illustrates the X-ray shadow images obtained for the FAS sample at two different voltage and current combinations. The linear attenuation coefficients of the asphalt are significantly lower than the aggregate coefficients. Therefore, X-rays can penetrate the asphalt at the low kilovoltage/current settings whereas they can not penetrate the aggregates in the lower portion of the image. The X-rays from the higher voltage/current setting can penetrate the aggregates and form an image of the aggregates section but they can not be stopped sufficiently to form an image of the asphalt section. Three distinct aggregate layers corresponding to three different particle sizes are clearly visible at higher voltage and current. InFig. 11, a sequence of X-ray micro-CT cross-sectional images captured at the interface between 300-mm and 600-mm particles in the FAS sample are displayed. Similar images were obtained for other samples but are not shown here due to space constraints.

5.2. Portland cement concrete

Research studies were also conducted at CNDE to analyse the air-void structure of concrete samples with computed tomography and projection radiography techniques. The objective of this study was to conduct initial investigations for the development of a simple and quick air-void analysis methodology based on the projection radiography images of hardened concrete. In cold climates, particularly where the temperature difference between daytime and night-time is high, concrete is subjected to repeated freeze—thaw cycles resulting in degradation of the cement matrix. To overcome this problem, homogeneously distributed air bubbles are introduced by employing chemicals called air entraining admixture. The air-void system generated in the concrete needs to be monitored carefully since low air content does not provide protection whereas excess air reduces the concrete strength. In addition to the quantity of air introduced, the quality of the air-void system is of great importance. For quality assurance purpose the characteristics of the entrained air-void system in concrete samples should be determined. ASTM C231 (Test Method for Air Content of Freshly Mixed Concrete by

the Pressure Method)41and ASTM C173 (Test Method for Air

Content of Freshly Mixed Concrete by the Volumetric Method)42 are two methods widely used for fresh concrete; however, both methods were found to be misleading in certain cases, especially

when the entrained air bubbles were very small.43_ASTM-C457

(Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete)44 was used to determine the characteristics of the air-void system; however, this method is laborious and is only applicable to hardened concrete. In the early years of the 1990s Danish researchers developed a system, the air-void analyser (AVA), which has been found to be accurate in determining the quantity, size, and spacing

Fig. 7. Front and back views of stripped AC sample used in this study

1 2 3

2 mm

factor of the entrained air bubbles and is accepted widely. New technologies have emerged to quantify and characterise the entrained air and these include the micro-CT scan.43,45

In both projection radiography and CT implementations, what determines the quality of the outcome is the contrast between the properties of the sought-out features and the matrix. The hardened concrete that will form the target material in this work, is composed of three different components, namely aggregates, cement and air-voids. Although hardened cement paste and aggregates are not too different from each other chemically, the aggregate density is significantly higher, making the aggregate radiographically quite distinct in comparison with the cement. A typical example of this is given by the CT image inFig. 12. The contrast between three phases inFig. 12forms the basis for air-void characterisation.

Figure 13provides a projection radiography image of a hardened concrete sample. The aggregates, cement and air-voids are clearly visible; however, they are lumped together. Projection radiography lumps all the properties along the photon path into one single value. As shown inFig. 13, the image reflects the distribution of the air-voids, aggregates and cement phases. Now,

researchers are faced with the challenge of distinguishing the air-void phase from the multiple phases in the materials tested. Another issue with air-void characterisation is the limit of detectability. Although magnification increases the chances of detecting smaller air-voids, the minimum void is possibly dependent on the overall sample thickness. The initial study performed by the researchers has indicated that these issues can be solved through X-ray-based test practices.

Thus, the use of X-ray micro-CT-based imaging

technologies can be very instrumental in the study of pavement construction materials. Asphalt concrete is a multi-phase composite containing aggregates spanning a two-decade range of particle size distribution. Using micro-CT, it is possible to obtain quantitative information regarding the fine aggregate particles, asphalt mortar, asphalt film thickness, etc., which is not possible using the conventional X-ray CT systems due to resolution limitations. Similarly, the X-ray technology can serve as

Fig. 9. Three-dimensional reconstructed image of the stripped AC sample 4·75-mm particles 300-µm particles 600-µm particles (a) (b) Asphalt Specimen support

Fig. 10. X-ray shadow images of the FAS sample: (a) 80 kVp, 0$10 mA; (b) 120 kVp, 0$40 mA (kVp,

peak kilo voltage—peak tube potential across the X-ray tube)

1 2 3

1 mm

Fig. 11. X-ray micro-CT cross-sectional images for FAS sample at the interface between 300-mm particles and 600-mm particles

one of the most effective methods for characterising the in situ air-void structures of concrete and other industrial materials in a reliable and speedy manner.

6. SUMMARY AND CONCLUSIONS

The importance of aggregate structure or internal structure in the performance of asphalt concrete pavements is well recognised. In recent years, there has been significant research on the qualitative or/and quantitative assessment of AC internal structure using 2D and 3D image analysis techniques.

X-ray computed tomography (X-ray CT or CT) is a viable, non-destructive, 3D imaging tool that has promising applications in characterisation, modelling and computational simulation to optimise AC mix design, predict performance and conduct investigative forensic studies. A significant benefit of using CT is that CT cross-sectional images can be used to reconstruct the 3D internal structure of a sample for computer simulation; meanwhile the sample remains intact and can be used for determining other macro properties.

This paper has reviewed some of the emerging imaging techniques related to AC internal structure characterisation based on the research studies reported in the literature. These methods have been applied recently in studying the differences among different laboratory compaction methods, improving the simulation of laboratory compaction to field compaction, segregation analysis, quantifying the effect of laboratory strength testing on AC internal structure properties in terms of damage evolution, characterising air-void distribution properties and predicting the permeability of AC mixtures. CT imaging has also been used to reconstruct virtual samples for applying computational simulation techniques in conjunction with mechanical modelling.

Preliminary studies were undertaken at the CNDE at Iowa State University to study asphalt and concrete materials using an X-ray micro-CT system. The results from two different studies related to asphalt materials, and one study related to concrete were presented. The objectives of the studies related to asphalt materials was to demonstrate the usefulness of X-ray micro-CT in conducting forensic investigative studies on distressed/failed pavements and to assess the accuracy of micro-CT in capturing coarse and fine aggregates in the presence of asphalt. The objective of the study related to concrete was to conduct initial investigations for developing a simple and quick air-void analysis methodology based on the projection radiography images of hardened concrete. These studies indicated the potential of micro-CT in the study of asphalt mortar (or mastic) properties, asphalt film thickness and for characterising the in situ air-void structures of PCC, which will be some of the areas for future research.

By quantifying the internal structure of pavements using the state-of-the-art imaging techniques and through mechanical modelling and numerical analysis that account for the internal structure, it will be possible quantitatively to relate the raw material properties to pavement performance. By thorough examination of the internal structure, aggregate matrices that exhibit enhanced performance can be identified and steps can be taken to develop processing techniques that can produce this structure. Imaging techniques may also be used for forensic investigations to determine the cause of premature failure of a pavement.

In summary, researchers at Iowa State University are taking advantage of the advanced imaging facilities available at the CNDE and the latest developments in image analysis techniques to develop a deeper understanding of the pavement internal structure, develop and optimise the various parameters that describe the internal structure and relate them to the performance of pavements in a scientific way. This will provide the

Aggregate

Air-voids

Cement paste

Fig. 12. A micro-CT image slice from a hardened concrete sample showing three distinct phases in the X-ray images (sample dimensions: 26 mm!26 mm!13 mm)

Fig. 13. A projection radiography image of a hardened concrete sample with all three phases, namely aggregate, cement paste and air bubbles (sample dimensions: 26 mm!26 mm!13 mm)

foundations for building more durable and long-lasting transportation infrastructure systems.

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