Peeling Behavior and Spalling Resistance of Bonded Bidirectional Fiber Reinforced Polymer Sheets

This paper presents the peeling behavior and spalling resistant effect of bidirectional fiber reinforced polymer (FRP) sheets externally bonded to concrete surfaces. Experimental investigations are carried out through a series of newly designed punching-peeling tests. A wide range of variables, such as FRP sheet layers and fiber direction, plate constraint, concrete strength, adhesives, bond length of FRP sheets, diameter of indenter, and types of fibers, are considered in the experimental investigation. Theoretical study is also conducted for the specimens. Interfacial fracture energy is calculated analytically using a membrane-peeling method. It is realized that only two material parameters, i.e., the interfacial fracture energy of the FRP-concrete interface and the tensile stiffness of FRP sheets, are necessary to represent the interfacial spalling resistant behavior. Finally, the theoretical results are validated by comparing with experimental results. Comparison of theoretical to experimental results shows that the proposed theoretical model is satisfactory in reasonably and accurately predicting the peeling behavior and spalling resistant capacity of bidirectional FRP sheets bonded to concrete surface.     

Development of the Nonlinear Bond Stress–Slip Model of Fiber Reinforced Plastics Sheet–Concrete Interfaces with a Simple Method

A new analytical method for defining the nonlinear bond stress–slip models of fiber reinforced plastics (FRP) sheet–concrete interfaces through pullout bond test is proposed. With this method, it is not necessary to attach many strain gauges on the FRP sheets for obtaining the strain distributions in FRP as well as the local bond stresses and slips. Instead, the local interfacial bond stress-slip models can be simply derived from the relationships between the pullout forces and loaded end slips. Based on a series of pullout tests, the bond stress–slip models of FRP sheet–concrete interfaces, in which different FRP stiffness, FRP materials (carbon FRP, aramid FRP, and glass FRP), and adhesives are used, have been derived. Only two parameters, the interfacial fracture energy and interfacial ductility index, which can take into account the effects of all interfacial components, are necessary in these models. Comparisons between analytical results and experimental ones show good accordance, indicating the reliability of the proposed method and the proposed bond stress–slip models.     

Experimental Investigation of the Influence of Moisture on the Bond Behavior of FRP to Concrete Interfaces

The effects of moisture on the initial and long-term bonding behavior of fiber reinforced polymer (FRP) sheets to concrete interfaces have been investigated by means of a two-year experimental exposure program. The research is focused on the effects of (1) moisture at the time of FRP installation, in this paper termed “construction moisture,” consisting of concrete substratum surface moisture and external air moisture; and (2) moisture, in this paper termed “service moisture,” which normally varies throughout the service life of concrete. Concrete beams with FRP bonded to their soffits were prepared. Before bonding, concrete substrates were preconditioned with different moisture contents and treated with different primers. The FRP bonded concrete beams were then cured under different humidity conditions before being subjected to combined wet/dry (WD) and thermal cycling regimes to accelerate the exposure effects. Adhesives with different elastic moduli were used to investigate the long-term durability of each adhesive when subjected to accelerated WD cycling. Pull-off tests and bending tests were conducted at the beginning of the cycling and then again after 8 months, 14 months, and 2 years of exposure so as to evaluate the tensile and shear performance of the FRP-to-concrete interfaces. It was found that the effect of the concrete substrate moisture content on short-term interfacial bond performance could be eliminated if an appropriate primer was used. All FRP-to-concrete bonded joints failed at the interface between the primer and concrete after exposure while those not exposed usually failed within the concrete substrate. After exposure to an environment of accelerated WD cycles, it was also found that the interfacial tensile bond strength degraded asymptotically with the exposure time while the flexural capacity of the FRP sheet bonded plain concrete beams even increased. The mechanism behind the above, which is an apparently contradictory phenomenon, is discussed.     

Analysis of Scaling and Instability in FRP-Concrete Shear Debonding for Beam-Strengthening Applications

The debonding mode of failure, which is observed in girders strengthened using externally attached fiber-reinforced polymer (FRP) sheets, is studied in this paper. A numerical analysis of the direct-shear response of FRP attached to concrete substrate is performed to study the initiation, formation, and propagation of an interfacial crack between the two adherents. The material response of the bimaterial interface, which includes postpeak softening, is incorporated into the numerical model. The load response obtained numerically is shown to be in close agreement with that determined experimentally from direct shear tests on concrete blocks strengthened with FRP sheets. An instability in the load response is predicted close to failure and the arc-length method is used to obtain the entire load response past the displacement-limit point. The instability in the load response is shown to be a result of snapback, where both the load and the displacement decrease simultaneously. The effect of the bonded length on the stress transfer between the FRP and concrete and on the ultimate failure is also analyzed. It is shown that there is a scaling in the load capacity when the bonded length does not allow for the establishment of the full stress-transfer zone associated with interface crack growth. From the results of the numerical analysis, a fundamental understanding of interfacial crack propagation and instability at failure in concrete members strengthened using externally bonded FRP is developed. Using a simple energy based formulation; it is shown that in strengthened girders, the instability at complete debonding of FRP from concrete translates into an explosive failure associated with a sudden release of energy.     

Numerical Cracking and Debonding Analysis of RC Beams Reinforced with FRP Sheet

Bonding a fiber reinforced polymer (FRP) sheet to the tension-side surface of reinforced concrete (RC) structures is often performed to upgrade the flexural capacity and stiffness. Except for upper concrete crushing, FRP sheet reinforcing RC structure may fail in sheet rupture, sheet peeloff failure due to opening of a critical diagonal crack, or concrete cover delamination failure from the sheet end. Accompanying the occurrence of these failure modes, reinforcing effects of the FRP sheet will be lost and load-carrying capacity of the RC structures will be decreased suddenly. This study is devoted to developing a numerical analysis method by using a three-dimensional elasto-plastic finite element method to simulate the load-carrying capacity of RC beams failed in the FRP sheet peeloff mode. Here, the discrete crack approach was employed to consider geometrical discontinuities such as opening of cracks, slipping of rebar, and debonding of the FRP sheet. Comparisons between analytical and experimental results confirm that the proposed numerical analysis method is appropriate for estimating the load-carrying capacity and failure behavior of RC beams flexurally reinforced with a FRP sheet.     

Experimental Investigation and Fracture Analysis of Debonding between Concrete and FRP Sheets

The last few years have witnessed a wide use of externally bonded fiber reinforced polymer (FRP) sheets for strengthening existing reinforced and prestressed concrete structures. The success of this strengthening method relies on the effectiveness of the load-transfer between the concrete and the FRP. Understanding the stress transfer and the failure of the concrete–FRP interface is essential for assessing the structural performance of strengthened beams and for evaluating the strength gain. This paper describes an experimental investigation of the interfacial bond behavior between concrete and FRP. The strain distributions in concrete and FRP are determined using an optical technique known as digital image correlation. The results confirm that the debonding process can be described in terms of crack propagation through the interface between concrete and FRP. The data obtained from the analysis of digital images was used to determine the interfacial material behavior for the concrete–FRP interface (stress versus relative displacement response) and the fracture parameter GF (fracture energy). The instability in the test response at failure is shown to be the result of snapback, which corresponds with the elastic unloading of the FRP as the load carrying ability of the interface decreases with increasing slip.     

Numerical Modeling of Concrete-FRP Debonding Using a Crack Band Approach

In this study, numerical procedures are proposed to predict the structural behavior of concrete members strengthened with fiber-reinforced polymeric (FRP) sheets or plates. The concept of damage band or crack band is introduced and used for predicting the debonding failure of the concrete-epoxy interface formed when FRP sheets or plates are externally bonded to a concrete substrate. In the crack band approach, all the processes taking place during the failure of a concrete-epoxy interface are smeared in a band of fixed width. This makes the approach attractive from a modeling point of view since continuum theories, along with softening relations, can be used to model the damage which causes debonding of the interface. In order to validate this approach, numerical predictions, using the concept of crack band, are compared against experimental results obtained from tests of concrete blocks and reinforced concrete beams strengthened with FRP. In particular, the capability of the proposed numerical approach to predict the load-displacement response, strain distributions, failure sequences, damage distribution, and failure mechanisms experimentally observed is verified. Results presented in this study indicate that the concept of crack band is appropriate when modeling concrete-epoxy interfaces under general states of stresses.     

Experimental Behavior of Carbon Fiber-Reinforced Polymer (CFRP) Sheets Attached to Concrete Surfaces Using CFRP Anchors

Fiber-reinforced polymer (FRP) composite sheets have gained popularity as a viable strengthening technique for existing reinforced concrete structures. The efficiency of the strengthening system largely depends on adequate bond between FRP sheets and the concrete substrate. In recent years, techniques to anchor FRP sheets have been proposed in applications that have limited distance to develop FRP sheet strength. One promising technique consists of fabricating and bonding FRP anchors during the FRP sheet saturation and embedding them into predrilled holes in the concrete substrate. This paper presents experimental results highlighting the complex behavior between FRP sheets and anchors. The primary failure modes that the sheet-anchor system can experience are identified. The experiments identify the main variables that influence the FRP anchor-sheet system behavior. This research contributes to the needed experimental database that will aid in future development of design recommendations of this anchorage system.     

Analytical Solution for Fracture Analysis of CFRP Sheet–Strengthened Cracked Concrete Beams

Fiber-reinforced polymer (FRP) composite materials have been widely used in the field of retrofitting. Theoretical analysis of FRP plate- or sheet-strengthened cracked concrete beams is necessary for estimating service reliability of the structural members. In previous studies, the effect of a perfectly bonded FRP plate or sheet was equivalent to a cohesive force acting at the bottom of crack to delay the crack propagation in concrete and reduce the crack width. However, delamination between FRP and cracked beam is inevitable due to interfacial shear stress concentration at the bottom of crack. The intention of this paper is to present an analytical solution for fracture analysis of carbon FRP (CFRP) sheet–strengthened cracked concrete beams by considering both vertical crack propagation in concrete and interfacial debonding at CFRP-concrete interface. The interfacial debonding is modeled as the interfacial shear crack propagation in this paper. Four different stages are discussed after initial cracking state of the concrete. At the first stage, only fictitious crack propagation occurs in the concrete. At the second stage, macrocrack propagates in the concrete without interfacial debonding. At the third stage, both vertical macrocrack propagation in the concrete and horizontal shear crack propagation at the CFRP-concrete interface occur in the strengthened beam. The tensile stress in the CFRP sheet and interfacial shear stress along the span are formulated based on the deformation compatibility condition at the CFRP-concrete interface at this stage. Finally, macroshear crack propagates at the interface until the CFRP sheet is completely peeled out from the beam, and then the member is fractured. The applied load is determined as a function of the referred two crack lengths at different stages. At the beginning, the applied load increases to one peak value with the full propagation of fictitious crack at the first stage. At the third stage, the applied load is improved to another peak value due to the relatively high cohesive effect of the CFRP sheet. Then the two peak values are determined by the Lagrange multiplier method. The validity of the proposed analytical solution is verified with the experimental results and numerical simulations. It can be concluded that the proposed analytical solution can predict the load-bearing capacity of CFRP sheet-strengthened cracked concrete beams with reasonable accuracy.     

Evaluation of Bending Strength of RC Beams Strengthened with FRP Sheets

This paper deals with reinforced concrete beams strengthened by means of externally bonded fiber-reinforced polymer (FRP) sheets. The scope of the work is to discuss and compare an exact and an approximate approach to the computation of the flexural load-carrying capacity of the strengthened beam. The two approaches differ from one another in the way they take into account the extent of the load already acting throughout strengthening operations. To achieve this goal a numerical model is presented and validated by comparing its output with that of 46 experimental tests taken from the literature. The numerical model is then adopted to perform a numerical parametric analysis of a wide range of practical applications, excluding all cases of FRP delamination, and useful conclusions are drawn.     

Confinement Effectiveness of FRP in Retrofitting Circular Concrete Columns under Simulated Seismic Load

The results of a research program that evaluated the confinement effectiveness of the type and the amount of fiber-reinforced polymer (FRP) used to retrofit circular concrete columns are presented. A total of 17 circular concrete columns were tested under combined lateral cyclic displacement excursions and constant axial load. It is demonstrated that a high axial load level has a detrimental effect and that a large aspect ratio has a positive effect on drift capacity. Compared with the performance of columns that are monotonically loaded until failure, three cycles of every displacement excursion significantly affect drift capacity. The energy dissipation capacity is controlled by FRP jacket confinement stiffness, especially under a high axial load level. The fracture strain of FRP material has no significant impact on the drift capacity of retrofitted circular concrete columns as long as the same confining pressure is provided, which differs from the common opinion that a larger FRP fracture strain is advantageous in seismic retrofitting. The amount of confining FRP greatly affects the length of the plastic hinge region and the drift capacity of FRP-retrofitted columns. A further increase in confinement after a critical value causes a reduction in the deformation capacity of the columns.     

Evaluation of Fracture Energy of Composite-Concrete Bonded Interfaces Using Three-Point Bend Tests

In this paper, a conventional test method using a notched three-point bending beam (3PBB) specimen is adapted to characterize Mode I fracture of composite-concrete bonded interfaces, and the interface fracture energy is evaluated based on a fictitious crack model. Two types of fiber fabrics—E-glass and carbon—are used, and a common epoxy resin is applied to bond the composite fabri?s to concrete. Mode I fracture tests of the 3PBB specimens for carbon fiber reinforced polymer (CFRP)- and glass fiber reinforced polymer (GFRP)-concrete bonded interfaces are performed to determine the applied load and load point displacement relationship from which the interface fracture energy is computed. The effects of loading rates, types of fiber fabrics, and curing time on the fracture energy of FRP-concrete bonded interfaces are studied and discussed. It is expected that the proposed experimental method can be used effectively to obtain fracture data for performing delamination studies under various environmental exposures and service loading.     

Uniaxial Tension Behavior of Reinforced Concrete Members Strengthened with Carbon Fiber Sheets

A two-dimensional (2D) nonlinear numerical analysis code by using the rigid body spring method (RBSM) was developed by the writers at Hokkaido University to simulate the behavior of reinforced concrete (RC) members strengthened with fiber-reinforced polymer (FRP) sheets. The code supports the nonlinear constitutive laws for the different materials and nonlinear bond stress-slip relationships for steel-concrete and FRP sheet-concrete interfaces. This study uses the aforementioned code to examine the uniaxial tension behavior of RC members strengthened with carbon fiber sheets (CFS). Experimental results are compared with relevant numerical outputs to validate the model and confirm its ability to simulate the experimental observations. This study also assesses the influence of the amount of CFS strengthening on the tension-stiffening behavior of strengthened members. Finally, this research also suggests new analytical expressions for the average stress-strain relationships of concrete and steel in tension in the presence of stiffening contributions from internal steel reinforcement bars and externally bonded CFS reinforcement.     

Front and Side View Image Correlation Measurements on FRP to Concrete Pull-Off Bond Tests

Understanding the transfer of force by bond between externally bonded fiber-reinforced polymer (FRP) reinforcement and concrete is an important step in formulating good models for predicting debonding failures observed in externally bonded reinforcement strengthened systems. In this paper, a 3D optical displacement measurement system was used to capture the full-field displacements from the front and side view in pull-off bond specimens. The experiments were carried using six specimens with carbon FRP (CFRP) strips having different axial stiffnesses but a constant bond length to the concrete substrate. Using the optical measurements, it was possible to obtain the in-plane displacement or slip and the out-of-plane displacement or separation between the CFRP strip and the concrete. It was demonstrated, that the usual assumption of pure shear stresses in such pull-off tests is not true and that the bond behavior is a two-dimensional problem involving shear and peeling stresses. The bond behavior in CFRP strip to concrete pull-off tests was characterized by three stages: (1) the initiation of the first crack; (2) the initiation of debonding; and (3) failure by complete debonding. Based on the test results it was found that there was a dependency between the maximum bond shear stress, the maximum fracture energy of the FRP-concrete interface, and the stiffness of the FRP. However, the slip values after initiation of debonding (Stage 2) were independent of the FRP stiffness. The measured anchorage force and anchorage length were in good agreement with predictions from existing code equations.     

Tensile Behavior of FRP Anchors in Concrete

Strengthening of concrete structures using fiber-reinforced polymer (FRP) systems has become a widely accepted technology in the construction industry over the past decade. Externally bonded FRP sheets are proven to be a feasible alternative to traditional methods for strengthening and stiffening deficient reinforced or prestressed concrete members. However, the delamination of FRP sheets from the concrete surface poses major concerns, as it usually leads to a brittle member failure. This paper reports on the development of FRP anchors to overcome delamination problems encountered in surface bonded FRP sheets. An experimental investigation was conducted on the performance of carbon FRP anchors that were embedded in normal- and high-strength concrete test specimens. A total of 81 anchors were tested under monotonic uniaxial loading. Test parameters included the length, diameter, and angle of inclination of the anchors and the compressive strength of the concrete. The experimental results indicate that FRP anchors can be designed to achieve high pullout capacities and hence can be used effectively to prevent or delay the delamination of externally bonded FRP sheets. The results also indicate that the diameter, length, and the angle of inclination of the anchors have a significant influence on the pullout capacity of FRP anchors.     

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. The analytical models from various sources are applied to this database and the results are presented.     

Behavior of Gravity Load-Designed Rectangular Concrete Columns Confined with Fiber Reinforced Polymer Sheets

This study concentrates on analytical evaluation of the effect of external confinement using fiber reinforced polymers (FRP) sheets on the response of concrete rectangular columns designed for gravity load only and having spliced longitudinal reinforcement at the column base. A general analytical scheme for evaluating the strength capacity and ductility of the columns under combined flexural–axial loads was developed. The analysis takes into account the bond strength degradation of the spliced reinforcement with increase in lateral load by incorporating a generalized bond stress–slip law, and considers the effect of FRP confinement on the stress–strain response of concrete material. Particular emphasis is placed in the analysis on the slip response of the spliced bars and the consequent fixed end rotation that develops at the column base. Results predicted by the analysis showed very good agreement with limited experimental data. A parametric evaluation was carried out to evaluate the effect of different design and strength parameters on the column response under lateral load. Without confinement, the columns suffered premature bond failure and, consequently, low flexural strength capacity. Confining the concrete in the columns end zone at the splice location with FRP sheets enhanced the bond strength capacity of the spliced reinforcement, increased the steel stress that can be mobilized before bond failure occurs, and consequently improved the flexural strength capacity and ductility of the columns. A general design equation, expressed as a function of the main parameters that influence the bond strength capacity between spliced steel bars and FRP confined concrete, is proposed to calculate the area of FRP sheets needed for strengthening of the subject columns.     

Concrete-Filled Square and Rectangular FRP Tubes under Axial Compression

This paper presents results of an experimental study on the behavior of square and rectangular concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs) under concentric compression. FRP tubes were designed as column confinement reinforcement and were manufactured using unidirectional carbon fiber sheets with fibers oriented in the hoop direction. The effects of the thickness and corner radius of the tube, sectional aspect ratio, and concrete strength on the axial behavior of CFFTs were investigated experimentally. Test results indicate that FRP confinement leads to substantial improvement in the ductility of both square and rectangular columns. Confinement provided by the FRP tube may also improve the axial load-carrying capacity of the square and rectangular columns if the confinement effectiveness of the FRP tube is sufficiently high. The results also indicate that the confinement effectiveness of FRP tubes is higher in square columns than in rectangular columns, and in both sections the effectiveness of confinement increases with the corner radius. Furthermore, for a given confinement level, improvement observed on the axial behavior of concrete due to confinement decreases with increasing concrete strength.     

Strengthening RC Beams in Flexure Using New Hybrid FRP Sheet/Ductile Anchor System

This paper explores a new hybrid fiber-reinforced polymer (FRP) sheet/ductile anchor system for rehabilitation of reinforced concrete (RC) beams. The advantages of the proposed strengthening method is that it overcomes the problem of low ductility that is associated with brittle failure mode in conventional methods of strengthening beams using epoxy-bonded FRP sheets. The proposed system leads to a ductile failure mode by triggering yielding to occur in a steel anchor system (steel links) rather than by rupture or debonding of FRP sheets, which is sudden in nature. Four half-scale RC T-beams were tested under four-point bending. Three retrofitted beams were strengthened using one layer of carbon FRP sheet. The results of the two beams that were strengthened with the new hybrid FRP sheet/ductile anchor system were compared with the results from the beam strengthened with conventional FRP bonding method and the control beam. The results show the effectiveness of the proposed strengthening system in increasing flexural capacity and ductility of RC beams.     

Fiber-Reinforced-Cementitious-Composites Plate for Anchoring FRP Sheet on Concrete Member

To improve the fiber-reinforced polymer (FRP)/concrete bond capacity, this paper presents a new anchoring approach with the gluing of precast fiber-reinforced cementitious composites (FRCC) plate on top of the FRP sheets. In order to measure the improvement in ultimate load and deformation capacity and to study the failure mechanisms around the anchored area, the direct shear bond test is performed on concrete prisms with bonded FRP. Several sets of tests have been carried out with anchoring plates of different FRCC compositions and lengths. Comparison with the control sample shows that the installation of FRCC plate can significantly increase both the bond and deformation capacities (by up to 100%). On the basis of the shear bond test, two types of FRCC plate materials were found to be particularly effective and were selected for strengthening of beam members to be tested under four-point bending. Comparison with control members (without anchor) and those with conventional U-shaped FRP anchors indicates that both the ultimate load and central deflection can be improved by the new anchoring method.