Externally Bonded Research Articles

Modeling the behavior of CFRP-strengthened RC slabs under fire exposure

This paper presents the development of a numerical model that simulates the fire response of reinforced concrete (RC) slabs externally strengthened with carbon fiber-reinforced polymer (CFRP)laminates using two different techniques, externally bonded (EB) and near-surface mounted (NSM). A three-dimensional (3D) nonlinear finite element (FE) model is developed to predict the thermal and structural behavior of strengthened RC slabs subjected to fire. The model incorporates temperature-dependent thermal and mechanical properties of concrete, steel reinforcement, and CFRP, as well as mechanical bond interaction between CFRP and concrete interfaces. The predicted temperature profiles, ultimate loads, and midspan deflections are compared with previously published experimental data. Results from the proposed model show a good correlation with the experimental data throughout the fire exposure duration. The validated model can be adapted to conduct parametric studies intended to inspect the effect of important factors that influence the behavior of strengthened RC slabs under fire.

Size Effect in FRP Shear-Strengthened RC Beams: Design Models versus Experimental Data

Numerous studies on the size effect have been devoted to reinforced concrete (RC) beams. They have shown that increasing the beam size leads to a decrease in ultimate shear strength (stress) at failure. This is reflected in the design model of most current international codes and guidelines, where the size effect is taken into consideration by reducing concrete contribution to the shear resistance (force). In contrast, the size effect of RC beams strengthened with externally bonded (EB) fibre-reinforced polymer (FRP) is not fully documented, and very few experimental studies have been devoted to the phenomenon. The objective of this study was to evaluate the accuracy of the current code and guideline models in terms of the size effect on the EB-FRP contribution to shear resistance. To this end, a database of experimental findings on the size effect in EB-FRP-strengthened beams was built based on the reported literature, as well as our own experimental tests. The data were analysed and compared with the predictions of six current codes and design guidelines to assess their accuracy. Experimental results clearly revealed the presence of a size effect related to EB-FRP as well as the existence of interaction between internal stirrups and EB-CFRP. Based on analysis of the collected experimental test results, the study clearly revealed that the predictions of current codes and guidelines overestimate the contribution of EB-FRP systems to shear resistance. The size effect tends to exacerbate this overestimation as the effective depth (d) of the beams increases. Therefore, until the size effect for RC beams strengthened in shear with EB-FRP is captured by the prediction models, current codes and design guidelines are to be used with caution.

Efficiency of different BFRP-based strengthening techniques in improving flexural behavior of RC slabs

This study aims to analyze the flexural behavior of one-way reinforced concrete (RC) slabs strengthened with basalt fiber-reinforced polymers (BFRP). A total of six slabs were constructed and subjected to four-point flexural experimental tests. One slab served as the control specimen, and two slabs were strengthened with externally bonded (EB) BFRP sheets and near-surface mounted (NSM) BFRP bars, respectively. The other three slabs were strengthened with a composite reinforced mortar (CRM) layer composed of BFRP grids and polymer cement mortar, of which the BFRP grids for one slab were nonprestressed, and those for the remaining two slabs were prestressed. The experimental results indicated that the types of strengthening techniques including EB, NSM and CRM affected the failure mode of the strengthened slab. In comparison with the EB and NSM techniques, the CRM technique was more efficient in improving the serviceability and ultimate behavior of RC slabs, and it was considerably more economical. Meanwhile, the application of prestress in the CRM technique could further improve the cracking load and flexural stiffness of the strengthened slabs. Additionally, an analytical model was presented to predict the cracking and yield loads of the strengthened slabs, and the predictions showed a good agreement with the experimental results.

Strain Transfer of Fiber Bragg Grating Sensor Externally Bonded to FRP Strip for Structural Monitoring after Reinforcement.

To date, a method of attaching a FRP (fiber-reinforced polymer) to concrete members with epoxy has been widely applied to increase the strength of the member. However, there are cases in which the adhesion of the epoxy deteriorates over time and the reinforcing effect of the FRP is gradually lost. Therefore, monitoring whether or not the reinforcing effect is properly maintained is needed in order to prevent a decrease in the structural performance of the member improved by FRP reinforcement. In this regard, this study examines FRP with OF (optical fiber) sensors to monitor the reinforcing effect of FRP in concrete structural members. In particular, this paper seeks to determine an appropriate adhesion length when FBG (fiber Bragg grating) based OF sensors are externally bonded to FRP strips with epoxy resin. To this end, a tensile test was carried out to evaluate the sensing performance according to the adhesion length. In addition, an analytical approach was performed and the result were compared with test result. The results of the experimental and analytical studies showed that the strain generated in the FRP is sufficiently transferred to the OF if the total adhesion length of it is 40 mm or more in consideration of the error in the epoxy thickness.

Flexural response and load capacity of reinforced concrete beams strengthened with reinforced mortar layer

Reinforced concrete (RC) elements can be strengthened using several techniques. Among these techniques are the externally bonded (EB), near-surface-mounted (NSM), textile-reinforced mortar (TRM), and ultra-high-performance concrete (UHPC). The NSM and EB methods are primarily based on the efficiency and thickness of the concrete cover, while the UHPC method demands excessive surface preparation. This study presents a new technique to strengthen RC beams using a limited steel-bar-reinforced mortar layer (SBRML). This method reduces the installation time and cost and combines the advantages of existing techniques. An experiment with eight concrete beams, one unstrengthened beam (CB), and seven strengthened beams is performed. The RC beams are tested until failure under a four-point bending load. The effect of the SBRML reinforcement area, internal reinforcement configuration, adhesive type, concrete cover condition (with or without), position (bottom or side), and application type (casting or bonding) on the failure mechanism and capacity of the RC beams are studied. The results show an increase in the bearing capacities of RC beams strengthened with epoxy-bonded SBRML and cast SBRML in the range of 152.5–213.9% and 122.7–158.1%, respectively, compared to those of CB. The values of yield and ultimate loads for RC beams strengthened with epoxy-bonded RML are analyzed. A numerical simulation is performed and verified against the experimental results. A parametric study was conducted to reveal the effect of fiber-reinforced polymer reinforced mortar layer FRP-RML on the behavior of the strengthened beams.

Analytical study on the effectiveness of hybrid FRP strengthening on behaviour of slender reinforced concrete square columns

Strengthening of reinforced concrete (RC) columns using external bonded (EB) fibre-reinforced polymer (FRP) composites has become popular since last three decades. The effectiveness of EB confinement reduces for slender columns due to the secondary moment arising from applied axial loads. The effectiveness of the hybrid FRP strengthening system for enhancing the performance of RC slender square columns is analytically investigated. Hybrid FRP strengthening uses a combination of EB FRP wrapping for confinement and longitudinal near-surface mounted (NSM) FRP laminates for bending. Hybrid FRP strengthening aims to reduce the second-order lateral displacement of the slender RC column and increase its axial capacity and flexural stiffness. A numerical modelling approach is presented to capture second-order effects in hybrid FRP strengthened slender RC square column based on Newmark's central difference scheme. Both material and geometric nonlinearities are accounted for in the calculations. The predictions are validated with test results available in the literature. After validation, a parametric study is carried out to evaluate the influence of slenderness ratio, the number of EB FRP layers, NSM laminate ratio, modulus of NSM laminate, and buckling of FRP composites. Results show that hybrid FRP strengthening is effective in improving the performance of slender RC columns.

Experimental investigation on low-velocity impact behavior of CFRP wraps in presence of concrete substrate

The externally bonded (EB) carbon fiber-reinforced polymer (CFRP) sheets widely used for retrofitting reinforced concrete structures might suffer severe damages under the levels of low-velocity impact far below those considered by current design codes. Among the different EB CFRP-retrofitting systems, the CFRP wraps used for enhancing the compressive strength of the concrete in the retrofitted columns are most likely to encounter various low-velocity impact events, and the consequence of their failure can be catastrophic. However, the in-depth understanding of this safety hazard has been lacking so far. In this study, a large-scale drop-weight apparatus with a horizontal quasi-static loading system was specially designed and manufactured. A comprehensive experimental investigation of sixty impact scenarios was performed to examine the effects of a series of parameters on the low-velocity impact behavior of CFRP wraps in the presence of concrete substrate, namely the incident kinetic energy of impactor, CFRP thickness, axial compressive stress-to-concrete strength ratio and the shape of impactor. Furthermore, the mechanical characteristics at the contact interfaces between CFRP wraps and impactors were disclosed, and the vulnerability of CFRP wraps was evaluated by quantifying impact damages. In addition, the analytical relationships between the impact damages of CFRP wraps and the incident kinetic energy of impactor were established.

Finite element modelling of aluminum alloy plated reinforced concrete beams

This study presents a nonlinear finite element (FE) model development of reinforced concrete (RC) beams externally strengthened with aluminum alloy (AA) plates. The aim of this numerical study was to elucidate the effects of different anchorage schemes on the capacity, ductility, and failure mode of AA plate strengthened beams reported in a published test. Three FE models were developed; namely, a reference RC beam, a beam externally bonded (EB) with an AA plate, and a beam EB with an AA plate with carbon fiber reinforced polymers (CFRP) U-wraps at the plate's end. Validation of the developed FE models was carried out by comparing their load-deflection plots, maximum attained loads, deflections at failure, and failure modes with those reported during the test. The results of each FE model yielded an absolute percentage error less than 5%. Moreover, premature failure modes like end-plate and intermediate crack debonding were simulated and closely agreed with those observed during the test. Finally, the validated models were used to employ a parametric study comprising of twelve beams varying in size of steel reinforcement, presence of AA plates, and end-anchorage. It was concluded that the developed FE models could serve as a design platform for assisting structural engineers during flexural retrofit applications using AA plates.

Nonlinear finite element models of reinforced concrete beams strengthened in bending with mechanically fastened aluminum alloy plates

This paper reports 3D nonlinear finite element (FE) models of reinforced concrete (RC) beams externally strengthened with mechanically fastened (MF) and externally bonded (EB) aluminum alloy (AA) plates. The measured and observed parameters of each specimen were obtained from an experiment conducted by the authors of this study. Each FE model adopted accurate material constitutive laws and appropriate element definitions to approximate the mechanical behavior observed during testing. A total of ten models were constructed based on the diameter, embedment depth, and arrangement of the expansion anchor bolt (EAB) as well as the presence or absence of epoxy. Results like the load-carrying capacity of the tested specimens as well as the failure modes like end-plate debonding (ED), local-plate debonding (LD), and plate rupture (PR) were accurately predicted. Additionally, Stress and strain contour plots were generated to investigate the FE models’ mechanical behaviors. Finally, a parametric study was conducted to illustrate the effects of the flexural steel reinforcement ratio and AA plate grades on the models’ structural response. It was concluded that the FE models could serve as a valid predictive platform for simulating the flexural behavior of RC beams strengthened with MF and EB AA plates.

Brickwork wall models strengthened with diagonal and horizontal GFRP strips

The use of fiber reinforced polymers (FRPs) as external bonded (EB) strengthening of unreinforced brickwork masonry has became a common intervention technique for restoration of masonry walls damaged by seismic actions with the aim to improve the capacity under in-plane shear loading. The exact knowledge of the behavior of system composed by masonry and EB FRP strips through the interaction mechanism between strengthening and substrate is essential for the effectiveness of interventions based on the use of FRP strips. This paper provides an investigation on the behaviour of unreinforced and reinforced masonry walls made with solid clay bricks in scale 1/3rd, considering the presence of different types of masonry, such as historic and modern, and different configuration of the GFRP strengthening. Three walls, characterized by double T shape, were tested under combined compression and shear loading. Two of them were strengthened after damage using, respectively, diagonal and horizontal EB GFRP strips; another one was strengthened with horizontal GFRP strips without damage and subjected to the same loading path until failure. The response of all models is illustrated and discussed considering the walls’ shear strength and effects on the failure mechanism due to diagonal and horizontal EB GFRP strips.

Multilayer versus Monolayer Externally Bonded CFRP Sheets for Shear Strengthening of Concrete T-Beams

Abstract The use of externally bonded (EB) carbon fiber–reinforced polymer (CFRP) fabrics for strengthening existing structures has received worldwide acceptance since the late 1990s. The composite...

Experimental and analytical study of flexural behaviour of BFRP sheets strengthened RC beams with new epoxy anchors

Intermediate crack (IC) debonding is a common failure mode of externally bonded (EB) FRP-strengthened RC beams. This debonding failure initiates at an intermediate crack and propagates towards a plate end. New epoxy anchor was proposed recently by the authors and has shown its effectiveness in enhancing interfacial bonding behaviour and therefore it might be effective to delay or suppress IC debonding failure in RC beams. This study is to experimentally investigate the efficiency of using epoxy anchors for mitigating the IC debonding under three-point bending tests. The application of the new proposed epoxy anchors has advantage of simple installation procedure including pre-drilling holes and then bonding FRP. Totally, five RC beams including one control specimen and four anchored ones were tested. Damage modes and structural response of unanchored and anchored RC beams were evaluated and discussed. The effects of various configurations of epoxy anchors were analysed and discussed. The experimental results show that the load-carrying capacity and the ductility of anchored beams increased by up to 13.12% and 53.31%, respectively, and the strain utilization of FRP can be significantly improved by 43.48% as compared to the control specimen.

Mixed-Mode Debonding Behavior between CFRP Plates and Concrete under Fatigue Loading

The cohesive zone model (CZM), which is capable of simulating the behavior of adhesively bonded joints subjected to static and fatigue loading, is widely accepted. In this study, fatigue peeling/shear of RC with externally bonded (EB) carbon fiber–reinforced polymer (CFRP) laminates under the mixed-mode implementation of the CZM is proposed. The procedure has been implemented in commercial software by programming the appropriate software-embedded subroutines. The main advantage of the proposed CZM is the use of coupled mixed-mode damage (shear-peeling), accounting for cumulative damage resulting from fatigue loading. Two-dimensional numerical simulations of the innovative double-lap-shear bonding test were performed by the proposed CZM. The excellent agreement between the simulated results and the experimental results are verified, and the good performance of the method as a predictive tool is demonstrated. Moreover, the peeling/shear propagation during loading can be effectively predicted.

Experimental Analysis of Continuous Beams Made of Self-Compacting Concrete (Scc) Strengthened with Fiber Reinforced Polymer (Frp) Materials

Strengthening of concrete structures is applied as a solution for various deterioration problems in civil engineering practice. This also refers to the structures made of self-compacting concrete (SCC), which is increasingly in use, but there is a lack of research in this field. This paper presents an experimental analysis of flexural behavior of reinforced concrete (RC) continuous beams made of SCC, strengthened with fiber reinforced polymer (FRP) materials (glass (GFRP) and carbon (CFRP) bars, CFRP laminates), by the use of near surface mounted (NSM) and externally bonded (EB) methods. Six two-span continuous beams of a total length of 3200 mm, with the span between supports of 1500 mm and 120/200 mm cross section, were subjected to short-term load and tested. The displacements of beams and the strains in concrete, steel reinforcement, FRP bars and tapes were recorded until failure under a monotonically increasing load. The ultimate load capacities of the strengthened beams were enhanced by 22% to 82% compared to the unstrengthened control beam. The ductility of beams strengthened with GFRP bars was satisfactory, while the ductility of beams strengthened with CFRP bars and tapes was very small, so the failure modes of these beams were brittle.

Effect of mechanical anchorage on the bond performance of double overlapped CFRP-to-steel joints

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.

Efficiency of shear strengthening of RC beams with externally bonded FRP materials – State-of-the-art in the experimental tests

This paper presents a study of the shear capacity of reinforced concrete (RC) beams strengthened in shear with externally bonded (EB) fiber reinforced polymer (FRP) composites, based on the performance of existing published tests. The main objective of this study is to create a comprehensive overview of the performance of different shear strengthening configurations and the failure modes observed in experimental tests. In addition, the influence of the following parameters on the strengthening efficiency has been analyzed: shear span to depth ratio (a/d), existing steel transverse reinforcement ratio (ρsw), concrete strength (fck), size effect (d), longitudinal reinforcement ratio (ρsL), strengthening configurations, type of FRP: sheets or laminates (CFRP, GFRP), and FRP axial stiffness (AfEf).

Enhancing the EB-FRP strengthening effectiveness by incorporating a cracking-control layer of ECC with different thicknesses

The multiple and tight cracking nature of engineered cementitious composite (ECC) under tension can significantly mitigate or eliminate the premature debonding of externally bonded (EB) fiber-reinforced polymer (FRP). This paper systematically investigates the flexural performance of concrete beams that were jointly strengthened by hybrid carbon FRP (CFRP) and ECC. The ECC layer realizes the dual-function of maximizing the strengthening effectiveness and contributing additional flexural strength. The test results indicate that the inclusion of an ECC layer changed the CFRP strip from debonding to rupture at the ultimate state, which in turn remarkably increased the member flexural capacity, the energy consumption capacity and ductility, compared to the EB-CFRP strengthened counterpart. The CFRP rupture occurred even though the ECC thickness was only half that of the concrete cover. Further increasing ECC thickness contributed marginally to the member load capacity and ductility until when the ECC total enwrapped longitudinal reinforcement. The optimal thickness of ECC was discussed. The crack width of the concrete block was considerably narrowed after applying the ECC layer. Finally, a model was developed to predict the flexural strength of CFRP-ECC jointly strengthened beams, providing good agreement between the predicted and experimental results.

Novel fiber reinforced polymers (FRP)-ultrahigh performance concrete (UHPC) hybrid beams with improved shear performance

Fiber reinforced polymer (FRP) and ultrahigh performance concrete (UHPC) are two durable and high strength construction materials. FRP-UHPC hybrid beam is a durable and efficient bridge superstructure, however, its capacity is limited by the low shear strength of FRP profiles. This paper investigated two ways to improve the shear performance of FRP-UHPC hybrid beams: externally bonded (EB) carbon FRP (CFRP) sheets to the webs of glass FRP (GFRP) profiles, and increasing the number of webs of FRP profiles. Nine FRP-UHPC hybrid beams were tested under bending loads, which indicates: (i) EB CFRP sheets to GFRP webs substantially increased the shear capacity and the rigidity of FRP-concrete hybrid beams. (ii) the built-up II-shaped FRP profiles had almost doubled the shear capacity of FRP-concrete hybrid beams using I-shaped profiles. (iii) With UHPC height increased from 30 mm to 80 mm, the failure mode changed from the shear fracture of UHPC slab to the interfacial disconnection. (iv) With the decrease of shear span length from 700 mm to 120 mm, the shear capacity was largely improved. In summary, EB CFRP sheets and using double web GFRP profiles are two effective ways to improve the shear performance of FRP-UHPC hybrid beams.

Behavior of cracked steel plates strengthened with adhesively bonded CFRP laminates under fatigue Loading: Experimental and analytical study

The strengthening of steel structures using externally bonded (EB) carbon fiber-reinforced polymer (CFRP) laminates has gained popularity due to the advantages such as their high strength-to-weight ratio and corrosion resistance. Even though previous studies showed the application of EB CFRP laminates can enhance the fatigue performance of cracked steel plates, little is known regarding the high-cycle fatigue performance of CFRP-to-steel bonded joints. As debonding of the CFRP laminate from the steel substrate is a commonly observed failure mode under fatigue loading, a sound understanding of the behavior of CFRP-to-steel bonded joints is crucial for a better understanding of the behavior of CFRP-strengthened cracked steel plates under fatigue loading. This study experimentally and theoretically investigates the fatigue performance of CFRP-strengthened cracked steel plates. Five pre-cracked steel plates were strengthened with CFRP laminates and tested under fatigue loading. The test results for the failure modes, the fatigue-life extension, and the behavior of the CFRP-to-steel bonded joint were discussed. A numerical modeling approach based on a recently developed bond-slip model for the behavior of the CFRP-to-steel bonded interface under fatigue loading is presented for modeling the behavior of the CFRP-strengthened cracked steel plate. Although the proposed theoretical model is conservative, this method accurately predicted the remaining fatigue life of CFRP-strengthened cracked steel plates.

Flexural strengthening of RC beams using aluminum alloy plates with mechanically-fastened anchorage systems: An experimental investigation

Externally bonded reinforcement (EBR) has emerged as one of the promising techniques for flexural strengthening of reinforced concrete (RC) members due to its practicality and structural effectiveness. However, shortcomings like premature failure modes, cost, labor, and installation time limited its use. Mechanically fastened (MF) Aluminum Alloy (AA) plates instead have the potential to overcome some of these drawbacks by providing strength and ductility, while influencing the failure modes. In this investigation, 16 RC beams were prepared, one beam was left un-strengthened (CB), one was strengthened with an externally bonded (EB) AA plate (CBE), and the remaining 14 beams were strengthened with MF AA plates that varied based on expansion anchor bolts (EAB) sizes, spacing, layout, and the presence or absence of epoxy. The test results indicated that all the specimens with MF AA plates exhibited approximately 30% increase in strength accompanied with drastic increase in ductility up to about 84% compared to specimen CBE. Analytical predictions were made to numerically assess the advantages of using MF systems where ACI 318-19 outperformed ACI 440.2R-17 due to the sufficient composite behavior that allows the AA plates to behave similar to steel reinforcement such that the section has an additional lever arm. It is concluded that the implementation of epoxy and EAB as an alternative anchorage technique serves as a viable approach in enhancing the strength and ductility of RC beams strengthened with AA plates.