FRP-concrete Bond Research Articles

Elevated Temperature Effects on FRP–Concrete Bond Behavior: A Comprehensive Review and Machine Learning-Based Bond Strength Prediction

Because of their improved properties, FRP composites are vastly used in the strengthening of aged concrete infrastructures. However, it has been observed that their performance is highly compromised when exposed to high temperatures, as expected during fire incidents, which critically affects FRP–concrete bond behavior, hence affecting the overall efficiency of the strengthening system. This paper critically presents the available literature concerning the degradation of bond strength between FRP systems with concrete substrates due to increased temperatures. Both analytical and numerical bond–slip models developed for the prediction of bond strength degradation under such conditions are reviewed. A generally confirmed fact is that exposure to high temperatures, especially those reaching glass transition temperature (Tg) for epoxy adhesives, leads to bond degradation. Therefore, cement mortar-bonded CFRP textiles display better performance in fire endurance. This present paper also utilizes machine learning algorithms for the prediction of bond strength under elevated temperatures based on an experimental database of 37 beams. The nonlinear relationships and variable interactions in the developed model provide a reliable method for the estimation of bond strength with reduced extensive experimental testing, where the critical role of temperature in bond behavior is identified. This paper emphasizes the use of advanced predictive models to ensure the durability and safety of FRP-strengthened concrete structures in thermally challenging environments.

Comprehensive FRP-concrete bond behavior: Impact of test methods and the innovative UBoT

The lack of a standardized bond test method for fiber-reinforced polymer (FRP)-concrete interfaces leads to variations in forms of the popular single-lap shear, double-lap shear, beam, mixed-mode, and pull-off tests. Recognizing the limitations of each bond test technique and the importance of analyzing all results as complementary, a detailed evaluation of these variations is essential for accurate bond behavior assessment prior to design and field deployment, to avoid subjective decisions and predictions. This study pioneers the evaluation of FRP-concrete bond test methods, offering the industry a consistent dataset for informed decision-making. Single- and double-lap shear tests yield trilinear bond-slip responses; however, the single-lap test’s lack of symmetry reduces bond strength by 15% compared to the double-lap test. The beam test, unsuitable for bond-slip modeling, shows comparable average bond strength to the single- and double-lap tests. The mixed-mode test better simulates field applications, while the pull-off test provides the most consistent results. Given that each test method reflects only one field scenario, the Universal Bond Tester (UBoT) was developed. This device can convert to all FRP-concrete bond test types. It also addresses the modified double-lap shear test’s limitations (e.g., inapplicability to pultruded laminate and FRP rupture in wet layup systems) and present a modified ASTM D7958 beam and mixed-mode tests cost-effectively. The UBoT captures full bond behavior and variabilities, overcoming the limitations of existing methods which fail to capture all failure modes in bond-critical applications. It reduces specimen sizes, weight, and data acquisition needs by half for most tests. This study allows test laboratories to use any preferred testing method with the UBoT. Link to video demonstrations for all tested specimens are provided in the supplementary material.

Nonuniformity in stress transfer across FRP width of FRP-concrete interface

The nonuniformity of stress and strain analysis is an important consideration that describes the bond behavior between concrete and externally bonded fiber reinforced polymer (FRP) laminates. The interfacial stress transfer between FRP and concrete generates a longitudinal strain gradient in the longitudinal (or FRP fiber) direction. Most of the available studies in this area have focused on nonuniformity in the longitudinal direction. In this paper, the nonuniformity in the FRP width direction is investigated by studying the strain distribution using digital image correlation (DIC) full-field optical techniques. In the direction along the FRP width, there is an area in the middle that has consistent strain and stress, while the edge regions have strain and stress gradients. During the complete debonding process, the width of the central area is found to be insignificantly affected by the concrete strength but significantly affected by the FRP stiffness. The fracture characteristics below the central region are purely Mode II fracture. However, the complex curved cracks and high in-plane shear strains at the interface near the FRP edge regions show a mixed mode with Modes II and III. This boundary effect results in a significant difference in the bond–slip characteristics of FRP along the width direction. The maximum bond stress of the central region is higher than that of the edge region. However, for FRP with higher stiffness, the inhomogeneity of the stress distribution is weakened. Moreover, it is reasonable to find that calculating the sum of the bearing capacity of the single FRP strips along the width direction is close to the experimental bond strength. Based on the research findings in this study, it is qualitatively recommended that the width factor model in calculating FRP-concrete bond strength should include the significant effect of FRP stiffness.

Machine learning based graphical interface for accurate estimation of FRP-concrete bond strength under diverse exposure conditions

Predicting FRP-to-concrete bond strength (FRP-CBS) under diverse exposure conditions is an intricate task influenced by multiple variables. Yet, existing pertinent models have several limitations. Accordingly, this study proposes a novel data driven machine learning (ML) methodology to predict the FRP-CBS considering various exposure conditions. A comprehensive database on single and double lap-shear strength tests on concrete specimens was meticulously compiled. Twenty-seven analytical models were used to appraise the developed ML models. Feature importance analysis was conducted to ascertain the influence of input parameters on bond strength. The proposed data-driven ML models attained exceptional accuracy and superior performance compared to existing analytical models. To enhance the accuracy of bond strength estimation and simplify the process for practicing engineers and FRP applicators, a user-friendly graphical interface was developed. It could eliminate the need for complex design procedures, making it easier to accurately estimate the FRP-CBS, thus improving overall efficiency in engineering practice.

Experimental and FE analysis of NSM FRP bond in low- and normal-strength natural and recycled aggregate concretes

Poor mechanical properties of construction materials constitute one of the major reasons for structural deterioration. Since the extent of the weakening in the material quality could reach an alarming threshold, their suitability for strengthening needs assessment prior to adopting any strengthening strategy. This paper experimentally investigates the bond behavior between near-surface mounted (NSM) fiber-reinforced polymer (FRP) strengthening in low- and normal-strength concrete with compressive strength in the range of 11.6 – 14.8 MPa for the former and 39.4 MPa for the latter. The use of two aggregate types – natural and recycled aggregates – was also investigated in the low-strength concretes. Steel fiber reinforcement was used in some of the mixes to check whether some enhancement is possible in low-quality concrete that can result from inferior recycled material, compromised workmanship, or material deterioration due to durability challenges. Ultimately, the variables of the study include recycled coarse aggregate incorporation ratio of 0%, 30%, 60%, and 100% replacement of the natural aggregate for each of the steel fiber’s percentages of 0%, 0.5%, and 1% of the specimen’s volume. The control NSM FRP-concrete specimen made from normal-strength concrete (39.4 MPa) was prepared and tested as a reference. The low-strength concrete mixtures demonstrated a decrease in the NSM FRP-concrete bond strength with an increase in the RCA incorporation. However, the addition of steel fibers marginally increased the bond failure capacity. Compared to the reference specimen of normal-strength concrete, at least, a 44% reduction in the NSM-concrete bond failure load was observed. Because some design guides (including the ACI 440) suggest a minimum concrete compressive strength for NSM FRP strengthening, a finite element model (FEM) was used to check the full spectrum of the mixture’s quality (both low and high-strength materials) and other key parameters affecting the NSM FRP-concrete bond behavior. The so-developed model is first validated against the experimental observations, followed by using it to run a series of parametric studies using the bar diameter, concrete compressive strength, FRP modulus, and steel fiber content as variables.

Experimental and numerical analysis of CFRP-concrete joint bond behavior after exposure to wet-dry cycles

Fiber-reinforced polymer (FRP) composites have been largely used for strengthening and retrofitting reinforced concrete members in the last few decades. However, limited information is available regarding their performance after prolonged exposure to different environmental conditions. In this paper, the effect of wet-dry cycles on the bond behavior of carbon FRP-concrete joints tested using a single-lap direct shear test set-up is investigated. Specimens were exposed to 50 wet-dry cycles, each comprised of 6 h of immersion in water at 25 °C followed by 18 h of drying at 50 °C. The digital image correlation technique was used to obtain the displacement field of the composite and determine the cohesive material law and fracture energy of the FRP-concrete interface. A finite element model was then used to reproduce the load response and reinforcement strain profile observed experimentally. The results show that wet-dry cycles affect the FRP-concrete bond properties and change the failure mode from a cohesive debonding within the concrete substrate to an adhesive debonding within the interface.

An Optimized Neuro-Bee Algorithm Approach to Predict the FRP-Concrete Bond Strength of RC Beams

Over the world, there is growing worry about the corrosion of reinforced concrete structures. Structure repair, rehabilitation, replacement, and new structures all require cost-effective and long-lasting technologies. Fiber Reinforced Polymer (FRP) has been widely employed in both retrofitting existing structures and building new ones. Due to its varied qualities in reinforced concrete and masonry constructions as a repair composite material, FRP have seen a rise in use over the last decade. This material have several advantages such as high stiffness-to-weight and strength-to-weight ratios, light weight, possibly high longevity, and relative ease of usage in the field. Among all the parameters the bond between concrete and FRP composite play an important role in the strengthening of structures. However, the bond behaviour of the FRP-concrete interface is complex, with several failure modes, making the bond strength difficult to forecast, resulting in the FRP strengthened concrete structure. To overcome such kind of issues machine learning models are sufficient to forecast the bond strength of FRP-concrete. In this article Artificial Neural Network (ANN), optimized Artificial Bee Colony (ABC)-ANN and Gaussian Process Regression (GPR) algorithms are deployed to predict the bond strength. The R-value of ABC-ANN and GPR models are 0.9514 and 0.9618 respectively. This research aids researchers in estimating bond strength in less time, at a lower cost, and with less experimental work.

Exploitation of FRP fabric Reinforcement: Innovation beyond FRP bars

This study provides combined experimental and numerical findings for a novel use of 3D Fiber Reinforced Polymer (FRP) fabrics as reinforcements in concrete beams. Experiments demonstrate that concrete beams reinforced with the multidirectional FRP fabrics not only possess adequate strength and ductility, but also have a great potential to achieve low-impact structures. In this study, a MATLAB-based code has been developed to determine the material and geometric design details of the 3D FRP reinforced concrete beams. This code includes the complex mechanical and geometric effects, e.g. concrete cracking, FRP-concrete bond, equilibrium, compatibility and boundary conditions. This design tool has been validated against experimental results, including load–strain and load–deflection curves. The validated code provides with a valuable tool to aid in achieving optimal reinforcement designs that ensure both strength and ductility in the 3D FRP reinforced beams casted by using notably less concrete material.

Effects of concrete heterogeneity on FRP-concrete bond behaviour: Experimental and mesoscale numerical studies

Extensive experimental and numerical studies have been conducted to understand the bond behaviour of FRP-concrete interfaces with the assumption of homogenous materials. This study is aimed at a better understanding of the effects of concrete heterogeneity on the FRP-concrete bond behaviour, by a combination of experiments and mesoscale numerical modelling. FRP-concrete and FRP-mortar bonded joints were tested using a four-point bending setup. The crack initiation and propagation process with interfacial debonding until failure was accurately captured by the digital image correlation (DIC) technique. The results show that the presence of coarse aggregates in the FRP-concrete joints leads to 19% higher bond strength, but much higher variations in the bond strength and the strain distribution across the width of FRP sheets, than the FRP-mortar joints. Monte Carlo simulations of mesoscale finite element models with random distribution of polygonal aggregates were then carried out for the FRP-concrete bond tests. It is found that the distribution of coarse aggregates significantly affects the overall force–deflection responses as well as the simulated fracture processes and final failure modes that are highly comparable with the DIC observations.

Durability assessment of FRP-concrete bond after sustained load for up to thirteen years

Small-scale plain concrete notched beams strengthened with carbon and glass fiber reinforced polymer (CFRP and GFRP) sheets were tested in flexure to assess the change in debond onset strain following sustained flexural loading in indoor and outdoor environments for as long as 13 years. The bond tests were all conducted quasi-statically in a normal laboratory atmosphere. For indoor conditioning, debond onset strain was practically unchanged in the CFRP beams over 13 years, while a decrease was seen in the GFRP beams. For outdoor conditioning, which included freeze-thaw cycles, rain, hot weather, and ultraviolet exposure, clear reductions in debond onset strain were observed in both GFRP and CFRP strengthened beams.

Ensemble learning based approach for FRP-concrete bond strength prediction

Nowadays, externally bonding fiber reinforced polymer (FRP) plates or sheets have become a major maintenance approach for aged reinforced concrete flexure structures. However, the capacity of strengthend structure cannot be precisely estimated as a result of the critical FRP-concrete interfacial (FCI) bond strength unpredictable. In order to solve this issue, many experimental studies have been carried out with corresponding emipirical models proposed. Due to limited experiment samples, these models were found more or less lacking the generalization ability. Under this circumstance, in this study, an ensemble learning algorithm “gradient boosted regression trees” (GBRT) was employed to develop a prediction model for FCI bond strength prediction based on a collected comprehensive database containing 520 tested samples. The model’s performance has been thoroughly compared with the representative empirical models and the common utilized machine learning algorithms. The rationality of this model has also been discussed through feature importance analysis. The results showed that the model in this study exhibits the highest accuracy and is proven to be feasible for predicting FCI bond strength in actual practice.

Experimental verification of 2- and 3-D numerical models for bond-slip behavior of CFRP-concrete

This paper studies the bond performance between fiber-reinforced polymer (FRP) and concrete using two variations of the double shear lap test setups. Experimental tests were conducted followed by a series of finite element simulations using cohesive zone model. The experiment utilized an improved double-pull shear setup with the tests conducted on specimens of varying FRP-concrete bond lengths. The numerical simulations were carried out for the improved as well as the traditional setups using two converged meshes with both 2- and 3-D spatial idealizations to study their relative performance. The results are discussed in light of the damage mechanism, load–displacement, strain and stress distribution, bond-slip behavior, and fracture energy. The tested specimens show no effect of bond length variation on the FRP-concrete interfacial strength so long as the bond length exceeds the effective bond length – Only the case of bond length lower than the effective bond length shows otherwise. The study also succeeds in validating the 2- and 3-D numerical simulations using the experiments in both the two cases of spatial idealizations and the two types of test setups in terms of strain and stress distributions, bond-slip curve, and fracture energy. However, while the 3-D models succeed in capturing the failure load accurately, the 2-D models fail to do so. Evidence of similar performance between the improved double shear test and traditional double shear test setups reaffirms the fact that the former could be used to replace the latter.

A coupled dynamic cohesive zone model for FRP-concrete mixed-mode separation

Cohesive zone models (CZMs) have been used to model composite fracture, metal adhesion failure, concrete cracking and other fracture scenarios. In these applications, CZMs are broadly classified into uncoupled models for pure mode fracture and coupled models incorporating mode I and mode II characteristics for mixed-mode fracture. Various coupled dynamic CZMs have been provided for dynamic fracture problems. However, a coupled CZM for FRP-concrete bond interface is still lacking. Such a bond interface contains diverse traction-separation performance in mode I and mode II fracture, as well as dynamic enhancing effect. To bridge this gap, current study proposed a coupled dynamic CZM to analyse FRP-concrete mixed-mode separation. The main objective was to evaluate model response under various mixed-mode ratios, including stress-separation curves and energy dissipation. The model was validated by various FRP-concrete dynamic separation tests, e.g. single-lap shear tests, three-point bending tests and FRP strengthened reinforced concrete (RC) beam subjected to a dynamic loading. Subsequently, the model was applied in finite element analysis (FEA) of an FRP strengthened RC wall under blast scenarios. From the comparison with the test results, the model was shown to be reliable and accurate in simulating the behaviour of FRP-concrete mixed-mode separation.

Behavior of FRP sheet-concrete bond in high strength concrete samples

The main goal of the present work is to investigate the effect of concrete compressive strength on the behavior of FRP sheet-concrete bond. For this purpose, a model of single shear test was selected and modeled using ANSYS program to study the “FRP-concrete bond”. The formulated model was used in the analysis process and it gave results of good correspond with the available actual test results. It was found that the increasing in concrete strength leads to increasing in the bond capacity and the greater concrete strength the better utility of the FRP sheet. All studied models were failed in the same way by debonding the FRP sheet due to concrete failure.

Experimental study of bond behavior of laminated bamboo plate to concrete interfaces

This research experimentally studied the bond behavior of laminated bamboo (LB) plate to concrete interface. Totally forty-five single shear pull-out specimens, which were categorized in nine groups, were prepared and tested to determine the bond characteristics. Plate thicknesses, plate widths and bond lengths were the three major parameters considered in the specimen preparations. It was found that debonding in concrete near the concrete-adhesive interface often occurred in the specimens with bond length less than 150 mm, and a mixed failure mode of debonding at the bamboo plate-adhesive interface and debonding in concrete was observed in most specimens with bond length equal to and over 150 mm. The coefficient of variation of the LB-concrete bond specimens was lower than such value of similar FRP-concrete bond specimens. This confirmed that the interfaces of LB plate to concrete can exhibit sufficient reliability for engineering applications. The models for predicting peak load, effective bond length and bond stress–slip curve, originally used for FRP bonded concrete specimens, were calibrated for LB bonded concrete specimens based on the test results. Compared with other models, the models proposed by Teng et al. and Zhou were more accurate for the prediction of the peak load and effective bond length of the LB bonded concrete specimens. Classical power function model with a linear descending branch was able to effectively describe the nonlinear bond stress–slip curves and achieved good agreement with test results.

FRP-concrete bond performance under accelerated hygrothermal conditions

Drawing on information obtained via double lap shear tests, this paper discusses FRP-concrete bond behavior following five weeks of exposure to several accelerated hygrothermal conditions. Material tests were also carried out for both dry carbon fibers and the epoxy used in the bond study under similar hygrothermal conditions. The bond-slip relationship for the hygrothermal conditions were developed, and fracture energy calculated for the FRP-concrete bond. The use of degraded material properties obtained from material test coupons under the same exposure conditions prevented overestimation of the FRP-concrete bond fracture energy. Testing revealed that the FRP-concrete bond strength increased as temperature increased at low humidity (dry heat) — a potential consequence of alteration in the epoxy glass transition temperature. The bond strength of concrete with steel slag incorporated was superior at all temperatures and low humidity. In response to continuous or periodic submergence in water, specimen failure mode transitioned from substrate failure to adhesive-concrete interface failure. Alternating two-day wet/dry cycles resulted in a 12% drop of FRP-concrete bond strength. Under low humidity, the FRP dry fiber tensile stress increased as temperature increased (dry heat), while the opposite dynamic was observed in the epoxy coupons.

A new and robust hybrid artificial bee colony algorithm – ANN model for FRP-concrete bond strength evaluation

The present study describes the bond strength estimation between the FRP strip and the substrate concrete using machine learning approaches. To provide a precise model, a database including 656 single and double-lap direct shear test results was compiled from the literature. Artificial neural networks (ANN) and the combination of ANN with the artificial bee colony optimization algorithm (ABC-ANN) were implemented for the model development. The concrete compressive strength, the elasticity modulus of FRP strip, FRP thickness, the width of the FRP strip, and the width of concrete block were considered as input parameters of the models. The results of the ANN and the hybrid ABC-ANN were compared with those of existing models and international procedures. It was indicated that the accuracy of the proposed models outperforms the existing models. The correlation coefficients of the ABC-ANN and ANN approaches are 0.97 and 0.93, respectively. The robustness of the models was investigated using the distribution of absolute error values. It was demonstrated that the proposed ABC-ANN model is robust and resulted in the least error values, distributed in small ranges (less than 10%), compared to ANN and other methods. Using the hybrid ABC-ANN, a straightforward formulation for bond strength evaluation was proposed.

3D acid diffusion model for FRP-strengthened reinforced concrete structures: Long-term durability prediction

Fibre-reinforced polymer (FRP) composites have been utilised as promising and popular solutions to strengthen existing reinforced concrete (RC) structures. Due to the increasing industrial activities, growing population and climate change, corrosive environments (e.g. acid rains) that diminish the durability of FRP-strengthened RC structures are becoming more common. The long-term durability of structures in these corrosive environments is central to the success of any strengthening technique. Despite this urgency, very limited research data is available regarding the degradation of RC structures strengthened with the FRP in acidic environments. This paper evaluates the impact of sulphuric acid on the performance of externally bonded FRP-strengthened RC structures and investigates the long-term behaviour of concrete, FRP composite, and the FRP-concrete interface. The acid diffusion mechanism within the FRP-concrete bond system is studied with the establishment of both one-dimensional (1D) and three-dimensional (3D) models. Moreover, a reduction factor (βα) accounting for the degrading effect of acid, is proposed to predict the retained bond strength at the FRP-concrete interface. The proposed models provide a theoretical basis for the durability design and the service life prediction of FRP-strengthened RC structures when subjected to acid attack.

Mode I fracture characterisation of FRP-concrete interfaces under dynamic loading

Tensile (mode I) fracture between fibre reinforced polymer (FRP) and concrete is found in the FRP strengthened reinforced concrete (RC) structures, especially for structures under dynamic loads. However, currently, there is a lack of studies on the mode I fracture under dynamic loads. The present programme consisted of two sets of experiments to bridge this gap and to obtain the principal interfacial properties of such fracture. Direct tension tests and notched three-point bending tests were used to determine tensile bond strength and fracture energy of the FRP-concrete interface bond, respectively. Digital image correlation (DIC) measurements were used to characterise the fracture process. It was found that the two bond properties were close to those of plain concrete in the quasi-static regime and showed significant dynamic enhancing effect at a loading rate of 20 mm/s. Dynamic increase factor (DIF) equations for the two bond properties were provided to predict the interfacial response of FRP-concrete bond under dynamic loads. As an application example, the two bond properties were used in finite element simulations of the three-point bending tests.

Shear Characteristics between FRP-Concrete Bonding Interface

At present, fiber reinforced composite materials (FRP) are widely used in the reinforcement of concrete structures. The bonding interface between FRP plates and concrete is the key part of the strengthening of concrete structures with FRP plates. The bonding performance directly determines the success or failure of structural reinforcement. Based on the self-developed test device, the development of FRP and concrete in direct shear debonding test specimens, with the aid of advanced digital image correlation DIC technology, accurate measurement of specimen strain distribution on the surface of the FRP plate, and the FRP plate surface strain along the plate long distribution rule and the bond strength of the specimens was well researched, it reveals that the stripping of FRP and concrete interface failure process, and provides technical guidance for the treatment of FRP-concrete bond interface in practical projects.