Flexural strengthening of reinforced concrete (RC) beams with externally bonded (EB) carbon fibre-reinforced polymers (CFRP) is frequently limited by intermediate crack debonding (ICD). Although some analytical and numerical studies have investigated ICD in conventional CFRP-to-concrete joints, the behaviour of hybrid bonded joints with transversely compressed mechanical anchorages remains poorly understood, particularly regarding the influence of the anchorage location. This study introduces a new Finite Difference Method (FDM) formulation that simulates the end debonding (ED) and ICD in mechanically anchored CFRP-to-concrete joints. Unlike existing bond-slip formulations, the proposed exponential relationship is based on an odd and continuous function, enabling stable modelling of opposite slip signs developed between adjacent cracks under variable load ratios. The model also allows the analysis of different anchorage locations along the bonded interface, which has barely been addressed in previous ICD studies available in the literature. Sixty numerical cases combining different crack spacings, load ratios, and anchorage configurations were analysed and validated against Finite Element Method (FEM) simulations. The results demonstrate that the proposed approach accurately reproduces the load-slip response and provides an efficient tool for assessing ICD mechanisms in hybrid CFRP-strengthened RC beams.

The increasing use of carbon fibre reinforced polymers (CFRPs) in aircraft introduces significant end-of-life (EOL) challenges, as these high-value and high embedded-energy materials frequently face disposal. This study explores reusing EOL aircraft CFRP components for strengthening reinforced concrete (RC) structures, promoting cross-sectoral circularity. The proposed segmentation strategies for fuselage and wing stiffeners were applied to fragments from a decommissioned Airbus A350 fuselage section. Although the reclaimed laminates exhibit lower stiffness and strength than common commercial unidirectional strips used to strengthen concrete, equivalent structural performance can be achieved by increasing the reinforcement cross-section. Experimental tests on RC beams strengthened with composite fragments from a decommissioned aircraft section showed that reused laminates enhanced the flexural response by delaying crack initiation and steel yielding and by increasing load capacity by 123.7%, with favourable bond performance. Moreover, reusing EOL laminates requires far less energy (roughly 400 times lower) than producing virgin or recycled CFRPs, making it a highly sustainable option. Findings confirm structural reuse as a technically viable and highly sustainable alternative for aerospace CFRP waste, linking aircraft decommissioning with civil infrastructure rehabilitation. Based on this initial validation, a more systematic investigation of the parameters governing bond behaviour and flexural performance is necessary before the results can be generalised.

In reinforced concrete (RC) elements externally bonded with fibre-reinforced polymer (FRP) materials, the bond behaviour between the external FRP reinforcement and the concrete has been largely studied from experimental, analytical and numerical points of view due to its influence on design of these elements under both serviceability and ultimate conditions. Under service loads, the bond behaviour determines the tension-stiffening contribution of concrete and crack formation and propagation. This study presents a numerical methodology to analytically simulate the tensile behaviour of a RC tie strengthened with externally bonded FRP materials. The problem is simplified to a one-dimension model aimed at calculating the stabilized cracking process given the linear bond-slip laws at both the steel-concrete and FRP-concrete interfaces. The solution, in terms of strains and stresses in materials and slips at interfaces, is found through a finite differences method that allows obtaining the tensile load in the tie for a given value of increasing slip at the end. Results also include the calculation of the difference between the mean strain in both reinforcements and concrete and the crack spacing at the stabilized condition. In the paper, a parametric study is performed to evaluate the influence of the tensile area of concrete and the bond-slip law of FRP reinforcement on the cracking and deformability of the tie, by means of comparisons with experimental results available in literature.

The aeronautic sector, driven by globalisation and the growing demand for advanced transportation, is experiencing a substantial increase in the use of Carbon Fibre Reinforced Polymer (CFRP) components due to their high specific properties. As the fleet of aircraft expands, the challenge of managing End-Of-Life (EOL) CFRP materials from decommissioned aircraft becomes imminent. Current disposal methods, primarily landfilling, are unsustainable and do not take advantage of the excellent material properties of composites, highlighting the need for innovative recycling solutions. On the other hand, CFRP materials are widely recognized in the construction industry for their ability to enhance the strength of existing structures. While CFRPs offer significant improvements in the flexural performance of Reinforced Concrete (RC) structures, their high cost and the environmental impact associated with their production limit a broader application. This research pioneers the application of EOL aircraft CFRP components for strengthening concrete structures, aligning with Sustainable Development Goals by promoting resource efficiency and waste reduction. The study investigates the feasibility of this approach through an experimental assessment of the bond and flexural behaviour of RC beams strengthened with reused aircraft CFRP parts. It includes a series of single-shear and four-point bending tests for RC beams with identical dimensions but with different reused CFRP rectangular parts. The results, in terms of bond and flexural capacity, load-deflection behaviour, CFRP strain, concrete strain and failure modes are presented and compared with predictions obtained from code formulations used for conventional commercial CFRP materials.

With the emergence and diversity of various strengthening methods in fibre-reinforced polymer (FRP) strengthened reinforced concrete (RC), understanding and comparing the bond behaviour and active bond mechanisms of these different methods is crucial before their application. In this context, a general method applicable to various strengthening techniques employed to develop accurate bond–slip models and identify the active bond mechanisms is proposed. This approach is based on experimental load–slip behaviour data and does not rely on a predefined model shape. To this end, this paper presents an experimental study on various strengthening techniques, including externally bonded reinforcement (EBR), externally bonded reinforcement on grooves (EBROG), hybrid bonded (HB), and near-surface mounted (NSM), all tested under a single lap-shear test. Here, the results of single-shear bond tests are analysed to examine the manifestation of different activated bond mechanics from various strengthening techniques in the bond–slip law and to compare their bond behaviours.

Structural repair and strengthening have always been challenging tasks in construction. One of the most commonly applied techniques for Reinforced Concrete (RC) structures is the Externally Bonded Reinforcement (EBR). However, a persistent challenge with EBR is the premature debonding of FRP laminates from the concrete surface. In response, researchers have explored alternative techniques to improve this issue, leading to techniques such as Near-Surface Mounted (NSM) reinforcement, Externally Bonded Reinforcement on Grooves (EBROG), Externally Bonded Reinforcement in Grooves (EBRIG), and the use of mechanical fasteners. This study introduces and evaluates a new strengthening technique designed to enhance the conventional EBR approach while requiring minimal alterations to surface preparation, saving both time and costs. The proposed technique, presented as Externally Bonded Reinforcement Side Extended (EBRSE), involves applying resin to both lateral sides of the precured laminate parallel to the loading direction. In this work, single shear tests were conducted on concrete specimens using both EBRSE and conventional EBR techniques to compare their performance. Additionally, a numerical approach was applied, combining the finite difference method with a metaheuristic optimization algorithm, to derive the bond-slip law governing the constitutive behavior of both systems. The findings revealed that the EBRSE technique significantly outperformed the conventional EBR method, with an increase of up to 80 % in ultimate load capacity and up to 86 % in ultimate slip. Furthermore, the comparison between numerical predictions and experimental findings, specifically in terms of bond-slip law, load-slip curves, and strain distribution along the bonded FRP, confirmed an increase in fracture energy for the EBRSE technique.

Hybrid-bonded (HB) systems are employed to anchor carbon fibre reinforced polymer (CFRP) laminates to concrete structures and delay their premature debonding failure mode by generating compressive stresses on the joint. However, although previous research on these strengthening systems exists, the optimum parameters that minimize the potential for premature debonding failure and maximize the ultimate load still need to be investigated.In this work, the feasibility of decoupling the effect of decohesion and friction of a HB CFRP-to-concrete joint is assessed through experimental and numerical analysis. To this end, single shear tests are performed on concrete specimens strengthened with externally bonded reinforcement (EBR) and HB-CFRP precured laminate. Besides, a numerical procedure, based on the finite difference method and a metaheuristic optimization algorithm, is utilized to obtain the bond-slip law that describes the constitutive behaviour of both systems. Moreover, the separated contributions of the cohesion of the adhesive joint and the friction induced by the external compressive stresses normal to the composite surface are analysed separately, and separated cohesive and friction stress-slip laws are obtained. The experimental behaviour of the full anchoring system is compared against that obtained by combination of the separated cohesive and friction contributions. The method represents progress in examining how different anchoring parameters, including the size (width and thickness) of the anchor plate and the torque applied to the bolts, influence the performance of the anchoring system in an efficient and systematic manner.

A near-surface-mounted (NSM) technique using fiber-reinforced polymer (FRP) reinforcement increases the load-bearing capacity and stiffness of reinforced-concrete (RC) beams and delays the yielding moment. In those cases, the verification of serviceability limit states becomes necessary to guarantee functionality and protection of steel reinforcement. At present, there is a lack of provisions for the crack width prediction, mainly because of the scarceness of experimental data. This work presents the results of an experimental program aiming at studying the effect of different NSM reinforcement arrangements on the midspan deflection, crack spacing, and crack width of NSM FRP RC beams. One RC beam and 11 NSM FRP RC beams were tested under a four-point bending configuration up to failure. Carbon -and glass-FRP rods were used. It was found that NSM FRP reinforcement provides an effective reduction in deflection, crack width, and spacing. Larger crack formation phases were observed in all strengthened specimens. Moreover, crack width decreases with the increase of the NSM FRP reinforcement ratio. Finally, cracks at the bottom of the beam are around 11%-25% wider than at the height of the steel internal reinforcement. DOI: 10.1061/JCCOF2.CCENG-3907. (c) 2023 American Society of Civil Engineers.

With the assessment of intermediate crack debonding (ICD) being a subject of main importance in the design of reinforced concrete (RC) beams strengthened in flexure with externally bonded fibre-reinforced polymer (FRP), several approaches to predict the debonding loads have been developed in recent decades considering different models and strategies. This study presents an analysis of formulations with different levels of approximation collected in the fib Bulletin 90 regarding this failure mode, comparing the theoretical predictions with experimental results. The carried-out experiments consisted of three RC beams strengthened with carbon FRP (CFRP) tested under a four-point bending configuration with different concrete strengths and internal steel reinforcement ratios. With failure after steel yielding, higher concrete strength, as well as a higher reinforcement ratio, lead to a higher bending capacity. In addition, the performance of the models is assessed through the experimental-to-predicted failure load ratios from an experimental database of 65 RC beams strengthened with CFRP gathered from the literature. The results of the comparative study show that the intermediate crack debonding failure mode is well predicted by all models with a mean experimental-to-predicted failure load ratio between 0.96 and 1.10 in beams tested under three- or four-point bending configurations.

The use of fiber reinforced polymer (FRP) for flexural strengthening of reinforced concrete (RC) beams has become a popular strengthening technique. Significant amount of work is available on the short-term flexural behavior of RC beams strengthened with near-surface mounted (NSM) technique. However, their time-dependent flexural behavior, specifically under high service temperature, has not yet been addressed. This paper presents an experimental work to evaluate the time-dependent behavior of NSM carbon FRP (CFRP)-strengthened RC beams. The experimental program included 23 beams, where the effect of different parameters such as strengthening (CFRP) area, steel reinforcement ratio and applied temperature (20 and 50 ?) have been considered. Experimental results show that the effect of strengthening area is significant on the flexural short-term response of the beams, while minor effects are found on the time-dependent deflections. On the other hand, increasing the service temperature has no significant effect on the short-term tests, but it produces a large increase in the time -dependent deflection of the specimens. Finally, an analytical procedure for the prediction of time-dependent deflections, which is based on the age-adjusted effective modulus method (AEMM), is presented. Good agreement between the experimental results and analytical predictions on time-dependent deflections is shown.