Evolution Mechanism of Carbon Fiber Anode Properties for Functionalized Applications: Impressed Current Cathodic Protection and Structural Strengthening
Ji-Hua Zhu
,
Qujian Li
,
Chun Pei
,
Hongtao Yu
,
Feng Xing
Evolution Mechanism of Carbon Fiber Anode Properties for Functionalized Applications: Impressed Current Cathodic Protection and Structural Strengthening
Carbon fibers have excellent properties, including high strength, light weight, corrosion resistance, and high durability; therefore, they are widely used in various fields. Carbon fibers possess excellent electrical conductivity and electrochemical stability, and they can be used as electrode materials for functionalized applications in civil engineering. This study explores the evolution mechanism of the electrochemical properties of carbon fibers and carbon fiber composites used as anodes. This study further focuses on the collaborative intervention technique of impressed current cathodic protection and structural strengthening (ICCP-SS) for reinforced concrete structures, as well as the non-destructive recycling of carbon fibers based on their electrochemical evolution mechanism. This study aims to provide new ideas for the functionalization of carbon fiber composites in civil engineering.
Carbon fiber is widely used in the fields of aerospace, defense, railway transportation, automobile manufacturing, medical equipment, and sports equipment owing to its excellent mechanical properties and corrosion resistance [1], [2], [3], [4], [5], [6], [7]. Recently, improvements in the carbon fiber preparation process and reductions in production costs have promoted the application of carbon fiber and its related products in the field of civil engineering [8,9]. Owing to the good mechanical properties of carbon fibers, carbon fiber-reinforced polymer composites (CFRPs) have been successfully applied for the structural strengthening of bridges, tunnels, and high-rise buildings to improve their structural load-carrying capacity [10,11].
Carbon fiber exhibits good conductivity and electrochemical stability, which make it suitable for use as an electrode material. The rapid development of carbon fiber electrode materials in the energy, environment, and life medicine fields has demonstrated their broad application prospects as electrode materials [12], [13], [14], [15]. Recently, carbon fibers have been applied as electrode materials in civil engineering, demonstrating that carbon fibers and their composites can be used as anode materials for the cathodic protection of reinforced concrete (RC) [16], [17], [18], [19] and the electrochemical dichlorination of concrete [20], [21], [22]. This can effectively improve the protection efficiency of steel reinforcements and prolong the service life of RC structures.
Despite the significant potential of carbon fibers and their composites as electrode materials in civil engineering, relatively few studies have investigated this topic [23]. This study aims to systematically explore the anode electrochemical behavior of carbon fibers in civil engineering environments, introduce impressed current cathodic protection and structural strengthening (ICCP-SS) technology for coastal concrete, and investigate a non-destructive electrochemical recycling technology for carbon fibers based on this behavior. This study provides a theoretical basis and technical support for further application and research on carbon fibers and promotes their innovative application in civil engineering.
1.1. Evolution mechanism of anode properties of carbon fibers in strongly alkaline and high-chlorine environments
With increasing demands for improved safety, lower carbon emissions, and intelligent development in concrete construction, the multi-functional development of building materials has become critical in civil engineering. Carbon fiber is a versatile material that is used for applications in many fields. Its electrochemical processes in specific environments can provide a theoretical basis for corresponding functionalized applications. A new approach for exploring the functionalization of carbon fibers for civil engineering applications focuses on improving their electrochemical properties. The use of these electrochemical processes in concrete can enable the stable and efficient application of carbon fiber electrode materials in the field of civil engineering.
1.2. Anode electrochemical behavior of carbon fibers
Carbon fiber is an advanced functionalized carbon material that has been widely used in energy, environmental engineering, and other related fields. To improve the stability and efficiency of carbon fiber anode materials, numerous studies have investigated their anodic degradation behaviors and mechanisms. Assuming a strongly alkaline and high-chloride environment leading to concrete erosion by chloride salt and based on relevant research in other fields, the mechanism of the influence of hydroxide and chloride ions on the deterioration of carbon fiber anodes can be described as follows.
Under anodic polarization, the polyacrylonitrile (PAN)-based carbon fiber surface first forms protonated carbon active sites (C+), which hydrolyze to form surface carbon oxides. In addition, the hydroxyl free radical (·OH) formed by the electrochemical oxidation of OH− ions [24] reacts with the alkyl side chains at the edge of the small graphite domain. These two oxidation processes lead to the formation of a series of oxidized functional groups on the surface of the carbon fiber, such as C-OH, C-O-C, C=O, -COOH, and CO32− [25]. These oxygen-containing functional groups weaken the van der Waals interactions between the graphene layers of the carbon fiber. As a result, the solvated anions in the electrolyte can be inserted between the graphene layers of the carbon fiber, resulting in expansion of the interlayer gap and spallation of the carbon layers, thereby destroying the skeleton structure of the carbon fiber (Fig. 1) [25], [26], [27], [28], [29], [30]. Simultaneously, the surface of the carbon fiber can undergo an oxygen evolution reaction (2·OH → O2 + 2Hads) at a certain potential, which plays a further role in the physical peeling of the carbon layers, Hads denote hydrogen atoms in the adsorbed state. As the external current density increases, the relevant electrochemical reactions on the surface of carbon fiber intensify, resulting in severe damage to the skeletal structure of the carbon fiber. In the presence of chlorine salts, chloride ions can react with ·OH to form free chloride radicals (·Cl) [31], which can alleviate the rate of the oxygen evolution reaction, thus inhibiting the physical peeling of the carbon layer. Simultaneously, ·Cl and perchlorate ions formed by the oxidation reaction of chloride ions can promote the oxidation reaction of carbon atoms on the surface of carbon fibers to form more active functional groups. This uniformly improves the electrochemical activity of the carbon fiber surface, thus promoting uniform oxidation degradation of the carbon fiber and improving the service efficiency of carbon fiber materials. Experimental results have demonstrated that in strongly alkaline and high-chlorine salt environments, carbon fibers exhibited an improved anode performance. The relevant reactions occurring on the surfaces of the carbon fibers are listed below.
The surface hydrophobicity/hydrophilicity, chemical functionality, surface area, and pore structure of the carbon fibers will be altered by a series of electrochemical oxidation reactions, which affect the performance of the carbon fibers as electrode materials. During this process, the degradation rate is influenced by various factors, including the initial properties of the carbon fibers (crystallinity, grain size, defect type and density, surface functionality, and pore structure), electrolyte environment, and electrolysis conditions (potential window and current density). Therefore, addressing the durability of carbon fibers under anodic polarization conditions is critical for their use in electrochemical functional applications in the civil engineering of RC building systems located in coastal or high-chlorine environments.
1.3. Anode electrochemical behavior of CFRP
In engineering applications, carbon fibers are typically combined with resins to form CFRPs with excellent mechanical properties; these CFRPs are commonly used for structural reinforcement [32,33]. Therefore, the anodic electrochemical behavior of CFRP materials is closely related to their practical application as electrode materials in civil engineering [34,35]. Because resins are insulating materials, the electrochemical properties of CFRPs depend mainly on their carbon fiber components. Previous research has indicated that under strongly alkaline conditions, the anodic electrochemical reaction occurring on the surface of carbon fibers not only leads to their oxidative degradation, but the oxidation products also exert a certain influence on the resin matrix, which can lead to the separation of the carbon fibers from the resin [34],[36], [37], [38]. As the external current density increases, this degradation gradually intensifies, eventually leading to destruction of the CFRP composite structure (Fig. 2(a) [37]). However, in the presence of chloride ions, although the anode performance of the carbon fibers in CFRP is improved, the chloride ion oxidation products (such as ClO− and ·Cl) that are formed are more likely to react with the resin matrix [24,39] and cause C-N bond breakage [40], leading to depolymerization of the polymer chain [37] and degradation of the resin matrix. As the concentration of chloride ions increases, the degradation of the resin matrix increases. An investigation of the mechanical behavior of CFRPs further demonstrated the effect of anode degradation on the degree of degradation of the mechanical properties of the CFRP. A larger polarization current density per unit time results in more significant degradation (Fig. 2(b) [37]). However, in this case, chloride ions can significantly reduce the degradation of the mechanical properties (Fig. 2(c) [41]) and cause the polarized CFRP to exhibit different tensile failure modes (Fig. 2(d)). Therefore, by optimizing the design of the CFRP material components, working environment, and conditions, the role of carbon fiber as an electrode can be effectively improved while maintaining the original mechanical properties, thus promoting the development of multifunctional CFRP materials in the construction industry.
In summary, the electrochemical reaction process of carbon fibers, the anode degradation mechanism, and the anode degradation behavior of CFRP materials provide a theoretical basis for the electrochemical application of carbon fiber materials in civil engineering.
2. Materials and methods: Carbon fiber functionalization for civil engineering applications
2.1. Integrated concrete structure technology for prolonged lifespan: ICCP-SS
Owing to the excellent anode stability of carbon fibers at low currents in chloride-salt environments, the design of carbon fiber cementitious composite anode materials with structural enhancement functions has led to the development of an integrated technology that extends the life of coastal RC structures: ICCP-SS. The ICCP-SS technology not only prevents the corrosion of steel reinforcements but also improves the mechanical properties of RC structures. On one hand, this technology can be used for durability repairs and performance enhancement of existing RC structures to extend their service lives [42], [43], [44], [45]. On the other hand, the technology can be used as a combined system for new RC structures, which has the unique advantage of utilizing seawater sea sand with a high chloride ion concentration. The ICCP-SS system (Fig. 3) consists of an existing RC building, carbon fiber-based multifunctional anode (CFMA), power supply, monitoring equipment, and wiring. The CFMA is the core component of the system, which not only acts as the anode but also as a structural strengthening material, can be considered as carbon fibre fabric reinforced cementitious composite material (C-FRCM).
2.1.1. ICCP-SS system design concepts
In the cathodic protection technology, carbon fiber has a higher potential than carbon steel and can act as an anode. In addition, both structural reinforcement and cathodic protection can be achieved by arranging functional materials on the concrete surface according to their designed functions. Based on this structural similarity and the excellent mechanical and electrochemical properties of carbon fibers, the design of CFMA composites with both anodic and structural functions have led to the development of the ICCP-SS integrated technology for extending the life of concrete material structures such as coastal concrete; the ICCP-SS system combines electrochemical protection and structural reinforcement. The core concept of the ICCP-SS technology is the design of CFMAs and the evolution of the derived interfacial properties. The ICCP-SS technology is not limited to a particular carbon fiber composite material and can thus utilize the diverse array of available materials. However, the design of CFMAs should focus on the following aspects: the cost of the anode material, the anode/cathode area ratio, and the organic/inorganic composite cementitious materials.
In engineering technology, the cost of functional anode materials has emerged as a critical factor in the top-level planning of ICCP-SS technology. In the context of structural materials, the cost and strength of CFRP increase with the carbon fiber content. Compared with other widely used reinforcement materials, CFRP has no pricing advantage. However, as an anode material in ICCP technology, CFRP exhibits superior cost-effectiveness compared with commonly employed alternatives, which are typically precious metals. Nonetheless, assessments of the anode performance cannot solely rely on the cost of carbon fibers but require a design approach that considers electrochemical reaction systems. Consequently, in the design of CFRP dual-function materials for practical implementation in ICCP-SS technology, paramount importance should be placed on fulfilling the requirements while judiciously considering the mechanical properties. The preferred attributes of the CFRP mesh are high cost-effectiveness and a convenient design. Simultaneously, a low resin content and economical nature can be effectively combined with the characteristics of conductive inorganic cementitious materials.
In ICCP technology, the most direct and effective means of improving the anode service performance is to increase the anode/cathode area ratio, which can reduce the anode current density and delay the deterioration of the anode material. Therefore, to obtain the maximum anode area, the anode is often placed on the surface of the concrete. When the material costs are the same (based on the carbon fiber content), a CFRP bar acts as a point anode, which has the smallest surface area and is the most unfavorable choice. In contrast, carbon fiber cloth and mesh act as face anodes that can be arranged on the surface of the concrete to obtain the largest nominal anode surface area. Carbon fiber mesh not only forms a face anode but can also be combined with conductive substrate materials to form a three-dimensional (3D) conductive network that can further expand the anode area, which is a more favorable design.
In structural strengthening techniques, CFRP is typically bonded to concrete surfaces using organic epoxy resins [46]. However, epoxy resins are not electrically conductive; therefore, the addition of conductive particles [47,48] may be a solution for achieving the ICCP function. However, this method only resolves the electrical conductivity of the ICCP system; it cannot transfer harmful ions inside the concrete to the anodic region to protect the reinforcement. In addition, epoxy resins exhibit poor aging resistance and durability, and the anode reaction product will rapidly deteriorate the interfacial properties of the anode and concrete. Therefore, this method cannot satisfy the performance and service life requirements of ICCP in engineering practice. Inorganic cementitious materials are a more appropriate choice, as these composites have good compatibility with concrete and low manufacturing costs, and their electrical conductivity and long-term polarization properties can be improved using various modification methods. This makes it possible to form a 3D conductive network of primary and secondary anode systems with CFRP materials and further expand the anode area, thereby achieving the ICCP effect more effectively. It may be difficult to fully utilize the mechanical properties of carbon fibers when using inorganic cementitious materials, which is a compromise that must be made to meet the technical requirements of ICCP; this tradeoff is also a key consideration for future ICCP-SS technology optimization.
2.1.2. Design and optimization of composite anode materials
By taking advantage of the high designability of carbon fiber composites, we constructed an innovative micro- and nano-carbon composite-modified CFMA using a continuous carbon fiber mesh as the reinforcing phase and main anode material, and modified inorganic cementitious materials as the matrix phase and secondary anode material (Fig. 4). CFMA can overcome the limitations of traditional resin materials, which cannot provide effective ion carrier; furthermore, improving the mechanical properties of RC structures is difficult when using precious metal anodes. As the core component of ICCP-SS technology, CFMA not only assumes the role of an auxiliary anode but also has the function of structural reinforcement. The electrochemical stability of the CFMA and long-term maintenance of the mechanical properties are key factors for improving the structural life of RC structures via ICCP-SS technology.
In ICCP-SS technology, the inorganic cementitious material employed as the secondary anode should have the dual functions of transferring loads and providing a phase boundary zone for electrochemical reactions. According to the standard of Cathodic Protection of Steel in Concrete (EN-ISO 12696: 2012), the bond strength of the cementitious outer anode material bonded to the concrete substrate should have an average value of 1.5 MPa, with a minimum value of 1.0 MPa. Furthermore, its electrical resistivity should not exceed 200% of the electrical resistivity of the concrete protective layer under normal operating conditions of the system [49]. Based on these requirements, we developed a carbon fiber-reinforced cementitious secondary anode material for ICCP-SS using short-cut carbon fibers. The material exhibited excellent flexural and compressive strengths, electrical conductivity, and shear strength. The results of the accelerated ICCP-SS experiments demonstrated that the composite developed based on the short-cut carbon fiber-reinforced cementitious secondary anode material could provide effective cathodic protection and transfer of the shear stress for RC structures subjected to chloride corrosion [50].
To further improve the mechanical properties and electrical conductivity of carbon fiber-reinforced cementitious composites, an original graphene/polyvinyl alcohol nanofluidic additive (GNA) [51] was used for the proposed nano/microscale carbon-modified cementitious composites [52,53]. Owing to the synergistic effect of the GNA and carbon fibers, this material exhibited excellent mechanical and electrical properties (Figs. 5(a)-(d) [52]). In addition, the accelerated acidification results demonstrated that the nano/microscale carbon-modified cementitious composites used as secondary anodes exhibited excellent resistance to acidification, which can significantly extend the lifetime of the composite anodes in the ICCP system. This improvement is realized because the nano/microscale carbon-modified cementitious materials and carbon fiber mesh collaboratively form a 3D primary and secondary anode system (Fig. 5(e) [53]). This combined system increases the anode area, reduces the anode polarization current density, and effectively improves the electrochemical stability of the composite anode materials.
The anodic and mechanical properties of the multifunctional anode system directly affect the life-extension effect of the ICCP-SS technology on RC structures. The cathodic protection of the reinforcing steel achieved in accelerated ICCP experiments demonstrated that the developed multifunctional composite anode material possessed long-term electrochemical stability and could effectively prevent corrosion of the reinforcing steel (Figs. 6(a) and (b)) [54,55]. The name of the test specimen in Figs. 6(a) and (b) [55] is composed of three parts: the first term, F0 and F2, indicates that the number of layers of carbon fiber mesh in the C-FRCM composite is 0 and 2, respectively; the second term, I20, I60, and I100, indicates that the current density in the ICCP procedure is 20, 60, and 100 mA·m−2, respectively; and the third term is denoted either by specific numerical values or by the letter “S.” Here, the number indicates the degree of corrosion of the reinforcement, while “S” denotes the beams treated with the ICCP-SS technique. When used as a reinforcement material, CFMA is essentially a carbon fabric-reinforced cementitious matrix composite. The results of mechanical property experiments demonstrated that the developed CFMA had excellent mechanical properties and could satisfy the requirements for structural reinforcement (Figs. 6(c) and (d)) [56], [57], [58].
In the ICCP-SS technique, numerous factors affect the mechanical properties of the CFMA, such as the content of short-cut carbon fibers, number of layers of carbon fiber mesh, and applied current density. Increasing the number of layers of carbon fiber mesh can enhance the safety margin of CFMA composites against external forces [58]. In addition, the short-cut carbon fiber content affects the mechanical properties of the CFMA. The experimental results demonstrated that the CFMA exhibited the best tensile properties at a cement weight ratio of 1.2% [59]. Furthermore, the charge density affects the mechanical properties of the CFMA. The tensile strength of CFMA composites decreases as the cumulative charge density increases [46,48], and the fatigue properties are weakened [49]. According to the experimental results [57], Eq. (7) is proposed to predict the ultimate tensile strength after polarization based on the amount of applied charge.
where σu is the ultimate tensile strength of the C-FRCM after polarization, q is the cumulative charge density, and σu_FRCM is the stress of the C-FRCM before polarization with respect to the total cross-sectional area of the C-FRCM.
2.1.3. ICCP-SS structure design and operation maintenance
(1) Interface behavior of CFMA. The interfacial performance of the CFMA is critical for the long-term performance of ICCP-SS. The CFMA interface consists of two parts: the interface between the carbon fiber mesh and cementitious matrix, and the interface between the CFMA and concrete. To evaluate the interface performance between the CFMA and concrete, core drilling, pulling, and single-shear experiments were conducted. The experimental results indicated that the interface between the CFMA and concrete was not damaged during the ICCP process, whereas the interface between the carbon fiber mesh and cement matrix was damaged [59]. The bonding behavior between the carbon fiber mesh and cement matrix has important implications for the interfacial properties of the CFMA.
The large particle size of the inorganic matrix material makes it difficult for the matrix to fully infiltrate the carbon fiber mesh, resulting in complex interfacial bonding within the CFMA. To thoroughly investigate this complex interfacial bonding behavior, we conducted fiber pull-out experiments and developed a new interfacial bonding model [60]. The proposed model considers the degree of matrix impregnation, divides the carbon fiber yarns into two regions (externally impregnated and internally unimpregnated), and introduces two different interfacial bonding slip relationships to characterize the interfacial behavior, as shown in Fig. 7 [60]. Based on the proposed interfacial bonding model, a method for analyzing the interfacial ontological model between the carbon fiber mesh and cementitious matrix was established, thus providing a theoretical basis for the design of ICCP-SS structures [61].
As a dual-functional material, the interfacial properties between the fabric and substrate of the CFMA are also affected by anodic polarization. A higher charge density results in a greater degree of impairment of the interfacial properties. The maximum pullout force of CFMA materials decreases as the electric charge density increases [54]. In addition, anodic acidification also affects the behavior of the interfacial adhesion fatigue of CFMA materials, such that specimens with the highest charge exhibit the weakest load-carrying capacity and fatigue life in fatigue pullout tests [62].
(2) Design methodology for RC components under ICCP-SS intervention. During their service life, RC members are typically subjected to three types of static loads: compression, shear, and bending. To investigate the effect of ICCP-SS technology modification on the static performance of RC members, mechanical and accelerated polarization experiments were conducted on ICCP-SS-modified RC beams and columns (Fig. 8 [63,64]). The experimental results showed that the ICCP-SS-reinforced beams and columns had a higher ultimate load-carrying capacity when no current was applied [65]; however, corrosion of the steel reinforcement could not be prevented. The accelerated polarization experiments revealed that the ultimate load-carrying capacity of the RC members under ICCP-SS intervention was generally higher than that of RC members treated with ICCP or SS alone [64,66,67]. This is because ICCP-SS not only effectively retards the corrosion of RC members but also restores the load-carrying capacity of RC members damaged by corrosion.
Comparison of the experimental results with the design codes for predicting the flexural load capacity of RC beams revealed that the existing design codes underestimate the strengthening effect of the CFMA [66]. Therefore, a new design method for RC beams with a C-FRCM reinforcement system is proposed, as shown in Eq. (8) [42].
$ M_{\mathrm{up}}=\theta M_{\mathrm{u} 3}$
where Mup represents the prediction of the design bending moment; Mu3 represents that of the current bending moment; and θ is the correction factor, which is derived by calibrating the results, that is, by comparing the experimental and numerical results to consider factors such as the carbon fiber mesh layer, fully wrapped layer, and degree of corrosion of the reinforcement. It can be calculated as shown in Eq. (9).
where LCF and LU are the number of carbon fiber mesh layers and number of fully clad layers, respectively, both of which range from 1 to 3; ρ is the degree of corrosion of the reinforcement, with values ranging from 0 to 10; and b0-b6 are coefficients determined from the multivariate regression analyses based on finite-element analyses and experimental results.
Fatigue load tests were conducted on RC beams under ICCP-SS intervention [55,57]. The results indicate that the fatigue life of the ICCP-SS-treated RC beams was increased by 202% compared with that of the untreated corroded beams. This improvement was attributed to the use of CFMA, which effectively inhibited crack propagation, reduced the stress concentration around the crack tip, and decreased the crack-opening displacement. Based on the fatigue tests [68] and numerical calculations, we developed a design approach for the fatigue performance of RC beams under ICCP-SS intervention [43], as shown in Eq. (10). A comparative analysis with the experimental results validated the approach as highly accurate in predicting and optimizing the fatigue performance of RC beams (Fig. 9 [68]).
where A1, B1, C1, D1, and G1 are material-related parameters obtained by fitting the experimental results; S1 is the stress amplitude, N is the fatigue life, and e is the charge density.
The seismic performance of RC columns with application of the ICCP-SS technology was investigated using cyclic loading experiments and numerical simulations [69,70]. The results demonstrated that the dual-function ICCP-SS system could significantly improve the load and energy dissipation capacities of the corroded circular RC columns (Fig. 10 [69]). This improvement occurred because ICCP-SS enhances the ductility, stiffness, strength, and energy dissipation properties of corroded circular RC columns owing to the tight bonding of CFMA with the column base [44]. Based on the experimental results and numerical simulations, formulae for estimating the ultimate capacity and displacement of CFMA-reinforced corroded RC columns were developed, which can be used for the seismic design of RC structures along with the application of ICCP-SS technology [70].
(3) ICCP-SS operational assessment methodology. The results of the ICCP acceleration experiments demonstrated that the ICCP-SS technology not only effectively prevented corrosion of the steel reinforcement but also significantly enhanced the mechanical properties of the structure. Based on the principle of equal charge, an evaluation method for the operating effect of the ICCP-SS system was established by considering the efficiency of the residual strength of carbon fiber as the control index (Eqs. (11), (12), (13), (14), (15)) [41]. For example, according to this evaluation method, ICCP-SS technology can extend the theoretical service life of RC structures by at least 40 years under the most unfavorable working conditions (cathodic protection current density of up to 20 mA∙m−2 and reinforcement rate of 5%) for concrete with a cross-section of 400 mm × 400 mm and a built-in component with eight steel bars.
where Qanode is the total charge through the carbon fiber, Qcathode is the total charge through the steel, qanode is the total power density applied to the anode, Aa is the nominal area of the carbon fiber anode, n is the number of roots of steel, ip is the applied cathodic protection current density, ttest is the cathodic protection time, tlife is the service life of the carbon fiber under the condition of a total charge equal to Qcathode, Asteel is the cross-sectional area of the steel, Ac is the cross-sectional area of the concrete, ia is the current density of the carbon fiber anode, and rs is the reinforcement ratio.
In summary, the composite ICCP-SS intervention technique with carbon fibers doubled the protection of the corrosion and mechanical properties of RC structures and effectively solved the problems of durability and safety. Carbon fiber cementitious composite anodes with both structural enhancement functions were successfully developed by utilizing the excellent mechanical and electrochemical properties of carbon fibers, and an organic combination of ICCP and traditional SS technology was achieved. By applying ICCP-SS technology to RC members, the mechanical properties and service life of RC structures were enhanced, and the corrosion of steel reinforcements in RC structures was effectively prevented. Based on numerous experimental studies and numerical simulations, a methodology for designing RC structures using ICCP-SS technology was developed. ICCP-SS is a new durable composite intervention technology for coastal concrete that can reduce maintenance costs, improve building safety, and extend the service life of buildings.
As we advance the field of ICCP-SS, several critical areas require further exploration to address existing scientific and technological challenges. First, the development of more efficient and durable materials for ICCP-SS systems is of paramount importance. This includes exploring novel composites and alloys that offer enhanced protection and longevity under diverse environmental conditions. Second, understanding the long-term interactions between ICCP-SS systems and various structural materials, such as RC and other cementitious composites, requires comprehensive longitudinal studies. Additionally, the integration of advanced monitoring technologies, such as real-time sensors and artificial intelligent (AI)-driven predictive maintenance systems, could revolutionize the management and optimization of ICCP-SS systems. Another key area for future research is the environmental impact and sustainability of ICCP-SS systems. Investigating eco-friendly and cost-effective methods for material production and system implementation is crucial for the widespread adoption of ICCP-SS systems. Finally, addressing the scalability and practical deployment of ICCP-SS in large-scale infrastructure projects remains a significant challenge. Collaborative efforts across disciplines, including materials science, civil engineering, and environmental science, will be essential to overcome these barriers and drive innovation in ICCP-SS technologies.
The development and promotion of ICCP-SS technology will further promote the large-scale application of carbon fiber composites in civil engineering. The waste disposal problem at the end of the service life of a structure was considered at the beginning of the design stage of the ICCP-SS system to facilitate sustainable development. During the dismantling of a building structure, separating carbon fibers from the matrix material is difficult; this significantly increases the difficulty and cost of recycling construction waste. Carbon fiber composites are also resistant to natural degradation. Traditional landfill and incineration treatments consume valuable land resources, cause serious environmental pollution, and waste resources. This is a common challenge for sustainable development in civil engineering, aerospace, railway transportation, and other composite applications. The construction of CFRP waste recycling and reuse technology systems can help promote the green and low-carbon development of ICCP-SS technology.
2.2.1. Evolution of carbon fiber recycling research
To investigate the research progress and cutting-edge dynamics in the field of carbon fiber recycling with a relatively long-term scope, CiteSpace (Drexel University, USA) was used to present a timeline cluster mapping of relevant keywords (Fig. 11) and analyze the evolution of trends in carbon fiber recycling research topics. A timeline clustering map was generated using the clustered keywords to outline the historical trajectory and timeline of the development of specific keywords in each cluster. Based on the timeline mapping information of the keywords, carbon fiber recycling research from 1995 to 2024 can be divided into three stages.
The first stage of relevant research involved the embryonic period of carbon fiber recycling (1995-2010); in this stage, the number of publications on this topic was small, and the relative keywords and clusters were limited, most of which were not generated. The relevant literature mainly focused on the CFRP material composition, mechanical properties, and exploratory studies of supercritical fluids, which played a foundational role in the subsequent study of carbon fiber recycling.
The second stage of carbon fiber research investigated the development of carbon fiber recycling (2010-2020). The number of articles published on this topic increased significantly compared with the previous stage, and the keywords and clusters were relatively complete, that is, most of the high-frequency keywords appeared in this stage. This research stage had an important leading and diffusing effect on the subsequent development of carbon fiber recycling. The main keywords included “thermoplastic composites,” “reaction kinetics,” “mechanism analysis,” “resin composition,” “chemical recycling,” “selective depolymerization,” “pyrolysis,” “thermosetting composites,” and “life cycle assessment.”
The third stage represents a deepening of carbon fiber recycling research (2020-2024). The number of studies on carbon fiber recycling increased rapidly along with the number of keywords compared with the previous stage. Based on the topics in the previous stage, studies on carbon fiber recycling gradually transitioned from focusing on recycling technology to considering the reuse of recycled carbon fibers. Numerous new research topics and keywords appeared, such as “recycled carbon fiber,” “3D printing,” “recycled CFRP concrete,” “additive manufacturing,” and “intelligent construction.”
The literature clustering analysis successfully identified clusters of keywords in the field of carbon fiber recycling that fluctuated over time, signifying academic interest in carbon fiber recycling issues and trends in the field. This analysis not only provides a structural understanding of the study of carbon fiber recycling but also demonstrates notable patterns and changes in research in this field. To provide a deeper understanding of the core issues in the field of carbon fiber recycling, an in-depth analysis of the current status of carbon fiber recycling technologies represented by each cluster was conducted to explore the specific research questions and methods involved. This analysis could reveal developing trends in the study of carbon fiber recycling technology and provide useful insights for subsequent research.
2.2.2. Status of carbon fiber recycling technology research
Currently, carbon fiber recycling methods can be divided into four main categories: physical, thermal, chemical, and electrochemical (Table 1) [38],[71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93].
(1) Physical recycling methods. Physical recycling methods include combustion and mechanical recycling (Fig. 12) [71],[72],[77], [78], [79],[94],[95]. The combustion method recovers the thermal energy generated by burning the carbon fibers, whereas the mechanical recovery method relies on mechanical means to decompose, crush, and grind the composite materials. Despite the low cost and simplicity of mechanical methods, raw materials usually need to be crushed into smaller sizes, which severely reduces the performance of the recycled fibers, limits their re-use value, and pollutes the environment.
(2) Thermal recycling methods. Thermal recycling methods recover carbon fibers by subjecting waste CFRP to high-temperature conditions, which degrades the macromolecular polymers in the resin into small molecular compounds (Fig. 13(a) [79,80]). Thermal recycling is the only CFRP recycling technology that is commercially operational, with methods including vacuum cracking, microwave pyrolysis, fluidized beds [83],[99], [100], [101], and multi-stage pyrolysis recovery system (Figs. 13(b) and Figs. (c)) [100], [101], [102], [103], [104], [105]. However, these methods have limitations that need to be solved, such as resin residues in vacuum cracking, fiber damage during microwave pyrolysis, and reduced CFRP size in the fluidized bed method resulting in the loss of fiber strength.
(3) Chemical recycling methods. Super/subcritical fluid [86,90,91,101,106] and atmospheric-pressure solvent [107], [108], [109], [110], [111], [112], [113] methods have been used for chemical recycling, as shown in Fig. 14 [86,93]. The former utilizes the compressible and flowable nature of supercritical fluids to degrade the resin matrix. A suitable solvent can be employed to break the corresponding chemical bonds in the resin matrix and recover the carbon fibers. The latter method uses chemical solvents to decompose the resin matrix in the CFRP under atmospheric pressure to obtain recycled carbon fibers. Although the chemical recycling method has a high recycling efficiency and maintenance rate of carbon fibers, it is difficult to employ in large-scale industrial applications because it involves the solvents and waste liquids require safety and environmental protection considerations. This, these methods are mostly in the exploratory research phase.
The recycling methods described above are generally limited by conditions such as high temperatures, high pressures, and specimen size. In addition, these methods require significant equipment and energy and may negatively affect the properties of recycled products. Therefore, based on the research results regarding the evolution mechanism of carbon fiber anode properties, we optimized the polarization conditions and regulated the electrochemical behavior of the carbon fiber surface to address the aforementioned technical challenges. This optimization simultaneously improved the recovery efficiency and maintenance rate of carbon fiber recycling to promote sustainable recycling and improved reusability of carbon fibers.
(4) Electrochemical recycling method. Based on the synergistic effect of strong permeation and surface activation of chlorine ions on carbon fibers under high-current conditions, a non-destructive recycling method for carbon fibers was developed by optimizing the polarization conditions to achieve efficient and mild separation of carbon fibers from matrix materials [38],[73], [74], [75], [76]. Rapid permeation and oxidative degradation of the matrix material were achieved through the optimal design of the ionic environment and polarization conditions and by utilizing the derivatives of the anodic reaction. The electrochemical recycling method utilizes the charge effect, which causes Cl− in the electrolyte to lose electrons to reduce the production of chlorine gas, while a further loss of electrons facilitates the production of HClO, which is partially ionized to ClO− in solution (Fig. 15 [73,75]).
At ambient temperature and pressure, electrochemical recycling of carbon fibers could achieve a recycled carbon fiber debonding rate of 99%, tensile strength of 95% of the original filament, and interfacial shear strength of 120% of the original filament [75]. This method promotes the degradation of the resin matrix via directional bond breaking, and the anodic oxidation surface modification of carbon fibers demonstrated significant results. Using this technique, the nanoscale microstructure of the recycled carbon fiber surface was significantly increased and optimized (Fig. 16(a)) [75,76]. The content of oxygen-containing functional groups on the surface of the recycled carbon fibers increased, improving the surface polarity and significantly enhancing their interfacial bonding properties (Fig. 16(b)) [73,74]. In addition, the crystal width and inter-crystalline spacing of the recycled carbon fibers were reduced (Fig. 16(c)), resulting in better performance maintenance. Moreover, the electrochemical recycling method has lower equipment and initial investment requirements, which reduces the cost by approximately 80% compared with conventional carbon fiber production methods.
The application of electrochemically recycled carbon fibers to organic composites produces recycled carbon fiber reinforced polymers (rCFRPs) [114]. After remanufacturing, the mechanical strengths of the rCFRP panels were similar to those of the original CFRP panels (Fig. 17(a)) [114]. Further applications of recycled carbon fibers in inorganic composites yielded recycled carbon fiber RC (rCFRC), which was comparable to raw carbon fiber filament RC (Fig. 17(b)) [114].
In summary, electrochemical recycling of carbon fibers is excellent in terms of recycling efficiency and performance maintenance. Moreover, it is low-cost and has low equipment requirements, making electrochemical recycling feasible for broad applications. By applying recycled carbon fiber in the remanufacturing of composite materials, various types of rCFRP and rCFRC have been successfully produced, providing a feasible method for sustainable resource utilization and environmentally friendly production. This innovative technology not only solves the problem of environmental pollution caused by waste in composite applications, such as civil engineering, aerospace, and railways, but also provides a new direction for the sustainable development of the carbon fiber industry chain and promotion of green manufacturing.
3. Conclusions
In this study, the continuous evolution of carbon fiber composite applications in civil engineering was explored, and the evolution mechanism of the anodic properties of carbon fibers was explained. Considering the special polarization performance of carbon fiber in a chloride salt environment as an entry point, research regarding functional applications based on the evolution of the anodic performance was discussed. The application of carbon fiber composites in civil engineering is promising; however, significant efforts are required to promote the development of high-quality carbon fiber composites and their functionalization. Utilizing the evolution mechanism of the carbon fiber polarization performance in chloride salt environments can expand the eco-design and life cycle of carbon fiber composites, thus promoting multidisciplinary synergistic innovation of civil engineering and composites to achieve favorable interactions between scientific and technological progress while facilitating green development.
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