Ultimate Condition of Fiber Reinforced Polymer-Confined Concrete

One important application of fiber reinforced polymer (FRP) composites is as a confining material for concrete in the retrofit of existing concrete columns by the provision of FRP jackets. Such jackets are commonly formed in a wet layup process, with the fibers being only or predominantly in the hoop direction. It has been well established in recent studies that the rupture strains/strengths of FRP measured in tests on such FRP-confined concrete cylinders fall substantially below those from flat coupon tensile tests, but the causes are unclear. This paper presents the results of a study that is aimed at clarifying these causes. To this end, the paper reports and compares the ultimate tensile strains of two types of FRP (carbon FRP and glass FRP) obtained from three types of tests—flat coupon tensile tests, ring splitting tests, and FRP-confined concrete cylinder tests. Based on comparisons of these test results, it can be concluded that the FRP hoop rupture strains in FRP-confined concrete cylinders are reduced below the ultimate tensile strains from flat coupon tests by at least three factors—(1) the curvature of the FRP jacket; (2) the deformation localization of the cracked concrete; and (3) the existence of an overlapping zone. While the first factor that reduces the in situ strain capacity of FRP on confined concrete is material dependent, the last two factors that result in a nonuniform strain distribution in the jacket are independent of the FRP material properties. The third effect reduces the average hoop rupture but does not affect the distribution of the confining pressure, as the FRP jacket is thicker in the overlapping zone.     

Variable Strain Ductility Ratio for Fiber-Reinforced Polymer-Confined Concrete

The encasement of concrete in fiber-reinforced polymer (FRP) composite jackets can significantly increase the compressive strength and strain ductility of concrete columns and the structural system of which the columns are a part, be it a building or a bridge. Due to the approximate bilinear compressive behavior of FRP-confined concrete, analysis and design of FRP-confined concrete members requires an accurate estimate of the performance enhancement due to the confinement provided by FRP composite jackets. An analytical model is presented for predicting the bilinear compressive behavior of concrete confined with either bonded or nonbonded FRP composite jackets. This article describes the basis of the model, which is a variable plastic strain ductility ratio. The variable plastic strain ductility ratio defines the increase in plastic compressive strain relative to the increase in the plastic compressive strength of the FRP-confined concrete, which is a function of the hoop stiffness of the confining FRP composite jacket, the plastic dilation rate, and the type of bond between the FRP composite and concrete.     

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

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

Performance Evaluation of Reinforced Concrete Bridge Columns Wrapped with Fiber Reinforced Polymers

This study investigates the performance of new bridge columns wrapped with fiber reinforced polymers (FRP) when exposed to aggressive environmental conditions. This has been accomplished through field monitoring and laboratory tests. As part of the field monitoring, temperature data were collected at various locations of bridge columns. In addition, visual inspection of two bridges was performed periodically for over a period of two years. No evidence of deterioration of the FRP wraps was detected during that period. Laboratory tests were performed to investigate how FRP wraps protect reinforced concrete columns from corrosion, and freeze–thaw laboratory tests were conducted to study the impact of temperature cycles on the mechanical behavior of FRP-wrapped columns. From the corrosion experimental tests, it was found that FRP provides excellent protection against aggressive agents (salty water or moisture) even when a single layer is used. Compression tests were conducted on specimens subjected to freeze–thaw cycles. It was found that minor thermal cycles have no effect on the performance of FRP-wrapped concrete specimens. However, for large thermal cycles, some degradation of ductility in the axial and the hoop directions was observed.     

Rectangular Filament-Wound Glass Fiber Reinforced Polymer Tubes Filled with Concrete under Flexural and Axial Loading: Analytical Modeling

This paper presents an analytical model to predict the behavior of concrete-filled rectangular fiber reinforced polymer (FRP) tubes (CFRFTs), subjected to bending and axial loads. The model accounts for different laminate structures of the flange and web of the tube. Gradual reduction of stiffness, resulting from progressive failure of FRP layers oriented at various angles is considered through the ultimate laminate failure approach. The model adopts cracked section analysis, using layer-by-layer approach and accounts for totally and partially filled tubes. The model predicts the moment–curvature responses of beams, load–strain responses of columns, and complete interaction curves of beam–columns. The model is verified using experimental results and is used to study the effects of laminate structure, hybrid laminates, thickness of the tube and optimization of partially filled tubes. Comparisons of CFRFT with conventional reinforced concrete (RC) sections showed that CFRFT could provide axial load–bending moment interaction curves comparable to those of RC sections of similar reinforcement index. Also, providing a small fraction of carbon fibers in the flanges could substantially improve flexural performance. The first ply failure approach could highly underestimate the strength of CFRFT.     

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.     

Axial Behavior of Reinforced Concrete Columns Confined with FRP Jackets

This paper presents the results of an experimental investigation of the axial behavior of small-scale circular and square plain concrete specimens and large-scale circular and square reinforced concrete columns confined with fiber reinforced polymer (FRP) composite jackets, subject to monotonic, concentric axial loads. Improvements in the axial load-carrying and deformation capacities of FRP jacketed concrete members over unjacketed members are reported. Factors influencing the axial stress-strain behavior of FRP confined concrete, such as transverse dilation and effectively confined regions and their relationship to jacket properties, are identified and discussed. Factors necessary to calibrate in situ jacket behavior and reported or measured FRP material properties are proposed and their interrelationships discussed.     

Use of Microwaves for Damage Detection of Fiber Reinforced Polymer-Wrapped Concrete Structures

Jacketing technology using fiber reinforced polymer (FRP) composites is being applied for seismic retrofit and rehabilitation of reinforced concrete (RC) columns designed and constructed under older specifications. In this study, the authors develop an electromagnetic (EM) imaging technology for detecting such damage as voids and debonding between the jacket and the column, which may significantly weaken the structural performance of the column otherwise attainable by jacketing. This technology is based on the reflection analysis of a continuous EM wave sent toward and reflected from layered FRP–adhesive-concrete medium: Voids and debonding areas will generate air gaps which produce additional reflections of the EM wave. In this study, dielectric properties of various materials involved in the FRP-jacketed RC column were first measured using a plane-wave reflectometer. The measured properties were then used for a computer simulation of the proposed EM imaging technology. The simulation demonstrated the difficulty in detecting damage by using plane waves, as the reflection contribution from the voids and debonding is very small compared to that from the jacketed column. In order to alleviate this difficulty, dielectric lenses were designed and fabricated, focusing the EM wave on the bonding interface. Finally, three concrete columns were constructed and wrapped with glass–FRP jackets with various voids and debonding conditions artificially introduced in the bonding interface. Using the proposed EM imaging technology involving the especially designed and properly installed lenses, these voids and debonding areas were successfully detected. This technology can be used to assess the jacket bonding quality during the initial jacket installation stage and to detect debonding between the column and the jacket caused by earthquake and other destructive loads.     

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

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

Experimental Study of Normal- and High-Strength Concrete Confined with Fiber-Reinforced Polymers

The experimental program reported here was conducted to gain insight into the behavior of concrete confined with fiber-reinforced polymers (FRPs). A total of 112 cylindrical concrete specimens, each 150 mm in diameter, 300 mm in height, and concrete strength up to 112 MPa, were tested under monotonic uniaxial compression. Test variables included amount of FRP, strength and stiffness of FRP, concrete strength, and the health of concrete at the time of strengthening. Results showed that, with an increase of the unconfined concrete strength, the strength enhancement, energy absorption capacity, ductility factor, and work (energy) index at rupture of FRP jackets all decreased remarkably. A positive correlation was found between concrete ductility and FRP rupture strain. A gradual post-peak failure of the specimens, observed previously from FRP-confined concrete columns tested at the University of Toronto, was also observed in some of the current tests. This ductile failure, attributed to the gradual unzipping failure of FRP jacket, is related to specimen size and is explained in terms of various confinement parameters.     

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

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

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.     

Modeling of Corroded FRP-Wrapped Reinforced Concrete Columns in Axial Compression

This paper discusses the mechanical behavior of reinforced concrete columns wrapped with fiber-reinforced polymer (FRP) sheets. A numerical routine was developed to predict the behavior of the columns using a step-by-step technique. The routine is based on an existing model and was modified to account for confinement provided by the traditional steel as well as the external FRP wraps. Several empirical equations for the confined concrete were calibrated with results from experimental tests from different published papers. The most accurate equation was incorporated into the routine to predict the stress-strain relation of the column up to failure. A different confinement to the outer concrete cover and the inner core was used to account for the FRP wraps and the transverse steel. The model was calibrated with experimental results from different experiments on FRP-wrapped reinforced concrete columns.The model was taken one step further by using it to predict the behavior of reinforced concrete columns, with a combination of steel corrosion and CFRP wraps. The columns modeled were subjected to harsh corrosive environment over 44 months. The model successfully predicted the load deformation in both axial and circumferential directions in corroded and intact columns, both wrapped and unwrapped, with good accuracy. The analysis forms a solid foundation for accurate evaluation of the effect of corrosion and wrapping on reinforced concrete columns.     

Behavior of FRP Jacketed Concrete Columns under Eccentric Loading

This paper describes a study on the behavior of fiber-reinforced polymer (FRP) jacketed square concrete columns subjected to eccentric loading. The effect of strain gradient on the behavior of concrete columns confined by the FRP jacket was investigated through experimental and numerical analysis methods. Nine (108 × 108 × 305 mm) square concrete column stubs with zero, one, and two plies of unidirectional carbon FRP fabric were tested under axial compressive loading. In addition to the FRP jacket thickness, the effects of various eccentricities were examined. The nonlinear finite-element analysis results were compared and validated against the experimental test results. The results show that the FRP jacket can greatly enhance the strength and ductility of concrete columns under eccentric loading and that the strain gradient reduces the retrofit efficiency of the FRP jacket for concrete columns. Therefore, a smaller enhancement factor should be used in designing FRP-jacketed columns under eccentric loading. Furthermore, the nonlinear finite-element models established in this study can be used as templates for future research work on FRP-confined concrete columns.     

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

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

Theoretical Model for Fiber-Reinforced Polymer-Confined Concrete

Fiber-reinforced polymer (FRP) composites have found increasingly wide applications in civil engineering due to their high strength-to-weight ratio and high corrosion resistance. One important application of FRP composites is as a confining material for concrete, particularly in the strengthening or seismic retrofit of existing reinforced concrete columns by the provision of a FRP jacket. FRP confinement can enhance both the compressive strength and the ultimate strain of concrete significantly. This paper presents a new stress–strain model for FRP-confined concrete in which the responses of the concrete core and the FRP jacket as well as their interaction are explicitly considered. Such a model is often referred to as an analysis-oriented model. The key novel feature of the proposed analysis-oriented model, compared to existing models of the same kind, is a more accurate and more widely applicable lateral strain equation based on a careful interpretation of the lateral deformation characteristics of unconfined, actively confined, and FRP-confined concrete. Through comparisons with independent test data, the proposed model is shown to be accurate not only for FRP-confined concrete but also for concrete confined with a steel tube, demonstrating the wide applicability of the model to concrete confined with different confining materials. The accuracy of the proposed model is also shown to be superior to existing analysis-oriented stress-strain models through comparisons with test data.     

Combined Effect of Loading and Cold Temperature on the Stiffness of Glass Fiber Composites

Because of the short construction season and cold winters in Alaska, the prestressed concrete decked bulb-tee bridge system is very popular. However, the concrete deck is an integral part of the bridge superstructure and cannot be easily replaced when it deteriorates. Obviously, there is merit in combining durable “premanufactured” fiber-reinforced polymer (FRP) composite deck with stiffer prestressed concrete girders in cold regions. However, the effects of long-term exposure to extreme temperature variations and various moisture conditions typical of cold regions on the performance of FRP composite materials are not fully understood. This paper summarizes the combined effect of low-temperature and deformation strain levels on the longitudinal modulus of glass fiber-reinforced polymer (GFRP) samples. The modulus of elasticity of GFRP laminate coupons was tested at various temperatures down to ?31 °F (?35°C) by temporarily subjecting the samples to three strain levels of 1,000, 2,000, or 3,000 microstrains. Both biaxial and uniaxial samples subjected to a deformation of 1,000 microstrains showed an increase in stiffness when tested at increasingly colder temperatures, and no noticeable change in stiffness was seen when the samples were retested after being equilibrated to room temperature. However, samples subjected to a predetermined elevated strain level did show significant stiffness degradation after room temperature equilibration. The degree of degradation was noticeably larger for samples subjected to the low temperatures than for control samples that were subjected to the equivalent number of cycles at room temperature. It was also noted that the degradation due to load cycles or temperature coupled with load cycles was noticeably less for uniaxial samples than for biaxial samples.     

Creep Analysis of Axially Loaded Fiber Reinforced Polymer-Confined Concrete Columns

An analytical model is developed to study the time-dependent behavior of concrete-filled fiber reinforced polymer (FRP) tubes (CFFT) and fiber-wrapped concrete columns (FWCC) under sustained axial loads. The model utilizes the double power law creep function for concrete in the framework of rate of flow method, and the linear viscoelastic creep model for FRP. It follows geometric compatibility and static equilibrium, and considers the effects of sealed concrete, multiaxial state of stresses, creep Poisson’s ratio, stress redistribution, variable creep stress history, and creep rupture. The model is verified against previous creep tests by the writers on FWCC and CFFT columns. It is then used to study the practical design parameters that may affect creep of FRP-confined concrete under service loads, or lead to creep rupture at high levels of sustained load. Creep of FWCC is shown to be close to that of sealed concrete of the same mix, as the effect of confinement on creep of concrete is not very significant. CFFT columns, on the other hand, creep much less than FWCC, mainly due to axial stress redistribution. As the stiffness of the tube increases relative to the concrete core, larger stress redistributions take place further reducing the creep. However, there is a threshold, beyond which, stiffer tubes would not significantly lower the creep of concrete. Creep rupture life expectancy of CFFT columns is shown to be quite acceptable.     

Punching Shear Behavior of Fiber Reinforced Polymers Reinforced Concrete Flat Slabs: Experimental Study

This paper presents the results of a two-phase experimental program investigating the punching shear behavior of fiber reinforced polymer reinforced concrete (FRP RC) flat slabs with and without carbon fiber reinforced polymer (CFRP) shear reinforcement. In the first phase, problems of bond slip and crack localization were identified. Decreasing the flexural bar spacing in the second phase successfully eliminated those problems and resulted in punching shear failure of the slabs. However, CFRP shear reinforcement was found to be inefficient in enhancing significantly the slab capacity due to its brittleness. A model, which accurately predicts the punching shear capacity of FRP RC slabs without shear reinforcement, is proposed and verified. For slabs with FRP shear reinforcement, it is proposed that the concrete shear resistance is reduced, but a strain limit of 0.0045 is recommended as maximum strain for the reinforcement. Comparisons of the slab capacities with ACI 318-95, ACI 440-98, and BS 8110 punching shear code equations, modified to incorporate FRP reinforcement, show either overestimated or conservative results.     

Experimental Investigation of Bonded Fiber Reinforced Polymer-Concrete Joints under Cyclic Loading

The strengthening of reinforced concrete structures by means of externally bonded fiber reinforced polymers (FRPs) is becoming an attractive technique for upgrading existing structures. Although previous laboratory investigations have shown that the bending capacities of beams can be increased considerably with this strengthening technique, premature failure by debonding of the FRP reinforcement can often limit its effectiveness. To gain insight into debonding phenomena, various experimental and analytical investigations of the behavior of bonded FRP-to-concrete joints have been carried out. However, such studies have generally been limited to monotonic (“static”) loading conditions. In this paper, we present results from an experimental investigation of bonded FRP-to-concrete joints under cyclic loading. First, we describe the experimental setup and test parameters. Next experimental results for the effects of cyclic loading on slip at the FRP–concrete interface, crack opening, and strain profiles along the bonded FRP joint are presented and discussed. A power-law expression for the so-called “S–N” curves (cyclic stress ranges versus numbers of cycles to failure) is proposed, and the parameters in this expression are determined from the experimental data. The influence of various parameters such as bond length, bond width, and cyclic bond stress levels on fatigue behavior are discussed.