Showing papers in "Journal of Composites for Construction in 2008"

Mechanisms of Shear Resistance of Concrete Beams Strengthened in Shear with Externally Bonded FRP

Abstract: In recent years, numerous investigations have addressed the shear strengthening of reinforced concrete (RC) beams with externally bonded fiber-reinforced polymer (FRP) composites. Despite this research effort, the mechanisms of shear resistance that are developed in such a strengthening system have not yet been fully documented and explained. This clearly inhibits the development of rational and reliable code specifications. This paper aims to contribute to the understanding of the shear resistance mechanisms involved in RC beams strengthened in shear with externally bonded FRP. It is based on results obtained from an experimental program, involving 17 tests, performed on full size T beams, and using a comprehensive and carefully optimized measuring device. The resistance mechanisms are studied by observing the evolution of the behavior of the strengthened beams as the applied loads are increased. The local behavior of the FRP and the transverse steel, in particular in the failure zones, are thoroughly examined. The operative resistance mechanisms are also studied through the load sharing among the concrete, the FRP, and the transverse steel, at increasing levels of applied load.

Flexural Strengthening of RC Beams with Externally Bonded CFRP Systems: Test Results and 3D Nonlinear FE Analysis

Abstract: This paper presents experimental results and a numerical analysis of the reinforced concrete RC beams strengthened in flexure with various externally bonded carbon fiber-reinforced polymer CFRP configurations. The aim of the experimental work was to investigate the parameters that may delay the intermediate crack debonding of the bottom CFRP laminate, and increase the load carrying capacity and CFRP strength utilization ratio. Ten rectangular RC specimens with a clear span of 4.2 m, categorized in two series, were tested to evaluate the effect of using the additional U-shaped CFRP systems on the intermediate crack debonding of the bottom laminate. Two different configurations of the additional systems were proposed, namely, continuous U-shaped wet layup sheets and spaced side-bonded CFRP L-shaped laminates. The fiber orientation effect of the side-bonded sheets was also investigated. A numerical analysis using an incremental nonlinear displacement-controlled 3D finite-element FE model was developed to investigate the flexural and CFRP/concrete interfacial responses of the tested beams. The finite-element model accounts for the orthotropic behavior of the CFRP laminates. An appropriate bond-slip model was adopted to characterize the behavior of the CFRP/concrete interface. Comparisons between the FE predictions and experimental results show very good agreement in terms of the load-deflection and load-strain relation- ships, ultimate capacities, and failure modes of the beams.

Design Considerations of Carbon Fiber Anchors

Abstract: Carbon fiber-reinforced polymer (CFRP) sheets can be used to strengthen existing reinforced concrete members However, debonding (separation of the CFRP sheet from the concrete surface) may occur at less than 50% of CFRP sheet’s tensile capacity, implying that half of the CFRP material is ineffective in increasing the strength of a concrete member The use of carbon fiber anchors can increase the amount of tension carried in the CFRP sheets Forty specimens were tested to develop initial design parameters of carbon fiber anchors Tests showed that by providing anchors with a total cross-sectional area at least two times greater than that of the longitudinal sheet, it was possible to fracture the CFRP sheets The best results were obtained using a greater number of smaller anchors Further, surface preparation is unimportant when the CFRP sheets were well anchored and a 1:4 transition slope can manage any offsets in surface level The general anchor design was then implemented on a series of long beams and

Fatigue Behavior of Externally Strengthened Concrete Beams with Fiber-Reinforced Polymers: State of the Art

Abstract: This paper presents the recent progress and achievement in the application of fiber-reinforced polymers (FRP) on strengthening reinforced/prestressed concrete beams subjected to fatigue loading. Although the performance of FRP-strengthened structures under monotonic loading has been intensively investigated, fatigue behavior is relatively less known to date. This paper summarizes most of the currently available literature, including the codes and design manuals, on reinforced/prestressed concrete beams externally strengthened with FRP. The review focuses specifically on the fatigue life as a function of the applied load range, bond behavior of externally bonded FRP, damage accumulation, crack propagation, size effects, residual strength, and failure modes. Research needs including considerations for design guidelines are presented.

Concrete-Filled Square and Rectangular FRP Tubes under Axial Compression

Abstract: This paper presents results of an experimental study on the behavior of square and rectangular concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs) under concentric compression. FRP tubes were designed as column confinement reinforcement and were manufactured using unidirectional carbon fiber sheets with fibers oriented in the hoop direction. The effects of the thickness and corner radius of the tube, sectional aspect ratio, and concrete strength on the axial behavior of CFFTs were investigated experimentally. Test results indicate that FRP confinement leads to substantial improvement in the ductility of both square and rectangular columns. Confinement provided by the FRP tube may also improve the axial load-carrying capacity of the square and rectangular columns if the confinement effectiveness of the FRP tube is sufficiently high. The results also indicate that the confinement effectiveness of FRP tubes is higher in square columns than in rectangular columns, and in both sections the effectiveness of confinement increases with the corner radius. Furthermore, for a given confinement level, improvement observed on the axial behavior of concrete due to confinement decreases with increasing concrete strength.

Experimental Study on Bond Behavior between Concrete and FRP Reinforcement

Abstract: 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 F...

Hybrid bonding of FRP to reinforced concrete structures

Abstract: The adhesive attachment of fiber-reinforced polymers (FRP) laminate to the external face of reinforced concrete structures is currently one of the most popular and effective methods for retrofitting and strengthening concrete structures. With this method, the additional strength of the attached reinforcement is transmitted into the concrete members through adhesion. However, the relatively weak adhesive interface fundamentally limits the efficacy of the method. Much effort has been made in the research community to improve the bond strength and develop bond models, but a satisfactory solution has yet to be found. Mechanical fastening is another more traditional technology that is used to bond one material to another. This paper introduces a new hybrid bonding technique that combines adhesive bonding and a new type of mechanical fastening. The new mechanical fastening technique does not rely on bearing to transmit the interfacial shear, but instead increases the interfacial bond by resisting the separation...

Structural Concept, Design, and Experimental Verification of a Glass Fiber-Reinforced Polymer Sandwich Roof Structure

Abstract: This paper reports on the design and construction of a lightweight glass fiber-reinforced polymer roof structure in Basel, Switzerland The sandwich construction allowed for an integration of static, building physical and architectural functions that enabled the prefabrication of the entire roof in only four lightweight elements that were easily transported to the site and rapidly installed Cutting of foam blocks with a computerized numerical control machine and adhesive bonding proved to be advantageous procedures for the fabrication of the complex roof shape, without the use of expensive molds The factor of safety of the design was adjusted during the design process through experimental verification Although some of the characteristic material properties were overestimated in the preliminary design, the design properties for the final design were higher than for the preliminary design as it was possible to reduce the resistance factors (according to Eurocode format) after testing by a factor of 20 Existing design models and test standards to determine material properties proved to be applicable © 2008 ASCE

Does the Use of FRP Reinforcement Change the One-Way Shear Behavior of Reinforced Concrete Slabs?

Abstract: For members with no transverse reinforcement, numerous models have been proposed for determining shear capacity, most often based on a statistical curve fit to experimental beam test results. The shear provisions of the Canadian code (CSA) for steel-reinforced concrete, by contrast, are based on a theoretical model, the modified compression field theory. This paper demonstrates that the CSA shear provisions for steel-reinforced members can be safely applied to members with internal fiber-reinforced polymer (FRP) bars by adjusting the term Es As in the method to Er Ar . A database of 146 shear failures of specimens reinforced with carbon, glass, or aramid FRP or steel is presented and gives an average test to predicted ratio of 1.38 with a coefficient of variation (COV) of 17.2%. The CSA code equations were optimized for the typical strain range of steel-reinforced concrete and when an equation appropriate for the wider range of strains associated with FRP is used, then a better statistical result can be a...

Evaluating and Proposing Models of Predicting IC Debonding Failure

Abstract: Debonding failure due to intermediate crack-induced (IC) fracture is one of the most dominant failure modes associated with the fiber-reinforced polymer (FRP) bonding technique. To date, extensive efforts have been paid by many researchers worldwide to study the debonding phenomenon for effective applications of FRP composites and rational design of FRP-strengthened structures. Based on these efforts and various relevant field applications, different models and code provisions have been proposed to predict IC debonding failure. Out of all the existing code provisions and models, five typical ones are investigated in the current paper. A comprehensive comparison among these code provisions and models is carried out in order to evaluate their performance and accuracy. Test results of 200 flexural specimens with IC debonding failures collected from the existing literature are used in the current comparison. The effectiveness and accuracy of each model have been evaluated based on these experimental results. Finally, based on a statistical analysis, a simple and more effective model for predicting the load-carrying capacity of FRP-strengthened flexural members due to IC debonding failure is proposed.

Seismic Rehabilitation of Reinforced Concrete Frame Interior Beam-Column Joints with FRP Composites

Abstract: An experimental research program is described regarding the use of externally applied carbon fiber-reinforced plastic (CFRP) jackets for seismic rehabilitation of reinforced concrete interior beam-column joints, which were designed for gravity loads The joints had steel reinforcement details that are known to be inadequate by current seismic codes in terms of joint shear capacity due to the absence of transverse steel hoops and bond capacity of beam bottom steel reinforcing bars at the joint Lap splicing of beam bottom steel reinforcement at the joint using externally applied longitudinal CFRP composite laminates is investigated Improvement of joint shear capacity using diagonal CFRP composite laminates is another strengthening scheme employed Concrete crack widths for the as-built specimens and the extent of CFRP delamination for the rehabilitated specimens at various drift ratios are reported The test results indicate that CFRP jackets are an effective rehabilitation measure for improving the seismic performance of existing beam-column joints with inadequate seismic details in terms of increased joint shear strength and inelastic rotation capacity In addition, CFRP laminates are effective rehabilitation measures for overcoming problems associated with beam bottom steel bars that have inadequate embedment into the beam-column joints

Flexural Behavior of Continuous GFRP Reinforced Concrete Beams

Abstract: The results of testing two simply and three continuously supported concrete beams reinforced with glass fiber-reinforced polymer (GFRP) bars are presented. The amount of GFRP reinforcement was the main parameter investigated. Over and under GFRP reinforcements were applied for the simply supported concrete beams. Three different GFRP reinforcement combinations of over and under reinforcement ratios were used for the top and bottom layers of the continuous concrete beams tested. A concrete continuous beam reinforced with steel bars was also tested for comparison purposes. The experimental results revealed that over-reinforcing the bottom layer of either the simply or continuously supported GFRP beams is a key factor in controlling the width and propagation of cracks, enhancing the load capacity, and reducing the deflection of such beams. Comparisons between experimental results and those obtained from simplified methods proposed by the ACI 440 Committee show that ACI 440.1R-06 equations can reasonably pred...

3D Finite-Element Analysis of Substandard RC Columns Strengthened by Fiber-Reinforced Polymer Sheets

Abstract: Numerical analyses are performed to predict the stress–strain behavior of square reinforced concrete columns strengthened by fiber-reinforced polymer (FRP) sheet confinement. The research focuses on the contribution of FRP sheets to the prevention of elastic buckling of longitudinal steel bars under compression, in cases of inadequate stirrup spacing. A new Drucker–Prager-type plasticity model is proposed for confined concrete and is used in constructed finite-element model. Suitable plasticity and elasticity models are used for steel reinforcing bars and fiber-reinforced polymers correspondingly. The finite-element analyses results are compared against published experimental results of columns subjected to axial compression, to validate the proposed finite-element model. Stress concentrations in concrete core and on FRP jacket are investigated considering circular or square sectioned, plain or reinforced concrete columns. Geometry of the section as well as the presence of steel bars and stirrups affect r...

Review of Design Guidelines for FRP Confinement of Reinforced Concrete Columns of Noncircular Cross Sections

Abstract: Current international design guidelines provide predictive design equations for the strengthening of reinforced concrete (RC) columns of both circular and prismatic cross sections by means of fiber-reinforced polymer (FRP) confinement and subjected to pure axial loading. Extensive studies (experimental and analytical) have been conducted on columns with circular cross sections, and limited studies have been conducted on members with noncircular cross sections. In fact, the majority of available research work has been on small-scale, plain concrete specimens. In this review paper, four design guidelines are introduced, and a comparative study is presented. This study is based on the increment of concrete compressive strength and ductility and includes the experimental results from six RC columns of different cross-sectional shapes. The observed outcomes are used to identify and remark upon the limits beyond the ones specifically stated by each of the guides and that reflect the absence of effects not consi...

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Development of Nonmetallic Anchor Systems

Abstract: This paper presents a novel anchoring technique for strengthening reinforced concrete beams with prestressed carbon fiber- reinforced polymer (CFRP) sheets. Permanent steel anchors are commonly used for the application of prestressed CFRP sheets. The steel anchors are, however, susceptible to corrosion and may not blend into the aesthetics of the original structure. As a result, it may be preferable to remove the steel anchors after transferring the required prestress to the structure with minimal losses of sustained prestress. A technique for replacing the steel anchors with nonmetallic anchors is investigated and reported herein. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The developed nonmetallic anchorages successfully transfer the sustained prestress in the CFRP sheets with insignificant prestress losses. A closed-form solution for the transfer of prestress is developed and compared to the experimental results.

Assessing the efficiency of CFRP discrete confinement systems for concrete cylinders

Abstract: Concrete columns requiring strengthening intervention always contain a certain percentage of steel hoops Applying strips of wet layup carbon fiber-reinforced polymer (CFRP) sheets inbetween the existent steel hoops might, therefore, be an appropriate confinement technique with both technical and economic advantages, when full wrapping of a concrete column is taken as a basis of comparison To assess the effectiveness of this discrete confinement strategy, circular cross-sectional concrete elements confined by distinct arrangements of strips of CFRP sheet are submitted to a direct compression load up to the failure point The influence of the width of the strip, distance between strips, number of CFRP layers per strip, CFRP stiffness, and concrete strength class on the increase of the load carrying capacity and ductility of concrete columns, is evaluated An analytical model is developed to predict the compressive stress-strain relationship of concrete columns confined by discrete and continuous CFRP arrangements The main results of the experimental program are presented and analyzed and used to assess the model performance

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

Abstract: 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 occur...

Experimental Response of Externally Retrofitted Masonry Walls Subjected to Shear Loading

Abstract: Recent earthquakes have produced extensive damage in a large number of existing masonry buildings, demonstrating the need for retrofitting masonry structures. Externally bonded carbon fiber is a retrofitting technique that has been used to increase the strength of reinforced concrete elements. Sixteen full-scale shear dominant clay brick masonry walls, six with wire-steel shear reinforcement, were retrofitted with two configurations of externally bonded carbon fiber strips and subjected to shear loading. The results of the experimental program showed that the strength of the walls could be increased 13–84%, whereas, their displacement capacity increased 51–146%. This paper presents an analysis of the experimental results and simple equations to estimate the cracking load and the maximum shear strength of clay brick masonry walls, retrofitted with carbon fiber.

Analytical Model for FRP-Confined Circular Reinforced Concrete Columns

Abstract: One disadvantage of most available stress–strain models for concrete confined with fiber-reinforced polymer (FRP) composites is that they do not take into consideration the interaction between the internal lateral steel reinforcement and the external FRP sheets. According to most structural concrete design codes, concrete columns must contain minimum amounts of longitudinal and transverse reinforcement. Therefore, concrete columns that have to be retrofitted (and therefore confined) with FRP sheets usually contain lateral steel. Hence, the retrofitted concrete column is under two actions of confinement: the action due to the FRP and that due to the steel ties. This paper presents a new designed-oriented confinement model for the axial and lateral behavior of circular concrete columns confined with steel ties, FRP composites, and both steel ties and FRP composites. Comparison with experimental results of confined concrete stress–strain curves shows good agreement between the test and predicted results.

Fire survivability of externally bonded FRP strengthening systems

Abstract: The use of externally-bonded fiber-reinforced polymers (FRPs) to strengthen reinforced concrete (RC) structures is now widely recognized. However, a concern that continues to discourage the use of FRPs in many applications is their susceptibility to high temperature and fire. Although recent studies have shown that the fire endurance of appropriately designed and insulated FRP strengthened RC members is satisfactory, the specific performance of FRP systems at, and after exposure to, high temperature remains largely unknown. The results of tests on the residual properties after high-temperature exposure of various available FRP strengthening systems for concrete are reported; these include: tension coupon tests, single-lap FRP-to-FRP bond tests, direct tension FRP-to-concrete bond tests, and pull-apart FRP-to-concrete shear bond tests after exposure to temperatures up to 400°C . The data show that the allowable exposure temperatures for residual performance of externally bonded FRP systems lie between the ...

Embedding NSM FRP Plates for Improved IC Debonding Resistance

Abstract: The use of near surface mounted (NSM) fiber reinforced polymer (FRP) strips for strengthening reinforced concrete structures shows great promise as the strains at intermediate crack (IC) debonding are generally much greater than for externally bonded FRP strips. In this research, the NSM technique is taken a step further by embedding the NSM strip, i.e., is by providing cover to the strip. It is shown that embedment can increase the IC debonding resistance by up to three times. Importantly, embedment allows: substantially larger strains at IC debonding and, hence, greater ductility; the use of larger cross sections of FRP plate; and, through providing cover to the NSM, may be the first step in enhancing the fire resistance. In this paper, 20 new pull tests are described from which mathematical expressions are developed for the effect of embedment on both the IC debonding resistance and its associated local bond stress–slip (τ–δ) relationship.

Efficient Strengthening Technique to Increase the Flexural Resistance of Existing RC Slabs

Abstract: Composite materials are being used with notable effectiveness to increase and upgrade the flexural load carrying capacity of reinforced concrete (RC) members. Near-surface mounted (NSM) is one of the most promising strengthening techniques, based on the use of carbon fiber-reinforced polymer (CFRP) laminates. According to NSM, the laminates are fixed with epoxy based adhesive into slits opened into the concrete cover on the tension face of the elements to strength. Laboratory tests have shown that the NSM technique is an adequate strengthening strategy to increase the flexural resistance of RC slabs. However, in RC slabs of low concrete strength, the increase of the flexural resistance that NSM can provide is limited by the maximum allowable compressive strain in the compressed part of the slab, in order to avoid concrete crushing. This restriction reduces the effectiveness of the strengthening, thus limiting the use of the NSM technique. A new thin layer of concrete bonded to the existing concrete at the...

Performance of FRP-Strengthened RC Beams with Different Concrete Surface Profiles

Abstract: Performance of fiber-reinforced polymer (FRP) composites in repair and retrofit of concrete structures depends to a great extent on the substrate condition to which it is bonded. The present research investigated the effect of concrete surface roughness on the bond behavior and general performance of FRP strengthening systems. The study included flexural testing of 26 specimens with two different carbon FRP systems (wet layup and precured) and three different levels of surface roughness. The influence of six different levels of anchorage by means of U-straps was also evaluated. Additionally, 10 bond specimens were tested in double shear for the wet layup FRP system to compare with the results of beam tests and to further investigate the debonding issue. Bond-dependent coefficient was analyzed with respect to different levels of surface roughness and shear span-to-depth ratio, and was then compared with other test results available in the literature. Surface roughness did not appear to have a significant i...

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Using Nonmetallic Anchor Systems

Abstract: This paper presents the flexural behavior of reinforced concrete beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) sheets using nonmetallic anchor systems. The developed nonmetallic anchor systems replace the permanent steel anchorage. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The flexural behavior of the tested beams, including detailed failure modes of each nonmetallic anchor system, is investigated. The study shows that the developed nonmetallic anchors are more effective in resisting peeling-off cracks compared to the permanent steel anchors and the beams strengthened with the nonmetallic anchors provide comparable load-carrying capacity with respect to the steel anchored control beam.

Repair of Cracked Steel Girders Connected to Concrete Slabs Using Carbon-Fiber-Reinforced Polymer Sheets

Abstract: This paper presents the results of an experimental study on the repair of artificially damaged steel – concrete composite beams repaired using adhesively bonded carbon-fiber-reinforced polymer (CFRP) sheets. Eleven, 2 m long, beams composed of W150 x 2 steel sections with 465 x 75 mm concrete slabs were tested in four-point bending. Severe damage was first introduced in ten beams by saw cutting the tension flange completely at mid span, to simulate a fatigue crack or a localized severe corrosion. Standard modulus (SM) and high modulus (HM) CFRP sheets were then used to repair nine damaged beams. The length and number of CFRP layers applied to the cracked flange on the underside, or on both sides, were varied. Results showed that the damage had reduced flexural strength and stiffness by 60 and 54%, respectively. Nevertheless, CFRP-repaired beams achieved various levels of recovery, and in some cases, exceeded the original capacities. The strength of beams repaired with sheets, ranging in length from 8 to 97% of the span, varied from 46 – 116% of the original undamaged strength, whereas the stiffness range was 86 – 126% of original stiffness. SM-CFRP failed by debonding whereas HM-CFRP was ruptured. Bonding the sheets to both sides of the flange was not very advantageous over bonding to the underside only.

Ductility and Cracking Behavior of Prestressed Concrete Beams Strengthened with Prestressed CFRP Sheets

Abstract: This paper investigates the flexure of prestressed concrete beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) sheets, focusing on ductility and cracking behavior. Structural ductility of a beam strengthened with CFRP sheets is critical, considering the abrupt and brittle failure of CFRP sheets themselves. Cracking may also affect serviceability of a strengthened beam, and may be especially important for durability. Midscale prestressed concrete beams (L=3.6 m) are constructed and a significant loss of prestress is simulated by reducing the reinforcement ratio to observe the strengthening effects. A nonlinear iterative analytical model, including tension of concrete, is developed and a nonlinear finite-element analysis is conducted to predict the flexural behavior of tested beams. The prestressed CFRP sheets result in less localized damage in the strengthened beam and the level of the prestress in the sheets significantly contributes to the ductility and cracking behavior of the st...

Performance of Mechanically Fastened FRP Strengthened Concrete Beams in Flexure

Abstract: The use of adhesively bonded fiber-reinforced polymer (FRP) materials has become widely accepted for use in flexural strengthening applications; however, the method of attachment presents drawbacks in application. These include extensive time and labor investments, as well as a tendency of the system to fail in a brittle manner. This paper presents a study of a series of reinforced concrete beams each strengthened in flexure with an FRP strip attached with large diameter concrete screws. The concrete screws were arranged in a variety of patterns. The effect of fastener number and spacing, as well as the effect of fastener pattern on the behavior of the beam, was investigated through the use of two groups of specimens. The beams in each group were tested to failure to verify the behavior of the strengthening system. Measured behavior was then used to determine an analytical approach for prediction of load response behavior of mechanically fastened systems. It was found that the strengthening method investigated improved the flexural capacity of the specimens 12 to 39% with little or no loss in ductility.

Structural performance of hybrid GFRP/steel concrete sandwich panels

Abstract: Precast/prestressed concrete sandwich panels consist of two concrete wythes separated by a rigid insulation foam layer and are generally used as walls or slabs in thermal insulation applications. Commonly used connectors between the two wythes, such as steel trusses or concrete stems, penetrate the insulation layer causing a thermal bridge effect, which reduces thermal efficiency. Glass fiber-reinforced polymer (GFRP) composite shell connectors between the two concrete wythes are used in this research as horizontal shear transfer reinforcement. The design criterion is to establish composite action, in which both wythes resist flexural loads as one unit, while maintaining insulation across the two concrete wythes of the panel. The experiments carried out in this research show that hybrid GFRP/steel reinforced sandwich panels can withstand out-of-plane loads while providing resistance to horizontal shear between the two concrete wythes. An analytical method is developed for modeling the horizontal shear tra...

Seismic Behavior of RC Columns with Bond-Critical Regions: Criteria for Bond Strengthening Using External FRP Jackets

Abstract: In 2003, an experimental research program was initiated at the American University of Beirut with the objectives of (1) evaluating the effectiveness of external fiber-reinforced polymer (FRP) confinement in improving the bond strength of spliced reinforcement in reinforced-concrete (RC) columns and its implications on the lateral load capacity and ductility of the columns under seismic loading; and (2) establishing rational design criteria for bond strengthening of spliced reinforcement using external FRP jackets. This paper presents a discussion of recent experimental results dealing with rectangular columns and the results of a pilot study conducted on circular columns with particular emphasis on aspects related to the bond strength of the spliced column reinforcement. A nonlinear analysis model is developed for predicting the envelope load–drift response, taking into account the effect of FRP confinement on the stress–strain behavior of concrete in compression. Results predicted by the model showed exc...

Experimental investigation on seismic retrofitting of square RC columns by carbon FRP sheet confinement combined with transverse short glass FRP bars in bored holes

Abstract: Jacketing is less effective to large square/rectangular RC columns due to the inability of the rectangular-shaped jacket in restraining the dilation of concrete in the middle of a straight side. A new retrofit method is proposed in this work by fiber reinforcing the surface concrete in the middle of a straight side. Fiber reinforcing is achieved by inserting small fiber-reinforced polymer (FRP) bars into the concrete in the plastic hinge zone. The inserted FRP bars act as horizontal reinforcement to increase the ductility of the concrete in a similar way as that in normal fiber-reinforced concrete. When this fiber reinforcing technique is combined with the conventional jacketing, the concrete in all parts of a cross section may be effectively confined. In this work, experimental tests were undertaken to investigate the effectiveness of this new retrofit technique. Six half-scaled columns were tested and the test results demonstrated the effectiveness of the method.