Tensile Capacity of GFRP Postinstalled Adhesive Anchors in Concrete

This paper presents the results of an experimental study conducted on the pullout capacity of glass fiber reinforced polymer (GFRP) postinstalled adhesive anchors embedded in concrete. A total of 90 adhesive anchors were installed using sand-coated GFRP reinforcing bars and tested under monotonic tension loading in accordance with ASTM E-488-96 in 1996. The test parameters were: (1) the GFRP bar diameter (25.4, 15.9, and 6.4?mm); (2) the embedment depth (5, 10, and 15 db where db=bar diameter); (3) the adhesive type (epoxy-based and cement-based adhesives); and (4) installation conditions (wet or partially submerged and dry holes). The tested GFRP adhesive anchors were installed in concrete slabs measuring 3,750?mm long, 1,750?mm wide, and 400?mm deep. The test specimens were kept outdoors for 7?months to be subjected to real environmental conditions including freeze-thaw cycles, wet and dry cycles, and temperature variations. The experimental results indicated the adequate performance of GFRP adhesive anchors installed in wet or partially submerged condition using epoxy-based adhesive. Similar behavior was observed for those installed with cement-based adhesive in dry conditions as well. The capacity of the GFRP bars installed with both adhesive types was achieved at an embedment depth ranging from 10 to 15 db.     

Pullout Strength Models for FRP Anchors in Uncracked Concrete

Mechanical anchorage can delay or even prevent premature debonding failure in externally bonded fiber-reinforced polymer (FRP) composite strengthening systems. A promising type of anchor made from FRP, which is known as a FRP spike anchor or FRP anchor among other names, is noncorrosive and can be applied to a wide range of structural elements and externally bonded FRP strengthening schemes. Experimental investigations have shown FRP anchors to be effective under tension (pullout) and shear loading, however, few analytical models exist to date. This paper in turn presents analytical models to quantify the pullout strength of FRP anchors. As existing research on the pullout behavior of metallic anchors is partially relevant to FRP anchors, this paper first presents a review of current pullout strength models for metallic anchors. These models are then assessed with experimental data of FRP anchors and modified and recalibrated where appropriate. As a result, simple and rational pullout strength models for FRP anchors are proposed which can also be used in design. Finally, parametric studies are undertaken and the influence of key variables is identified.     

Hybrid Bonding of FRP to Reinforced Concrete Structures

The adhesive attachment of fiber-reinforced polymers (FRP) laminate to the external face of reinforced concrete structures is currently one of the most popular and effective methods for retrofitting and strengthening concrete structures. With this method, the additional strength of the attached reinforcement is transmitted into the concrete members through adhesion. However, the relatively weak adhesive interface fundamentally limits the efficacy of the method. Much effort has been made in the research community to improve the bond strength and develop bond models, but a satisfactory solution has yet to be found. Mechanical fastening is another more traditional technology that is used to bond one material to another. This paper introduces a new hybrid bonding technique that combines adhesive bonding and a new type of mechanical fastening. The new mechanical fastening technique does not rely on bearing to transmit the interfacial shear, but instead increases the interfacial bond by resisting the separation of the FRP laminate from the concrete substrate. Experimental tests demonstrated that the bond strength with this new hybrid bonding technology was 7.5 times that of conventional adhesive bonding. Furthermore, the new bonding technique is applicable to all types of commercially available FRP laminate (fabric, sheet, plate, and strip), and in principle is also applicable to materials other than FRP.     

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 FRP stiffness, are proposed.     

Analysis of the Flexural Behavior of Partially Bonded FRP Strengthened Concrete Beams

An investigation was conducted on the flexural behavior of partially bonded fiber-reinforced polymer (FRP) strengthened concrete beams focusing on the improvement of ductility. An analytical model was developed based on the curvature approach to predict the behavior of beams strengthened with partially bonded FRP systems. The result of the analysis showed that ductility of the partially bonded system was improved while sustaining high load carrying capacity in comparison to the fully bonded system. To verify the analytical model, an experimental program was carried out with reinforced concrete beams strengthened with the externally bonded FRP system. A comparison of the analytical prediction and experimental results showed good agreement.     

Experimental Investigation of Nonconventional Confinement for Concrete Using FRP

The present study investigates experimentally the behavior of concrete confined with fiber reinforced polymers (FRP) in the form of jackets which are applied according to a number of nonconventional techniques. First, the effectiveness of various jacketing configurations combined with anchors as a measure of increasing the strength and deformability of L-shaped columns is investigated. It is concluded that easy to install and low-cost anchors made of resin impregnated fibers properly placed at the reentrant corner of L-shaped columns enable excellent mobilization of confining stresses supplied by the FRP jackets. Next, a number of alternative confinement methods are investigated on concrete cylinders, aimed at quantifying the effectiveness of (1) unbonded jacketing, (2) spirally applied strips attached only at their ends, and (3) jacketing directly on concrete with mortar plastering. Although the study may be regarded as preliminary, it provides useful experimental support to a number of techniques which have the potential to open new horizons in the field of externally applied FRP for enhancing concrete confinement.     

Modeling of Moisture Diffusion in FRP Strengthened Concrete Specimens

Modeling the movement and distribution of moisture in the fiber-reinforced polymer (FRP) composites strengthened concrete structure is important because the interfacial adhesion between FRP and concrete is susceptible to moisture attack. Using relative humidity as the global variable, the moisture diffusion governing equation was derived for the multilayered system in this study. The moisture diffusivity (diffusion coefficient) and the isotherm curve, which correlates the moisture content to environmental relative humidity, of each constitutive material (concrete, epoxy, and FRP) were experimentally determined. A multilinear diffusivity model was developed for concrete based on desorption test, and a linear diffusivity model was proposed for epoxy adhesive based on absorption test. A simple method was developed to directly measure the FRP/concrete interface region relative humidity (IRRH). Finite-element analysis was performed to study the moisture diffusion in the FRP-adhesive-concrete system. The IRRH values were obtained for different environmental relative humidity in the numerical study. The error between the experimental and numerical results of IRRH at test locations was less than 5% RH. The good agreement between experimental and numerical results indicates that the approach developed in this study worked well.     

Moment Redistribution in FRP and Steel-Plated Reinforced Concrete Beams

Research on retrofitting reinforced concrete (RC) beams and slabs using externally bonded (EB) fiber reinforced polymer (FRP) or steel plates has reached the stage where the flexural strength can be determined with confidence. Research has also shown that EB plated structures tend to debond at relatively low strains and to such an extent that guidelines often preclude moment redistribution which can severely restrict the use of plating. However, recent research on retrofitting using FRP and steel near surface mounted plates (NSM) has shown that NSM plates tend to debond at high strains which can allow substantial amounts of moment redistribution. A moment redistribution approach has been developed for both NSM and EB plated beams that allows for the wide range of debonding strains that can occur. This allows RC beams to be retrofitted for both strength and ductility which should help expand the use of this convenient and inexpensive form of retrofitting.     

Design Considerations of Carbon Fiber Anchors

Carbon fiber-reinforced polymer (CFRP) sheets can be used to strengthen existing reinforced concrete members. However, debonding (separation of the CFRP sheet from the concrete surface) may occur at less than 50% of CFRP sheet’s tensile capacity, implying that half of the CFRP material is ineffective in increasing the strength of a concrete member. The use of carbon fiber anchors can increase the amount of tension carried in the CFRP sheets. Forty specimens were tested to develop initial design parameters of carbon fiber anchors. Tests showed that by providing anchors with a total cross-sectional area at least two times greater than that of the longitudinal sheet, it was possible to fracture the CFRP sheets. The best results were obtained using a greater number of smaller anchors. Further, surface preparation is unimportant when the CFRP sheets were well anchored and a 1:4 transition slope can manage any offsets in surface level. The general anchor design was then implemented on a series of long beams and demonstrated that the full CFRP sheet tensile capacity can be realized without incurring limitations due to debonding.     

Simulation of Crack Growth in FRP Reinforced Concrete

Experimental results show that the crack growth of fiber-reinforced polymer (FRP) reinforced concrete flexural elements experience a crack development stage followed by crack stabilization. The crack length and elastic crack mouth opening displacement (CMOD) increase during the crack development stage until reaching the crack stabilization stage. A finite-element representation was proposed to predict the initial CMOD. A debonded length was specified to account for the bond-slip between FRP bar and concrete. It was assumed that there was no tangential displacement between the reinforcement and concrete outside of the debonded length. A fatigue model was created using the Paris equation to simulate the growth of elastic CMOD. The model displayed good agreement with the test results. A size effect was also observed for the exponential parameter in the Paris equation.     

Effect of FRP Confinement on Bond Strength of Hooked Bars: Normal-Strength Concrete Structures

To assess the viability of the external confinement of normal-strength concrete beam–column joints with carbon fiber-reinforced polymer (CFRP) sheets in increasing the bond strength of hooked bars anchored in the joints, 12 hooked bar specimens were tested. The variables were beam tensile bar size, anchorage length, mode of confinement of the beam hooked bars in the beam–column joint (whether the hooked bars were anchored within or outside the column reinforcement cage, denoted as “confined specimens” or “unconfined specimens”), and presence or absence of FRP wraps. The specimen simulated the rigid connection of a cantilever beam to a column. The tensile beam reinforcement consisted of two bars anchored in the base column using hooked-bar anchorages. Test results indicated that FRP sheets were effective in increasing the anchorage capacity and the ductility of the load–deflection history for both unconfined and confined specimens. However, FRP sheets had a more significant influence on unconfined specimens than companion confined specimens. As compared with unconfined specimens without FRP wrapping, unconfined FRP specimens had an average of a 23% increase in bond strength, confined non-FRP specimens had an average 30% increase in bond strength, and confined FRP specimens had an increase of 54%.     

Design and Evaluation of Fiber-Reinforced Polymer Bond-Type Anchorages and Ground Anchors

This paper presents laboratory tests on a bond-type anchorage system and a full-scale ground anchor with fiber-reinforced polymer 9-bar tendons. A cement-based mortar filled steel tube was used to simulate a rock mass environment. Resistance strain gauges, fiber optic sensors, embedded strain gauges, and linear variable displacement transducers were used to monitor the behavior of the anchor and to study load transfer mechanism of the tendon to surrounding rock mass. The test results show that the developed bond-type anchorage performs well for post-tensing applications with the tendons. The tested anchor presents an acceptable tensile behavior for each loading stage investigated (up to a load level of 0.6ffu, where ffu=guaranteed capacity of the tendon) in accordance with existing codes. The anchor is similar in strain distribution profile to conventional steel anchors, except giving a shorter load transfer length. It is expected that the anchor require a minimum anchor bonded length of 2,000?mm with plain cement grouts.     

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.     

Mineral-Based Bonding of Carbon FRP to Strengthen Concrete Structures

The advantages of fiber-reinforced polymer (FRP)-strengthening have been shown time and again during the last decade. Several thousand structures retrofitted with FRPs exist worldwide. There are various reasons why the retrofit is needed, but it is not uncommon for the demands on the structure to change with time, as buildings and civil structures usually have a very long life. The structures may have to eventually carry larger loads or fulfill new standards. In extreme cases, a structure may need repair due to an accident or to errors made during the design or construction phases, and must therefore be strengthened before it can be used. Different methods to retrofit with FRPs also exist, such as bonding of plates or sheets, with their use of epoxy as the bonding agent being the commonality. Epoxy provides very good bond to concrete and is durable and resistant to most environments in the building industry. However, epoxy may also create problems in the working environment, needs a minimum application temperature, and creates diffusion-closed surfaces. These drawbacks can be overcome if the epoxy can be replaced with a cementitious bonding agent. In this paper tests are presented where the epoxy has been replaced with a cement based bonding agent for retrofitting. Pilot tests show that very good composite action can be achieved and that only minor changes in the design procedure need to be taken.     

Behavior of Bond-Critical Regions Wrapped with Fiber-Reinforced Polymer Sheets in Normal and High-Strength Concrete

This paper reports on the third phase of a multiphase study undertaken at the American University of Beirut (AUB) to examine the effect of fiber-reinforced polymer (FRP) sheets in confining tension lap splice regions in reinforced concrete beams. Results of the first two phases showed that glass and carbon fiber-reinforced polymer (GFRP and CFRP) sheets were effective in increasing the bond strength and improving the ductility of the mode of failure of tension lap splices in high-strength concrete (HSC) beams with nominal concrete strength of 70 MPa. The experimental results of the two phases were used to propose a new FRP confinement parameter, Ktr,f, that accounts for the bond strength contribution of FRP sheets wrapping tension lap splice regions in HSC beams. In this third phase of the AUB study, the trend of the results of phases 1 and 2 and the validity of the analytical model proposed were verified if normal-strength concrete (NSC) is used instead of HSC. Seven beams with nominal concrete strength of 27.58 MPa (4 ksi) were tested in positive bending. Each beam was designed with a tension lap splice in a constant moment region in the midspan of the beam. The main test variables were the configuration (1 strip, 2 strips, or a continuous strip) and the number of layers (1 layer or 2 layers) of the CFRP sheets wrapping the splice region. The test results demonstrated that CFRP sheets were effective in enhancing the bond strength and ductility of failure mode of tension lap splices in NSC in a very similar way to HSC. In addition, the FRP confinement index proposed earlier for HSC was proven to be valid in the case of NSC.     

Investigation of Bond in Concrete Structures Strengthened with Near Surface Mounted Carbon Fiber Reinforced Polymer Strips

Fiber reinforced polymer (FRP) materials are currently produced in different configurations and are widely used for the strengthening and retrofitting of concrete structures and bridges. Recently, considerable research has been directed to characterize the use of FRP bars and strips as near surface mounted reinforcement, primarily for strengthening applications. Nevertheless, in-depth understanding of the bond mechanism is still a challenging issue. This paper presents both experimental and analytical investigations undertaken to evaluate bond characteristics of near surface mounted carbon FRP (CFRP) strips. A total of nine concrete beams, strengthened with near surface mounted CFRP strips were constructed and tested under monotonic static loading. Different embedment lengths were used to evaluate the development length needed for effective use of near surface mounted CFRP strips. A closed-form analytical solution is proposed to predict the interfacial shear stresses. The model is validated by comparing the predicted values with test results as well as nonlinear finite element modeling. A quantitative criterion governing the debonding failure of near surface mounted CFRP strips is established. The influence of various parameters including internal steel reinforcement ratio, concrete compressive strength, and groove width is discussed.     

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.     

Determination of Nonlinear Softening Behavior at FRP Composite/Concrete Interface

For concrete beams and slabs, the bonding of fiber reinforced plastic (FRP) plates to the bottom surface is an effective and efficient technique for flexural strengthening. Failure of strengthened members often occurs due to stress concentrations at the FRP/concrete interface. For debonding failure initiated at the bottom of shear or shear/flexural cracks in the concrete, experimental results clearly indicate a progressive failure process accompanied by gradual reduction in shear transfer capability at the interface. Several existing models for FRP debonding have taken interfacial shear softening into account. However, the assumed shear stress versus slip relations employed in the models have never been properly measured. In this investigation, a combined experimental/theoretical approach for the extraction of interfacial stress versus slip relation is developed. With loading applied to a bonded FRP plate, strain is measured at various points along its length. Based on the strain measurements, the interfacial softening curve is derived from a finite element analysis. The present paper will present the proposed approach in detail, demonstrate its application to typical experimental data, and discuss the implications of the results.     

Does the Use of FRP Reinforcement Change the One-Way Shear Behavior of Reinforced Concrete Slabs?

For members with no transverse reinforcement, numerous models have been proposed for determining shear capacity, most often based on a statistical curve fit to experimental beam test results. The shear provisions of the Canadian code (CSA) for steel-reinforced concrete, by contrast, are based on a theoretical model, the modified compression field theory. This paper demonstrates that the CSA shear provisions for steel-reinforced members can be safely applied to members with internal fiber-reinforced polymer (FRP) bars by adjusting the term EsAs in the method to ErAr. A database of 146 shear failures of specimens reinforced with carbon, glass, or aramid FRP or steel is presented and gives an average test to predicted ratio of 1.38 with a coefficient of variation (COV) of 17.2%. The CSA code equations were optimized for the typical strain range of steel-reinforced concrete and when an equation appropriate for the wider range of strains associated with FRP is used, then a better statistical result can be achieved. Application of this expression to the database resulted in an average test to predicted strength ratio of 1.15 with a COV of 14.9%. As both methods are based on a theoretical shear model that was derived for steel-reinforced concrete and since both methods work safely, it can be concluded that the use of internal FRP bars does not change the one-way shear behavior of reinforced concrete beams and slabs without stirrups.     

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.