Tensile Behavior of FRP Tendons for Prestressed Ground Anchors

The research work reported in this paper involves investigation of the tensile behavior of fiber-reinforced polymer (FRP) ground anchors. Variables of the tests on the anchor models were anchor fixed length, tendon type, and tendon constituent. Sixteen monorod and four multirod grouted aramid FRP (AFRP) (Arapree and Technora) and carbon FRP (CFRP) (CFCC and Leadline) anchors were tested according to standard methods of tensile tests and sustained load tests under different load levels. Test results indicated that AFRP Arapree and Technora monorod anchors showed higher displacement and slip in comparison with CFRP CFCC and Leadline anchors. Technora anchors failed because of the detaching of winding fibers from the core of the rod. CFRP anchors had a higher tensile capacity and lower creep displacement than AFRP anchors. All the tested CFRP monorod and FRP multirod anchors with a 1,000-mm fixed length appeared to have an acceptable tensile behavior according to existing codes. Creep behavior appeared to control the long-term tensile capacity of prestressed FRP ground anchors. The recommended working load for prestressed FRP ground anchors is 0.40fpu for AFRP rods and 0.50fpu for CFRP rods, where fpu is the ultimate load or strength of anchor tendons.     

CFRP Tendons for the Repair of Posttensioned, Unbonded Concrete Buildings

The deterioration attributable to corrosion of concrete structures reinforced with unbonded, posttensioned tendons is a costly problem. Recent research has shown composite materials such as fiber-reinforced polymers (FRP) to be suitable alternatives to steel because they provide similar strength without susceptibility to electrochemical corrosion. Carbon-FRP (CFRP) in particular has great promise for prestressed applications because it shows resistance to corrosion in environments that might be encountered in concrete and experiences less relaxation than steel. This paper outlines the testing and implementation of a posttensioned system that uses CFRP tendons to replace corroded, unbonded posttensioned steel tendons. This system was then implemented in a parking garage in downtown Toronto. To the writers’ knowledge, this is the first example of an unbonded, posttensioned tendon replacement using FRP tendons. The system used split-wedge anchors designed specifically for CFRP tendons. The dead end was anchored by directly bonding the tendon to the concrete slab. The CFRP tendon was successfully inserted in the opening created by the removal of the corroded tendon and stressed. Although the system was shown to be feasible, the current anchorage configuration results in load losses of up to 60% during the transfer. Changing the orientation of the anchor was found to reduce the load loss to an acceptable range of 1–9%.     

Bond Strength of Fiber Reinforced Polymer Rebars in Normal Strength Concrete

The bond behavior of reinforcing bars in concrete is a critical issue in the design of reinforced concrete structures. This study focuses on the bond strength of fiber reinforced polymer (FRP) rebars in normal strength concrete. Four different types of rebars were tested using the pullout method: aramid FRP (AFRP); carbon FRP (CFRP); glass FRP (GFRP), and steel. This involved a total of 151 specimens containing 6, 8, 10, 16, and 19?mm rebars embedded in a 203?mm concrete cube. The test embedment lengths were five, seven, and nine times the rebar diameter (db). For each rebar, the test results include the bond stress–slip response and the mode of failure. The test results showed that the bond strength of an FRP rebar is, on average, 40–100% the bond strength on a steel rebar for pullout failure mode. Based on this research, a proposal for the average bond strength of straight FRP rebars in normal strength concrete is made, which verifies an existing bond strength relationship (GFRP) and extends its application to AFRP and CFRP. It is an expression that is a function of the rebar diameter, and the concrete compressive strength.     

Constitutive Model for Time-Dependent Behavior of FRP–Concrete Interface

A fundamental understanding of fiber-reinforced polymer (FRP) laminate bonding behavior, including bond strength and effective bonding length, is of primary importance for the development of design guidelines and codes for concrete structures strengthened with externally bonded FRP reinforcement as a bond-critical application. However, the long-term serviceability of such FRP-strengthened structures is still a concern due to a lack of both long-term performance data and a suitable model to represent these performances. This study aims at presenting a viscoelastic model describing the time-dependent behavior of the FRP–concrete interface. The proposed model has been calibrated using strain measurements of the designed specimen for the experimental investigation of the time-dependent behavior of the FRP–concrete interface, including the development of the effective bonding length. Afterward, the proposed model satisfactorily predicts the time-dependent bonding length of the FRP sheet in comparison with the experimental results. The effects, both of creep of the adhesive layer and of creep and shrinkage of the concrete, on the changes in the effective bonding length of the PFRP sheet are also discussed.     

Axial Load-Bending Moment Diagrams of Carbon FRP Wrapped Hollow Core Reinforced Concrete Columns

Hollow core reinforced concrete columns are generally preferred in use to decrease the cost and weight/stiffnesss ratio of members, such as bridge columns and piles. With a simplified stress state assumption, strengthening a hollow core reinforced concrete column with fiber-reinforced polymer (FRP) wrapping provides a biaxial confinement to the concrete, which leads to a need of defining the effect of FRP wrapping on the strength and ductility of the hollow core reinforced concrete columns. In this study, two groups of four hollow core reinforced concrete columns (205?mm outer diameter, 56?mm hollow core diameter, and 925?mm height) were tested under concentric, eccentric (25 and 50?mm eccentricity) and bending loads to observe the effect of carbon FRP (CFRP) wrapping. All the columns had internal steel reinforcement. Half of the columns had three layers of circumferential CFRP wrapping, whereas the other half had no external confinement. Axial load-bending moment (P–M) diagrams of each group were drawn using the obtained experimental results for both groups. It was observed that, CFRP wrapped columns had higher load and moment carrying capacities than the other group. An analytical model is proposed for drawing the P–M diagram of CFRP wrapped hollow core reinforced concrete columns.     

Prediction of Tensile Capacity of Bond Anchorages for FRP Tendons

Past test data show that the bond stress distribution of bond anchorages is nonuniform along the bonded length and that the point of the peak bond strength shifts from the entry of the tendon to an inside point of the anchorage as the applied load increases. Based on these results, this paper analyzes the working mechanism of bond anchorages for fiber-reinforced polymer (FRP) tendons and presents a conceptual model to calculate the bond stress at the tendon-grout interface and the tensile capacity of bond anchorages for FRP tendons. Experimental and analytical results show that the geometry of FRP tendon and steel sleeve and the mechanical properties of filling grout are the relevant parameters in the development of tendon-grout interface stresses. The characteristic bond strength depends mainly on the properties of the bonding agent-cement grout, the geometry and surface conditions of the tendon, and the radial stiffness of the confining medium. A comparison of the calculated and experimental results showed good agreement.     

Theoretical Study on Short-Term and Long-Term Deflections of Fiber Reinforced Polymer Prestressed Concrete Beams

This paper presents the methods for predicting the short-term and time-dependent deflections of fully or partially prestressed concrete beams with fiber reinforced polymer (FRP) tendons under sustained bending moment and axial force. The age-adjusted effective modulus method is used to model the creep behavior in the concrete and the relaxation in the FRP prestressing tendons. A tension-stiffening model is proposed to evaluate the stiffness of the section after cracking. The analytical values are compared to the test results and it is found that the analytical values are in good agreement with the experimental results.     

Tensile Lap Splicing of Bundled CFRP Reinforcing Bars in Concrete

In the case of heavily reinforced concrete structural members, bundled bars are required rather than spaced bars. The use of spliced bundled bars is necessary when available bar lengths are limited. No design recommendations regarding the use of bundled or spliced bundled FRP bars are available. The results of four-point flexural testing of nine concrete beams reinforced with spliced bundled CFRP bars are presented herein. The effects of the type of bundle and splice length on the bond strength of bundled CFRP bars are investigated. Based on the experimental results, a procedure for determining the critical splice length of FRP bars is presented and the corresponding values of bond stresses can be predicted. Moreover, the ultimate strength analysis method is used to predict the maximum stress in spliced bundled CFRP bars. Finally, comparisons with the existing recommendations regarding the use of bundled steel bars and the recommended modifications for bundled CFRP bars are presented.     

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 and Computational Studies on High-Strength Concrete Circular Columns Confined by Aramid Fiber-Reinforced Polymer Sheets

The aim of this paper is to study the properties of high-strength concrete (HSC) circular columns confined by aramid fiber-reinforced polymer (AFRP) sheets under axial compression. A total of 60 specimens were tested, considering the following parameters: the compressive strength of concrete, the number of AFRP layers, and the form of AFRP wrapping. In addition, an analytical model for predicting the stress–strain curves is proposed based on the experimental results. Meanwhile, a three-dimensional nonlinear finite-element model with a Drucker–Prager plasticity model for the concrete core and an elastic model for the AFRP is developed by using the finite-element code ANSYS. It is demonstrated that the strength and ductility of the columns with continuous AFRP wrapping are increased greatly; whereas the strength of the columns with discontinuous AFRP wrapping is also increased, but the ductility is not always increased notably. The analytical model and the finite-element model are validated against the experimental results.     

Effect of Fiber-Reinforced Polymer Confinement on Bond Strength of Reinforcement in Beam Anchorage Specimens

This paper reports on the fourth 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 bond-critical regions in reinforced concrete beams. Results of the first three 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) and normal-strength concrete (NSC) beams. The main objective of the fourth phase of the AUB study was to assess the effect of CFRP sheets in improving the serviceability and ultimate response of beam anchorage specimens. The added experimental data and the improved knowledge of the bond behavior of FRP confined concrete members will encourage the use of FRP technology to strengthen and retrofit bond anchorage zones. Ten beam anchorage specimens were tested in positive bending in two series. The variables were bar size, anchorage length, and concrete strength. For each bar size, anchorage length, and concrete strength, two companion specimens—identical except for whether the anchorage zone was wrapped with FRP sheets or not wrapped—were tested. The test results demonstrated that CFRP sheets were effective in enhancing the bond strength and ductility of anchorage zones in beam anchorage specimens where splitting failures were imminent.     

Long-Term Deflection and Cracking Behavior of Concrete Beams Prestressed with Carbon Fiber-Reinforced Polymer Tendons

This paper discusses the experimental result on the long-term deflection and cracking behavior of concrete beams prestressed with carbon fiber-reinforced polymer (CFRP) tendons, under sustained long-term service load, including cracked and uncracked sections. Six full-scale beams were cast and tested. The experimental parameters included the level of prestress, the level of sustained service loading, and concrete strengths. The experimental results showed that the performance of concrete beams prestressed with CFRP tendons meets the serviceability criteria in terms of deflection and cracking. The test results also showed that the long-term performance of concrete beams prestressed with CFRP tendons was comparable to those prestressed with steel tendons. Furthermore, the test results showed that with the increase of concrete strength, the serviceability performance also improved with concrete beams prestressed with CFRP tendons.     

Size Effect of Concrete Short Columns Confined with Aramid FRP Jackets

Most previous studies on concrete short columns confined with fiber-reinforced polymer (FRP) composites were based on small-scale testing, and size effect of the columns still has not been studied thoroughly. In this study, 99 confined concrete short columns wrapped with aramid FRP (AFRP) jackets and 36 unconfined concrete short columns with circular and square cross sections were tested under axial compressive loading. The circular specimens were divided into six groups, and the square specimens were divided into five groups, with each group containing different levels of the AFRP’s confinement. In each group, the specimens were geometrically similar to one another and had three different scaling dimensions. Statistical analyses were used to evaluate the size and interaction effects between the specimen size and the AFRP’s confinement, and a size-dependent model for predicting the strength of the columns was developed by modifying Baz?nt’s size-effect law. The experimental results showed that the size of a specimen had a significant effect on the strength of AFRP-confined concrete short columns, lesser effect on the axial stress-strain curves, and slight effect on the failure modes. The modified Baz?nt model was in good agreement with the experimental data.     

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.     

ESPI Measurement of Bond-Slip Relationships of FRP-Concrete Interface

Fiber-reinforced polymer (FRP) composites are widely used for strengthening reinforced concrete structures because of their superior properties. Reliable performance of the bond between the external FRP and the concrete in maintaining the composite action between them is crucial for this strengthening technique to be effective. To fully understand and model this bond behavior, a rigorous bond-slip law is essential. This paper presents an experimental study in which the displacement fields in a carbon fiber-reinforced polymer (CFRP)-to-concrete double shear test were measured using the nondestructive and noncontact electronic speckle pattern interferometry (ESPI) technique. Full-field in-plane displacements were measured in 33 CFRP specimens with concrete strengths varying from 23 to 69?MPa. The measurement results were used to infer the bond-slip behavior between the FRP and the concrete. The inferred bond-slip curves include a nonlinear ascending branch and a descending branch. The strength of concrete is found to have significant effect on the peak bond stress but little effect on the slip corresponding to the peak bond stress. A logarithmic model and a simpler parabolic model are proposed to represent the experimental bond-slip constitutive curves.     

Analytical Model for Circular Normal- and High-Strength Concrete Columns Confined with FRP

This paper presents a new incremental stress-strain model for fiber-reinforced polymer (FRP)-confined concrete. The model, able to accommodate concrete with a wide range of strength (25–110 MPa), is based on material properties, force equilibrium, and strain compatibility, and uses newly developed models for constantly confined concrete. An expression is proposed to calculate a FRP jacket rupture strain in columns. Beyond the initiation of rupture, gradual failure of a FRP jacket is modeled to account for the size effect on the FRP-confined concrete columns. This proposed constitutive model is unique in that it accommodates a wide range of concrete strength and uses an analytical rupture strain of a FRP jacket to predict the complete stress-strain curve. Small and large specimens tested by the authors and other researchers are used to validate the proposed model. Very good to excellent agreements have been achieved between the analytical and experimental responses.     

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.     

Flexural Behavior and Deformability of Fiber Reinforced Polymer Prestressed Concrete Beams

After a brief review of the ductility and deformability indices currently used in the design of concrete beams reinforced or prestressed with steel or fiber reinforced polymer (FRP) tendons, a new definition of a deformability index (factor) for prestressed concrete beams is proposed. The new factor is defined in terms of both a deflection factor and a strength factor. The deflection factor is the ratio of the deflection at failure to the deflection at first cracking, while the strength factor is the ratio of the ultimate moment (or load) to the cracking moment (or load). The proposed deformability factor is verified not only by test results obtained by the writer, but also by other test results available in the literature and it appears to be a suitable measurement of the deformability of concrete beams prestressed with either FRP tendons or steel tendons.     

Creep-Effect on Mechanical Behavior of Concrete Confined by FRP under Axial Compression

Despite many successes in concrete creep studies, its effect on the mechanical behavior of concrete members is far from a thorough case-specific understanding. For the members that have been subjected to a long-term load, the classical stress-strain models describing the short-term behavior of either confined or unconfined concrete are unsuitable. In order to investigate this creep-effect, an experiment on eight concrete cylindrical columns confined by fiber-reinforced polymer (FRP) is carried out. Based on the theory of plasticity for concrete, a constitutive model that takes into account the effect of creep on mechanical behavior of concrete confined by FRP is presented. In the model, the creep law inspired in the microprestress-solidification theory is generalized to triaxial stress condition for the calculation of the creep of the concrete columns confined by FRP. The predictions of the model agree well with the experimental results. The present study indicates that the creep increases the elastic modulus, slightly decreases the compressive strength, and degrades the deformation capability of the concrete confined by FRP.     

Strengthening of Eccentrically Loaded Reinforced Concrete Columns with Fiber-Reinforced Polymer Wrapping System: Experimental Investigation and Analytical Modeling

This paper presents the results of experimental program and analytical modeling for performance evaluation of a fiber-reinforced polymer (FRP) wrapping system to upgrade eccentrically loaded reinforced concrete (RC) columns. A total of 12 RC columns with end corbels were tested. The test specimen had an overall length of 1,200?mm. Each end corbel had a cross section of 250×250?mm and a length of 350?mm. The specimen in the test region was 125×125?mm having a longitudinal steel ratio of 1.9%. Test parameters included confinement condition (no wrapping, full FRP wrapping, and partial FRP wrapping), and eccentricity-to-section height (e/h) ratio (0.3, 0.43, 0.57, and 0.86). Research findings indicated that the strength gain caused by FRP wrapping decreased as e/h was increased. Full FRP wrapping resulted in about 37% enhancement in compression strength at a nominal e/h of 0.3, whereas only 3% strength gain was recorded at a nominal e/h of 0.86. The compression strengths of the partially wrapped columns were on average 5% lower than those of the fully wrapped columns. A nonlinear, second-order analysis that accounts for the change in eccentricity caused by the lateral deformation was proposed to predict the columns strength. A comparison between analytical and experimental results of the present study in addition to data published in the literature demonstrated the accuracy and validity of the proposed analysis.