Hugo C. Biscaia

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

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

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

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

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.

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

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

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.

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.

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

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

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

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

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.

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

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

The monitoring of structures has undergone important advances with the improvements of digital cameras available on the market. Thus, the Digital Image Correlation (DIC) technique has become a viable way of studying engineering problems. Recently it has been used in the debonding failure process between the reinforcement and the substrate. The methods or methodologies that should be followed to obtain the results associated to the debonding phenomenon using the DIC technique need to be better understood and studies on this topic are scarce. The present work therefore proposes a new and inexpensive method to monitor the interfacial behaviour between a reinforcement material and a substrate by combining the use of the DIC technique and a simplified nonlinear bond-slip model. For the validation of the proposed method, a series of single-lap shear tests with a sufficient long bond length carried out by the authors are used. Based on the slip distribution obtained from the DIC technique, it was found that a third-degree polynomial function can be used to approximate the interfacial bond-slip curve of the joint. The validation of the model is made with several analytical solutions using the proposed bond-slip model. © 2018 Politechnika Wrocławska

This work presents the results and the main conclusions of a series of experimental tests carried out to evaluate the efficiency of post-installed stainless steel reinforcement on the flexural strengthening of Reinforced Concrete (RC) T-beams when the bonding techniques EBR (Externally Bonded Reinforcement), NSM (Near Surface Mounted) and MA-EBR (EBR with Mechanical Anchors) are used. The RC T-beams were also modelled using a commercial Finite Element (FE) software in order to predict their behaviour until the rupture. For this purpose, a set of single-lap shear tests were also carried out to evaluate the local bond-slip relationships developed within the Stainless Steel (SS)-to-concrete interface. Due to the experimental bond-slip relationships, the numerical simulations were able to predict, with good accuracy, the different behaviours of the RC T-beams until their rupture. Moreover, the different rupture modes observed on all the RC T-beams herein tested were very well estimated by the numerical analyses. The tests of the RC T-beams showed that all the strengthening techniques allowed their flexural stiffness to be increased. Nevertheless, the RC T–beams strengthened with the EBR and NSM techniques had premature ruptures, i.e. the rupture in the RC T-beams occurred even before the yielding of their steel reinforcements. The RC T-beam strengthened with the MA-EBR technique showed good ductility and the highest load bearing capacity, which means that the MA-EBR technique is the best bonding technique herein used. © 2018 Elsevier Ltd

The durability performance of stainless steel makes it an interesting alternative for the structural strengthening of reinforced concrete. Like external steel plates or fibre reinforced polymers, stainless steel can be applied using externally bonded reinforcement (EBR) or the near surface mounted (NSM) bonding techniques. In the present work, a set of single-lap shear tests were carried out using the EBR and NSM bonding techniques. The evaluation of the performance of the bonding interfaces was done with the help of the digital image correlation (DIC) technique. The tests showed that the measurements gathered with DIC should be used with caution, since there is noise in the distribution of the slips and only the slips greater than one-tenth of a millimetre were fairly well predicted. For this reason, the slips had to be smoothed out to make it easier to determine the strains in the stainless steel and the bond stress transfer between materials, which helps to determine the bond–slip relationship of the interface. Moreover, the DIC technique allowed to identify all the states developed within the interface through the load–slip responses which were also closely predicted with other monitoring devices. Considering the NSM and the EBR samples with the same bonded lengths, it can be stated that the NSM system has the best performance due to their higher strength, being observed the rupture of the stainless steel in the samples with bond lengths of 200 and 300 mm. Associated with this higher strength, the NSM specimens had an effective bond length of 168 mm which is 71.5% of that obtained for the EBR specimens (235 mm). A trapezoidal and a power functions are the proposed shapes to describe the interfacial bond–slip relationships of the NSM and EBR systems, respectively, where the maximum bond stress in the former system is 1.8 times the maximum bond stress of the latter one. © 2018, The Author(s).

The effective stress transfer between the fiber reinforced polymers (FRP) and the steel substrate is crucial for the successful retrofit of existing steel structures with FRP composites. However, there are no standard tests for FRP-to-steel interfaces, wherefore different test configurations have been used in recent years to assess the bond behaviour in these interfaces. The present study shows that the choice of test configuration is highly important and leads to different transfer stresses between the FRP and steel composites and consequently, has a direct influence on the strength of the bonded joint. Therefore, it is important to understand the debonding process that occurs in each test and avoid misinterpretations, erroneous analyses and dangerous characterizations of the interfacial behaviour of these interfaces. The current study presents a new analytical approach for the prediction of the debonding of FRP-to-steel interfaces when double-lap pull or double-strap tests are used. © 2017 Elsevier Ltd