Bond Durability of Glass Fiber-Reinforced Polymer Bars Embedded in Concrete Beams

An experimental and analytical investigation of bond durability of E-glass fiber-reinforced polymer reinforcement bars in concrete beams is presented. Beams were conditioned with sustained flexural loads in indoor, outdoor, 60°C alkaline solution, or freeze/thaw environments for up to 3?years, after which they were subjected to eccentric three-point flexure tests to evaluate bond. Experimental bar force and slip were used to draw direct conclusions on bond durability, and also to calibrate a proposed local bond–slip model that incorporates concrete cover splitting. Experimental bar force at the onset of free-end slip varied little after any of the conditionings, although the characteristic of bond failure was noted to be less ductile in the moister environments. The interfacial fracture energy associated with bond–slip did not change with conditioning time in any of the environments except freeze/thaw, where a monotonic reduction versus time was seen. The effective bond length of the bar under various conditionings varied roughly in proportion to the local slip at complete local bond failure.     

Shear Strength of One-Way Concrete Slabs Reinforced with Fiber-Reinforced Polymer Composite Bars

This paper evaluates the shear strength of one-way concrete slabs reinforced with different types of fiber-reinforced polymer (FRP) bars. A total of eight full-size slabs were constructed and tested. The slabs were 3,100?mm?long×?1,000?mm?wide×200?mm?deep. The test parameters were the type and size of FRP reinforcing bars and the reinforcement ratio. Five slabs were reinforced with glass FRP and three were reinforced with carbon FRP bars. The slabs were tested under four-point bending over a simply supported clear span of 2,500 mm and a shear span of 1,000 mm. All the test slabs failed in shear before reaching the design flexural capacity. The experimental shear strengths were compared with some theoretical predictions, including the JSCE recommendations, the CAN/CSA-S806-02 code, and the ACI 440.1R-03 design guidelines. The results indicated that the ACI 440.1R-03 design method for predicting the concrete shear strength of FRP slabs is very conservative. Better predictions were obtained by both the CAN/CSA-S806-02 code and the JSCE design recommendations.     

Durability Prediction for GFRP Reinforcing Bars Using Short-Term Data of Accelerated Aging Tests

This paper presents a procedure based on the Arrhenius relation to predict the long-term behavior of glass fiber-reinforced polymer (GFRP) bars in concrete structures, based on short-term data from accelerated aging tests. GFRP reinforcing bars were exposed to simulated concrete pore solutions at 20, 40, and 60°C. The tensile strengths of the bars determined before and after exposure were considered a measure of the durability performance of the specimens. Based on the short-term data, a detailed procedure is developed and verified to predict the long-term durability performance of GFRP bars. A modified Arrhenius analysis is included in the procedure to evaluate the validity of accelerated aging tests before the prediction is made. The accelerated test and prediction procedure used in this study can be a reliable method to evaluate the durability performance of FRP composites exposed to solutions or in contact with concrete.     

Durability of GFRP Reinforcing Bars Embedded in Moist Concrete

This paper presents mechanical, microstructural, and physical characterization of glass fiber-reinforced polymer (GFRP) bars exposed to concrete environment. GFRP bars were embedded in concrete and exposed to tap water at 23, 40, and 50°C to accelerate the effect of the concrete environment. The measured tensile strengths of the bars before and after exposure were considered as a measure of the durability performance of the specimens and were used for long-term properties prediction based on the Arrhenius theory. In addition, Fourier transform infrared spectroscopy, differential scanning calorimetry, and scanning electron microscopy were used to characterize the aging effect on the GFRP reinforcing bars. The results showed that the durability of mortar-wrapped GFRP bars and exposed to tap water was less affected by accelerated aging than the bars exposed to simulated pore-water solution. These results confirmed that the concerns about the durability of GFRP bars in concrete, based on simulated laboratory studies in alkaline solutions, do not properly correspond to the actual service life in concrete environments.     

Fatigue Life Evaluation of Concrete Bridge Deck Slabs Reinforced with Glass FRP Composite Bars

Since bridge deck slabs directly sustain repeated moving wheel loads, they are one of the most bridge elements susceptible to fatigue failure. Recently, glass fiber-reinforced polymer (FRP) composites have been widely used as internal reinforcement for concrete bridge deck slabs as they are less expensive compared to the other kinds of FRPs (carbon and aramid). However, there is still a lack of information on the performance of FRP–reinforced concrete elements subjected to cyclic fatigue loading. This research is designed to investigate the fatigue behavior and fatigue life of concrete bridge deck slabs reinforced with glass FRP bars. A total of five full-scale deck slabs were constructed and tested under concentrated cyclic loading until failure. Different reinforcement types (steel and glass FRP), ratios, and configurations were used. Different schemes of cyclic loading (accelerated variable amplitude fatigue loading) were applied. Results are presented in terms of deflections, strains in concrete and FRP bars, and crack widths at different levels of cyclic loading. The results showed the superior fatigue performance and longer fatigue life of concrete bridge deck slabs reinforced with glass FRP composite bars.     

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.     

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.     

Transverse Thermal Expansion of FRP Bars Embedded in Concrete

Fiber reinforced polymers (FRPs) have a thermal expansion in the transverse direction much higher than in the longitudinal direction and also higher than the thermal expansion of hardened concrete. The difference between the transverse coefficient of thermal expansion of FRP bars and concrete may cause splitting cracks within the concrete under temperature increase and, ultimately, failure of the concrete cover if the confining action of concrete is insufficient. This paper presents the results of an experimental investigation to analyze the effect of the ratio of concrete cover thickness to FRP bar diameter (c/db) on the strain distributions in concrete and FRP bars, using concrete cylindrical specimens reinforced with a glass FRP bar and subjected to thermal loading from ?30?to?+80°C. The experimental results show that the transverse coefficient of thermal expansion of the glass FRP bars tested in this study is found to be equal to 33 (×10?6?mm/mm/°C), on average and the ratio between the transverse and longitudinal coefficients of thermal expansion of these FRP bars is equal to 4. Also, the cracks induced by high temperature start to develop on the surface of concrete cylinders at a temperature varying between +50 and +60°C for specimens having a ratio of concrete cover thickness to bar diameter c/db less than or equal to 1.5. A ratio of concrete cover thickness to glass fiber reinforced polymers (GFRP) bar diameter c/db greater than or equal to 2.0 is sufficient to avoid cracking of concrete under high temperature up to +80°C. The analytical model, presented in this paper, is in good agreement with the experimental results, particularly for negative temperature variations.     

Durability Gap Analysis for Fiber-Reinforced Polymer Composites in Civil Infrastructure

The lack of a comprehensive, validated, and easily accessible data base for the durability of fiber-reinforced polymer (FRP) composites as related to civil infrastructure applications has been identified as a critical barrier to widespread acceptance of these materials by structural designers and civil engineers. This concern is emphasized since the structures of interest are primarily load bearing and are expected to remain in service over extended periods of time without significant inspection or maintenance. This paper presents a synopsis of a gap analysis study undertaken under the aegis of the Civil Engineering Research Foundation and the Federal Highway Administration to identify and prioritize critical gaps in durability data. The study focuses on the use of FRP in internal reinforcement, external strengthening, seismic retrofit, bridge decks, structural profiles, and panels. Environments of interest are moisture/solution, alkalinity, creep/relaxation, fatigue, fire, thermal effects (including freeze-thaw), and ultraviolet exposure.     

Long-Term Performance of a Glass Fiber-Reinforced Polymer Truss Bridge

In the summer of 2005, after eight years of use as a temporary bridge during the winter, the Pontresina Bridge for pedestrians was transported to the Swiss Federal Institute of Technology Lausanne for a detailed assessment of the structural safety, serviceability, and long-term durability of the bridge. The assessment included a visual inspection, quasistatic testing identical to that performed in 1997, and detailed investigations of material degradation. The visual inspection showed a variety of different local defects and damage such as local crushing caused by impact, local cracks due to inappropriate storage and lifting of the structure, fiber blooming, degradation of cut surfaces, and damage due to vandalism. Comparisons between load tests performed in 1997 and 2005 showed, however, that the structural safety and serviceability of the bridge have not been affected by these local damages. The stiffness of the pultruded shapes remained unchanged, whereas a slight decrease in strength between 13 and 18% was measured, which, however, is not critical when taking into consideration the high effective safety factors. In view of a further service period of 5 years until the next inspection, the visible damages were repaired. This experience showed that the durability is primarily affected by inappropriate constructive detailing and that pultruded glass fiber-reinforced polymer shapes, if correctly manufactured and processed, can offer good long-term performance and durability.     

Residual Tensile Properties of GFRP Reinforcing Bars after Loading in Severe Environments

The long-term behavior of glass fiber-reinforced polymer (GFRP) reinforcing bars is one of the most critical issues for the acceptance of these materials as reinforcement for concrete structures. There is a high demand for experimental studies to investigate the stability of the tensile strength, ultimate elongation, and elasticity modulus. GFRP reinforcing bars inherently have a low elasticity modulus, which must not significantly decrease over time under loading or the serviceability behavior of the concrete element containing them will be jeopardized. This paper evaluates the residual tensile properties of three sizes of sand-coated GFRP reinforcing bars in alkaline and water environments combined with sustained loading and elevated temperature. Bar diameters of 15.9 (No. 5), 12.7 (No. 4), and 9.5?mm (No. 3) were loaded for different durations, then tested in axial tension for residual tensile properties. The test periods varied from 1?to?4?months under elevated temperature to hasten degradation and simulate extended service periods. The reduction in tensile strength was found to be 7–13% of the guaranteed strength for the three bar sizes under elevated temperature, which is at least 26% higher than the specified design strength as recommended by ACI 440.1R-03. More importantly, no significant change in the elastic modulus was observed.     

Bond between Carbon Fiber Reinforced Polymer Bars and Concrete. II: Computational Modeling

Computational modeling of the behavior of concrete reinforced with fiber reinforced polymer (FRP) bars requires models for the constitutive behavior of concrete and FRP and a model for their interaction, bond. This study focuses upon the application of an elastoplastic bond model to represent the behavior of the type A bar experimentally examined in Part I. By fully coupling the tangent and radial response, the model is sensitive to the stress state around the bar and can induce longitudinal cracking in the adjacent concrete. The model is calibrated and then applied to predict the splitting behavior of two types of specimens. Though the two specimens differ significantly, the estimated splitting loads fall within the experimental scatter. The model is then used to obtain preliminary design data for the needed cover thickness, development length, and transfer lengths. Computational models, when calibrated with limited experimental data, allow the behavior of several different specimens to be estimated. This approach is an effective means of obtaining preliminary design data and of planning further experimental verification.     

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).     

External Prestressed Carbon Fiber-Reinforced Polymer Straps for Shear Enhancement of Concrete

The use of fiber-reinforced polymers (FRPs) for the strengthening and repair of existing concrete structures is a field with tremendous potential. The materials are very durable and, hence, ideally suited for use as external reinforcement. Although extensive work has been carried out investigating the use of FRPs for flexural strengthening, a fairly recent development is the use of these materials for the shear strength enhancement of concrete. The current system investigates the use of posttensioned, nonlaminated, carbon fiber-reinforced polymer (CFRP) straps as external shear reinforcement for concrete. Experiments were carried out on an unstrengthened control beam and beams strengthened with external CFRP straps. It was found that the ultimate load capacity of the strengthened beams was significantly higher than that of the control specimen. Existing design codes and analysis methods were found to underestimate the ultimate resistance of the control specimen and the strengthened beams. Nevertheless, the modified compression field theory provided insight into possible failure mechanisms and the influence of the strap prestress level on the structural behavior. It is concluded that the use of these novel stressed elements could represent a viable and durable means of strengthening existing concrete infrastructure.     

Deflection Response of Glass Fiber-Reinforced Pultruded Components in Hot Weather Climates

Presented in this paper are the experimental results pertaining to the deflection response of E-glass/polyester pultruded structural elements when subjected to bending and temperature profiles comparable to those encountered in hot weather conditions. Experiments were conducted on tubular components subjected to an applied force resulting in a maximum stress corresponding to 4% of the composite material strength. In one case, five component tests were carried out in an environmental condition in which the air temperature was first gradually increased from 20°±2°C?to?72°±2°C, then decreased to 60°±2°C in 1?h. In another case, one component was tested under an air temperature that was first gradually increased from 20?to?60°C, then kept at 60°±2°C for 215?h. Test results are compared with those obtained in another study at laboratory conditions (22°±2°C). It is concluded that the increase in the deflection of glass-reinforced pultruded components in hot climates is substantial and must always be accounted for in the design of pultruded structures.     

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.     

Barrier Wall Impact Simulation of Reinforced Concrete Decks with Steel and Glass Fiber Reinforced Polymer Bars

Many experimental studies have been performed to evaluate the behavior of noncorroding glass fiber reinforced polymer (GFRP) rebars in reinforced concrete (RC) flexural members. Relatively few studies have focused on the behavior of bridge deck overhangs in the event of a barrier wall impact, which subjects this region to a combination of flexure, shear, and axial tension. The objective of this investigation is to evaluate deck overhangs under these forces. Three bridge deck reinforcing schemes were considered in the study: all epoxy-coated steel (ECS), all GFRP, and hybrid made up of a top mat of GFRP rebars and a bottom mat of ECS rebars. Laboratory testing of nine RC specimens was performed. Results showed that all three reinforcing schemes meet the AASHTO requirements.     

Stress-Strain Model for Fiber-Reinforced Polymer-Confined Concrete

The design of fiber-reinforced polymer (FRP)-confined concrete members requires accurate evaluation of the performance enhancement due to the confinement provided by FRP composite jackets. A strain ductility-based model is developed for predicting the compressive behavior of normal strength concrete confined with FRP composite jackets. The model is applicable to both bonded and nonbonded FRP-confined concrete and can be separated into two components: a strain-softening component, which accounts for unrestrained internal crack propagation in the concrete core, and a strain-hardening component, which accounts for strength increase due to confinement provided by the FRP composite jacket. A variable strain ductility ratio described in a companion paper is used to develop the proposed stress-strain model. Equilibrium and strain compatibility are used to obtain the ultimate compressive strength and strain of FRP-confined concrete as a function of the confining stiffness and ultimate strain of the FRP jacket.     

Field Monitoring and Repair of a Glass Fiber-Reinforced Polymer Bridge Deck

This paper reports on the monitoring and repair of a pilot field deployment of a glass fiber-reinforced polymer (GFRP) deck on a small steel girder bridge in the Washington State. Deck deflections were monitored periodically over a 10-month period and were found to increase significantly over that time. The GFRP deck is an adhesively bonded assembly of GFRP tubes and top and bottom plates. After 9 months of service, wearing surface cracking was observed, and upon closer inspection, the top GFRP plate was found to be delaminated from the tubes over a fairly large area. Deck deflections in the area of delamination were found to be considerably larger than those observed during previous monitoring in undamaged locations. A retrofit solution was employed where the top plate was reconnected to the tubes using screws coated with a two-part epoxy that mixed when they were driven. At the time of writing the retrofit was successful in reattaching the delaminated top plate.     

Reinforced Concrete T-Beams Strengthened in Shear with Fiber Reinforced Polymer Sheets

This research studies the interaction of concrete, steel stirrups, and external fiber reinforced polymer (FRP) sheets in carrying shear loads in reinforced concrete beams. A total of eight tests were conducted on four laboratory-controlled concrete T-beams. The beams were subjected to a four-point loading. Each end of each beam was tested separately. Three types of FRP, uniaxial glass fiber, uniaxial carbon fiber, and triaxial glass fiber, were applied externally to strengthen the web of the T-beams, while some ends were left without FRP. The test results show that FRP reinforcement increases the maximum shear strengths between 15.4 and 42.2% over beams with no FRP. The magnitude of the increased shear capacity is dependent not only on the type of FRP but also on the amount of internal shear reinforcement. The triaxial glass fiber reinforced beam exhibited more ductile failure than the other FRP reinforced beams. This paper also presents a test model that is based on a rational mechanism and can predict the experimental results with excellent accuracy.