Fiber Reinforced Polymer Concrete Bond Research Articles

Bond Strength Evaluation of FRP–Concrete Interfaces Affected by Hygrothermal and Salt Attack Using Improved Meta-Learning Neural Network

Fiber-reinforced polymer (FRP) laminates are popular in the strengthening of concrete structures, but the durability of the strengthened structures is of great concern. Due to the susceptibility of the epoxy resin used for bonding and the deterioration of materials, the bond performance of the FRP–concrete interface could be degraded due to environmental exposure. This paper aimed to establish a data-driven method for bond strength prediction using existing test results. Therefore, a method composed of a Back Prorogation Net (BPNN) and Meta-learning Net was proposed, which can be used to solve the implicit regression problems in few-shot learning and can obtain the deteriorated bond strength and the impact weight of each parameter. First, the pretraining database Meta1, a database of material strength degradation, was established from the existing results and used in the meta-learning network. Then, the database Meta2 was built and used in the meta-learning network for model fine-tuning. Finally, combining all prior knowledge, not only the degradation of the FRP–concrete bond’s strength was predicted, but the respective weights of the environment parameters were also obtained. This method can accurately predict the degradation of the bond performance of FRP–concrete interfaces in complex environments, thus facilitating the further assessment of the remaining service life of FRP-reinforced structures.

A Review on the Physical Parameters Affecting the Bond Behavior of FRP Bars Embedded in Concrete

The present study is a detailed literal survey on the bond behavior of FRP (Fiber Reinforced Polymer) reinforcing bars embedded in concrete. There is an urgent need for the accurate assessment of the parameters affecting the FRP–concrete bond and quantification of these effects. A significant majority of the previous studies could not derive precise and comprehensive conclusions on the effects of each of these parameters. The present study aimed at listing all of the physical parameters affecting the concrete-FRP bond, presenting the effects of each of these parameters based on the common opinions of the previous researchers and giving reasonable justifications on these effects. The studies on each of the parameters are presented in detailed tables. Among all listed parameters, the surface texture was established to have the most pronounced effect on the FRP–concrete bond strength. The bond strength values of the bars with coarse sand-coating exceeded the respective values of the fine sand-coated ones. However, increasing the concrete strength was found to result in a greater improvement in bond behavior of fine sand-coated bars due to the penetration of concrete particles into the fine sand-coating layer. The effects of fiber type, bar diameter and concrete compressive strength on the bar bond strength was shown to primarily originate from the relative slip of fibers inside the resin of the bar, also known as the shear lag effect.

Ensemble Machine Learning-Based Approach for Predicting of FRP–Concrete Interfacial Bonding

Developments in fiber-reinforced polymer (FRP) composite materials have created a huge impact on civil engineering techniques. Bonding properties of FRP led to its wide usage with concrete structures for interfacial bonding. FRP materials show great promise for rehabilitation of existing infrastructure by strengthening concrete structures. Existing machine learning-based models for predicting the FRP–concrete bond strength have not attained maximum performance in evaluating the bond strength. This paper presents an ensemble machine learning approach capable of predicting the FRP–concrete interfacial bond strength. In this work, a dataset holding details of 855 single-lap shear tests on FRP–concrete interfacial bonds extracted from the literature is used to build a bond strength prediction model. Test results hold data of different material properties and geometrical parameters influencing the FRP–concrete interfacial bond. This study employs CatBoost algorithm, an improved ensemble machine learning approach used to accurately predict bond strength of FRP–concrete interface. The algorithm performance is compared with those of other ensemble methods (i.e., histogram gradient boosting algorithm, extreme gradient boosting algorithm, and random forest). The CatBoost algorithm outperforms other ensemble methods with various performance metrics (i.e., lower root mean square error (2.310), lower covariance (21.8%), lower integral absolute error (8.8%), and higher R-square (96.1%)). A comparative study is performed between the proposed model and best performing bond strength prediction models in the literature. The results show that FRP–concrete interfacial bonding can be effectively predicted using proposed ensemble method.

Quantitative Non-Destructive Evaluation of FRP Laminate-Concrete Bond Strength Using Ultrasound Tomography

Abstract Fiber-reinforced polymer (FRP) laminate is one of the most popular and practical solutions for the strengthening and retrofitting of deteriorated or under-strengthened concrete structures, adding strength, ductility, and durability. The quality of the FRP–concrete bond is critical in stress transfer and composite action at the interface, and in situ evaluation of the bond strength is a challenging issue. To quantitatively evaluate the bond strength, nondestructive evaluation (NDE) with ultrasound tomography was utilized on 32 small-scale concrete beams strengthened with carbon FRP laminates. Various parameters that may affect the bond strength, such as surface roughness, surface voids, epoxy type and thickness, and FRP type, were considered. The associated bond capacities were found through a bending test and correlated through finite element modeling. Quantitative relationships involving the NDE output values and associated bond strengths, with parametric inputs, were developed. Simple numeric examples are provided, showing application of the developed equations, which will be very useful in estimating the in-service bond conditions of applied FRP laminates on concrete structures. This may lead to quantitative estimation of the expected strength contribution of the laminates to the overall flexural capacity of structural members.

Predicting fiber‐reinforced polymer–concrete bond strength using artificial neural networks: A comparative analysis study

AbstractThe repair efficiency of fiber‐reinforced polymer (FRP) is crucially linked to bond strength between FRP and concrete. Artificial neural networks (ANNs) technique is employed for the prediction of FRP–concrete bond strength based on more than 440 data points collected from literature work for training and testing of the proposed ANNs model. Such a model facilitates investigating the effect of various key parameters in controlling the bond. These are concrete compressive strength, maximum aggregate size, FRP thickness and modulus of elasticity, FRP‐to‐concrete length and width ratios, and adhesive tensile strength. The proposed ANNs model shows high fitting and prediction capability of training and testing data, respectively, with low mean square errors. Its accuracy of prediction far exceeds that of literature empirical models. Furthermore, the present comparative and sensitivity study of the predicted bond strength promotes the understanding of the impact of the above key parameters.

Fatigue behavior of FRP–concrete bond under hygrothermal environment

In subtropical coastal regions, fiber reinforced polymer (FRP) strengthened concrete bridge structures are typically subject to harsh environmental attack such as temperature and humidity variations during their service (i.e. under cyclic vehicle loading). For such FRP strengthened concrete structures, fatigue performance of the FRP-to-concrete bond (referred to as interface hereafter for simplicity) is extremely important to the durability of the strengthened structures. In order to investigate the fatigue performance of the interface subjected to temperature and humidity variations, 17 double shear test specimens with carbon fiber laminate (CFL) were prepared, and experimental study was carried out on the fatigue performance of the interface in a simulated environment with constant temperature and relative humidity (RH) (60°C, 95%). The results showed that the adopted temperature and RH adversely affected the bond behavior of CFL–concrete interface. In particular, the fatigue life of specimens under the above temperature and RH is significantly less than that of reference specimens without hygrothermal pretreatment. The test results also showed that the fatigue life can be reduced by a higher stress level. Based on the analysis on the bond-slip behavior of the interface, a three-phase fatigue damage model was presented to describe the fatigue damage process of the CFL-to-concrete bond under fatigue load while exposure to a certain hygrothermal environment.

SRG/SRP–concrete bond–slip laws for externally strengthened RC beams

The goal of the paper is to assess the effectiveness of some Fiber Reinforced Polymer (FRP)–concrete bond–slip laws, widely used in the numerical analyses, to model Steel Reinforced Grout (SRG)–concrete and/or Steel Reinforced Polymer (SRP)–concrete interfaces. The assessment was carried out through a suitable Finite Element (FE) study developed on a set of externally strengthened Reinforced Concrete (RC) beams. For this purpose a database of experimental results, available in literature, was collected.A two-dimensional FE model was defined for each tested beam. The linear and nonlinear behavior of all materials was adequately modeled, whereas the connection between concrete surface and external reinforcing layer was simulated by a cohesive element with a suitable damage model.The numerical and experimental comparisons were used to evaluate the reliability and adaptability of the FRP–concrete bond–slip laws to predict the structural behavior of RC beams externally strengthened to flexure with SRG and SRP innovative systems.The study showed that among a set of bi-linear FRP–concrete bond–slip laws one was also suitable to simulate the behavior of SRG–concrete interface. For the SRP–concrete interface a modification of the same bond–slip law was proposed, calibrated and validated against the available experimental data.

Evaluation of Pull-Off Tests as a FRP–Concrete Bond Testing Method in the Laboratory and Field

The direct tension pull-off test has gained popularity as a test method used in the laboratory and especially in the field to assess the quality of bond between externally bonded fiber-reinforced polymer (FRP) repair materials and concrete owing to its low cost and simple procedure. However, the small scale and localized nature of the test can lead to high variability in the results. Furthermore, the test does not load the bond in a way that is representative of service conditions. Because of these limitations, satisfactory interpretation of results can be difficult. This paper examines the challenges that may be encountered when analyzing the data obtained during testing, including the high variances. Past research efforts using pull-off tests, and results of recent field and laboratory testing by the authors, are discussed to evaluate the benefits and weaknesses of pull-off tests as a quality control and research tool. The authors conclude that pull-off tests have many drawbacks, and if they are used, great care is needed in their application. It is recommended that if this method is to be used as a quality control procedure in the field, pull-off tests on the concrete alone should be conducted to obtain baseline values, and supplementary tests such as acoustic sounding also be used. For laboratory conditions, it is advised that alternative testing procedures be considered and large sample sizes be used to make statistical validation possible.

Ductility of FRP–concrete systems: Investigations at different length scales

Fiber reinforced polymer (FRP) materials have been increasingly used in the last two decades to improve various structural characteristics of reinforced concrete (RC) bridges, buildings and other structures. Ductility of the resulting FRP–concrete system plays an important role in structural performance, especially in certain applications such as earthquake resistant design of structures, where ductility and energy dissipation play a vital role. Wrapping RC columns with FRP has been shown to generally result in significant increase in ductility due to the confinement of concrete by the FRP. Other applications such as flexural strengthening of beams involve tradeoffs between ductility and the desired load capacity. Furthermore, environmental factors may adversely affect the FRP–concrete bond raising concerns about the ductility of the system due to possible premature failure modes. Characterization of these effects requires the use of more involved mechanics concepts other than the simple elastic or ultimate strength analyses. This paper focuses on characterizing ductility of the FRP–concrete systems at different length scales using a combined experimental/computational mechanics approach. Effects of several parameters on ductility, including constituent material properties and their interfaces, FRP reinforcement geometry at the macro- and meso-level, and atomistic structure at the molecular level are discussed. Integration of this knowledge will provide the basis for improved design strategies considering the ductility of FRP–concrete systems from a global as well as local perspective including interface bond behavior under various mechanical and environmental conditions.

Fire Endurance and Residual Strength of Insulated Concrete Beams Strengthened with Near-Surface Mounted Reinforcement

The comparatively weak performance of fiber-reinforced polymer (FRP) strengthened members under fire exposure is a primary factor hindering the application of FRP strengthening in buildings in which fire risk is not negligible. As part of a larger ongoing study investigating the behavior of FRP strengthening systems under fire exposure, an investigation was set up to examine and document the performance of six full-scale near-surface mounted (NSM) FRP-reinforced concrete beams exposed to 1 h of fire. All the specimens were preloaded to the service load of the strengthened member. Tests results indicated that, if appropriately insulated, the NSM FRP-strengthened beams can achieve a satisfactory fire endurance of 1 h as per fire resistance test specifications. Moreover, this paper also presents a study on the residual performance of fire-tested beams. Results of this study suggest that if the insulation system is able to maintain the adhesive temperature at relatively low value (tadhesive≤1.6tg for the beam configuration in this test program), the FRP concrete bond degradation under fire is limited, and the FRP-strengthened beam can retain a large part (up to 92% in this test program) of its original strength.

Durability of CFRP–concrete joints under freeze–thaw cycling

Abstract The long-term durability of fiber reinforced polymer (FRP) strengthening systems under freeze–thaw cycling is crucial to the safety of structures in cold climates. The durability of the FRP–concrete bond interface under freeze–thaw cycling was investigated in the study reported here, with exposure condition, concrete grade, and number of freeze–thaw cycles as the parameters considered. The behavior of the carbon FRP (CFRP)–concrete bond interface was investigated with single-face shear tests. The results indicate that the bond strength, bond stiffness, interfacial fracture energy, and maximum slip of the joints decrease with increases in the number of freeze–thaw cycles, and they are also affected by the exposure environment. The depth of cracking and effective bond length increase with increases in cycle number, thus affecting bond stiffness and strength. The deterioration of bond strength can be attributed to the damage caused to the concrete by the freeze–thaw cycling.

Improvement in Fiber-Reinforced Polymer–Concrete Bond by External Pressure

An experimental study evaluated the improvement in the bond of fiber-reinforced polymer (FRP) and concrete to dry and wet substrates from applied pressure. In the study, 12 full-sized pile specimens, including four controls, were wrapped by using two different glass FRP systems–- a prepreg and a wet layup–-and two different layouts typically used for unidirectional and bidirectional fibers. Wrapping was conducted inside a partially filled tank to ensure that the bonded areas of the dry and wet regions were identical. Sustained pressure was maintained during curing by using pressure or vacuum bagging. Bond improvement was evaluated from more than 400 pull out tests. Results showed that external pressure led to improved bond in both the dry and submerged regions. However, vacuum bagging was better for prepreg systems, whereas pressure bagging was better for wet layups. Transverse fiber layout typically used with bidirectional fibers gave better bond in controls where no external pressure was applied.

Experimental Study on Bond Behavior between Concrete and FRP Reinforcement

Bonding between fiber-reinforced polymer (FRP) sheets and concrete supports is essential in shear and flexural applications for transfer of stress between concrete structure and reinforcement. This paper aims at better understanding FRP–concrete bond behavior and at assessing some of the common formulations for effective bond length and bond–slip models (τ-s) by means of an extensive experimental program on 39 concrete specimens strengthened with various types and amounts of FRP strips and covering a wide range of FRP axial rigidities, subjected to both double-shear and bending tests. Effective bond length, maximum bond/shear stress, slip when bond stress peaks, and slip when bond stress falls to zero, were all experimentally measured. The influence of FRP stiffness on effective bond length and bond–slip behavior was observed. New expressions for (1) effective bond length; (2) maximum shear/bond stress; (3) slip at peak value of bond stress; and (4) slip at ultimate, taking into account the influence of F...

Prediction of Interfacial Bond Failure of FRP–Concrete Surface

The use of fiber-reinforced polymer (FRP) for strengthening concrete structures has grown remarkably during the past few years. In spite of exhibiting superior properties, the safety of usage is questionable as FRP undergoes brittle debonding failure. The aim of this study is to review and compare the existing research on bond failure between FRP and concrete substrates. Among the different failure modes, there has been little research in terms of intermediate crack-induced interfacial debonding and fewer strength models are developed for predicting such failures. Conducting a simple shear test on the FRP bonded to a concrete substrate can simulate this type of failure mode. Twelve specimens were tested to study the influence of concrete strength and the amount of FRP on the ultimate load capacity of a FRP–concrete bond under direct shear. Existing experimental work was collected from the literature and consists of an extensive database of 351 concrete prisms bonded to FRP and tested in direct shear tests...

Resistance to Freezing and Thawing of Fiber-Reinforced Polymer-Concrete Bond

This paper presents results from an experimental and theoretical study into the effects of freeze-thaw cycling on the fiber reinforced polymer (FRP)-concrete bond. The results of flexural tests on 39 small-scale flexural beams, reinforced in tension with externally bonded FRP sheets, are presented and discussed. The beams were subjected to from 0-300 freeze-thaw cycles and were plated with 4 different FRP materials. Results indicate that little, if any, damage to the FRP-concrete bond results from freeze-thaw cycling. A simple analytical model to predict bond behavior and the potential for bond damage due to thermal cycling is discussed.