**4. Conclusions**

This paper analyzed the internal pore morphology of two different mixtures through the use of CT scan technology. Both of them had the same matrix (same type and amount of cement, fine aggregate, nanosilica, water, and superplasticizer); the only difference was that one mixture, which was called SFRC, included steel fibers by 0.1% volume, while the other mixture, which was called PC, did not include fibers. The pore morphology was measured during the first week (when most of the curing process occurs) at five different ages, i.e., one day, two days, three days, four days, and seven days.

All of the specimens were kept in a curing room at 20 ◦C and 100% humidity. The information provided by the CT scan was post-processed using DIP software. Each individual void was identified and isolated, and its main parameters were extracted, which were the X, Y, and Z coordinates of the center of gravity, the volume, and the length, respectively.

Some worthy and interesting results are summarized below:


closer to the exposed surface, less stiffness and a smaller amount of voids are retained. Even in the case of SFRC, fibers do not provide the extra stiffness in an efficient way.


The modification of the pore morphology caused by the fiber can potentially affect the macroscopic response of hardened concrete, especially in those properties more directly related to porosity, such as frost resistance, fire resistance, creep, shrinkage, and fatigue, among others.

**Author Contributions:** J.M. carried out the scanning process and the images analysis, D.C.G. worked on the design of the experiment and on the results analysis and M.A.V. worked on the analysis of the results and wrote the paper.

**Funding:** This study was funded by the Ministerio de Economía y Competitividad, Gobierno de España (Spain) (grant number BIA2015-68678-C2-R).

**Conflicts of Interest:** The authors declare that they have no conflict of interest.
