## 3 Experimental Results and Discussion
### Porosity and Porosimetric Curves  
Post-processing the data provided by the CT scanner allows the ability to obtain the shape of each individual void and all of their related parameters. Particularly, the length and the volume of each void is of interest. The pore length is defined as the maximum distance between two voxels of the same void. Using these data from all the voids, we can plot the pore volume curves and the porosimetric curves. A pore volume curve is defined as the graph correlating the void's length and the sum of the volumes of the voids with a length equal or lower than that one. On the other hand, the porosimetric  
Figure 6: Variation of the porosity along the depth. curve is defined as the graph correlating the void's length and the sum of the volumes of the voids with a length equal or lower than that one divided by the total volume of pores. In both cases, these graphs represent the pore size distribution. Figure 7 shows the pore volume curves and the porosimetric curves belonging to the different mixtures and curing conditions. The horizontal axis is limited to 10 mm pore length, since greater pores are residual and not of interest. Figure 7(a) shows that the fiber-reinforced concrete specimens show a porosity greater than that of the plain concrete. Moreover, the curves belonging to fiber-reinforced concrete are over the ones belonging to plain concrete, which means that those ones show more voids in all the lengths. The differences between air-cured and moist-cured concretes are very small. In the case of fiber-reinforced concrete, it seems that the larger voids are more dominant than the smaller voids. On the contrary, in the case of plain concrete, it seems that smaller voids are more dominant than the larger ones. Figure 7(b) provides information complementary to the curve shown in Figure 7(a). It highlights that fiber-reinforced concrete curves are to the right of the plain concrete curves, which means that fiber-reinforced concretes are able to retain a greater percentage of large-size voids. This can be explained by the mechanisms that occur while concrete paste remains fresh. Because of the hydrostatic pressure that fresh concrete paste exerts on the pore walls, it tends to collapse. The void's stability depends on the viscosity of fresh concrete, as well as its stiffness and the void's diameter. The larger the diameter, the less stable the void is. In addition, the greater the fluidity of the fresh concrete paste and the smaller the stiffness, the less stable the void is. While concrete remains fresh, the fiber-reinforced concrete matrix is less fluid and stiffer. In consequence, it is able to retain larger voids. On the contrary, the plain concrete matrix is more fluid and less stiff. In consequence, it is not able to retain larger voids, and the voids tend to collapse, becoming smaller voids. Additionally, a part of the air of the collapsed pore gets off, and the global porosity reduces. In all cases, the porosimetric curves show two parts clearly different. In the first part, the curves are approximately straight. It goes from 0 to approximately 1.5 mm in the case of plain concrete, and to around 3.0 mm in the case of fiber-reinforced concrete. This pore length can be named as a \"critical pore length\", and it seems to be related to the stability of the voids. Beyond the critical pore length, the slopes of the curves are progressively reduced. During the first part, the pore volume shows a uniform distribution, i.e., the percentage of pore volume belonging to, for example, the pore length range 0.0-0.5 mm is the same that the one belonging to, for example, the pore length range 1.0-1.5 mm. On the contrary, during the second part of the curve, the participation of the different pore length ranges decreases, while the pore length increases. This means, for example, that the pore length range 3.0-3.5 mm retains more pore volume than the pore length range 4.0-4.5 mm. Figure 7: (**a**) Pore volume curves and (**b**) porosimetric curves.