## 1 Introduction  
Fiber-reinforced concrete is a structural solution of growing use. In most current uses, fibers are used to improve the mechanical and/or structural behavior of hardened concrete. The most common use of fibers is to reduce the cracking in concrete, thus directly improving the durability of concrete [1; 2; 3], although it cannot be strictly considered as a structural use of fibers. From a structural point of view, fibers are currently being used to improve the tensile strength of concrete, replacing totally or partially the conventional rebars. The research reveals that fibers have a positive impact on the post-peak behavior of concrete, although it strongly depends on fiber amount and orientation [4; 5; 6; 7; 8; 9; 10; 11]. Other current uses are to improve the fatigue behavior of concrete [11; 12; 13; 14; 15; 16; 17], the behavior under impact loads [18] or freeze-thaw cycles [19; 20; 21], among others. Moreover, during the past few years, a lot of research has been conducted regarding new types of fibers, beyond the most conventional ones (i.e., steel, polypropylene, carbon, glass, etc.) [22; 23; 24; 25]. It has been commonly assumed that, in general, the fibers do not modify the concrete matrix, except for the interface between concrete matrix and fiber, which tends to be more porous. However, some research has been conducted in the last years showing that fibers modify the concrete matrix [26; 27; 28]. This work aims to demonstrate that steel fibers modify the concrete matrix because they significantly alter the main geometrical parameters of the internal voids. This fact could have an impact on certain mechanical behaviors directly related to porosity, e.g., freeze-thaw cycles or fatigue. Computer tomography (CT) scan technology was used to observe the different geometrical patterns of internal voids due to presence of fibers inside the concrete specimens. This is a relatively new technology for concrete, although in some other fields, like medicine, it has been used since more than 40 years ago. Most of the previous research mainly focused on the fiber orientation inside the concrete [29; 30; 31; 32; 33; 34; 35]. However, in recent years, there is an increasing interest on the study of concrete porosity and several interesting works have been published in this field. Most of them are focused on plain concrete and, more particularly, on pervious concrete [36; 37; 38; 39; 40; 41; 42; 43; 44; 45]. Most of this research analyzes the correlation between porosity and macroscopic behavior, like fatigue or freeze-thaw cycles, among others. Hardened concrete always includes voids which play a vital role for concrete property. Some of the voids are unavoidable, as they are filled by air due to the loss of water during the curing process. In other cases, concrete is designed with a preset amount of internal voids, depending on functional needs [35; 46; 47; 48; 49; 50]. In these cases, the shape of the voids, their size distribution, etc., influence the macroscopic behavior of hardened concrete. According to the International Union of Pure and Applied Chemistry (IUPAC), pores can be classified according to their size into micropores (smaller than 2 nm), mesopores (from 2 to 50 nm) and macropores (larger than 50 nm) [51; 52; 53]. Several methods to analyze the voids can be found in the literature. The most traditional ones are mercury-intrusion porosimetry (MIP) and nitrogen absorption. These methods have 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 the open porosity and not about the closed porosity. In this situation, CT-Scan technology can overcome the limitations of the traditional methods and is able to provide information about the shape of all the voids inside the concrete specimen. In this study, two different concrete mixtures were studied: plain concrete (PC) and steel-fiber-reinforced concrete (SFRC). In both cases the concrete mixture is identical.