## 3 Experimental Results and Discussion
### Total Volume of Voids and Porosity  
The first parameter to be studied in all the specimens is the total volume of voids and the porosity. This last parameter is defined as the quotient between the total volume of voids and the volume of the specimen. Table 2 shows the average values of the total volume of voids and the porosity for the different mixtures and curing conditions. The values in brackets are the standard deviation.  
The values of porosity shown in Table 2 are, in general, low, since not all the porosity was measured; as explained before, only voids with a length between 75 \(\upmu\)m and 10 mm were considered.  
Table 2 shows that fiber-reinforced concrete specimens have significantly higher total volume of voids and porosity compared to plain concrete specimens, of around two or three times higher. This happens in both moist-cured and air-cured concrete. It is concluded that, in this case, steel fibers help to increase the internal porosity.  
Beyond the porosity that is trapped during the concreting process, most of the air voids in hardened concrete are because of the loss of water during the curing process. Water consumption during the hydration process and evaporation of water are the two main causes of air voids inside concrete.  
In the case of plain concrete, simultaneously with the water consumption and the appearance of voids, they tend to collapse (at least partially) because of the hydrostatic pressure of fresh concrete, as well as its low stiffness. This space is filled by the fresh concrete paste. This process leads to a reduction of the porosity, and the macroscopic effects are the well-known phenomena of plastic settlement of concrete and initial plastic shrinkage.  
In the case of steel-fiber-reinforced concrete, fibers provide higher viscosity to fresh concrete paste and also greater stiffness, which implies a less tendency to the voids' collapse.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Mixture and Curing Condition** & **Total Volume of Voids (mm\({}^{3}\))** & **Porosity (\%)** \\ \hline SFRC. Moist-Cured & 408.14 (40.3) & 0.63\% (0.06\%) \\ SFRC. Air-Cured & 424.60 (44.1) & 0.65\% (0.07\%) \\ PC. Moist-Cured & 112.18 (11.8) & 0.17\% (0.02\%) \\ PC. Air-Cured & 179.23 (16.6) & 0.28\% (0.03\%) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Total volume of voids and porosity.  
Figure 5: Three-dimensional views of the fibers belonging to different curing conditions.  
Additionally, plain concrete specimens subjected to air curing show a slight greater volume of voids and porosity. This is directly related to the moisture of the room where the specimens were cured. In the case of air-cured specimens, more evaporation occurs compared to moist-cured specimens. The lower loss of water in this last case implies a lower porosity. This phenomenon is less clear in the case of fiber-reinforced concrete.  
The difference between air-cured concretes and moist-cured concrete is small, since the concrete-exposed surface is small. In addition, in the case of steel-fiber-reinforced concrete, fibers help to mitigate the voids' collapse, even close to the exposed surface.