# Mechanical Behavior of Carbon Fiber-Reinforced Concrete Structures After Structural Failure
## 1 Introduction  
Over the past 30 years, fiber-reinforced polymers (FRPs) have been continuously used in the repair, strengthening, reconstruction, and new construction of structures [1, 2, 3]. The use of this material offers advantages such as high strength, durability, corrosion resistance, lightweight properties, ease of installation, and fatigue resistance, making it an attractive option for improving the capacity and performance of structures [4, 5]. The use of fiber-reinforced polymers (FRPs) to strengthen concrete columns through confinement is an effective solution to address the deterioration of mechanical properties of columns over time or to enhance their mechanical capacity to support higher loads [6]. The scientific literature has demonstrated the benefits of confining concrete columns with FRPs. Columns confined with FRPs show significant improvements in compressive strength, ductility, stiffness, and durability compared to unrein-forced concrete columns [7, 8, 9, 10]. These improvements are particularly relevant in the context of rehabilitating existing structures, where the aim is to enhance the load-bearing capacity and structural behavior of structural elements without the need for major interventions [11, 12, 13]. There are some concerns and limitations associated with the strengthening of fully confined concrete columns with FRPs, such as the adhesion between the FRP and the concrete surface and the high associated costs [14, 15, 16]. Moreover, in certain cases, full reinforcement of the columns may not be necessary in terms of compressive strength. Instead, they can be partially confined with FRP bands, achieving sufficient performance with a reduced amount of reinforcement fiber [17, 18, 19, 20]. Another important aspect of this type of reinforcement is the minimum requirements that the substrate must meet. The surfaces must be clean and free of anything that could compromise adhesion, must have sufficient flatness, and must have a moisture content below 4%. Regarding mechanical requirements, the minimum compressive strength of the concrete must exceed 15 N/mm\({}^{2}\) and the substrate's pull-off strength must exceed 15 N/mm\({}^{2}\)[21]. In recent years, there has been an increase in the incorporation of fibers into concrete. Among the fibers used, synthetic and polymeric fibers have been widely employed to improve the crack resistance of concrete. In particular, polypropylene macrofibers have gained significant importance due to their strength capacity and good adhesion to the concrete matrix [22; 23; 24]. The incorporation of polypropylene (PP) fibers, or their hybridization with other types of fibers, provides concrete with various benefits, such as toughness, lightness, improved fire performance by reducing the \"spalling\" effect, and enhanced chemical resistance in alkaline environments [25; 26; 27]. Studies have also examined the parameters that determine the behavior of fiber-reinforced concretes, with the most important being fiber geometry, fiber content, and the orientation and distribution of the fibers [28; 29; 30; 31; 32]. In the proposed research context, some studies have investigated the influence of structural synthetic discontinuous polypropylene fibers (DSSPFs) on the adhesion between fiber-reinforced concrete and carbon fiber-reinforced polymers (CFRPs). These studies have shown that the addition of fibers slightly improves adhesion strength with CFRP composites [33]. In concrete elements subjected to compression and reinforced with CFRPs, with the addition of fibers, it was observed that lightweight concretes achieved lower strengths than normal-weight concrete [33; 34]. Furthermore, the addition of carbon fibers or hybridization with polypropylene fibers barely changed the axial compression behavior of lightweight concrete confined with CFRPs [34]. The compressive strength of the concrete to which the reinforcement is applied was investigated in concretes with hybrid fibers, focusing on low-strength concretes and considering the number of reinforcement layers.