# Mechanical Behavior of Carbon Fiber-Reinforced Concrete Structures After Structural Failure
## 4 Conclusions  
This article presents an experimental study on the compressive behavior of cylindrical concrete specimens with and without polypropylene fibers, reinforced with CFRP. Forty-eight specimens were tested under axial compression load with stresses below 15 N/mm\({}^{2}\) and above 15 N/mm\({}^{2}\). From the analysis of the results, the following conclusions can be drawn:  
* The greatest compressive strength increases obtained by confining concrete were achieved in concretes with unconfined strengths below 15 N/mm\({}^{2}\) (Type II), reinforced with unidirectional fabrics of 600 g/m\({}^{2}\) weight. Specifically, a 464% increase was obtained in reference concretes (SA-5-U600) (Specimen 5 confined with MapeWrap C UNI-AX 600 carbon fibers without the addition of polypropylene fibers) and a 393% increase in concretes with polypropylene fibers (PP-4-U600) (Specimen 4 confined with MapeWrap C UNI-AX 600 carbon fibers with the addition of polypropylene fibers). * The reinforcement of CFRP with the unidirectional fabric of 600 g/m\({}^{2}\) results in a greater increase in strength compared to unreinforced concrete than when using a unidirectional fabric reinforcement of 300 g/m\({}^{2}\). When reinforcement is carried out with bidirectional fabrics of 230 and 360 g/m\({}^{2}\), the strength increases are significantly lower than those achieved with unidirectional fabrics of 300 and 600 g/m\({}^{2}\), regardless of whether the concrete contains polypropylene fibers or not. * As shown in Figures 2 and 3, the strain energy density (area under the stress-strain curve) increases substantially when the concrete is reinforced with carbon fiber fabrics, reaching the highest values when unidirectional reinforcements with a weight of 600 g/m\({}^{2}\) are used. This is observed both in reference concretes and in concretes with polypropylene fibers, significantly enhancing the ductility of the concrete. * The confinement reinforcement of concrete with bidirectional carbon fibers increases the strength compared to unreinforced concrete, but in a similar manner regardless of the fabric weight used (230 and 360 g/m\({}^{2}\)) or whether the concrete contains polypropylene fibers or not. Higher stress levels were only achieved when the unreinforced concrete had compressive strengths below 15 N/mm\({}^{2}\) (Type II), using the reinforcement with the higher fabric weight. * The addition of polypropylene fibers to the concrete does not improve its compressive strength but enhances the cohesion of the concrete, thereby preventing explosive failure of the specimens. * Type II stresses (\(\sigma<15\) N/mm\({}^{2}\)) have achieved the highest values compared to Type I stresses. Additionally, there is a closer alignment of stress values for bidirectional fibers compared to unidirectional fibers. * The mechanical behavior of confined reference concretes reinforced with carbon fiber is on the safe side compared to the theoretical values formulated by specialized standards (ACI [48] and FIB [49]). However, the same cannot be said for concretes with polypropylene fibers. * Low-performance concretes with strengths below 15 N/mm\({}^{2}\) can achieve significant increases in compressive strength when confined with unidirectional carbon fiber fabrics of 600 g/m\({}^{2}\) weight. The same type of reinforcement in concretes with the addition of polypropylene fibers also generates significant improvements in compressive strength. However, the values obtained are not always on the safe side concerning the existing standards (ACI [48] and FIB [49]). Conceptualization, T.G.P. and M.I.P. ; methodology, T.G.P. and M.I.P. ; software, A.L. ; validation, D.F., M.I.P. and T.G.P. ; formal analysis, A.L. ; investigation, M.I.P. ; resources, D.F. ; data curation, A.L. ; writing--original draft preparation, T.G.P. ; writing--review and editing, M.I.P. ; visualization, D.F. ; supervision, A.L. ; project administration, D.F. ; funding acquisition, D.F. All authors have read and agreed to the published version of the manuscript.