# Mechanical Behavior of Carbon Fiber-Reinforced Concrete Structures After Structural Failure
## 2 Materials and Methods  
Based on the objective of the study, a mix design was developed to produce low-performance concrete [37; 38], resulting in the preparation of 24 reference concrete specimens and 24 specimens with polypropylene fibers. The 48 specimens were tested to failure, separating each type into two groups based on whether their compressive strength was greater or less than 15 N/mm\({}^{2}\), the threshold recommended by manufacturers of this type of reinforcement and the reference value for the study. Within each group, the specimens were randomly reinforced with four types of reinforcements (unidirectional and bidirectional, used for compression confinement and flexural-compression reinforcements, respectively, with two fabric weights for each type to evaluate the improvement in structural performance). The reinforced specimens were then tested to failure under compression to evaluate the increase in stress and strain that reinforced concrete can achieve. The materials used for the experimental development of the research are as follows:  
* Cement type CEM II/B--L 32.5 N, in accordance with UNE-EN 197-1:2011 (Cement. Part 1: Composition, specifications, and conformity criteria for common cements) [39] and RC-16(Code for the design and execution of plain or reinforced concrete structures) [40]. * Coarse aggregate: Rolled aggregate with a granulometric fraction of 4-12 mm and a maximum aggregate size of 12 mm, siliceous in nature, in compliance with UNE-EN 12620:2003+A1:2009 (Aggregates for concrete) [41]. * Fine sand: Washed sand with a granulometric fraction of 0-4 mm, in accordance with standard UNE-EN 13139/AC:2004 [42]. * Potable water supplied by the Canal de Isabel II in the community of Madrid, meeting the technical requirements established for structural concrete. * Plasticizer additive Sikament-165 ES, commonly used for all types of concrete. * Polypropylene fibers: Extruded monofilaments with a diameter < 0.30 mm and a length of 12 mm (Sikafiber M-12). * Unidirectional carbon fiber fabrics (MapeWrap C UNI-AX) with weights of 300 and 600 g/m\({}^{2}\), equivalent dry fabric thickness of 0.164 mm, tensile strength > 4900 N/mm\({}^{2}\), tensile modulus of 252 GPa, and elongation at break of 2%. * Bidirectional carbon fiber fabrics (MapeWrap C BI-AX), with weights of 230 and 360 g/m\({}^{2}\), equivalent dry fabric thicknesses of 0.064 mm and 0.1 mm, respectively, tensile strength > 4800 N/mm\({}^{2}\), tensile modulus of 230 GPa, and elongation at break of 2.1%. * Epoxy resin MapeWrap 31, a two-component resin used to bond the carbon fiber fabrics to concrete [43]. A total of 48 cylindrical specimens with a diameter of 100 mm and a height of 200 mm were prepared, in accordance with UNE-EN 12390-1:2022 [44]. Half of the specimens were produced without any additions (SA), while the other half were produced with the addition of 3 kg/m\({}^{3}\) of polypropylene fibers (PP), with a cement-sand-gravel-water ratio of 1:2:3:0.5. All specimens were subjected to compressive failure testing in accordance with UNE-EN 12390-3:2020 [45] and were divided into two groups based on the results of the compressive test, using the following nomenclature: Type I specimens, those with a compressive strength greater than 15 N/mm\({}^{2}\), and Type II specimens, those with a compressive strength less than 15 N/mm\({}^{2}\). Once tested to failure, the specimens were reinforced with the four types of carbon fiber fabric. The experimental phase was conducted in accordance with applicable international standards (CNR-DT 200 R1/2013; EN 14889-2:2006) [46; 47]. The nomenclature for specimens with and without polypropylene fibers, reinforced with the various types of fabric, is presented in Table 1.  
Once the materials were prepared in the laboratory of the Escuela Tecnica Superior de Edificacion de Madrid, they were mixed in the IBERTEST planetary vertical-axis mixer, model CIB-701, updated to IB32-040V0. The cement, gravel, sand, and fibers were dry-mixed for two minutes to achieve material homogenization.