Showing papers in "Journal of Composites for Construction in 2011"

Behavior and Modeling of Concrete Confined with FRP Composites of Large Deformability

Abstract: This paper presents the results of an experimental study on the behavior of concrete confined by fiber reinforced polymer (FRP) jackets with a large rupture strain (LRS). The FRP composites considered herein are formed by embedding polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) fibers in a suitable epoxy resin matrix. The PEN and PET fibers are usually made from recycled materials (e.g., PET bottles) and have a strain capacity greater than 5%. They are ideal for use in seismic retrofit applications where increases in ductility and energy absorption capacity are of prime concern. The present study has two specific objectives: (1) to develop a good understanding of the compressive stress-strain behavior of concrete confined with LRS FRP; and (2) to examine whether existing confinement models developed for conventional FRPs are applicable to LRS FRPs. As the existing models have been developed and verified mainly based on test data for CFRP and GFRP, which have a jacket hoop rupture stra...

Flexural Strengthening of RC Beams with Cement-Based Composites

Abstract: In this paper, the effectiveness of fiber-reinforced cementitious matrix (FRCM) materials for the strengthening of reinforced concrete (RC) beams is experimentally investigated. Bending tests on RC beams strengthened with different FRCM materials, made out of (1) carbon fiber nets; and (2) poliparafenilenbenzobisoxazole (PBO) fiber nets embedded in cement-based matrix, are performed. For case (2), different net shapes, cementitious matrices, and a number of net layers were considered. Depending on the type of fibers and matrix, different flexural debonding failure modes are identified. The fiber strain at debonding is evaluated by comparing the experimental results with those obtained with two different theoretical models. The results obtained in this study confirm the effectiveness of FRCM materials for the strengthening of RC structures and encourage further experimental and theoretical work on the topic. A better understanding of the debonding phenomenon is crucial for an optimal design of the strength...

Bond Efficiency of EBR and NSM FRP Systems for Strengthening Concrete Members

Abstract: This paper reports the results of an experimental program to investigate the bonding behavior of two different types of fiber-reinforced polymer (FRP) systems for strengthening RC members: externally bonded carbon (EBR) plates and bars or strips externally applied with the near-surface-mounted (NSM) technique. The overall experimental program consisted of 18 bond tests on concrete specimens strengthened with EBR carbon plates and 24 bond tests on concrete specimens strengthened with NSM systems (carbon, basalt, and glass bars, and carbon strips). Single shear tests (SST) were carried out on concrete prisms with low compressive strengths to investigate the bonding behavior of existing RC structures strengthened with different types of FRP systems. The performance of each reinforcement system is presented, discussed, and compared in terms of failure mode, debonding load, load-slip relationship, and strain distribution. The findings indicate that the NSM technique could represent a sound alternative to EBR systems because it allows debonding to be delayed, and hence FRP tensile strength to be better exploited.

FRP-confined circular concrete-filled thin steel tubes under axial compression

Abstract: Concrete-filled steel tubes are widely used as columns in many structural systems, and a common failure mode of such tubular columns is inelastic outward local buckling near a column end. The use of fiber reinforced polymer (FRP) jackets/wraps for the suppression of such local buckling has recently been proposed and has been proven by limited test results to possess great potential in both retrofit/strengthening and new construction. Against this background, this paper presents the results of an experimental study into the behavior of FRP-confined circular concrete-filled steel tubes (CCFTs) under axial compression, as part of a larger study undertaken at the Hong Kong Polytechnic University aimed at the development of a good understanding of the structural behavior of and reliable theoretical models for CCFTs. The experimental program included three series of tests in which the main parameters examined were the thickness of the steel tube and the stiffness of the FRP wrap. The test results showed that th...

Evaluation of Flexural Behavior and Serviceability Performance of Concrete Beams Reinforced with FRP Bars

Abstract: Flexural behavior and serviceability performance of 24 full-scale concrete beams reinforced with carbon-, glass-, and aramid-fiber-reinforced-polymer (FRP) bars are investigated. The beams were 3,300 mm long with a rectangular cross section of 200 mm in width and 300 mm in depth. Sixteen beams were reinforced with carbon-FRP bars, four beams were reinforced with glass-FRP bars, two beams were reinforced with aramid-FRP bars, and two were reinforced with steel, serving as control specimens. Two types of FRP bars with different surface textures were considered: sand-coated bars and ribbed-deformed bars. The beams were tested to failure in four-point bending over a clear span of 2,750 mm. The test results are reported in terms of deflection, crack-width, strains in concrete and reinforcement, flexural capacity, and mode of failure. The experimental results were compared to the available design codes.

Structural Evaluation of Full-Scale FRP-Confined Reinforced Concrete Columns

Abstract: The external confinement of RC columns by means of externally bonded fiber-reinforced polymer (FRP) laminates is a well established technique for strengthening and retrofitting purposes. This paper presents a pilot research that includes laboratory testing of full-scale square and rectangular RC columns externally confined with glass and basalt-glass FRP laminates and subjected to pure axial load. Specimens that are representative of full-scale building columns were designed according to a dated American Concrete Institute (ACI) 318 code (i.e., prior to 1970) for gravity loads only. The study was conducted to investigate how the external confinement affects peak axial strength and deformation of a prismatic RC column. The results showed that the FRP confinement increases concrete axial strength, but it is more effective in enhancing concrete strain capacity. The discussion of the results includes a comparison with the values obtained using existing constitutive models.

Finite-element modeling of intermediate crack debonding in FRP-plated RC beams

Abstract: Intermediate crack-induced debonding (IC debonding) is a common failure mode of RC beams strengthened with externally bonded fiber-reinforced polymer (FRP) reinforcement. Although extensive research has been carried out on IC debonding, much work is still needed to develop a better understanding of the failure mode and a more reliable strength model. This paper presents an advanced finite-element (FE) model on the basis of the smeared-crack approach for predicting IC debonding failure. Existing FE models of the same type are generally deficient in capturing localized cracks (both their pattern and widths). This deficiency is overcome in the proposed FE model through the accurate modeling of interfaces between the concrete and both the internal steel and the external FRP reinforcements. The capability and accuracy of the proposed model are demonstrated through comparisons of its predictions with selected test results. The importance of accurate modeling of localized cracking is also explained using numeric...

Textile-Reinforced Mortar versus FRP as Strengthening Material for Seismically Deficient RC Beam-Column Joints

Abstract: In this paper, efficiency and effectiveness of textile-reinforced mortars (TRM) on upgrading the shear strength and ductility of a seismically deficient exterior beam-column joint has been studied. The results are then compared with that of carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP)-strengthened joint specimens. Five as-built joint specimens were constructed with nonoptimal design parameters (inadequate joint shear strength with no transverse reinforcement) representing an extreme case of preseismic code design construction practice of joints and encompassing the vast majority of existing beam-column connections. Out of these five as-built specimens, two specimens were used as baseline specimens (control specimens) and the other three were strengthened with TRM, CFRP, and GFRP sheets, respectively. All five subassemblages were subjected to quasi-static cyclic lateral load histories to provide the equivalent of severe earthquake damage. The response histories of control and strengthened specimens were then compared. The test results demonstrated that TRM can effectively improve both the shear strength and deformation capacity of seismically deficient beam-column joints to an extent that is comparable to the strength and ductility achieved by well-established CFRP, and GFRP-strengthening of joints.

Embedded Through-Section FRP Rod Method for Shear Strengthening of RC Beams: Performance and Comparison with Existing Techniques

Abstract: Embedded through-section (ETS) technique is a recently developed method to increase the shear capacity of reinforced concrete (RC) using fiber-reinforced polymer (FRP) rods. The ETS method presents many advantages over existing methods, such as externally bonded FRP sheets (EB FRP) and near-surface mounted FRP rods (NSM FRP). Unlike EB and NSM methods where the FRP relies on the concrete cover of RC beams, in the ETS method, the FRP relies on the concrete core of the RC beam, which offers a greater confine- ment and hence improves bonding performance. Additionally, the ETS method requires less concrete preparation compared with EB and NSM methods. The objective of this paper is to present results of an experimental investigation that studies the effectiveness of the ETS method and compares the performance of the ETS method with both EB and NSM methods. In total, 12 tests are performed on 4,520-mm- long T-beams. The parameters investigated are as follows: (1) the effectiveness of the ETS method, compared with EB FRP sheet and NSM FRP rod methods; (2) the presence of the internal steel; and (3) the internal transverse steel reinforcement ratio (i.e., spacing). The test results confirm the feasibility of the ETS method and reveal that the performance of the beams strengthened in shear using this method is signifi- cantly superior compared with that of the beams strengthened with EB and NSM methods. DOI: 10.1061/(ASCE)CC.1943-5614.0000174. © 2011 American Society of Civil Engineers. CE Database subject headings: Bonding; Concrete beams; Fiber reinforced polymer; Rehabilitation; Shear resistance; Epoxy. Author keywords: Bonding strength; Concrete beams; Fiber-reinforced polymers; Retrofitting; Shear resistance; Epoxy bonded; Near- surface mounted; Embedded through-section.

Experimental Bond Behavior of FRP Sheets Glued on Brick Masonry

Abstract: This paper deals with the experimental characterization of the mechanical tensile and shear bond behavior of fiber-reinforced polymer (FRP) sheets externally glued on masonry prisms, in terms of load capacity and stress distribution along the bonded length. The brick masonry adopted tries to replicate ancient brick masonry, by using handmade low-strength solids bricks and low-strength lime-based mortar. Key parameters relative to the FRP-masonry interface response, particularly bonded length, FRP materials, anchor scheme adopted, and shape of masonry substrate, were studied. Finally, an analytical bond stress-slip formulation was developed, allowing deducing local bond stress-slip curves directly from the experiments.

Bond Performance of Near-Surface-Mounted FRP Bars

Abstract: Near-surface-mounted (NSM) reinforcement has become a well-known method for strengthening existing concrete structures. The bond between the NSM reinforcing bars and concrete is the key factor in the NSM technique. In the NSM technique, there are two bond interfaces: one between the NSM bar and the adhesive, and the other between the adhesive and the concrete. For this technique to perform efficiently, these two interfaces need to be investigated. On the other hand, concrete structures that require rehabilitation are often exposed to aggressive environments. Many of these environments are related to cold-climate conditions as can be found in Canada. Environmental factors including freeze/thaw action, exposure to deicing salts, and sustained low temperatures combine to attack the integrity of repaired structures. Consequently, repair materials for the Canadian infrastructure must be able to withstand these harsh conditions for prolonged periods of time. A total of 80 NSM-fiber-reinforced polymer (FRP) bars installed in C-shaped concrete specimens were tested in pull-out setup to failure. Sixty specimens were tested at normal room temperature, while the remaining 20 specimens were tested after conditioning in an environmentally controlled chamber for 200 freeze/thaw cycles. The dimensions of the specimens were designed, upon a preliminary phase of testing, to ensure that no transverse cracking would occur in the specimen before bond failure of the NSM bar. The results are presented in term of failure load, average bond stress, strains in FRP bar, end slip, and mode of failure. A bond-slip model was proposed for the used FRP bars.

Prestressed CFRP for Strengthening of Reinforced Concrete Structures: Recent Developments at Empa, Switzerland

Abstract: In civil engineering today, only 20 to 30% of the strength of carbon-fiber-reinforced polymer (CFRP) strips is used when they are applied as externally bonded strips for flexural and shear strengthening or in confinement of reinforced concrete (RC) structural elements. The strips are better used when the CFRP material is prestressed. This offers several advantages, including reduced crack widths, reduced deflections, reduced stress in the internal steel, and possibly increased fatigue resistance. In this paper, recent developments in the field of RC strengthening using prestressed CFRP are presented. The paper focuses on developments in flexural and shear strengthening and column confinement made at the Swiss Federal Laboratory for Materials Testing and Research (Empa). Several innovative ideas have been successfully realized in the laboratory. For example, a gradient prestressing technique without end anchorage plates was developed and successfully applied to a 17 m RC bridge girder. A confinement techni...

Diagonal Compression Testing of FRP-Retrofitted Unreinforced Clay Brick Masonry Wallettes

Abstract: To address concerns regarding the seismic vulnerability of New Zealand unreinforced masonry (URM) buildings, a research program was undertaken to investigate the effectiveness of fiber-reinforced polymer (FRP) systems as a seismic retrofit intervention for in-plane loaded URM walls that are prone to fail in a shear mode during earthquakes. Seventeen URM wallettes were retrofitted with either externally bonded (EB) glass FRP fabrics, EB pultruded carbon FRP (CFRP) plates, or near-surface mounted pultruded CFRP rectangular bars. The wallettes were tested by the application of a diagonal compressive force, and data were recorded for applied force and corresponding wall drift. Results were compared with five nominally identical unretrofitted wallettes. It was determined that the FRP systems substantially increased the shear strength of the wallettes. Significant increases in the pseudoductility and the toughness were also obtained, which are all considered to be important goals of any seismic retrofit interve...

Shear Strengthening of RC Beams with EB FRP: Influencing Factors and Conceptual Debonding Model

Abstract: This paper deals with the shear strengthening of RC beams using externally bonded (EB) fiber-reinforced polymers (FRP). Current code provisions and design guidelines related to shear strengthening of RC beams with FRP are discussed in this paper. The findings of research studies, including recent work, have been collected and analyzed. The parameters that have the greatest influence on the shear behavior of RC members strengthened with EB FRP and the role of these parameters in current design codes are reviewed. This study reveals that the effect of transverse steel on the shear contribution of FRP is important and yet is not considered by any existing codes or guidelines. Therefore, a new design method is proposed to consider the effect of transverse steel in addition to other influencing factors on the shear contribution of FRP ( Vf ) . Separate design equations are proposed for U-wrap and side-bonded FRP configurations. The accuracy of the proposed equations has been verified by predicting the shear st...

Effects of Ratio of CFRP Plate Length to Shear Span and End Anchorage on Flexural Behavior of SCC RC Beams

Abstract: The aim of this experimental investigation is to study the effect of the ratio of carbon fiber-reinforced polymer (CFRP) precured laminate length to shear span and different end plate anchorage systems on the flexural behavior of reinforced concrete (RC) beams cast with self-consolidating concrete (SCC). SCC with a grade of 54 MPa has been used throughout this research to ensure consistent high quality and high concrete compressive strength in all beams and to eliminate the need of any compaction. Ten RC beams strengthened with CFRP plate lengths to shear span ratio of 0, 25, 70, and 85% with and without end anchorages, were tested under monotonic loading. In particular, a single layer of U-wrap sheet and two layers of U-wrap sheets with one layer in the longitudinal direction and the other in the transverse direction were used as end anchorages (double wrap). The results were compared with each other and with those of the same test conducted on an unstrengthened control beam specimen. The load-deflection...

Size Effect of Concrete Short Columns Confined with Aramid FRP Jackets

Abstract: Most previous studies on concrete short columns confined with fiber-reinforced polymer (FRP) composites were based on small-scale testing, and size effect of the columns still has not been studied thoroughly. In this study, 99 confined concrete short columns wrapped with aramid FRP (AFRP) jackets and 36 unconfined concrete short columns with circular and square cross sections were tested under axial compressive loading. The circular specimens were divided into six groups, and the square specimens were divided into five groups, with each group containing different levels of the AFRP’s confinement. In each group, the specimens were geometrically similar to one another and had three different scaling dimensions. Statistical analyses were used to evaluate the size and interaction effects between the specimen size and the AFRP’s confinement, and a size-dependent model for predicting the strength of the columns was developed by modifying Bazant’s size-effect law. The experimental results showed that the size of...

Durability of GFRP Rebars in Simulated Concrete Solutions under Accelerated Aging Conditions

Abstract: This paper discusses the potential ingress and effects of simulated concrete pore solution species on glass fiber-reinforced polymer (GFRP) rebars. Knowing that diffusion of moisture into the fiber-matrix interphase in a composite could cause fiber-matrix debonding and that the presence of alkalis at the location of the glass surface would lead to fiber degradation, particular attention was devoted to investigating whether GFRP rebars allow both species to penetrate or allow only water, while blocking the alkalis. To investigate this scenario, GFRP rebars were immersed in five types of simulated concrete pore solutions at elevated temperatures. Penetration of alkalis was assessed using X-ray mapping of backscattered electron images (BEI) and crosschecked by line and point energy dispersive spectroscopy (EDS) techniques. The effects of diffusion at the fiber-matrix interphase were observed by investigating the degradation of constituents at the interphase. Degradation of polymer matrix was assessed by a Fourier transform infrared (FTIR) spectroscopy. Degradation of fiber was investigated by observing highly magnified scanning electron microscopy (SEM) images and point EDS analyses close to the interphase. Debonding of fibers from the matrix was investigated using a SEM technique. Fiber-matrix debonding took place in a few samples, despite the fact that the glass fibers and polymer matrix remained essentially intact and that no penetration of alkalis into the GFRP rebars was observed. This debonding, which occurred only for samples exposed to 75°C, is believed to be caused by hydrolysis of fiber sizing at high temperatures. Finally, this study shows that the vinyl ester (VE) polymer matrix used acts as an effective semipermeable membrane by allowing the penetration of water, while blocking alkali ions.

In-Plane Lateral Response of a Full-Scale Masonry Subassemblage with and without an Inorganic Matrix-Grid Strengthening System

Abstract: A full-scale unreinforced masonry (URM) wall with an opening was tested under in-plane lateral loading. The wall was first subjected to monotonically increasing displacements until a moderate damage level was reached. The damaged specimen was then cyclically tested up to almost the same maximum drift attained during the monotonic test to investigate the effects of previous damage on its nonlinear response. Finally, the masonry wall was repaired with inorganic matrix-grid (IMG) composites and subjected to a cyclic displacement-controlled test up to a near-collapse state. Most of the observed damage developed in the spandrel panel affecting both lateral resistance and strength degradation. Rocking of piers governed lateral stiffness and hysteretic response, which was characterized by low residual displacements and recentering behavior. The comparison between the experimental force-displacement curves demonstrated that the IMG strengthening system was able to provide energy dissipation capacity to the spandr...

Design of Concrete-Filled FRP Tubular Columns: Provisions in the Chinese Technical Code for Infrastructure Application of FRP Composites

Abstract: In recent years, fiber-reinforced polymer (FRP) composites have found wide applications in new construction. A popular type of hybrid member incorporating FRP is the concrete-filled FRP tube (CFFT), consisting of an FRP tube filled with plain or steel-reinforced concrete. CFFTs have several advantages over traditional column forms, including their excellent corrosion resistance and ductility. Much research has been conducted on CFFTs over recent years, but no systematic procedure for designing such members has been developed, which has been one of the factors preventing CFFTs from becoming more widely accepted in practice. This paper presents a summary of the design provisions for CFFTs given in the Chinese Technical Code for Infrastructure Application of FRP Composites and explains the rationale behind these provisions. These provisions have been developed on the basis of available research and work undertaken at The Hong Kong Polytechnic University.

Shear Strengthening of RC Beams with Externally Bonded FRP Composites: Effect of Strip-Width-to-Strip-Spacing Ratio

Abstract: The results of an experimental and analytical investigation of shear strengthening of reinforced concrete (RC) beams with externally bonded (EB) fiber-reinforced polymer (FRP) strips and sheets are presented, with emphasis on the effect of the strip-width-to-strip-spacing ratio on the contribution of FRP (Vf). In all, 14 tests were performed on 4,520-mm-long T-beams. RC beams strengthened in shear using carbon FRP (CFRP) strips with different width-to-spacing ratios were considered, and their performance was investigated. In addition, these results are compared with those obtained for RC beams strengthened with various numbers of layers of continuous CFRP sheet. Moreover, various existing equations that express the effect of FRP strip width and concrete-member width and that have been proposed based on single or double FRP-to-concrete direct pullout tests are checked for RC beams strengthened in shear with CFRP strips. The objectives of this study are to investigate the following: (1) the effectiveness of...

Fatigue Flexural Behavior of Corroded Reinforced Concrete Beams Repaired with CFRP Sheets

Abstract: This study investigated the flexural behavior of corroded steel reinforced concrete beams repaired with carbon-fiber-reinforced polymer (CFRP) sheets under repeated loading. Thirty beams ( 152×254×2,000 mm ) were constructed and tested. Fatigue flexural failure occurred in 29 of these beams. The study showed that pitting of the steel reinforcement due to corrosion occurred only after about a 7% actual mass loss which coincided with a decrease in the fatigue performance of the beam. The controlling factor for the fatigue strength of the beams is the fatigue strength of the steel bars. Repairing with CFRP sheets increased the fatigue capacity of the beams with corroded steel reinforcement beyond that of the control unrepaired beams with uncorroded steel reinforcement. Beams repaired with CFRP at a medium corrosion level and then further corroded to a high corrosion level before testing had a comparable fatigue performance to those that were repaired and tested after corroding directly to a high corrosion level.

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

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

RC Slabs Strengthened with Prestressed and Gradually Anchored CFRP Strips under Monotonic and Cyclic Loading

Abstract: This paper presents the test results of reinforced concrete slabs strengthened with prestressed and gradually anchored carbon fiber–reinforced polymer (CFRP) strips under monotonic and cyclic loading. To take full advantage of the externally bonded CFRP technique, it is beneficial to apply the laminates in a prestressed state, which relieves the stress in the steel reinforcement and reduces crack widths and deflection. The aim of the monotonic tests was to determine the strengthening efficiency of the new prestressing technique and to investigate serviceability and ultimate states. The cyclic tests were performed to identify the fatigue behavior of the strengthened slabs and to investigate the influence of long-term cyclic loading and elevated temperature on the bond properties of the prestressed CFRP laminates and the ductility and flexural strength of the strengthened slabs. A nonlinear analytical model of reinforced concrete members strengthened with passive and prestressed CFRP strips under static loading is proposed in the paper. A comparison of the experimental and predicted results reveals an excellent agreement in the full range of loading.

Equivalent Moment of Inertia Based on Integration of Curvature

Abstract: This paper evaluates the benefits of computing deflection with an equivalent moment of inertia based on integration of curvature to account for changes in member stiffness along the span. Results are evaluated for steel and fiber-reinforced polymer reinforced (FRP-reinforced) concrete flexural members with different loading arrangements and support conditions. Closed-form solutions of integrated expressions for deflection are expressed in terms of an equivalent moment of inertia Ie′ and compared to deflection computed with an effective moment of inertia Ie based on the stiffness at the critical section. Results from this comparison are validated with measured deflections from an experimental database for FRP-reinforced concrete. Current code-related approaches are also compared to the experimental database. It is shown herein that the use of an integration-based expression for the moment of inertia can lead to improved prediction of deflection, though the use of an effective moment of inertia based on mem...

Investigation of RC Beams Strengthened with Prestressed NSM CFRP Laminates

Abstract: In this research, concrete beams were strengthened with carbon-fiber-reinforced polymer (CFRP) laminate strips that were placed in precut grooves on the concrete surface. Five simple-span reinforced concrete (RC) beams were tested. Among these specimens, one served as a reference beam and was not strengthened, one was tested after being strengthened with nonprestressed near-surface-mounted (NSM) CFRP laminates, and the remaining three were strengthened with NSM CFRP laminates that had been prestressed to 5, 20, and 30% of the nominal ultimate strain of the strips. The test results indicate that the prestressed beams more effectively improved the cracking and yielding loads than the nonprestressed strengthened specimen. In addition to a higher cracking load, the prestressed strengthened specimens had cracks with a more limited distribution and width. Furthermore, the prestressed strengthened beams had up to a 15% higher ultimate load-carrying capacity than the nonprestressed specimen, and their failures corresponded to lower deflections.

Bond Strength of Lap-Spliced Bars in Concrete Confined with Composite Jackets

Abstract: The effectiveness of fiber-reinforced polymer (FRP) and textile-reinforced mortar (TRM) jackets was investigated experimentally and analytically in this study to confine old-type reinforced concrete (RC) columns with limited capacity because of bond failure at lap-splice regions. The local bond strength between lap-spliced bars and concrete was measured experimentally along the lap-splice region of six full-scale RC columns subjected to cyclic uniaxial flexure under constant axial load. The bond strength of the two column specimens tested without retrofitting was found to be in good agreement with the predictions given by two existing bond models. These models were modified to account for the contribution of composite material jacketing to the bond resistance between lap-spliced bars and concrete. The effectiveness of FRP and TRM jackets against splitting at lap splices was quantified as a function of jacket properties and geometry as well as in terms of the jacket effective strain, which was found to depend on the ratio of lap-splice length to bar diameter. Consequently, simple equations for calculating the bond strength of lap splices in members confined with composite materials (FRP or TRM) are proposed.

Behavior of FRP-Retrofitted Joints Built with Plain Bars and Low-Strength Concrete

Abstract: Two series of tests on eight full-scale exterior beam-column joint subassemblages built with plain bars and low-strength concrete were conducted. No transverse reinforcement was present in the joint cores. In the first series of tests, which included three specimens, the behavior of joints before fiber-reinforced polymer (FRP) retrofitting was investigated. In the second series, which included five specimens, the behavior of the FRP-retrofitted joints was investigated. The six specimens consisted of a column, an in-plane beam, a transverse beam, and a slab part, and two specimens were plane members without transverse beams and slabs. The utilized retrofitting scheme is easily applicable for actual exterior beam-column joints, even in the presence of a transverse beam and a slab. Two types of strength limitation were observed for specimens in the first series. The strength of the specimen with beam longitudinal bars sufficiently anchored to the joint core was limited by the shear strength of the joint. The strengths of the other two specimens were limited by the slip of the beams’ longitudinal bars at their anchorages. In the second series of tests, significantly better performance was obtained both in terms of shear strength and ductility, provided that the slip of the beam bars was prevented. Furthermore, by using a simple theoretical algorithm based on truss analogy, the strength and deformability characteristics of the tested reference and FRP-retrofitted joints are predicted with reasonable accuracy. The same algorithm is used for predicting the joint shear strength of specimens tested by other researchers, and satisfactory agreement is obtained between the predictions and test results.

Nonlinear Behavior of a Masonry Subassemblage Before and After Strengthening with Inorganic Matrix-Grid Composites

Abstract: Past experimental tests on a full-scale masonry wall with an opening evidenced the key role of the spandrel panel in the in-plane nonlinear response of the system. Recent seismic codes do not provide specific criteria to assess and to strengthen existing masonry spandrel panels with inorganic matrix-grid (IMG) composites. Numerical finite-element (FE) analyses are used to deepen the knowledge about the nonlinear response of masonry walls and the role of the IMG strengthening system. The comparison of experimental and numerical results contributes to the development of a simplified analytical model to assess the influence of the external reinforcement system on the in-plane seismic response of masonry wall systems. Some hints about the strengthening design that could change the failure mode from brittle shear to ductile flexure are given. Finally, a further enhancement of the IMG strengthening system is proposed to avoid the undesirable splitting phenomena attributable to compression forces and to exploit ...

Small Beam Bond Test Method for CFRP Composites Applied to Concrete

Abstract: This paper presents the development of a test method that can be used to test the bond capacity of carbon fiber-reinforced polymer (CFRP) composites bonded to concrete. The rationale for the selection of the test method is described along with the results of the experimental work used to refine the test configuration and procedures. The research objectives were to develop a test method that (1) can be used to evaluate the durability of the FRP-concrete bond (adhesion failure mode); (2) facilitate multiple replicate for statistical validation; (3) is simple to conduct; and (4) provides comparative results that are easy to interpret. The method utilizes a small concrete beam modeled after the modulus of rupture test, which is typically used to measure concrete tensile strength. A number of small beam sizes and loading configurations were considered during the investigation. The final recommended specimen configuration is 4×4×14 in. ( 100 mm×100 mm×356 mm ) beam with a half-depth saw cut at midspan. A 1 in. ...

Bar Buckling in RC Columns Confined with Composite Materials

Abstract: The onset and evolution of bar buckling at the plastic hinge of old-type reinforced concrete (RC) columns confined with composite material [fiber-reinforced polymer (FRP) and textile-reinforced mortar (TRM)] jackets was investigated experimentally and analytically in this study. The interaction between composite jacket (or concrete cover, for unconfined concrete) and embedded longitudinal compression reinforcement at the onset and evolution of bar buckling was achieved through strain measurements of the compression reinforcement. Moreover, the implementation of a recent stress-strain confinement model, which relates lateral with axial strains, allowed the description and monitoring of the axial-lateral strain relationship at the base of the columns throughout the evolution of bar buckling. Based on the aforementioned model and the experimental measurements, the postbuckling behavior of columns was related to the jacket stiffness. Finally, a semiempirical expression that gives the drift ratio of RC columns at the onset of bar buckling was modified herein for the case of FRP- or TRM-jacketed columns.