Tests on the Ductility of Reinforced Concrete Beams Retrofitted with FRP and Steel Near-Surface Mounted Plates

Reinforced concrete beams are now commonly retrofitted using externally bonded (EB) fiber reinforced polymer (FRP) plates as the technique is both inexpensive and unobtrusive. However, tests have shown that EB carbon FRP plates tend to debond at low strains, which can severely limit the ductility or moment redistribution to such an extent that guidelines often preclude moment redistribution. This paper reports the moment redistribution achieved in tests on nine near full-scale two-span continuous reinforced concrete beams that were retrofitted with near-surface mounted (NSM) plates. The plates were either carbon FRP or high yield steel strips which were adhesively bonded within saw grooves cut into the concrete cover on the tension face or sides of the beam. It was found that the debonding strains of these NSM plates were considerably larger than those associated with EB plates and that substantial amounts of moment redistribution occurred. These tests suggest that NSM plates can be used to increase the strength of reinforced concrete structures with little, if any, loss of ductility.     

Bond Between Near-Surface Mounted Carbon-Fiber-Reinforced Polymer Laminate Strips and Concrete

In recent years, a strengthening technique based on near-surface mounted (NSM) laminate strips of carbon-fiber-reinforced polymer (CFRP) has been used to increase the load-carrying capacity of concrete and masonry structures by introducing laminate strips into precut grooves on the concrete cover of the elements to be strengthened. The high experimentally derived levels of strength efficacy with concrete columns, beams, and masonry panels have presented NSM as a viable and promising technique. This practice requires no surface preparation work and, after cutting the groove, requires minimal installation time compared to the externally bonded reinforcing technique. A further advantage associated with NSM CFRP is its ability to significantly reduce the probability of harm resulting from fire, acts of vandalism, mechanical damage, and aging effects. To assess the bond behavior of CFRP to concrete, pullout-bending tests have been carried out. The influences of bond length and concrete strength on bond behavior are analyzed, the tests are described, and the results are presented and discussed in detail. Finally, a local stress-slip relationship is determined based on both experimental results and a numerical strategy.     

Experimental and Analytical Behavior of Carbon Fiber-Based Rods as Flexural Reinforcement

This paper presents the results of an experimental and analytical comparison of a study on the flexural behavior of concrete beams reinforced with sandblasted carbon fiber-based composite rods. Twelve beams, including three control beams reinforced with steel, were tested for strength, deformation, and failure characteristics. Analytical comparisons included the generation of the theoretical strength and moment curvature relations. Experimental data from pullout tests indicated that bonding of sandblasted rods is not a major concern. However, excessive deformation in achieving the predicted moment capacity could be a limiting factor in the design of these beams.     

Bond Behavior of Near-Surface Mounted FRP Strips Bonded to Modern Clay Brick Masonry Prisms: Influence of Strip Orientation and Compression Perpendicular to the Strip

In this paper the results of 18 pull tests performed on clay brick masonry prisms strengthened with near-surface mounted carbon fiber-reinforced polymer (CFRP) strips are presented. The pull tests were designed to add to the existing database and investigate variables significant to masonry construction. FRP was bonded to solid clay brick masonry; FRP aligned both perpendicular and parallel to the bed joint; and in the case of FRP reinforcement aligned parallel to the bed joint, compression applied perpendicular to the strip was used to simulate vertical compression load in masonry walls. Results including bond strength, critical bond length, and the local bond-slip relationship are presented as well as a discussion on the effect of the new variables on these results.     

Bond Behavior of Near-Surface Mounted CFRP Laminate Strips under Monotonic and Cyclic Loading

Near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) laminate strips are used to increase the load-carrying capacity of concrete structures. This is done by inserting the CFRP strips into slits made in the concrete cover of the elements to be strengthened and gluing the strips to the concrete with an epoxy adhesive. In several cases the NSM technique has substantial advantages when compared with externally bonded laminates. To assess the bond behavior between the CFRP and concrete under monotonic and cyclic loading, an experimental program was carried out based on a series of pullout-bending tests. The influence of the bond length and loading history on the bond behavior was investigated. In this work the details of the tests are described and the obtained results discussed. Using the experimental data and an analytical-numerical strategy, a local bond stress-slip relationship was determined. A finite-element analysis was performed to evaluate the influence of the adhesive on the global response observed in the pullout-bending tests.     

Shear Strengthening of T Cross Section Reinforced Concrete Beams by Near-Surface Mounted Technique

With the purpose of evaluating the influence of both the percentage and inclination of the carbon fiber-reinforced polymer (CFRP) laminates on the effectiveness of the near-surface mounted technique for the shear strengthening of reinforced concrete T beams, an experimental program was carried out, using three percentages of laminates and, for each one, three inclinations: 90, 60, and 45°. The CFRP-strengthened beams had a steel stirrup reinforcement ratio (ρsw) of 0.1%. The highest CFRP percentage was designed to provide a maximum load similar to the one of a reference beam reinforced with ρsw equal to 0.24%. Although these beams have had a similar maximum load, the beams with CFRP presented higher stiffness. Laminates at 60° was the most effective shear strengthening configuration, having provided a maximum increase in the load capacity of 33%. The contribution of the CFRP strengthening systems was limited by the concrete tensile strength. Below certain spacing between laminates, a group effect occurs due to the interference between consecutive concrete failure surfaces, leading to the detachment of “two lateral walls” from the underlying beam core.     

Local Bond-Slip Relationship for FRP Reinforcement in Concrete

The objective of this paper is to define a rigorous numerical method to calibrate parameters of a given local bond-slip relationship using experimental results of pullout tests, taking into account the distribution of the slip and bond shear stress throughout the bar. The proposed method involves finding parameters of a given bond-slip relationship, such that results of pullout tests can be predicted in terms of applied pullout force and consequent slip at the loaded end and slip at the free end. The method is applied to some experimental data, and the results are discussed. For the application of the proposed method, two analytical expressions of the bond-slip relationship are selected, even though it could be applied to any analytical expression. An example of determination of anchorage length starting from the knowledge of the local bond-slip relationship is given.     

Development of FRP Reinforcement Guidelines for Prestressed Concrete Structures

Several national programs define the testing protocols and design guidelines for fiber reinforced polymer (FRP) reinforcement in concrete structures. This paper offers a review of these documents, comparing the materials testing and design philosophies for FRP reinforcement of different working groups. The work references Canadian, European, and Japanese efforts to codify these materials and assess the relative merits of each approach. The emphasis is on prestressing applications since the demands for sustained load capacity and full bond are more severe than for reinforced concrete.     

Shear Strength of Large Concrete Members with FRP Reinforcement

Increasing interest in the use of fiber-reinforced polymer (FRP) reinforcement for reinforced concrete structures has made it clear that insufficient information about the shear performance of such members is currently available to practicing engineers. This paper summarizes the results of 11 large shear tests of reinforced concrete beams with glass FRP (GFRP) longitudinal reinforcement and with or without GFRP stirrups. Test variables were the member depth, the member flexural reinforcement ratio, and the amount of shear reinforcement provided. Results showed that the equations of the Canadian CSA shear provisions provide conservative estimates of the shear strength of FRP-reinforced members. Recommendations are given along with a worked example on how to apply these provisions including to members with FRP stirrups. It was found that members with multiple layers of longitudinal bars appear to perform better than those with a single layer of longitudinal reinforcing bars. Overall, it was concluded that the fundamental shear behavior of FRP-reinforced beams is similar to that of steel-reinforced beams despite the brittle nature of the reinforcement.     

Constructability and Economics of FRP Reinforcement Cages for Concrete Beams

This paper presents a quantitative economic analysis and a qualitative constructability analysis of three-dimensional fiber-reinforced plastic (FRP) reinforcement cages for concrete beams. Material, labor, and life-cycle costs are provided, and construction practice aspects are discussed. The results of the analyses indicate that prefabricated FRP cages can offer benefits to the construction industry. Although the initial costs of the FRP materials are likely to be higher than those of steel rebar, there is a significant potential for cost savings due to reduced maintenance and labor costs, as a result of the corrosion resistance of the FRP and the increased construction productivity. When direct life-cycle costs are considered, FRP reinforcements already constitute, in many cases, an economically competitive alternative to conventional steel reinforcement in adverse environments. If, in addition, the indirect cost savings as well as quality and safety issues are considered, the FRP reinforcement may be even more competitive.     

Performance of Glued-Laminated Timbers with FRP Shear and Flexural Reinforcement

Glued-laminated wood beams (glulams) have low allowable shear stresses relative to competitive engineering wood products such as parallel strand lumber and laminated veneer lumber. For heavily loaded applications such as garage door headers, the lower shear allowable stresses typically necessitate the use of larger glulam members relative to other engineered lumber. This paper reports on experimental research aimed at increasing the shear capacity of glulams. To increase the shear strength, a series of fiber-reinforced polymeric (FRP) pins are inserted into holes drilled transversely across the plies of the glulam. These pins are epoxy-bonded into place after the glulam has been produced. The test results show that the shear strength scatter in the pinned set of glulams is significantly reduced as compared to the unpinned specimens. Two- and three-parameter Weibull estimates of the service-level allowables increases between 40% and 100% for specimen sets reinforced by the FRP pins. The transverse reinforcing scheme also shows promise for aiding in engagement of composite flexural reinforcement plies into the laminate stack. Previous research has shown that the bonding between wood and FRP plies has potential long-term durability problems. Therefore, the pinning technology reported on in this work shows promise for providing a means for direct engagement (through bearing of the pins) of the FRP plies to the wood plies. This behavior has been demonstrated though the testing of larger-scale specimens with both transverse shear reinforcements and longitudinal FRP plies. The reinforcing strategies show potential structural and economic advantages, especially when the FRP is coupled with low-grade, low-value finger-jointed lumber.     

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.     

Evaluation of Shear Design Equations of Concrete Beams with FRP Reinforcement

Several codes and design guidelines addressing fiber-reinforced polymer (FRP) bars as primary reinforcement for structural concrete have been recently published worldwide. This reflects the great progress in FRP research area that has been conducted by the research community over the past two decades. Most of these design provisions follow the traditional approach of Vc+Vs for shear design. Nevertheless, both equations of concrete contribution Vc and FRP stirrup contribution Vs to shear strength in these guidelines are different in the manner that they are calculated. In this paper, five methods for FRP shear design, currently used in design practice, were reviewed. These methods include the American Concrete Institute design guide, ACI 440.1R-06; the Canadian Standards Association, CAN/CSA-S806-02; the ISIS Canada design manual, ISIS-M03-07; the British Institution of Structural Engineers guidelines; and the design recommendations of the Japan Society of Civil Engineers. The five methods for shear design prescribed in these guidelines were compared with experimental database obtained from the literature. In addition, the modified compression field theory approach was reviewed and compared with the experimental database.     

Flexural Reinforcement of Glulam Timber Beams and Joints with Carbon Fiber-Reinforced Polymer Rods

Fiber-reinforced polymer (FRP) composites have largely been used in combination with masonry and concrete structural elements in the last decade. Recent applications showed that new advantages may also be achieved in the field of timber structures, even if currently steel fasteners are used mainly in connecting systems. This study investigated the possibility of using carbon FRP (CFRP) rods as glued-in reinforcement of glulam beams and as glued-in connectors for glulam timber head joints that should transfer flexural moment between two adjacent beams. Half-scale beams were tested both with and without the presence of FRP reinforcement. Flexural behavior of CFRP-reinforced beams was compared with unreinforced beams that were used as control specimens. Two different amounts of CFRP reinforcement were used in the beam section. Experimental results showed a significant influence of the CFRP rods, because the reinforced beams demonstrated an increase in ultimate capacity and stiffness. Experimental results were also compared with numerical analysis, which showed good accordance with regard to the load and deflection values. Full-size head joints were prepared and tested. Flexural behavior of the joints was compared with the mechanical properties of monopiece beams that were used as reference specimens. Three different force transfer lengths were used for the construction of CFRP-timber joints. Experimental results showed that the use of CFRP rods in timber joints was successful, because the capacity of the CFRP-jointed beams was almost the same as that of the monolithic beams for the longest bond length that was adopted. This result is important in order to find an adequate alternative to traditional joints made with steel bolts and plates, which are unable to create rigid connections, increase dramatically the weight of timber structures, and may be subjected to corrosion in an aggressive environment. A numerical modeling based on the virtual work principle was also conducted and theoretical results were found in good accordance with the experimental results for the tested joint.     

Connection of Concrete Railing Post and Bridge Deck with Internal FRP Reinforcement

The use of fiber-reinforced polymer (FRP) reinforcement is a practical alternative to conventional steel bars in concrete bridge decks, safety appurtenances, and connections thereof, as it eliminates corrosion of the steel reinforcement. Due to their tailorability and light weight, FRP materials also lend themselves to the development of prefabricated systems that improve constructability and speed of installation. These advantages have been demonstrated in the construction of an off-system bridge, where prefabricated cages of glass FRP bars were used for the open-post railings. This paper presents the results of full-scale static tests on two candidate post–deck connections to assess compliance with strength criteria at the component (connection) level, as mandated by the AASHTO Standard Specifications, which were used to design the bridge. Strength and stiffness until failure are shown to be accurately predictable. Structural adequacy was then studied at the system (post-and-beam) level by numerically modeling the nonlinear response of the railing under equivalent static transverse load, pursuant to well-established structural analysis principles of FRP RC, and consistent with the AASHTO LRFD Bridge Design Specifications. As moment redistribution cannot be accounted for in the analysis and design of indeterminate FRP RC structures, a methodology that imposes equilibrium and compatibility conditions was implemented in lieu of yield line analysis. Transverse strength and failure modes are determined and discussed on the basis of specification mandated requirements.     

New Approach for Estimating the Deflection of Beams Reinforced with FRP Reinforcement

Due to concerns with corrosion, the use of fiber-reinforced polymer (FRP) as a replacement to conventional steel reinforcement has greatly increased over the last decade. Researchers have identified the distinctive mechanical and bond properties of FRP reinforcement that prevent the use of existing relationships to establish serviceability of concrete structures reinforced with such products. Although studies have modified these empirical relationships to describe the behavior of structures reinforced with FRP reinforcement, this paper will provide a new approach to estimate deflection of concrete beams by considering material properties of the reinforcement and incorporating the effects of tension stiffening. Accuracy and precision of the approach was established by performing a statistical analysis on a database containing 171 FRP-reinforced concrete beams. Results were compared to those from existing proposed relationships and indicate the potential of the method to estimate deflection at various service conditions.     

Acoustic Emission Characterization of Damage in Hybrid Fiber-Reinforced Polymer Rods

The development and testing of a pseudoductile hybrid fiber-reinforced polymer rod consisting of glass and carbon fibers is described. Two different hybrid forms were considered for the development of the hybrid rods, including (1) random dispersion of the glass and carbon fibers; and (2) a core-shell design with a glass fiber core protected by the carbon fiber shells. The mechanical properties of the hybrids were evaluated through uniaxial tension tests. The hybrid rods developed by way of the core-shell approach exhibited pseudoductile properties that included a yield point of 1,153 MPa and an ultimate strength of 1,191 MPa, with a strain of 3.5% at ultimate. The experimental program also included damage assessment by way of a single-channel fiber optic acoustic emission (AE) sensor. The acoustic emission technique was employed for real-time determination of the progressive damage due to rupture of fibers. Moreover, the spectral energies of the frequency spectrum from the AE signals were employed for assessment of the state of damage in the rods. The spectral energies distinguish between the carbon and glass fiber ruptures. This differentiation of signals provides information about the condition and the state of health of the hybrid rod.     

Effects of Corrosion of Steel Reinforcement on RC Columns Wrapped with FRP Sheets

This study investigated the effectiveness of carbon fiber-reinforced polymer (CFRP) sheets in protecting reinforced concrete (RC) columns from corrosion of steel reinforcement. Thirty small-scale RC columns and four midscale RC columns were used in this study. The small-scale columns were used for a comprehensive parametric study, whereas the midscale columns were used to evaluate design guidelines proposed based on the results of the small-scale column tests. The test columns were conditioned under an accelerated corrosion process and then tested under uniaxial compression up to failure. The test results showed that although CFRP sheet wrapping decreased the corrosion rate, the corrosion of steel reinforcement could continue to occur, eventually showing a decrease in ultimate axial compression capacity. Design guidelines were proposed based on the small-scale RC column tests and evaluated through a comparison with the test results of midscale RC columns. The proposed design guidelines introduced a concept of effective area to account for the corrosion damage, such as internal cracking and cross-sectional loss of steel reinforcement.     

Analytical Model for FRP Confinement of Concrete Columns with and without Internal Steel Reinforcement

The paper aims to contribute to a better understanding of the behavior of reinforced concrete columns confined with fiber-reinforced polymer (FRP) sheets. In particular, some new insights on interaction mechanisms between internal steel reinforcement and external FRP strengthening and their influence on efficiency of FRP confinement technique are given. In this context a procedure to generate the complete stress-strain response including new analytical proposals for (1) effective confinement pressure at failure; (2) peak stress; (3) ultimate stress; (4) ultimate axial strain; and (5) axial strain corresponding to peak stress for FRP confined elements with circular and rectangular cross sections, with and without internal steel reinforcement, is presented. Interaction mechanisms between internal steel reinforcement and external FRP strengthening, shown by some experimental results obtained at the University of Padova with accurate measurements, are taken into account in the analytical model. Four experimental databases regarding FRP confined concrete columns, with circular and rectangular cross section with and without steel reinforcement, are gathered for the assessment of some of the confinement models shown in literature and the new proposed model. The proposed model shows a good performance and analytical stress-strain curves approximate some available test results quite well.     

Effect of Transverse Reinforcement on the Flexural Behavior of Continuous Concrete Beams Reinforced with FRP

Continuous concrete beams are structural elements commonly used in structures that might be exposed to extreme weather conditions and the application of deicing salts, such as bridge overpasses and parking garages. In such structures, reinforcing continuous concrete beams with the noncorrodible fiber-reinforced polymer (FRP) bars is beneficial to avoid steel corrosion. However, the linear-elastic behavior of FRP materials makes the ability of continuous beams to redistribute loads and moments questionable. A total of seven full-scale continuous concrete beams were tested to failure. Six beams were reinforced with glass fiber-reinforced polymer (GFRP) longitudinal bars, whereas one was reinforced with steel as control. The specimens have rectangular cross section of 200×300??mm and are continuous over two spans of 2,800?mm each. Both steel and GFRP stirrups were used as transverse reinforcement. The material, spacing, and amount of transverse reinforcement were the primary investigated parameters in this study. In addition, the experimental results were compared with the code equations to calculate the ultimate capacity. The experimental results showed that moment redistribution in FRP-reinforced continuous concrete beams is possible and is improved by increasing the amount of transverse reinforcement. Also, beams reinforced with GFRP stirrups illustrated similar performance compared with their steel-reinforced counterparts.