## 1 Introduction  
Concrete is the most commonly used building material, having been used for hundreds of years; needless to say, it has been the subject of many research studies. From the very beginning of its use, it was clear that concrete has high compressive strength and low tensile strength. In order to address the weakness of concrete under tension, concrete started being reinforced with steel bars (reinforcement) in the tensile zone. In this way, the concrete tensile properties are improved, and this also enables the concrete to be used at high tensile stresses. However, high tensile stresses cause cracking in concrete, which, in an aggressive environment, can cause steel corrosion, thus endangering the durability of reinforced concrete. Adding steel bars to concrete also increases the structure weight, especially for structures requiring a significant amount of reinforcement, such as bridges. In this way, a self-weight load, which decreases the bearing capacity of structures, is significant. To avoid these shortcomings caused by adding steel bars to concrete, reinforcing concrete with other materials (e.g., natural fibers [1]), instead of steel bars, has been considered. One such material is carbon fibers. There are many ways of using carbon fibers as a building material, and this is the subject of numerous research studies. If the steel reinforcements were replaced with carbon-fiber-reinforced polymer (CFRP) bars, then the weight of the reinforced concrete structure would be significantly reduced, and the corrosion risk would be eliminated [2]. By reinforcing concrete in this way, a high strength-to-weight ratio would be achieved [3], while the displacement would increase up to 1.86 times compared to that of the concrete reinforced with steel bars, and the ductile performance would be 50% higher [4]. The bond between the CFRP bars and the concrete has a huge influence on the behavior of the structure, which was researched in [2; 5]. Carbon fiber laminate sheets present another way of implementing carbon fiber into building materials [6; 7; 8; 9; 10]. The longitudinal carbon fiber sheets (CFSs) wrapped around the bottom surface of the reinforced concrete beams act together with the bottom longitudinal reinforcement to resist bending moments, consequently reducing the development of strain on the bottom longitudinal reinforcement [6]. They also improve the torsional capacity of beams [7] and increase the structural stiffness, strength, and energy dissipation capacity of a beam-column joint [8; 9]. Wrapping concrete cylinders with CFS also increases their compressive axial strength [10]. The use of CFS is especially suitable for structure rehabilitation and renovation. The third application of carbon fibers in civil engineering is reinforcing concrete with short carbon fibers, hereinafter called carbon short-fiber-reinforced concrete (CSFRC). The efficiency of this type of concrete reinforcement depends on the angle between the carbon fiber direction and the stress direction and percentage of carbon fibers in the concrete mixture [11; 12; 13]. Fiber orientation can be directed as desired using a special technique. Thus, for 1 vol% of carbon fibers in the cement paste aligned in the stress direction, there is a 5.6-foldincrease in the strength of flexural cement paste compared with plain cement paste, while for 3 vol% of carbon fibers aligned in the stress direction, the increase in flexural cement paste strength is around 14.40-fold. In contrast to the flexural strength, the compressive strength decreases around 1.17 times for 1 vol% of carbon fibers and 1.22 times for 3 vol% of carbon fibers [11; 14]. For 1 vol% of randomly dispersed carbon fibers, there is around a 2.45-fold increase in strength for the flexural cement paste compared with the plain cement paste [11]. In hardened concrete, the flexural concrete strength increase is around 2.70- and 4.65-fold for the specimen with 1 vol% and 3 vol% of carbon fibers, respectively, compared with unreinforced concrete.