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Zhou, H., W. - Y. Gao, H. C. Biscaia, X. - J. Wei, and J. - G. Dai. "Debonding analysis of FRP-to-concrete interfaces between two adjacent cracks in plated beams under temperature variations." Engineering Fracture Mechanics. 263 (2022). AbstractWebsite

Externally bonded fiber-reinforced polymer (FRP) composites have been widely used for the strengthening and repairing of reinforced concrete (RC) beams. Existing studies have denstrated that the full-range behavior and the associated debonding mechanism of the FRP-to-concrete interface between two adjacent cracks in the FRP-plated RC beam are different from those of the pull-off bonded joint. Moreover, the bond behavior between the FRP and the concrete may be affected by interfacial thermal stresses induced by the service temperature variations (i.e., the thermal loadings). Based on a fully reversible bilinear bond-slip model, this paper presents an analytical study to investigate the full-range deformation behavior of the FRP-to-concrete interface between two adjacent cracks under combined mechanical and thermal loadings. The analytical results have indicated that the thermal loadings may significantly influence the full-range deformation behavior and the axial stress distribution of the FRP plate, although the material properties of concrete, adhesive, and FRP are assumed to be not affected by the service temperature variations. A temperature increase leads to an increase in the ultimate load of the bond interface and vice versa. A finite element (FE) model with different considerations of the bondline damage is developed to verify the proposed analytical solution. The reliability of the proposed analytical solution is then validated by the comparisons between the analytical results and the corresponding predictions provided by the FE model. © 2022 Elsevier Ltd

Zhang, S. S., Y. Ke, E. Chen, H. Biscaia, and W. G. Li. "Effect of load distribution on the behaviour of RC beams strengthened in flexure with near-surface mounted (NSM) FRP." Composite Structures. 279 (2022). AbstractWebsite

Debonding failures of FRP have been frequently observed in laboratory tests of reinforced concrete (RC) beams flexurally-strengthened with near-surface mounted (NSM) fibre-reinforced polymer (FRP). A number of numerical and theoretical studies have been carried out to predict debonding failures in NSM FRP-strengthened beams, and several strength models have also been proposed. The existing studies, however, were all based on the scenario of a simply supported beam tested under one or two-point loading, while the influence of load distribution has not yet been investigated. This paper presents the first ever study into the effect of load distribution on the behaviour of NSM FRP-strengthened RC beams. A series of large-scale RC beams flexurally-strengthened with NSM FRP strips were first tested under different load uniformities; then a finite element (FE) model, which can give close predictions to the behaviour of such strengthened beams, was developed; finally, the proposed FE model was utilized to investigate the influence of bond length of NSM FRP on the load uniformity effect. It was found that the load uniformity has a significant effect on the beam behaviour, and the degree of this effect varies with the bond length of NSM FRP. © 2021 Elsevier Ltd

Biscaia, H. C., and R. Micaelo. "Emerging anchored FRP systems bonded to steel subjected to monotonic and cyclic loading: A numerical study." Engineering Fracture Mechanics. 261 (2022). AbstractWebsite

Recently, the study of the bond behaviour of hybrid joints has increased due to their application in different industries. Their main purpose is to obtain lightweight but strong, durable structures. In addition, in some industries such as the automotive industry, those requirements may facilitate the construction of vehicles that have lower carbon dioxide emissions. Although the knowledge on the bond behaviour of hybrid bonded joints under monotonic loading is sufficient, the knowledge on the cyclic bond behaviour needs to be improved. The present work aims to mitigate that gap by proposing a numerical model in which the cyclic bond performance between a Carbon Fiber Reinforced Polymer (CFRP) bonded to a steel substrate can be analysed. The results provided by the Externally Bonded Reinforcement (EBR) are used as reference data. These other simulated bonding techniques cover cases where the CFRP is anchored to the substrate in different ways, which are becoming more popular, namely by: (i) linearly increasing the width of the CFRP; (ii) using the mixed adhesive concept (two solutions were considered); (iii) using a steel plate on top of the CFRP strip; and (iv) assuming no interfacial slips at the CFRP unpulled end, which is intended to simulate a perfect anchorage. Compared to the simulations carried out under monotonic loading, the simulations with the adopted cyclic loading history (loading/unloading cycles), allowed us to observe a degradation of the bond strength of the joints with the number of cycles. However, if the overlapped bonded joint is long enough, the strength of the CFRP-to-steel joints is not affected. Excluding the numerical specimens with the perfectly anchored CFRP, i.e. with an “ideal” anchorage, the length of the other adopted anchorages affected either the strength or the ductility of the joint, whether subjected to a monotonic or to the adopted cyclic loading protocol. © 2022 Elsevier Ltd

Azevedo, A. S., J. P. Firmo, J. R. Correia, C. Chastre, H. Biscaia, and N. Franco. "Fire behaviour of CFRP-strengthened RC slabs using different techniques – EBR, NSM and CREatE." Composites Part B: Engineering. 230 (2022). AbstractWebsite

This paper presents an experimental study about the fire behaviour of reinforced concrete (RC) slabs strengthened with carbon fibre reinforced polymer (CFRP) strips, applied according to three different techniques: externally bonded reinforcement (EBR); near-surface mounted (NSM), and continuous reinforcement embedded at the ends (CREatE), a new technique that prevents premature CFRP debonding. The main goals of this study were three-fold: to understand and compare the fire behaviour of the strengthening techniques, namely the CREatE technique (yet to be studied); to assess the efficiency of the fire protection schemes (constant thickness vs. increased thickness at the CFRP anchorage zones) in extending the fire resistance of the CFRP systems; and, based on the experimental results and data available in the literature, to propose “critical” temperatures for the fire design of CFRP-strengthened RC members. The results obtained show that: (i) without protection, the CREatE technique presented higher fire resistance than the alternative NSM and EBR techniques (24 min vs. 16 min and 2 min); (ii) with fire protection, regardless of its geometry, the NSM and CREatE techniques presented a similar fire resistance (both above 120 min), higher than the EBR technique (less than 60 min); and (iii) the “critical” temperatures for each technique were defined as 1.0Tg, 2.5Tg and 3.0Tg for EBR, NSM and CREatE, respectively, with Tg being the glass transition temperature of the adhesive, defined based on the onset of the storage modulus curve decay from dynamic mechanical analysis. © 2021 Elsevier Ltd

Yang, Y., J. Zhao, S. Zhang, Z. Yang, and H. Biscaia. "Influence of salt fog and ambient condition exposure on CFRP-to-steel bonded joints." Composite Structures. 280 (2022). AbstractWebsite

In this paper, the influence of salt fog and ambient condition exposure on CFRP-to-steel bonded joints (CSJs) with a near end mechanical anchorage was studied. The tests of the CSJs were carried out with a monotonic loading history respectively with and without a near end mechanical anchorage both unaged and after being exposed to the ageing conditions. The results revealed failures in the adhesive for some CSJs with a near end mechanical anchorage, among which also exhibited the best bond performance both in aged and unaged specimens. Due to the high state of degradation after ageing exposure, local damage of the CFRP located at the gap position of the CSJs was induced by a torsion action during the fixation of the specimen in the tensile machine even prior to the testing, which caused, although locally, severe delamination of the CFRP of the aged CSJs. From the torsion test results, a decrease in the torsional capacity of the CFRP after aging revealed the degradation of the carbon fiber-to-matrix interface. Additional results reflected that a drop in the initial stiffness of the load vs. slip relationship can be observed, caused by a pre-crack with a length of 30 mm in the CSJs. © 2021 Elsevier Ltd

Biscaia, H. C., M. S. Carvalho, A. P. Martins, and R. Micaelo. "Interfacial failure of circular or tubular hybrid bonded joints: A theoretical description." Engineering Failure Analysis. 132 (2022). AbstractWebsite

In different industries, the bonding technique has gained several advances in recent years. However, due to the specificity of each industry, the bonded joints may present different configurations. For instance, in the case of metallic truss bridges, the use of Carbon Fibre Reinforced Polymers (CFRP) bonded on the steel surface members may require circular or even tubular transitions between these materials. Although the bonded transitions between a metal and a composite material have been deeply studied with flat surfaces the information on circular or tubular hybrid bonded joints is still scarce. Therefore, the present study aims to mitigate some of this lack of knowledge by proposing an analytical solution able to describe the interfacial debonding process between a circular or tubular bonded transition between two materials. The proposed model also aims to simulate the interfacial debonding of double butt (or stepped) lap joints. Under these circumstances, a bilinear local adhesive model is adopted which required the quantification of the elastic and the softened stiffnesses as well as the pure Mode II fracture energy. The Finite Element Method (FEM) is used for the validation of the proposed model. The behaviour of the adhesive joint between materials is numerically modelled through the Cohesive Zone Modelling (CZM) in which the same bilinear shape used in the analytical solutions is adopted. Different situations were analyzed thoroughly and the numerical simulations tracked very closely the analytical results obtained from the proposed closed-form solutions. © 2021 Elsevier Ltd

Micaelo, R., M. Carvalho, R. Almeida, W. - Y. Gao, and H. Biscaia. "Numerical Analysis on the Bond Performance of Different Anchored Joints under Monotonic and Cyclic Pull-push Loading." Journal of Applied and Computational Mechanics. 8 (2022): 388-404. AbstractWebsite

This study aims to mitigate the gap of knowledge on the cyclic bond behaviour of Carbon Fiber Reinforced Polymer (CFRP) bonded onto a steel substrate. The Distinct Element Method was used to model different bonding techniques such asExternally Bonded Reinforcement (for reference purposes); the linear increase of the width of the CFRP composite; theassumption of a mixed adhesive; and using an additional steel plate bonded on the top of the CFRP. Compared with themonotonic loading simulations, the load capacity and ductility of the joints with the lowest overlapped bonded lengths decreasedwith the number of cycles. However, the strength of the CFRP-to-steel joints was not affected if the overlapping bonded joint hada long length © 2022. Published by Shahid Chamran University of Ahvaz

Biscaia, H. C., J. Canejo, S. Zhang, and R. Almeida. "Using digital image correlation to evaluate the bond between carbon fibre-reinforced polymers and timber." Structural Health Monitoring. 21 (2022): 534-557. AbstractWebsite

The use of optic measurements such as digital image correlation to take strain measurements of fibre-reinforced polymers bonded to a substrate has been on the increase recently. This technique has proven to be useful to fully characterize the bond behaviour between two materials. Although modern digital cameras can take high-definition photos, this task is far from simple due to the tiny displacements that need to be measured. Consequently, digital image correlation measurements lead to relative errors that, at an initial stage of the debonding process, are higher than those calculated close to the debonding of the fibre-reinforced polymer from the substrate. This study aims to evaluate and analyse the use of the digital image correlation technique on the bond between carbon fibre-reinforced polymer laminates and timber when subjected to a pull-out load consistent with fracture Mode II. To allow the quantification of the relative errors obtained from the digital image correlation measurements during the full debonding process, several strain gauges were used to measure the strains in the carbon fibre-reinforced polymer composite. The accuracy of the digital image correlation measurements is analysed by comparing it with those obtained from the strain gauges, which is a very well-established measuring technique. Another contribution of this study is to check the versatility of the digital image correlation measurements on a broader range of situations. To that end, several timber prisms were bonded with seven different bonding techniques with and without the installation of a mechanical anchorage at the carbon fibre-reinforced polymer unpulled end. The results showed that the digital image correlation technique was able to track the slips calculated from the strain gauge measurements until the debonding load, but after that, some difficulties to measure the displacements of the anchored carbon fibre-reinforced polymer-to-timber joints were detected. The digital image correlation technique also over predicted bond stresses when compared with those taken from the strain gauges, which led to bond–slip relationships with higher bond stresses. © The Author(s) 2021.

Yang, Y., J. Zhao, S. Zhang, C. Chastre, and H. Biscaia. "Effect of mechanical anchorage on the bond performance of double overlapped CFRP-to-steel joints." Composite Structures. 267 (2021). AbstractWebsite

Mechanical anchorage devices have the potential to efficiently improve the bond behavior of Carbon Fiber-Reinforced Polymers (CFRP) Externally Bonded (EB) onto steel with an adhesive. Existing relevant studies, however, have been very limited. Against this background, experimental and numerical studies on the double overlapped CFRP-to-steel bonded joints with an end mechanical anchorage were carried out in the present study. Two types of CFRP laminates (i.e., SIKA CFRP and the other type made in Harbin Institute of Technology – HIT) were used in the bond tests, with or without end mechanical anchorage devices. The test results showed that the end mechanical anchorage had marginal effect on the bond-slip relationship between EB CFRP and steel as well as debonding load, but could increase the ultimate load. It was also found that CFRP laminate from HIT had a better bond performance than that from SIKA, in terms of both debonding load and ultimate load. Using ABAQUS, Finite Element (FE) model was established on such bonded joints and the accuracy was verified with test results. The verified FE model was adopted in a parametric study to further clarify the effect of key parameters on the bond behavior of such bonded joints. © 2021 Elsevier Ltd

Biscaia, H. C., R. Almeida, S. Zhang, and J. Canejo. "Experimental calibration of the bond-slip relationship of different CFRP-to-timber joints through digital image correlation measurements." Composites Part C: Open Access. 4 (2021). AbstractWebsite

Nowadays, the use of the Digital Image Correlation (DIC) technique has spread and it is being used in several engineering areas to measure displacements. The available data obtained from the DIC measurement to evaluate the bond performance between a Carbon fibre Reinforced Polymer (CFRP) externally bonded to a timber substrate is scarce. From the existing data obtained with other materials, this contactless technique revealed to be quite useful but its accuracy with other well-established techniques, such as the use of electric strain gauges is not well understood yet. Therefore, the current work aims to study the accuracy of 2D DIC measurements with the measurements obtained from the use of strain gauges within a low-cost perspective. To that end, several CFRP-to-timber bonded joints were tested under the single-lap shear test and different bonding techniques were considered as well. Some flaws intrinsically derived from the DIC measurements that complicate the bond assessment, such as the fluctuations in the generated displacements field, are identified, and to bypass this problem a new methodology is proposed. This new methodology is based on two different closed-form solutions that, after defining the local and global bond behaviours of different CFRP-to-timber bonded joints, allowed to eliminate the fluctuations found from the DIC measurements, facilitating the estimation and the comprehension of the full debonding process of the CFRP-to-timber joints, which was achieved with a good proximity to the homologous debonding process derived from the strain gauge measurements. © 2020

Azevedo, A., J. Firmo, J. Correia, C. Chastre, H. Biscaia, and N. Franco. "Fire behaviour of rc slab strips strengthened with advanced cfrp strengthening systems." fib Symposium. Vol. 2021-June. 2021. 1306-1315. Abstract

Carbon fibre reinforced polymer (CFRP) composite systems are widely used to strengthen reinforced concrete (RC) structures through bonding strips/sheets on the concrete surface – externally bonded reinforcement (EBR) technique, or through strips/rods bonded inside slits in the concrete cover – near-surface mounted (NSM) technique. Although both techniques provide high strength increases, it is usually not possible to use the CFRPs’ full strength due to premature debonding, especially with EBR. This limitation can be overcome when using CREatE (continuous reinforcement embedded at ends) technique (developed by the last three authors), which consists of (i) bonding the central part of the CFRP strip (as in EBR), or the CFRP rod (as in NSM), and (ii) anchoring both ends of the strip/rod inside the concrete section, after a transition curve, enhancing its anchorage capacity. However, all these techniques are susceptible to fire, due to the polymeric nature of CFRP materials and epoxy adhesives. This paper presents the results of an experimental study regarding the fire behaviour of RC slab strips strengthened with EBR, NSM and CREatE techniques, in which the influence of applying different fire protection systems was investigated. The specimens were strengthened with those systems and simultaneously subjected to a service load and the ISO 834 fire curve. The following main results were obtained: (i) without fire protection, the CREatE technique presented the highest fire resistance due to the better anchorage of the CFRP; (ii) when protected, the NSM technique presented higher fire resistance compared to EBR and CREatE techniques; and (iii) “critical” temperatures were proposed for each technique, 1.0Tg, 2.5Tg, and 3.0Tg for EBR, NSM and CREatE techniques, respectively. © Fédération Internationale du Béton (fib) – International Federation for Structural Concrete.

Biscaia, H. C., and S. Soares. "Adherence prediction between ribbed steel rebars and concrete: A new perspective and comparison with codes." Structures. 25 (2020): 979-999. AbstractWebsite

The interfacial behaviour between ribbed steel rebars and concrete has been extensively studied because the contribution of the adherence between these two materials is of the utmost importance for the behaviour of Reinforced Concrete (RC) structures. The majority of the available studies on this topic indicate that the local adherence between these two materials can be defined through a bond-slip relationship, which is obtained with short embedded lengths. Although this seems to be widely accepted, some incoherencies, mainly regarding the local and the global detachment process in that “conventional theory”, are identified in the present work. To facilitate the understanding of the detachment process between a ribbed steel rebar and concrete, an analytical solution is developed. An experimental program with 33 pull-out tests covering three different ribbed steel rebars with different diameters and embedded lengths was carried out. Based on the experimental load–displacement at the pulled end responses, a new local bond-slip relationship with friction is proposed. In the end, the new bond-slip relationship, as well as other known relationships, is implemented into a Finite Element (FE) commercial code and the results are compared to the experimental data. Based on these preliminary results and up to the yielding point of the steel rebars, the new proposed local bond-slip model is the only one able to clearly distinguish and simulate either the local and the global performances of the tested specimens. © 2020 Institution of Structural Engineers

Biscaia, H. C. "Closed-form solutions for modellingthe response of adhesively bonded joints under thermal loading through exponential softening laws." Mechanics of Materials. 148 (2020). AbstractWebsite

The bonding technique has received extensive attention in the recent decades. Unlike the use of metallic screws, screws or rivets, some of the advantages of this bonding technique include the elimination of stress concentration, lighter weight and the extension of the life cycle of the adhesively bonded structures. The stiffness and strength of a material are increased after bonding with another reinforced material, but it can only be effective if the interface is able to transfer bond stresses. However, how the bond stress transfer is carried out when the joint, reinforcement and substrate are subjected to a thermal loading is not yet well understood. One way to facilitate and improve such knowledge is to develop analytical solutions that, despite their simplicity, allow parameters to be identified that may directly influence the bond between materials. Therefore, the present study aims to develop a series of closed-form solutions able to describe the debonding process of bonded joints with different characteristics using two different exponential softening laws. The results mainly describe the interfacial slips, bond stresses and strains in both materials during the temperature increase process. In total, 108 examples with different bonding conditions are studied and, at the same time, numerically modelled through the Finite Element Method (FEM). The numerical results were compared to the proposed closed-form solutions and a satisfactory agreement was obtained between the numerical and analytical results, validating the latter. When considered in the simulations, the glass transition temperature (Tg) of the adhesive greatly affected the bond stress transfer between materials and compromising the initial integrity of the adhesively bonded structure. © 2020

Biscaia, H. C., and P. Diogo. "Experimental analysis of different anchorage solutions for laminated carbon fiber-reinforced polymers adhesively bonded to timber." Composite Structures. 243 (2020). AbstractWebsite

The Externally Bonded Reinforcement (EBR) of timber structures with Carbon Fiber Reinforced Polymers (CFRP) is a solution that has been increasing. Like CFRP strengthened concrete structures, it is known that the premature debonding of the CFRP also occurs on timber structures. To study other alternative solutions to the EBR technique, the Near Surface Mounted (NSM) and six other situations with an additional anchorage were considered. A total of 33 single-lap shear tests were carried out. The experimentally obtained load-slip responses, the strains developed in the CFRP strips or the local bond-slip relationships are all reported and discussed. Compared to the EBR technique, the efficiency of each anchorage system and/or bonding technique is also analysed. The current preliminary results suggest that using two superposed metallic L-shape profiles or embedding the CFRP strip into the timber substrate are the best anchorage systems for the reinforcement of a timber structure with CFRP strips. The NSM technique has also led to high load capacities of the CFRP-to-timber interface. © 2020 Elsevier Ltd

Franco, N., C. Chastre, and H. Biscaia. "Strengthening RC Beams Using Stainless Steel Continuous Reinforcement Embedded at Ends." Journal of Structural Engineering (United States). 146 (2020). AbstractWebsite

An innovative system for the flexural strengthening of RC structures designated continuous reinforcement embedded at ends (CREatE) is presented in this research work. The main characteristics and procedures for the application of this new strengthening technique were described. To evaluate the performance and efficiency of this technique, a set of RC T-beams was subjected to a four-point bending test setup. The reference RC T-beam was not strengthened; all other RC T-beams were strengthened with postinstalled stainless steel bars. Different application arrangements and different amounts of reinforcement were considered, and the CREatE technique was tested under monotonic and cyclic loading histories. The tests were modeled using the nonlinear finite-element method (FEM) to predict the performance of the RC T-beams, which allowed analyzing, in detail and with good agreement with the experiments, the influence of the CREatE technique on the (1) strains developed in the concrete, (2) cracking patterns, and (3) strains developed in the stirrups. Apart from the expected increases in the flexural stiffness and load-bearing capacity of the T-beams, the results showed that the use of the CREatE technique led to higher ductility indexes in the displacement compared with traditional techniques. Moreover, with the CREatE technique, premature debonding of the reinforcement material from the concrete tensioned surface - commonly observed in externally bonded reinforcement (EBR) strengthening systems - was eliminated. © 2020 American Society of Civil Engineers.

Yang, Y., M. A. G. Silva, H. Biscaia, and C. Chastre. "Bond durability of CFRP laminates-to-steel joints subjected to freeze-thaw." Composite Structures. 212 (2019): 243-258. AbstractWebsite

The degradation mechanisms of bonded joints between CFRP laminates and steel substrates under severe environmental conditions require more durability data and studies to increase the database and better understand their causes. Studies on bond properties of double-strap CFRP-to-steel bonded joints with two different composite materials as well as adhesive coupons subjected to freeze-thaw cycles for 10,000 h were conducted to reduce that gap. In addition, the equivalent to the number of thermal cycles and their slips induced in the CFRP laminates was replicated by an equivalent (mechanical) loading-unloading history condition imposed by a static tensile machine. The mechanical properties of the adhesive coupons and the strength capacity of the bonded joints were only slightly changed by the artificial aging. It was confirmed that the interfacial bond strength between CFRP and adhesive is critically related to the maximum shear stress and failure mode. The interfacial bond strength between adhesive and steel degraded with the aging. However, the equivalent thermal cyclic bond stress caused no detectable damage on the bond because only the interfacial elastic regime was actually mobilized, which confirmed that pure thermal cycles aging, per se, at the level imposed, have a low impact on the degradation of CFRP-to-steel bonded joints. © 2019 Elsevier Ltd

Biscaia, H. C., R. Micaelo, and C. Chastre. "Cyclic performance of adhesively bonded joints using the Distinct Element Method: Damage and parametric analysis." Composites Part B: Engineering. 178 (2019). AbstractWebsite

Adhesively bonded joints have been widely used by engineers to solve problems in different industries. Despite recent studies which have helped with the design and application of these joints, there are still several uncertainties that justify empirical implementation in practice. One main aspect is the rigorous understanding of the performance of these joints when subjected to cyclic loading. Although the monotonic performance of adhesively bonded joints is well known, the cyclic behaviour raises several issues. Therefore, a numerical strategy based on the Distinct Element Method (DEM) was implemented in this work to mitigate the lack of knowledge about the cyclic behaviour between two materials adhesively bonded to each other. For that purpose, the single-lap pull-push test was modelled and due to the diversity of the existing cohesive models, four different bond-slip relationships and two unloading paths (with ductile or “cleavage unloading”) were considered. The results suggest that if the bond-slip relationship has an elastic stage, then the interfacial bond degradation is delayed. On the other hand, the interfacial damage of the joints with the number of cycles increases rapidly in those cases where the bond-slip relationships have no elastic stage and, at the same time, the slip accumulation is allowed. In addition, the results of two different tests available in the literature were implemented and fairly reproduced by the DEM. © 2019 Elsevier Ltd

Biscaia, H. C., C. Chastre, and M. A. G. Silva. "Estimations of the debonding process of aged joints through a new analytical method." Composite Structures. 211 (2019): 577-595. AbstractWebsite

The estimation of the long-term durability of adhesively bonded interfaces between Fiber Reinforced Polymers (FRP) and concrete substrates is crucial because degradation potentiates FRP premature debonding. One of the main reasons for mistrusting the use of FRP composites is the premature debonding phenomenon, which, associated to degradation, has been preventing their widespread use. In this research work, an analytical model is proposed that introduces ageing to estimate the effects of degradation of Glass (G) FRP externally bonded to concrete. Cycles were used to experimentally accelerate ageing of beam specimens, namely, (i) salt fog cycles; (ii) wet-dry cycles with salted water; (iii) temperature cycles between −10 °C and +30 °C; and (iv) temperature cycles between +7.5 °C and +47.5 °C. Based on the experimental results obtained and a corresponding bond-slip curve, the analytical model predicts the complete debonding process between FRP composites and a substrate. Consequently, the temporal evolution of the degradation of the bonded interfaces can be calculated and compared with the initial situation prior to exposure. The effects of the environmental conditions are reported and compared. © 2018 Elsevier Ltd

Biscaia, H. C. "The influence of temperature variations on adhesively bonded structures: A non-linear theoretical perspective." International Journal of Non-Linear Mechanics. 113 (2019): 67-85. AbstractWebsite

Nowadays, adhesively bonded structures have received exhaustive attention mainly because, contrary to mechanical joints, they are able to avoid stress concentration. When a material is externally bonded to another structural member to improve the strength or stiffness of the latter, the adhesive joint is supposed to perform well for a long time, independently of the type of loading the bonded joint will be subjected to. However, studies dedicated to this topic are scarce when it comes to the influence of thermal action. The influence of temperature variations on bonded joints is not yet well understood, so more studies are needed to improve the current level of knowledge. The present study aims to develop an analytical solution capable of simulating the interfacial bond behaviour between two structural materials subjected to thermal loading. The complete debonding processes of such adhesively bonded joints are estimated based on a bi-linear bond–slip relationship. The proposed analytical model is validated by the numerical simulation of several examples, where some parameters previously identified as potentially affecting the bond behaviour are investigated. A commercial software based on the Finite Element Method (FEM) is used to support those examples in which either the analytical or the numerical simulations agreed very well. © 2019 Elsevier Ltd

Yang, Y., H. Biscaia, M. A. G. Silva, and C. Chastre. "Monotonic and quasi-static cyclic bond response of CFRP-to-steel joints after salt fog exposure." Composites Part B: Engineering. 168 (2019): 532-549. AbstractWebsite

Deterioration of adhesively bonded CFRP/steel systems in salt fog environment, i.e., deicing salts and ocean environments, has to be taken into account in the design of steel strengthened structures. In the present work, monotonic and quasi-static cyclic loading were applied to CFRP-to-steel double strap joints for two kinds of CFRP laminates after being aged for a period of 5000 h to evaluate the bond behavior. The bonded joints exposed to salt fog had a different failure mode than that observed in the control specimens (0 h of exposure). The severe reduction of the maximum bond stress resulted from damage initiation that occurred in the corrosion region of the steel substrate, associated with final partial rupture on the corroded steel substrate around the edge of the bonded area: it was also correlated with reduced load carrying capacity. Results of pseudo-cyclic tests showed that the relationship between a local damage parameter (D) and normalized local dissipated energy (W d /G f ) and the normalized slip increment (ΔS/ΔS ult ) exhibited almost the same trend in the un-aged and aged bonded joints. The normalized slip increment can be seen as a direct indicator for the local and global damage for the un-aged and aged bonded joints. However, monotonic and quasi-static cyclic tests results revealed that the stress concentration due to local corrosion of steel substrate could lead to brittle rupture or accelerated cumulative damage once the aged bonded interface had become weaker. The bonded joints have exhibited also a smaller relative deformation capacity between CFRP and steel. © 2019 Elsevier Ltd

Silva, M. A. G., H. Biscaia, and P. Ribeiro. "On factors affecting CFRP-steel bonded joints." Construction and Building Materials. 226 (2019): 360-375. AbstractWebsite

Failure of structural steel members strengthened with Carbon Fibre Reinforced Polymers (CFRP) may occur at the joints CFRP-steel and this study examines variables that alter or explain the corresponding reduction of load capacity for a specific CFRP laminate, adhesive and steel. Factors and parameters likely to be influential like surface treatment prior to bonding, the bonded length, the glass transition temperature (Tg) of the adhesive, the exposure to aggressive environment, the temperature at service and different types of loading were examined. The experimental program selected double strap CFRP-steel bonded joints under shear for the analysis. The steel surfaces to be bonded were subjected to sand blasting (6.3 bar) or abrasive grinding (6.9 bar) corresponding to thorough blast cleaning Sa2; surfaces rusted after exposure to salt fog at 35 °C were also considered. Differences detected in responses of specimens treated by sand or steel spheres blasting were relatively minor. Tests made at increasing ambient temperatures confirmed that service temperature near and above adhesive Tg caused rapid deterioration of ultimate capacity and change of failure modes. Salt fog cycles (SF) originated the most significant losses of joint capacity. Application of cyclic static loading above the critical loading threshold obtained for unaged joints did not reduce the capacity of joints previously aged by freeze-thaw. The same cyclic loading after salt fog cycles, reduced bond capacity and increase the ultimate slip, suggesting larger effective length. Despite the losses of capacity, microscopic changes of structural nature could not be identified. © 2019 Elsevier Ltd

Biscaia, H. C., C. Chastre, and M. A. G. Silva. "A Simple Method for the Determination of the Bond-Slip Model of Artificially Aged Joints." Journal of Composites for Construction. 23 (2019). AbstractWebsite

The durability of adhesively bonded fiber-reinforced polymers (FRP) and concrete substrates has been the subject of recent studies. The degradation of bonded interfaces conjugated with other factors that affect the interface strength may compromise the potentialities of using FRP in externally bonded reinforced (EBR) concrete structures. However, the estimation of the effects of degradation on these bonded interfaces and the analytical methodologies to quantify them are not fully understood. The present work focuses on a local bond-slip model characterized by two parameters for which the values are obtained experimentally. Then, the determination of the local bond-slip relationship of a glass (G) FRP-to-concrete interface can be estimated. The assessment of the degradation of the bonded interface when subjected to cycles of (1) salt fog; (2) wet-dry environments with salt water; (3) temperatures between -10°C and +30°C; and (4) temperatures between +7.5°C and +47.5°C is presented. The results obtained using the proposed bond-slip model led to the conclusion that after 10,000 h of exposure to temperature cycles between -10°C and +30°C, there was a small change in the GFRP-to-concrete interface performance, whereas the effects on the bonded interface for the specimens subjected to temperature cycles between +7.5°C and +47.5°C were far more most severe. © 2019 American Society of Civil Engineers.

Biscaia, H. C., and P. Ribeiro. "A temperature-dependent bond-slip model for CFRP-to-steel joints." Composite Structures. 217 (2019): 186-205. AbstractWebsite

It is supposed that the adhesively bonded structures would perform well during their lifetime, but the action of high temperatures may affect the initial integrity of the joints, as recognized by some researchers. Still, there are few studies proposing a model to locally predict the interfacial bond behaviour of Carbon Fibre Reinforced Polymers (CFRP) bonded to a steel substrate when subjected to temperature changes. The influence of temperature on CFRP-to-steel bonded joints is, therefore, not very well understood yet and more studies are needed to better understand how these joints behave under such circumstances. The present work aims to contribute to the mitigation of the existing lack of knowledge on the performance of CFRP-to-steel bonded joints under high temperatures. Therefore, an experimental program was considered and specimens were tested at different temperatures: 20 °C, 35 °C, 50 °C, 65 °C, 80 °C, and 95 °C. To help the interpretation of the results, an analytical model is proposed to predict the load capacity of the CFRP-to-steel joints. The local bond-slip behaviour of the tested specimens is also analyzed and, based on a literature review, a temperature-dependent bond-slip model with a bi-linear shape is proposed and implemented into a commercial software based on the Finite Element Method (FEM). © 2019 Elsevier Ltd

Yang, Y., M. A. G. Silva, H. Biscaia, and C. Chastre. "CFRP-to-steel bonded joints subjected to cyclic loading: An experimental study." Composites Part B: Engineering. 146 (2018): 28-41. AbstractWebsite

Pseudo-cyclic and cyclic loading were applied to CFRP-to-steel bonded joints built with two different CFRP laminates. In this paper, the strength capacity and bond-slip curves are presented and compared. The modes of failure are also described and associated with the types of material used, and the observed performances are correlated. The analysis of the results showed a threshold value for loading and amplitude level, below which the cyclic loading caused no detectable damage. For cycles above that limit, the region of the joints around the loaded end presented degradation reflected on the bond-slip stiffness and on the increase of residual deformation. It was found that the normalized dissipated energies either obtained from the bond-slip relationship or from the load-slip response had the same trend. The experimental data allowed also to establish a relationship between the damage developed within the interface and the normalized slip. A preliminary estimate of fatigue limit based on those data is suggested. © 2018 Elsevier Ltd

Biscaia, H. C., and C. Chastre. "Design method and verification of steel plate anchorages for FRP-to-concrete bonded interfaces." Composite Structures. 192 (2018): 52-66. AbstractWebsite

Concrete structures Externally Bonded Reinforced (EBR) with Fibre Reinforced Polymers (FRP) have been studied and used since the end of the last century. However, several issues need to be better studied in order to improve performance. The influence of size of anchorage plates used on Reinforced Concrete (RC) structures strengthened with EBR FRP composites, the external compressive stress to be applied on the anchorage plate and the numerical simulation of this region are some of the topics that need to be more carefully studied in order to clarify the performance of the FRP-to-concrete interface within the anchorage plate region. This study proposes a design methodology to estimate the amount of external compressive stress necessary to be applied on the anchorage plate of EBR systems with FRP composites, in order to avoid premature debonding. The external compressive stress imposed on the FRP composite is intended to simulate the effect produced by a mechanical anchorage system tightened to the EBR system. The results from the design proposal, when compared with the numerical ones, were efficient enough on the prediction of the bond strength improvement of FRP-to-concrete interfaces. © 2018 Elsevier Ltd