FRP-strengthened Concrete Beams Research Articles

Shear strength prediction of FRP-strengthened concrete beams using interpretable machine learning

This study identified the factors affecting the contribution of externally bonded fiber reinforced polymer (FRP) composite (Vf) to the shear strength of reinforced concrete (RC) beams with internal shear reinforcement through the use of interpretable machine learning (ML). A comprehensive database with 442 RC beams strengthened in shear with FRP was established and subjected to data anomaly detection using the isolation forest algorithm. Six ML models (artificial neural networks (ANN), XGBoost, random forest, CatBoost, LightGBM, AdaBoost algorithms) were trained to predict Vf. The ML models outperform six traditional explicit equations commonly used in design codes or the literature in terms of accuracy and variation. Based on the interpretable ML, the important variables were revealed; in particular the effective height of FRP, shear span ratio, and reinforcement method (U-wrap, full wrapping, side bonding) had significant influence on Vf. Finally, driven by results of the ML models, an explicit equation was derived to predict Vf, considering the physical meaning of the key influencing parameters. This study combines ML models and traditional physical models to achieve a novel, interpretable ML method to predict Vf.

Multiscale viscoelastic analysis of FRP-strengthened concrete beams

A multiscale framework with an elastic reinforced concrete (RC) beam bonded to a fiber reinforced polymer (FRP) plate (connected by an adhesive layer with viscoelastic properties) is established in this research; this framework is proved to be fast and efficient in avoiding time integration and complex meshing in existing FE models. The FRP plate is composed of elastic fibers and surrounded by viscoelastic polymers whose effective properties are generated by the locally exact homogenization theory. According to the correspondence principle, the viscoelasticity problem can be solved in the Laplace domain as an elasticity problem. This leads to solutions that can be transformed back to the time domain through Zakian's numerical method. To validate the present method, comparisons with the simulations reported in literature are presented. The effects of cracks, prestress in FRP plates, microscale fiber content, and temperature on the long-term behavior of FRP-strengthened RC beams are effectively investigated using the present method. Through the recovery of stress relaxation within the adhesive layer, the present technique is found capable of conveniently predicting the long-term behavior of FRP-strengthened RC beams, providing a robust tool for structural design and maintenance.

Finite element analyses of FRP-strengthened concrete beams with corroded reinforcement

Existing deteriorated reinforced concrete (RC) structures need strengthening to extend service life. Fibre reinforced polymer (FRP) has been widely used to strengthen sound structures, but its application on damaged concrete structures still needs to be investigated. This paper presents non-linear finite element analyses conducted to assess the flexural behaviour of corrosion-damaged RC beams strengthened with externally bonded FRP. Beams in four different categories were analysed: a reference beam, a corroded but non-strengthened beam, and corroded beams strengthened with glass FRP (GFRP) and carbon FRP (CFRP) respectively. Furthermore, the strengthened beams were modelled with different modelling choices to investigate the effectiveness of FRP applied to the beam soffit and as U-jackets. Pre-loading and corrosion-induced cracks were incorporated by reducing the tensile strength of concrete elements at crack locations. Average and pitting corrosion were incorporated by reducing the cross-sectional area of the reinforcement corresponding to the measured corrosion levels. Interface elements were used to simulate the bond between FRP and concrete. The modelling methods were validated against experimental results. It was found that modelling of pitting corrosion, especially the location of pits, lengths and number of pits considered, were influential in predicting the load and deformation capacity of beams. A CFRP plate at the beam soffit, combined with inclined U-jackets at its ends of the CFRP plate provided sufficient flexural strengthening. Thus, intermediate U-jackets did not further increase the load-bearing capacity for the studied beam geometry and corrosion damages. However, with a GFRP sheet at the beam soffit, both inclined and intermediate U-jackets were needed to provide full utilisation of the GFRP sheet for the studied beam geometry. In further studies of the effectiveness of the strengthening methods, it is recommended to investigate beams of varying dimensions, corrosion patterns and levels, and FRP spacing and dimensions.

Edge and Crack-Induced Debonding Analysis of FRP-Strengthened Concrete Beams Using Innovative Beam Finite Elements

In order to analyze the edge debonding and intermediate flexural crack–induced debonding of fiber-reinforced polymer (FRP)-strengthened concrete beams, several innovative beam finite elements were developed in this paper. The proposed elements were capable of modeling the concrete, adhesive layer, and FRP sheet in one macroscopic element. The explicit formulations of the proposed element stiffness matrices were given to simulate the adhesive layer in different loading stages. Comparisons against several numerical examples available in the literature were given to validate the proposed elements. The effects of the FRP tapering ends, load, and crack location on the interfacial shear stress within adhesive layers were effectively investigated using the proposed elements. It was found that the proposed elements could conveniently predict the edge and flexural crack–induced debonding behavior of FRP-strengthened beams with accuracy and computational efficiency.

Experimental study of FRP-strengthened concrete beams with corroded reinforcement

Corrosion of steel reinforcement is the major cause of deterioration in reinforced concrete structures. Strengthening of concrete structures has been widely studied. However, most research was conducted on sound structures without considering the effects of corrosion. This paper presents an experimental study of the feasibility of using externally bonded FRP laminates combined with U-jackets, applied directly without repairing the deteriorated concrete cover, to strengthen beams with corroded reinforcement. Ten beams were tested in four-point bending. Two beams were not deteriorated and non-strengthened; these served as references. The other eight were pre-loaded to induce flexural cracks and then exposed to accelerated corrosion. Two of the deteriorated beams were not strengthened, three were strengthened with glass-FRP (GFRP) laminates and three with carbon-FRP (CFRP) plates on the beam soffits. On the six strengthened beams, CFRP U-jackets were installed along the span. Local corrosion levels were evaluated with a 3D-scanning technique. Pitting corrosion significantly reduced the load-carrying and deformation capacity of the deteriorated beams. Despite average corrosion levels of 20%, local corrosion levels up to 57% and corrosion-induced cracks up to 1.9 mm wide, the FRP-strengthening method (applied directly to the beams without repairing the deteriorated concrete cover) was effective in upgrading the load-carrying capacity and flexural stiffness. The applied U-jackets effectively suppressed the delamination of the concrete cover and led to the rupture of GFRP laminates and a utilisation ratio of CFRP plates up to 64%. However, improvement in the deformation capacity was not noticeable; this requires further research.

A Moving Interface Finite Element Formulation to Predict Dynamic Edge Debonding in FRP-Strengthened Concrete Beams in Service Conditions

A new methodology to predict interfacial debonding phenomena in fibre-reinforced polymer (FRP) concrete beams in the serviceability load condition is proposed. The numerical model, formulated in a bi-dimensional context, incorporates moving mesh modelling of cohesive interfaces in order to simulate crack initiation and propagation between concrete and FRP strengthening. Interface elements are used to predict debonding mechanisms. The concrete beams, as well as the FRP strengthening, follow a one-dimensional model based on Timoshenko beam kinematics theory, whereas the adhesive layer is simulated by using a 2D plane stress formulation. The implementation, which is developed in the framework of a finite element (FE) formulation, as well as the solution scheme and a numerical case study are presented.

Experimental study on anchoring of FRP-strengthened concrete beams

This study aims at determining an effective technique for anchoring reinforced concrete beams strengthened using FRP composites. Fourteen reinforced concrete beams were cast and strengthened with FRP. Twelve of them were anchored using three different techniques: FRP anchor; bolted steel plates ancho, and FRP sheet and bolted steel plate’s anchors, and two beams were unanchored and left as a control. The beams then were tested under four-point bending to assess their structural performance in terms of failure modes, and load-displacement relations. The experimental results have clearly shown that the unanchored beams suffered from premature debonding failure, while the FRP-anchored beams experienced concrete cover separation failure, and the presence of end anchorages in the bolted steel plates and FRP sheet and bolted steel plate’s anchor's beams had shifted the failure mode to a less critical one. The anchored beams showed an increase in the ultimate load-carrying capacity accompanied by a reduction in mid-span deflection in different percentage with respect to the control beam. It was also observed that the FRP and/or steel U-jackets increase the shear capacity of the anchorage zone, therefore result in higher anchorage efficiency of FRP-strengthened concrete beams. The results also revealed that the FRP sheet and bolted steel plate’s anchors U configuration was the best anchorage system in terms of the structural performance factor. Developing an anchor system to prevent plate end debonding failure is vital for the successful design of strengthening systems using FRP composites.

Flexural performance evaluation of NSM basalt FRP-strengthened concrete beams using digital image correlation system

This study investigates the flexural behavior of reinforced concrete beams strengthened with near surface mounted (NSM) basalt fiber reinforced polymer (BFRP) bars. A total of five concrete beams with a length of 2100mm were prepared with different internal longitudinal reinforcement and strengthening reinforcement ratios. Four-point bending tests were conducted under monotonic loading. A digital image correlation (DIC) technique was used to monitor full-field displacement and strain contours as well as mid-span deflections. The strains in the BFRP bars were recorded using strain gauges. The force-displacement curves, longitudinal strains in NSM reinforcement, and crack widths were computed and used to evaluate the performance of the strengthened beams. Results indicate that the NSM BFRP bars can successfully restore the original capacity of a concrete beam with a corroded internal reinforcement while providing a more ductile behavior. In addition, the ultimate load capacity of the reinforced concrete beams can be augmented using NSM BFRP bars but in an expense of smaller deflection capacity.

Rational Approach for Evaluating Fire Resistance of FRP-Strengthened Concrete Beams

AbstractThis paper presents a rational approach for evaluating the fire resistance of reinforced concrete (RC) beams strengthened with different types of fiber-reinforced polymers (FRP). This approach is based on conventional fire design principles for RC beams, but incorporates the contribution of FRP and fire insulation in fire resistance calculations. Simplified equations are proposed for evaluating cross-sectional temperatures in fire-exposed FRP-strengthened RC beams with and without fire insulation. These temperatures are used to determine the loss of strength in steel, FRP, and concrete using temperature-dependent strength properties. Employing the strength in different materials, the moment capacity of FRP-strengthened beams is calculated, at any given fire exposure time, by applying force equilibrium and strain compatibility principles. At each time step, the limit state is applied to define the failure of an FRP-strengthened RC beam, and the time to failure is taken as the fire resistance of the...

Finite element modeling of debonding failures in FRP-strengthened RC beams: A dynamic approach

This paper is concerned with the finite element simulation of debonding failures in FRP-strengthened concrete beams. A key challenge for such simulations is that common solution techniques such as the Newton–Raphson method and the arc-length method often fail to converge. This paper examines the effectiveness of using a dynamic analysis approach in such FE simulations, in which debonding failure is treated as a dynamic problem and solved using an appropriate time integration method. Numerical results are presented to show that an appropriate dynamic approach effectively overcomes the convergence problem and provides accurate predictions of test results.

Modeling of debonding and fracture process of FRP-strengthened concrete beams via fracture mechanics approach

Fiber-reinforced polymer (FRP) composite materials have been widely used in the field of retrofitting. To estimate their service reliability, it is necessary to conduct a theoretical analysis of FRP-strengthened concrete beams. In this paper, a fracture mechanics approach is presented to model the cracking behavior of and interfacial debonding process in FRP-strengthened concrete beams. The K-superposition method is adopted to calculate the net stress intensity factor at the crack tip. After the validity of the proposed approach is verified with experimental results obtained from the literature, the effects of various factors on the load-bearing capacity and crack growth resistance of FRP-reinforced concrete beams are quantitatively evaluated. It is found that the first peak load increases as the beam height and compressive strength of concrete increase or the ratio of the initial crack length to beam height decreases, whereas the second peak load is an increasing function of the FRP sheet thickness and width. It is also found that the crack growth resistance increases with an increase in ratio of the initial crack length to beam height and FRP sheet thickness and width but is seldom influenced by the beam height and compressive strength of concrete.

Analytical Solution of Interface Shear Stresses in Externally Bonded FRP-Strengthened Concrete Beams

AbstractThe fiber-reinforced polymer (FRP) plate or sheet debonding or cover delamination (concrete cover separation) failure mode in externally strengthened reinforced concrete beams has attracted a lot of attention. In this paper, a closed-form analytical solution is developed to determine the nonlinear shear stress distribution along the laminate interface and cover area for any load stage assuming a perfect bond. Trilinear moment-curvature and moment-extreme compression fiber strain is assumed to realize the analytical results. By differentiating the FRP axial tension force with respect to position along the beam, closed-form derivatives in terms of curvature and extreme compressive fiber strain are obtained. The results show three distinct regions of constant or stepwise linear shear distribution in each. These correspond to the uncracked, postcracked, and postyielded zones of the shear span. The results are shown to yield an exact match to those numerically obtained by dividing the shear spans into ...

Deflection Prediction of FRP-Strengthened Reinforced Concrete Beams

Bonding fiber reinforced polymer (FRP) laminates to the tension face of RC members has been proven to be an effective method to improve the flexural strength. However, structural members are not only needed to have adequate strength, but also to have adequate performance of deformation at service load levels. To evaluate the deflection of externally FRP-strengthened RC beams, a total of 18 RC beams, including 16 beams strengthened with CFRP laminate under different preload levels and 2 control beams, were tested. Based on the assumption that the section of the beam behaves a tri-linear moment-curvature response characterized by pre-crack stage, post-crack stage and failure stage and the test results, this paper presents a modified model to evaluate the deflection of CFRP-strengthened RC beams. The present modified model was verified by the similar test results, and shows a good agreement with the test results.

Recent Developments in Long-Term Performance of FRP Composites and FRP-Concrete Interface

This paper presents recent developments in long-term behavior of fiber reinforced polymer (FRP) composites and FRP-strengthened concrete beams with special emphasis on the FRP-concrete interface subjected to creep and fatigue loads. Although short-term behavior of FRP-strengthened structures is intensively reported thus far, their long-term performance has not been thoroughly elucidated yet. This state-of-the-art paper provides a synthesis of recent findings on the long-term performance of FRP composites and their time-dependent bond behavior for strengthening concrete structures, including externally-bonded FRP sheets and near-surface-mounted FRP strips. The review examines the bond-stress slip response, damage accumulation associated with progressive crack propagation, and failure modes, including descriptions of the predictive models. The identified research needs to further advance FRP-strengthening technologies are addressed.

Effect of Fiber-Reinforced Polymer Configuration on Reliability of Flexurally Strengthened Concrete Beams

External bonding of composite materials in the form of strips, plates, and laminates to add tensile capacity in deficient zones has proved to be an efficient strengthening technique. The performance of composite materials in strengthening ailing infrastructure components has made them a mainstream alternative in some applications such as strengthening concrete structures. The technique is mature enough that design standards have been published. Few researchers have investigated the reliability of concrete structures strengthened with fiber-reinforced polymer (FRP). The results of these reliability investigations were incorporated in some of the recently published design guidelines. It is a well-known fact that FRP debonding is the dominant mode of failure. Nevertheless, the impact of different FRP configurations on the reliability of strengthened concrete beams in flexure has not yet been investigated. An analytical study is presented of the reliability of FRP-strengthened concrete beams in two of the most popular configurations: U-wrap and soffit-only. Material, fabrication, and model uncertainties are accounted for in a strength limit state function to analyze the reliability of strengthened reinforced concrete beams by using the first-order reliability method. Results from this study show that the reliability of U-wrap configurations is higher than that of soffit-only configurations. This finding can be used in establishing different resistance factors for each case such that the use of more materials in the U-wrap configuration is rewarded by the ability to predict its performance better.

Creep behaviour of CFRP-strengthened reinforced concrete beams

The strengthening of reinforced concrete structures with externally bonded fibre reinforced polymer (FRP) laminates has shown excellent performance and, as a result, this technology is rapidly replacing steel plate bonding techniques. The numerous studies that have been carried out to date on FRP-strengthened concrete elements have mainly focussed on the static and short-term responses; very little work has been done regarding the long-term performance. This paper addresses this issue, and presents results from a series of experiments on the time-dependent behaviour of carbon FRP-strengthened concrete beams. Twenty-six reinforced concrete beams with dimensions 100 × 150 × 1800 mm, with and without bonded CFRP laminates, were investigated for their creep behaviour. Different reinforcement ratios were used to evaluate the contribution of the external reinforcement on the creep resistance of the beams. High levels of sustained load were used in order to determine the maximum sustained load that can be applied without any risk of creep failure. The applied sustained loads varied from 59% to 78% of the ultimate static capacities of the un-strengthened beams. For most of the long-term tests, the applied sustained loads were higher than the service loads. This was done to account for the fact that strengthening is typically required when a structure is expected to carry increased service loads. The main parameters of this study were (i) the level of sustained load and (ii) the strengthening scheme. The results confirm that FRP strengthening is effective for increasing the ultimate capacities of the beams; however, there is virtually no improvement in performance with regard to the long-term deflections.

Treatments with Fracture Mechanics in Debonding of FRP-Strengthened Concrete Beams

External bonding fiber reinforced polymers (FRP) to the tension faces of reinforced concrete (RC) beam as an effective approach of rehabilitation and strengthening of reinforced structures have attracted significant interests of researchers in the past decade. Since the load-carrying capacity of RC beams strengthened with the FRP is dominated by interfacial delaminated failure, the anchorage strength in FRP to concrete bonded joints under shear and the debonding stress transferring behavior at the end parts of flexural concrete members bonded with FRP have been deeply studied. Recently, methods of fracture mechanics have been introduced into analyzing the above issues. Compared with the traditional methods, the analytical models of existing fracture mechanics are reviewed and analyzed. Based on them the debonding bearing capacity of FRP-strengthened RC beams is calculated and the corresponding finite element models are created. These models of fracture mechanics are then evaluated with the finite element results and the experimental data from author’s and the literature. Some conclusions with engineering significance are drawn.

Effect of freeze–thaw cycling on the behaviour of reinforced concrete beams strengthened in flexure with fibre reinforced polymer sheets

Externally bonded fibre reinforced polymer (FRP) plates and sheets for strengthening and rehabilitating existing reinforced concrete structures have recently received a great deal of attention within the civil engineering community. Many tests have shown the benefits of FRP, but more information is required on their behaviour in cold regions. Twenty-seven small-scale concrete beams (100 mm × 150 mm × 1220 mm) were strengthened with FRP in flexure (and in some cases also in shear), subjected to up to 200 freeze–thaw cycles, and tested to failure in four-point bending. Test results were compared with those predicted by theoretical models and reasonable agreement between the tests and the models was obtained. Current design guidelines for FRP-strengthened beams were compared against the test data and were found to be adequate for the artificially aged beams. The test data also indicated that no significant damage to the glass or carbon FRP-strengthened concrete beams had occurred because of freeze–thaw cycling.Key words: concrete, rehabilitation, fibre reinforced polymers, FRP, beams, freeze–thaw, cold region engineering, flexure, external strengthening.

Structural performances of short steel-fiber reinforced concrete beams with externally bonded FRP sheets

The application of externally bonding FRP sheets to concrete structures has become a popular strengthening procedure. However, such FRP strengthening effect sometimes could not be well achieved due to the premature interfacial debonding along FRP–concrete bond interface. In FRP-reinforced plain concrete members, the rapid propagation of localized flexural cracks in concrete is one of the primary reasons that cause the concentration of interfacial shear stress around where concrete crack happens, thus resulting in the debonding initiation. Therefore, an effective control of crack localization and propagation in concrete might be a solution to avoid or delay the debonding. This article presents an approach to improve the FRP strengthening performance to concrete beams by mixing short steel-fibers into the concrete matrix. To investigate the enhancement of FRP strengthening effect, a series of experiments are carried out, which include a standard JIS test of short four-point bending beams without FRP strengthening and a test of three-point bending FRP-strengthened concrete beams with different volume fractions of mixed short steel-fibers. The control of crack propagation and the increase of concrete toughness through mixing short steel-fibers are achieved. In the experiment of three-point bending, FRP-strengthened concrete beams increasing steel-fiber volume fraction, leads to a smeared crack distribution in the concrete. The ultimate failure mode also changes from peeling-induced debonding to FRP rupture so that the FRP sheet can exert its strengthening effect sufficiently. In addition, a finite element analysis is performed to compare the experimental results, in which the increase of concrete toughness is described by fracture energy. The simulation basically reproduces the experiments. The validity of the proposed approach is demonstrated.

Strengthening of RC beams with epoxy-bonded fibre-composite materials

Strengthening of concrete beams with externally bonded fibre-reinforced plastic (FRP) materials appears to be a feasible way of increasing the load-carrying capacity and stiffness characteristics of existing structures. FRP-strengthened concrete beams can fail in several ways when loaded in bending. The following collapse mechanisms are identified and analysed in this study: steel yield-FRP rupture, steel yield-concrete crushing, compressive failure, and debonding. Here we obtain equations describing each failure mechanism using the strain compatibility method, concepts of fracture mechanics and a simple model for the FRP peeling-off debonding mechanism due to the development of shear cracks. We then produce diagrams showing the beam designs for which each failure mechanism is dominant, examine the effect of FRP sheets on the ductility and stiffness of strengthened components, and give results of four-point bending tests confirming our analysis. The analytical results obtained can be used in establishing an FRP selection procedure for external strengthening of reinforced concrete members with lightweight and durable materials.