Realistic Bond Strength of FRP Rebars in NSC from Beam Specimens

The bond strength of reinforcing bars in concrete is a prerequisite for the evaluation of the development length in reinforced concrete structures. This study concerns these phenomena for fiber reinforced polymer (FRP) rebars in normal strength concrete (NSC). Three different types of rebars were tested using the beam specimen: Carbon, glass, and steel. This involved a total of 26 beam specimens containing 10, 16, and 19?mm rebars. The test embedment lengths were 10, 15, and 20 times the rebar diameter (db). For each rebar tested, the results concern load deflection curves, bond stress-slip responses, and the mode of failure. The results showed that the bond strength of a FRP rebar is, generally, lower than that of steel rebar. Based on this and previous research, proposals for the average bond strength and for the development length of straight FRP rebars under tension in NSC are made.     

Bond Performance of GFRP Bars in Tension: Experimental Evaluation and Assessment of ACI 440 Guidelines

The development/splice strength and the pullout local bond stress-slip response of glass fiber-reinforced polymer (GFRP) bars in tension were experimentally investigated using beam specimens and pullout specimens, respectively. Two types of 12-mm (0.47-in.)-diameter GFRP bars were evaluated, namely, thread wrapped and ribbed. The test parameters included the concrete cover, the splice length, and the area of steel confinement for the beam specimens, and the concrete compressive strength for the pullout specimens. Companion steel reinforced beams were also tested for comparison. All beam specimens reinforced with thread-wrapped GFRP bars experienced pullout mode of bond failure, while all specimens reinforced with ribbed GFRP bars or steel bars experienced splitting mode of bond failure. It was found that the bond strength of FRP bars is largely dependent on the surface conditions of the bars. The pullout local bond stress-slip response of ribbed GFRP bars is intrinsically similar to that of steel bars reported in the literature. The bond strength of both types of GFRP bars investigated was about two to three times lower than that of steel bars. Predictions of the development/splice strength of GFRP bars in accordance with the ACI Committee 440 guidelines were unconservative in comparison with the test data. Also, in contradiction with the current ACI 440 report, the use of transverse confining reinforcement increased the bond strength by a sizable 15–30%.     

Flexural Load Testing of Concrete-Filled FRP Tubes with Longitudinal Steel and FRP Rebar

The flexural performance of reinforced concrete-filled glass-fiber reinforced polymer (GFRP) tubes (CFFTs) has been investigated using seven specimens, 220?mm in diameter and 2.43?m long. Specimens were reinforced with either steel, GFRP, or carbon–fiber reinforced polymer (FRP) rebar of various sizes. Prefabricated GFRP tubes with most of the fibers oriented in the hoop direction were used in five specimens. One control specimen included conventional steel spirals of stiffness comparable to the GFRP tube and the other had no transverse reinforcement. Test results have shown that CFFT beams performed substantially better than beams with a steel spiral. Unlike CFFTs with FRP rebar, CFFTs with steel rebar failed in a sequential progressive manner, leading to considerable ductility. An analytical model capable of predicting the full response of reinforced CFFT beams, including the sequential progressive failure, has been developed, verified, and used in a parametric study. It is shown that laminate structure of the tube affects the behavior, only after yielding of the steel rebar. Steel reinforcement ratio significantly affects stiffness and strength, whereas concrete strength has an insignificant effect on the overall performance.     

Physical Properties of Glass Fiber Reinforced Polymer Rebars in Compression

Forty-five glass fiber reinforced polymer (GFRP) rebars were tested in compression to determine their ultimate strength and Young’s modulus. The rebars (or C-bars), produced by Marshall Industries Composites, Inc., had an outside diameter of 15 mm (#15 rebar), and unbraced lengths varying from 50 to 380 mm. A compression test method was developed to conduct the experiments. Three failure modes, that are directly related to the unbraced length of the rebar, are identified as crushing, buckling, and combined buckling and crushing. The crushing region represents the failure mode a GFRP rebar would experience when confined in concrete under compression. The experimental results showed that the ultimate compressive strength of the #15 GFRP rebar failing by crushing is approximately 50% of the ultimate tensile strength. Based on a very limited number of tests, in which strain readings were acceptable, Young’s modulus in compression was found to be approximately the same as in tension.     

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.     

Bond between Carbon Fiber Reinforced Polymer Bars and Concrete. I: Experimental Study

The bond characteristics of four different types of carbon fiber reinforced polymer (CFRP) rebars (or tendons) with different surface deformations embedded in lightweight concrete were analyzed experimentally. In a first series of tests, local bond stress-slip data, as well as bond stress-radial deformation data, needed for interface modeling of the bond mechanics, were obtained for varying levels of confining pressure. In addition to bond stress and slip, radial stress and radial deformation were considered fundamental variables needed to provide for configuration-independent relationships. Each test specimen consisted of a CFRP rebar embedded in a 76-mm-(3 in.)-diam, 102-mm-(4 in.)-long, precracked lightweight concrete cylinder subjected to a constant level of pressure on the outer surface. Only 76 mm (3 in.) of contact were allowed between the rebar and the concrete. For each rebar type, bond stress-slip and bond stress-radial deformation relationships were obtained for four levels of confining axisymmetric radial pressure. It was found that small surface indentations were sufficient to yield bond strengths comparable to that of steel bars. It was also shown that radial pressure is an important parameter that can increase the bond strength almost threefold for the range studied. In a second series of tests, the rebars were pulled out from 152-mm-(6 in.)-diam, 610-mm-(24 in.)-long lightweight concrete specimens. These tests were conduced to provide preliminary data for development length assessment and model validation (Part II).     

Long-Term Properties and Transfer Length of Fiber-Reinforced Polymers

This paper discusses the experimental results on properties and transfer length of the common types of fiber-reinforced polymer (FRP) tendons. Based on the experimental results, an equation is proposed for predicting the creep coefficient of aramid-based FRP (AFRP) tendons. The results show that the creep of carbon-based FRP (CFRP) is less than 0.2%. The test results show that the transfer length of CFRP is in the range of 300–800 mm and the concrete strength at transfer is one of the major factors affecting the transfer length of CFRP. A new factor accounting for the concrete strength is proposed for estimating the transfer length of CFRP tendons, and the verification is made for this equation. The transfer length was found to vary from 170 to 270 mm, which was 20–30 times the tendon diameter for these AFRP tendons. Despite the creep and shrinkage of concrete and the relaxation of the tendon itself, the range for transfer length did not vary with time.     

Bond Behavior of Fiber Reinforced Polymer Bars under Direct Pullout Conditions

This paper examines the behavior of Eurocrete fiber-reinforced polymer (FRP) bars (glass, carbon, aramid, and hybrid) in concrete under direct pullout conditions. More than 130 cube specimens were tested in direct pullout where no splitting was allowed to develop. In normal concrete, the mode of bond failure of FRP bars was found to differ substantially from that of deformed steel bars because of damage to the resin rich surface of the bar when pullout takes place. Bond strengths developed by carbon fiber-reinforced polymer and glass fiber-reinforced polymer bars appear to be very similar and just below what is expected from deformed steel bars under similar experimental conditions. The load slip curves highlight some of the fundamental differences between steel and FRP materials. This paper reports in detail on the influence of various parameters that affect bond strength and development such as the embedment length, type, shape, surface characteristics, and diameter of the bar as well as concrete strength. The testing arrangement is also shown to influence bond strength because of the “wedging effect” of the bars.     

Bond Strength of Tension Lap Splices in High-Strength Concrete Beams Strengthened with Glass Fiber Reinforced Polymer Wraps

This paper presents the experimental results of the first phase of a study undertaken at the American University of Beirut to examine the effectiveness of fiber reinforced polymer (FRP) wraps to confine steel reinforcement in a tension lap splice region anchored in high-strength reinforced-concrete beams. Seven beam specimens were constructed. The specimens were reinforced on the tension side with three deformed bars spliced at midspan. The splice region was devoid of any transverse reinforcement to allow a full examination of the FRP wrap contribution. Glass fiber reinforced polymer (GFRP) sheets were used. The main test variables were the GFRP configuration in the splice region (one strip, two strips, or a continuous strip), and the number of layers of the GFRP wraps placed around the splice region (one layer or two layers). All GFRP wraps were U-shaped. Except for the epoxy adhesive, no other anchorage mechanism or bonding procedure was applied for the GFRP wraps on the concrete beam. Following the application of the GFRP wraps, the beams were tested in positive bending. The test results demonstrated that GFRP wraps were effective in enhancing the bond strength and ductility of failure mode of the tension lap splices, especially when continuous strips were applied over the splice region.     

Designing and Testing of Concrete Bridge Decks Reinforced with Glass FRP Bars

In addition to their high strength and light weight, fiber-reinforced polymer (FRP) composite reinforcing bars offer corrosion resistance, making them a promising alternative to traditional steel reinforcing bars in concrete bridge decks. FRP reinforcement has been used in several bridge decks recently constructed in North America. The Morristown Bridge, which is located in Vermont, United States, is a single span steel girder bridge with integral abutments spanning 43.90 m. The deck is a 230 mm thick concrete continuous slab over girders spaced at 2.36 m. The entire concrete deck slab was reinforced with glass FRP (GFRP) bars in two identical layers at the top and the bottom. The bridge is well instrumented at critical locations for internal temperature and strain data collection with fiber-optic sensors. The bridge was tested for service performance using standard truck loads. The construction procedure and field test results under actual service conditions revealed that GFRP rebar provides very good and promising performance.     

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.     

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.     

Bond of GFRP Bars in Concrete: Experimental Study and Analytical Interpretation

The local bond mechanics of glass-fiber reinforced polymer (GFRP) bars in normal strength concrete was investigated through experimental testing and analytical modeling. The experimental program was comprised of 30 direct tension pullout specimens with short anchorages. A novel test setup, specially designed so as to minimize the spurious influence of testing conditions on measured bond properties was adopted in the study. Parameters considered were the bar roughness and diameter, the size effect expressed by the constant cover to bar diameter ratio, and the external confining pressure exerted over the anchorage length by transverse externally bonded FRP sheets. Results of the study were summarized in the form of local bond-slip curves, whereby performance limit states were quantified by the amount of loaded end slip and bond strength. An analytical model of the bond stress-slip response of a GFRP bar was derived from first principles and calibrated against the test data of the present investigation. Using the calibrated model, design values for bond and slip were estimated with reference to the code limit state model for bond.     

Energy Release in Fiber-Reinforced Plastic Reinforced Concrete Beams

A set of 30 concrete beams reinforced with carbon/epoxy FRP (fiber-reinforced plastic) and four reinforced with comparable size steel rebars were subjected to static bending tests. Adequate bond between the FRP and the concrete was obtained, due to the use of carbon fiber overwrap on the smooth pultruded FRP rods. With adequate bond, the large strain to failure (>2%) of the FRP determines the ductility and failure mode of the FRP reinforced beams. An analytical evaluation of the fracture energy in these experiments shows that there is ductility due to the large fraction of the total strain energy that is absorbed in the concrete, because of the formation of distributed cracking. Variations in overwrap configuration, addition of steel stirrups, addition of polypropylene fibers, and comparison with four beams reinforced with equivalent steel reinforcement were also analyzed.     

Durability of GFRP Bars’ Bond to Concrete under Different Loading and Environmental Conditions

In the last decade, noncorrodible fiber-reinforced polymer (FRP) reinforcing bars have been increasingly used as the main reinforcement for concrete structures in harsh environments. Also, owing to their lower cost compared with other types of FRP bars, glass-FRP (GFRP) bars are more attractive to the construction industry, especially for implementation in bridge deck slabs. In North America, bridge deck slabs are exposed to severe environmental conditions, such as freeze-thaw action, in addition to traffic fatigue loads. Although the bond strength of GFRP bars has been proved to be satisfactory, their durability performance under the dual effects of fatigue-type loading and freeze-thaw action is still not well understood. Few experimental test data are available on the bond characteristics of FRP bars in concrete elements under different loading and environmental conditions. This research investigates the individual and combined effects of freeze-thaw cycles along with sustained axial load and fatigue loading on the bond characteristics of GFRP bars embedded in concrete. An FRP-reinforced concrete specimen was developed to apply axial-tension fatigue or sustained loads to GFRP bars within a concrete environment. A total of thirty-six test specimens was constructed and tested. The test parameters included bar diameter, concrete cover thickness, loading scheme, and environmental conditioning. After conditioning, each specimen was sectioned into two halves for pullout testing. Test results showed that fatigue load cycles resulted in approximately 50% loss in the bond strength of sand-coated GFRP bars to concrete, while freeze-thaw cycles enhanced their bond to concrete by approximately 40%. Larger concrete covers were found more important in cases of larger bar sizes simultaneously subjected to fatigue load and freeze-thaw cycles.     

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.     

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.     

Behavior and Modeling of Bond of FRP Rebars to Concrete

In the field of reinforced-concrete (RC) structures, the use of fiber reinforced plastic rebars (FRP rebars) as an alternative to the steel reinforcements appears very promising, especially if such structures are exposed to corrosive environments. However, a better understanding of the mechanical behavior of FRP reinforcements—in particular bond behavior—is needed in order to use them for practical purposes. For this reason, in the last few years a number of tests on several types of FRP rebars has been conducted in order to evaluate the interaction phenomena between FRP rebars and the concrete matrix and to evidence behavioral differences with respect to the deformed steel rods. In this paper a state-of-the-art report on the bond of FRP bars to concrete is presented. Numerous tests are analyzed to better understand bond mechanisms and the influence of type of fiber, outer surface (shape and type of matrix), and other significant parameters (i.e., confining pressure, bar diameter, compressive concrete strength) on bond performances. Furthermore, some analytical models of bond-slip behavior are examined to assess their adequacy to reproduce the experimental bond behavior. In particular, the investigation focuses on the reliability of the well-known model by Malvar (the first one dedicated to FRP reinforcements) as well as on the model by Eligehausen, Popov, and Bertero, developed for steel reinforcements but successfully applied to FRP ones. In addition, the effectiveness of two analytical formulations proposed by the authors, the first one representing the ascending branch of the bond-slip curve and the second the entire curve, is demonstrated.     

Flexural Strength of RC Beams with GFRP Laminates

The results of an experimental and numerical study of the flexural behavior of reinforced concrete beams strengthened with glass-fiber-reinforced-polymer (GFRP) laminates are presented in this paper. In the experimental program, ten strengthened beams and two unstrengthened beams are tested to failure under monotonic loading. A number of external GFRP laminate layers and bond length of GFRP laminates in shear span are taken as the test variables. Longitudinal GFRP strain development and interfacial shear stress distribution from the tests are examined. The experimental results generally showed that both flexural strength and stiffness of reinforced concrete beams could be increased by such a bonding technique. In the numerical study, an eight-node interface element is developed to simulate the interface behavior between the concrete and GFRP laminates. This element is implemented into the MARC software package for the finite-element analyses of GFRP laminate strengthened reinforced concrete beams. Reasonably good correlations between experimental and numerical results are achieved.     

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.