**1. Introduction**

Concrete is a porous material in nature. The content of pores is very wide between different concrete mixtures. In most cases, the concrete elements show an inevitable porosity, which is what remains inside concrete mixtures after being vibrated to eliminate all the entrained air (using, for example, surface or needle vibrators, etc.). In other cases, the concrete mixture is specially designed to include a certain porosity level, depending on particular needs.

Porosity has a strong influence on the behavior of fresh concrete, because it modifies the rheology of the mixture [1–3]. Moreover, porosity modifies the mechanical properties and the durability of hardened concrete. Among other effects, porosity has a relevant impact on permeability [4–7], the behavior under freeze-thaw cycles [8,9], the behavior under fire [10], and the behavior under fatigue loading [11–13].

The porosity of concrete must not necessarily be a problem. In fact, in some cases, porosity is a sought property. For example, an increase in porosity leads to an increase in the flowability of concrete, which is a needed requisite for pumpable concretes [14]. Pervious concrete is a good example of how porosity is a sought-after property. In this case, concrete pavement is designed with an extremely high volume of pores, in order to assure that roads remain dry when it is raining, even during extreme downpour, increasing the safety of the road [4,6]. This same idea can be used for ultralight concretes [15].

Another situation where air voids are useful is in the case of concrete elements subjected to freeze–thaw cycles [8,9]. Under these cycles, porosity improves the resistance of the concrete. In fact, concrete elements placed in regions with extreme freeze–thaw cycles events must be designed with a minimum threshold of porosity.

On the other hand, there has been an increasing use of fiber-reinforced concrete. It is a very suggestive solution because of the reduction of the labor cost, especially if it is combined with self-compacting concrete. In most cases, fibers are used to improve the mechanical behavior of concrete: they reduce cracking [16–18], improve the fatigue life [19–22], increase the tension strength capacity of concrete [23,24], improve the behavior under freeze–thaw cycles [25,26], and extend the fatigue life [27,28].

A different case involved the use of plastic fibers to improve the behavior of concrete under fire [29,30]. In this case, the strategy is that plastic fibers melt under a relatively low temperature, which is significantly below the temperature that fire typically starts to result in spalling in conventional concrete, and thus the internal overpressure caused by water vapor is dissipated.

However, in all the research works mentioned above, it has been implicitly assumed that fibers do not modify the concrete matrix, i.e., the microstructure of concrete matrix and, in particular, the pore morphology (voids content, pore size distribution, shape of the voids, etc.) is not affected by the presence of fibers.

The voids inside concrete can be classified into micropores (size less than 1 μm), mesopores (size between 1–10 mm), and macropores (size greater than 10 mm) [31]. Several methods can be found in the literature to analyze the pore structure. The traditional ones are nitrogen absorption and mercury intrusion porosimetry (MIP) [32,33]. These methods show two main limitations. First, they can only provide the pore size distribution, but not the pore distribution, shape, etc. Second, these techniques can only provide information about open porosity, and not about closed porosity.

Currently, the use of computed tomography (CT) scan technology is being used to analyze, in general, the microstructure of concrete and, in particular, the pore structure. Most of the research conducted has focused on fiber-reinforced concrete, and hence fiber orientation [12,34,35]. However, in the last years, there has been a growing interest in the internal pore structure, and several works have been published in this area [4,36–43].

Using CT scan technology, it is possible to visualize all the pores of the concrete samples, and not only the open porosity, but also the closed porosity. CT scan technology provides a lot of useful information of each individual void, such as the position, volume, length, etc. With this information, it is possible to determine several geometrical parameters, such as the shape factor, among others. Moreover, it is possible to obtain several correlations, such as the spatial distribution, among others.

In addition, when the scanning process is carried out daily during the first curing week of the specimens, it is possible to analyze the evolution of all the geometrical parameters of the voids over time.

This information can be used as a basis to establish the correlation between the porosity of the concrete and its macroscopic response.

The CT scan technology is a powerful tool; to date, no other technology can provide this information about the internal microstructure of concrete.

In this paper, the CT scan is used in order to detect the voids inside two different concrete mixtures: plain and fiber-reinforced concrete, and also to study the evolution of the voids over time, during the first curing week. In both cases, the concrete paste is the same, and the only difference is the presence of steel fibers. Using post-processing routines especially developed by the authors, it is possible to analyze the pore morphology in both cases: porosity, pore size distribution, pore shape, etc., and its variations with time during the early ages of concrete. The results show that plain and fiber-reinforced concrete mixtures have initial differences in pore morphology, and they also exhibit a different variation with time. The final result is that both concretes have very different pore morphology at the end of the studied time. The results also reveal the two mechanisms behind the differences between the final pore morphology of plain and steel fiber-reinforced concrete.

This paper is structured as follows: the experimental procedure is presented in Section 2, the results of the tests are described and discussed in Section 3, and finally the conclusions are found in Section 4.

## **2. Experimental Program**

In this section, the materials, the manufacturing procedure, and the scanning procedure are described.
