## 2 Experimental Program  
For this work, a total of 6 cubic specimens of 40 mm sides were scanned. These specimens were cut from a prismatic piece of 150 \(\times\) 150 \(\times\) 600 mm (Figure 2). Therefore, the microstructure of all the specimens is expected to be very similar.  
The material used in the study was a self-compacting concrete reinforced with steel fibers. The proportion of fibers was 0.3% by volume and its type was Dramix OL 13/.20 straight fibers from Bekaert (Zwevegem, Belgium), 13 mm in length and with an aspect ratio of 65. This length was chosen so that the fibers would be short enough with respect to the smallest specimen size to maintain the homogeneity of mechanical properties in this volume of material. In the same vein, the maximum aggregate size was limited to 8 mm.  
The concrete mix was designed for a compressive strength of 30 MPa in 100 mm cubes. The dosage of the concrete mix is presented in Table 1.  
With the hardened concrete, the average compressive strength obtained at 28 days was 26.4 \(\pm\) 1 MPa in cylinders of 75 mm diameter and 150 mm height, and 28.2 \(\pm\) 1 MPa in cubes of 100 mm side. The elastic modulus and Poisson's ratio were also measured using the cylindrical specimens, resulting in 17.4 \(\pm\) 1 GPa and 0.16 \(\pm\) 0.01, respectively. The properties were very homogeneous as evidenced by the reduced standard deviations.  
A MicroCT Phoenix V! Tome! X s 240 unit from GE Sensing & Inspections Technologies (Boston, MA, USA), equipped with a high-power nanofocus X-ray tube (180 kV/20 W) and a high-power microfocus tube (240 kV/320 W), was used for scanning.  
Figure 2: Specimens cut from the prism. Dimensions in mm.  
By using a post-processing software package, a total of 1334 2D slices with pixel sizes of 2048 \(\times\) 2048 (Figure 3) were obtained throughout the height of each specimen from the CT-scan data. The resolution, understood as voxel size, was approximately 30 \(\times\) 30 \(\times\) 30 \(\upmu\)m\({}^{3}\). The specimens were scanned one after each other and the geometry of the irradiation was cone beam. In this case, only the high-power nanofocus was used.  
A 3D image of each entire specimen was generated by using all the above-mentioned 2D images (Figure 4). The post-processing software assigned a grey value to each voxel, from 0 (which belongs to the least dense voxel, i.e., void) to 65,535 (which belongs to the densest voxel).  
The data from the scanning process were saved in a file that included the X, Y, and Z Cartesian coordinates of the center of gravity of all voxels and an integer, from 0 to 65,535, with regard to their density. The total number of voxels in each specimen was approximately 2.4 \(\times\) 10\({}^{9}\).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Component** & **Content (kg/m\({}^{3}\))** \\ \hline CEM II/B-L 32.5 N cement & 286 \\ \hline Water & 202 \\ \hline MasterEase 5025 superplasticizer & 2.86 \\ \hline MasterCast 212 air reducer & 1.00 \\ \hline Dramix OL 13/.20 steel fibers & 23.55 \\ \hline
12 \(\upmu\)m OMYA filler & 195 \\ \hline
0.5 mm feldspathic sand & 921 \\ \hline
4–8 mm coarse aggregate & 712 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Dosage of the concrete mix.  
Figure 3: Example of one 2D slice.