## 2 Experimental Program
### Scanning Process  
In this study, concrete specimens were scanned by a GE Phoenix v|tome|x device equipped with a tube of 300 kV/500 W. Using the post-processing software VGSTUDIO MAX (Volume Graphics, Heidelberg, Germany), 2000 2D slices of 2048 \(\times\) 2048 pixels (Figure 2) were obtained throughout the height of the specimen from the CT scanner machine. In this case, for a section of 45.2 mm diameter, the horizontal resolution is \(25\times 25\) um\({}^{2}\), and the vertical distance between the slices was 25 um. After that, a 3D image of the whole specimens was generated, using all the 2D images mentioned above. The post-processing software provides a gray value to each voxel (volumetric pixel), which is an integer number between 0 and 255, where 0 means pure black and 255 means pure white. A value of 255 is assigned to the densest voxel, and a value of 0 is assigned to the less dense voxel. For the rest of the voxels, a linear relation is considered. The conclusion of the scanning process is a file including X, Y and Z Cartesian coordinates of the voxel center of gravity and an integer number, from 0 to 255, regarding the density. The total amount of voxels in a specimen is around \(4.3\times 10^{9}\). A detailed explanation of the scanning process was previously described by [PERSON] et al. [34; 42; 58]. The next step in this process was to identify and isolate each individual void inside the specimen, with image analysis software. To do this, the digital image processing (DIP) software AVIZO (FEI Visualization Sciences Group, Hillsboro, OR, USA) was used. First, the software identified the voxels belonging to voids, which are the ones showing a gray value inside a range. In the case of steel-fiber-reinforced concrete, the voxels belonging to fibers were also identified and isolated. In this case, once the histogram of gray scale was studied, the considered range for voids was between 0 and 65, and for fibers between 200 and 255 (Figure 3). The voxels between 65 and 200 belong to the concrete matrix. It can be observed that a great number of the dark voxels belong to the ones placed outside the mold. In consequence, the diagrams in Figure 3 are not valid to obtain the percentage of voids in the specimen. Next, all the voxels placed outside the inner face of the mold were deleted, because they did not belong to the concrete specimen. Then, all the voxels in contact were merged, since they come from the same void. The software identifies and isolates the different voids. The final result of this image analysis is a dot matrix containing the Cartesian coordinates X, Y and Z of the center of gravity of each void, as well as some other geometrical parameters of the voids (length and volume among others). Figure 4 shows an isometric view of one specimen of each mixture and each mixture condition. Figure 2: Slices belonging to different mixtures and curing conditions. Analogously, in the case of the SFRC specimens, the voxels belonging to the fibers were identified and isolated. A new dot matrix was performed to each specimen, including the X, Y and Z Cartesian coordinates to the center of gravity of each fiber, as well as many other geometrical parameters of the fibers (Figure 5). It can be observed in Figure 2 that some of the largest voids are in contact to the inner face of the mold. In order to avoid these facts on the analysis, the whole specimen was not studied, but an inner cylinder of the specimen with a diameter 90% of the nominal diameter of the concrete specimen, i.e., 40.7 mm. This fact can be explained by the wall-effect of concrete and also by the shrinkage. In this work, only voids with less than three voxels in the largest direction (i.e., 75 mm length, approximately) were discarded, because they are too small, and the CT scan does not provide enough sharpness to clearly identify them. Additionally, voids larger than 10 mm in the largest direction were discarded, since they are non-representative voids. The results shown in this paper are the average values of the three specimens.