## 1 Introduction
These two values correspond to the 0\({}^{\circ}\) alignment angle between the direction of fibers and stress. At an alignment angle of 30\({}^{\circ}\), the flexural concrete strength increase is around 1.13-fold for 1 vol% and around 1.57-fold for 3 vol% of carbon fibers compared with unreinforced concrete [13]. These results show that the influence of carbon fibers on concrete strength decreases with increasing angular deviation between the directions of fiber and stress. This influence can be considered effective for angular deviations up to 20\({}^{\circ}\), while there are hardly any effects at 30\({}^{\circ}\), and no effects can be seen at 40\({}^{\circ}\)[13]. In order to achieve a suitable alignment angle between the directions of carbon fiber and stress, 3D concrete printing is the most efficient tool [15], while an even distribution of carbon fibers in cement paste can be achieved using silica fume (SF) [16]. However, it is necessary to be careful when applying SF because, besides the positive influence on the concrete mechanical parameters (strength, elasticity modulus, etc. ), it can also have a negative influence, e.g., increasing concrete shrinkage [16; 17; 18; 19; 20]. Reinforcing cement paste or concrete with short carbon fibers causes changes in its behavior, of which only some changes will be mentioned here. For example, shrinkage can cause cracking of cement paste at an early age. However, adding carbon fibers to cement paste reduces the shrinkage and prevents cracking at an early age [17; 19; 20; 21]. The concrete behavior changes as well. It is known that unreinforced concrete subjected to tensile load or bending suddenly undergoes brittle failure without any prior notice. Unlike unreinforced concrete, CSFRC shows pseudo-ductile behavior, so the concrete no longer experiences brittle failure [13]. Between the time of appearance of the first crack and concrete failure, a dense network of cracks will develop, which causes large energy dissipation. These changes in concrete (appearance and connection of micro-cracks) during the loading process also cause changes in its thermo-electrical features. Thus, there is high concrete electrical conductivity before loading and appearance of the first crack that decreases during the process of loading and damage. The electrical resistivity is inversely proportional to the electrical conductivity, so it is lower at the beginning of the loading process and increases during loading and damage of the structure. Based on this fact, it is possible to detect damage in the CSFRC over time in a nondestructive way. Because of its ability to be used as a self-sensor, the CSFRC is also considered a smart concrete [22; 23; 24; 25; 26; 27; 28; 29; 30]. It is obvious from the previous brief description that CSFRC has very complex behavior; hence, the main aim of this paper is to contribute to understanding its behavior. For that purpose, 24 CSFRC specimens were tested under tension, whereby strain gauges and digital image correlation (DIC) were used. The obtained results enabled the determination of the constitutive stress-strain law and damage processes in the specimens. Specimens of different ages were tested with short-term static tensile load. Unlike the articles from the literature, where the tensile strength and other mechanical parameters were mostly tested for specimens of only a single age (mostly 28 days), the ages of specimens in this work were 9, 14, 21, 28, 35, 49, 70, and 91 days. In this way, it is possible to estimate the influence of the specimens' age on their behavior. Based on those results, the influence of concrete shrinkage was investigated with the aim of highlighting its importance.