Transverse Thermal Expansion of FRP Bars Embedded in Concrete

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

Flexural Behavior of Hybrid FRP-Concrete-Steel Double-Skin Tubular Members

This paper presents the results of an experimental study on the flexural behavior of a new type of hybrid FRP-concrete-steel member as well as results from a corresponding theoretical model based on the plane section assumption and the fiber element approach. This new type of hybrid member is in the form of a double-skin tube, composed of a steel inner tube and an FRP outer tube with a concrete infill between the two tubes, and may be employed as columns or beams. The parameters examined in this study include the section configuration, the concrete strength, and the thicknesses of the steel tube and the FRP tube, respectively. The results presented in this paper show that these hybrid beams have a very ductile response because the compressive concrete is confined by the FRP tube and the steel tube provides ductile longitudinal reinforcement. The beams' flexural response, including the flexural stiffness, ultimate load, and cracking, can be substantially improved by shifting the inner steel tube toward the tension zone or by providing FRP bars as additional longitudinal reinforcement. The predictions from the theoretical model are in reasonably close agreement with the test results. Differences between the test and predicted results arise from factors not considered in the theoretical model, including the existence of a strain gradient in the confined concrete, concentrations of cracks and the slips between the concrete and the two tubes; these are issues to be accounted for in the development of a more accurate model in the future.     

Creep Deformation of Fiber Reinforced Plastics-Plated Reinforced Concrete Tensile Members

The problem of long-term creep deformation of reinforced concrete tensile elements strengthened by external fiber reinforced plastic (FRP) plates is studied. Formation of discrete cracks in concrete under tension is taken into account. A kinematic model is used, where relative slips between concrete, steel bars, and FRP plates are considered, governed by viscous interface shear stress–slip laws. Bazant’ solidification theory and exponential algorithm are used to obtain incremental constitutive equations for concrete as well as for steel-concrete and FRP-concrete interface laws. Moreover, cohesive normal stresses across transverse cracks in concrete are considered. The incremental differential system of equations is transformed into a nonlinear algebraic system by a finite difference discretization with respect to axial coordinate. Several numerical examples are presented, concerning both short-term and long-term loadings. It is shown that reinforcing by means of FRP plates or sheets has significant beneficial effects on the behavior of reinforced concrete elements under service loadings because (1) it increases concrete tension stiffening effect and (2) it strongly reduces crack width. The present study shows that these beneficial effects are preserved also in the case of long-term loadings.     

Numerical Cracking and Debonding Analysis of RC Beams Reinforced with FRP Sheet

Bonding a fiber reinforced polymer (FRP) sheet to the tension-side surface of reinforced concrete (RC) structures is often performed to upgrade the flexural capacity and stiffness. Except for upper concrete crushing, FRP sheet reinforcing RC structure may fail in sheet rupture, sheet peeloff failure due to opening of a critical diagonal crack, or concrete cover delamination failure from the sheet end. Accompanying the occurrence of these failure modes, reinforcing effects of the FRP sheet will be lost and load-carrying capacity of the RC structures will be decreased suddenly. This study is devoted to developing a numerical analysis method by using a three-dimensional elasto-plastic finite element method to simulate the load-carrying capacity of RC beams failed in the FRP sheet peeloff mode. Here, the discrete crack approach was employed to consider geometrical discontinuities such as opening of cracks, slipping of rebar, and debonding of the FRP sheet. Comparisons between analytical and experimental results confirm that the proposed numerical analysis method is appropriate for estimating the load-carrying capacity and failure behavior of RC beams flexurally reinforced with a FRP sheet.     

Punching Shear Behavior of Flat Slabs Strengthened with Fiber Reinforced Polymer Laminates

The present work reports the test results of seven full-scale reinforced concrete slab-column edge connections strengthened against punching shear using different methods. In this study, three slabs contained openings in the vicinity of the column, and the other four were without openings. The dimensions of the slabs were 1,540×1,020×120 mm with square columns (250×250 mm). The openings in the specimens were square (150×150 mm) with the sides parallel to the sides of the column. The slabs were reinforced with an average reinforcement ratio of 0.75%. Except for the two reference slabs, two different strengthening techniques were considered. Technique I applies externally bonded fiber reinforced polymer (FRP) flexible sheets on the slab around the column in two schemes with one or two layers of FRP sheets glued to the tension face or both tension and compression faces of the slab. Both glass and carbon FRP sheets were considered. Technique II applies externally bonded FRP sheets using either the first or second scheme combined with installing steel bolts through holes across the slab thickness around the column. Based on the test results, it is concluded that the presence of FRP sheets and steel bolts substantially increased the punching capacity of the connections. Code design expressions were conservative in predicting the experimental results.     

Grooving as Alternative Method of Surface Preparation to Postpone Debonding of FRP Laminates in Concrete Beams

Structural repair and strengthening have long been dynamic and challenging activities in construction work. One of the most commonly used methods for such repairs is the application of fiber-reinforced polymer (FRP) sheets to strengthen RC or even steel structure members. A major issue of concern in flexural strengthening of RC beams with FRP laminates is the debonding of the concrete substrate, which leads to premature failure of the structural member thus strengthened. One reason for this premature rupture may be the lack of proper preparation of the concrete surface in contact with the FRP sheet. Surface preparation is typically associated with such constraints as adverse environmental impacts, economic losses due to stoppage of activities, repair costs, or even inaccessibility of the member(s) to be strengthened. This study aims to investigate surface preparation for application of FRP sheets in an attempt to develop substitute methods for conventional surface preparation methods. The experimental specimens used for the purposes of this study included a minimum of 100 prism specimens of dimensions 100×100×500?mm subjected to four-point flexural loading. The specimens were divided into the two control and experimental groups. The control group lacked FRP sheets, while the experimental one had FRP sheets tested for their ultimate failure strength as a result of both surface preparation and transverse, longitudinal, and diagonal grooves as substitutes for surface preparation. The results indicated that surface preparation prior to bonding of FRP sheets increased ultimate rupture strength. It was also found that the substitute preparation methods greatly compensated for the lack of conventional surface preparation such that they changed, in some cases, the ultimate failure behavior of the member.     

Design Philosophy Issues of Fiber Reinfored Polymer Reinforced Concrete Structures

The conventional design philosophy for reinforced concrete (RC) relies heavily on the ductile properties of steel. These ductile properties are used as a “fuse” and conceal the large uncertainty in the determination of modes of failure caused directly by concrete. Current design guidelines for fiber reinforced polymer (FRP) RC structures have inappropriately adopted the same design philosophy used for steel RC, leading either to the adoption of conservative safety factors or reduced structural reliability. A reliability-based analysis of FRP RC beams shows that the current, very conservative partial safety factors for FRP reinforcement on their own do not influence the structural safety of overreinforced concrete elements. Proposals are made for the modification of the material partial safety factors to achieve target safety levels.     

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

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.     

Modeling Debonding Failure in FRP Flexurally Strengthened RC Members Using a Local Deformation Model

Reinforced concrete (RC) beams and slabs can be strengthened by bonding fiber-reinforced polymer (FRP) composites to their tension face. The performance of such flexurally strengthened members can be compromised by debonding of the FRP, with debonding initiating near an intermediate crack (IC) in the member away from the end of the FRP. Despite considerable research over the last decade, reliable IC debonding strength models still do not exist. The current paper attempts to correct this situation by presenting a local deformation model that can simulate IC debonding. The progressive formation of flexural cracks, and the associated crack spacings and crack widths are modelled from initial cracking to the onset of debonding. The bond characteristics between the longitudinal steel reinforcement and concrete, and the FRP and concrete, as well as the tension stiffening effect of the reinforcement and FRP to the concrete, are considered. The FRP-to-concrete bond-slip relation is used to determine the onset of debonding. The analytical predictions compare well with experimental results of FRP-strengthened RC cantilever slabs.     

Effect of Concrete Composition on FRP/Concrete Bond Capacity

External bonding of fiber-reinforced plastics (FRP) to concrete members has been established as an efficient and effective method for structural strengthening and retrofitting. Direct shear test is often employed to study the crack-induced debonding failure in reinforced concrete members flexurally strengthened with FRP composites. In many existing models, the bond capacity (which defines ultimate load capacity of the specimen in the direct shear test) is considered to be strongly dependent on the compressive or tensile strength of the concrete. However, since debonding behavior is affected by interfacial friction due to aggregate interlocking within the debonded zone, the concrete composition should also play an important role in determining the bond capacity. In this study, the direct shear test is performed with 10 different compositions of concrete. The test results indicate that the bond capacity has little correlation with either the concrete compressive or splitting tensile strength. On the other hand, the bond capacity is found to have reasonable correlation with the concrete surface tensile strength but correlates very well with the aggregate content. As a geometry independent parameter corresponding to bond capacity, the interfacial fracture energy is empirically proposed to relate to these two parameters. The consideration of aggregate content leads to much better agreement between predicted bond capacity and test result. Hence, the effect of concrete composition on the FRP/concrete bond should be considered in practical design.     

Seismic Behavior of High-Strength Concrete Columns Confined by Fiber-Reinforced Polymer Tubes

The use of high-strength concrete (HSC) in seismically active regions poses a major concern because of the brittle nature of material. The confinement requirements for HSC columns may be prohibitively stringent when ordinary grade transverse steel reinforcement is used. An alternative to conventional confinement reinforcement is the use of fiber-reinforced polymer (FRP) tubes in the form of stay-in-place formwork which can fulfill multiple functions of: (1) formwork; (2) confinement reinforcement; and (3) protective shell against corrosion, weathering and chemical attacks. The use of stay-in-place FRP formwork is investigated as concrete confinement reinforcement for HSC and normal strength concrete (NSC) columns with circular cross sections. Large-scale specimens with 270?mm circular cross-sections and different concrete strengths were tested under constant axial compression and incrementally increasing lateral deformation reversals. FRP tubes were manufactured from carbon fiber sheets and epoxy resin. The results indicate that inelastic deformability of HSC and NSC columns can be improved significantly by using FRP tubes, beyond the performance level usually expected of comparable columns confined with conventional steel reinforcement.     

Stress Transfer and Fracture Propagation in Different Kinds of Adhesive Joints

To effectively and efficiently utilize fiber-reinforced plastic (FRP) laminates (plates or sheets) in strengthening civil infrastructures, a design strategy integrating the properties of FRP reinforcement and composite structural behavior needs to be adopted. The interfacial stress transfer behavior including debonding should be considered to be one of the most important effects on the composite structural behavior. In this paper, two kinds of nonlinear interfacial constitutive laws describing the pre- and postinterfacial microdebonding behavior are introduced to solve the nonlinear interfacial stress transfer and fracture propagation problems for different kinds of adhesive joints in FRP/steel-strengthened concrete or steel structures. Expressions for the maximum transferable load, interfacial shear stress distribution, and initiation and propagation of interfacial cracks (debonding) are derived analytically. In addition, numerical simulations are performed to discuss the factors influencing the interfacial behavior and the theoretical derivations are validated by finite-element analysis.     

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

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 influence on the overall performance of the FRP strengthening system with or without adequate anchorage, and whether failure was by debonding or rupture of FRP.     

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

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

Slenderness Effects on Circular CFRP Confined Reinforced Concrete Columns

External bonding of circumferential fiber-reinforced polymer (FRP) wraps is a widely accepted technique to strengthen circular RC columns. To date, most of the tests performed on FRP strengthened columns have considered short, unreinforced, small-scale concrete cylinders, with height-to-diameter ratios of less than three, tested under concentric, monotonic, and axial load. In practice, most RC columns have height-to-diameter ratios considerably larger than three and are subjected to loads with at least minimal eccentricity. Results of an experimental program performed to study the effects of slenderness on carbon FRP (CFRP) wrapped circular RC columns under eccentric axial loads are presented. It is shown that CFRP wraps increase the strength and deformation capacity of slender columns, although the beneficial confining effects are proportionally greater for short columns, and that theoretical axial-flexural interaction diagrams developed using conventional sectional analysis (but incorporating a simple FRP confined concrete stress-strain model) provide conservative predictions for nonslender CFRP wrapped columns under eccentric loads. The use of longitudinal CFRP wraps to reduce lateral deflections and allow slender columns to achieve higher strengths, similar to otherwise identical nonslender columns, is also demonstrated.     

Comparison of Three Flexural Retrofit Systems under Monotonic and Fatigue Loads

The majority of experimental work involving the flexural retrofit of concrete bridge girders has been conducted on beam specimens with adhesive-applied, soffit-mounted, fiber-reinforced polymer (FRP) composite systems, referred to in this study as conventional adhesive application (CAA). It has been observed that the performance of such girders is often controlled by the quality of the bond between the FRP and the concrete substrate and the substrate’s ability to transfer stress from the steel to the FRP. With the goal of improving the performance of bonded FRP in mind, two additional soffit-mounted retrofit schemes are investigated: near-surface mounted (NSM), where the FRP strips are embedded in adhesive within slots cut into the substrate concrete, and, powder-actuated fastener-applied (PAF) FRP, which uses a powder-actuated nail gun to install mechanical fasteners through predrilled holes in the FRP into the concrete substrate, “nailing” the FRP in place. The PAF application is a recent development, and little work has been done on it other than by the proprietors of the system. This study reports on a comparative study of the static and fatigue performance of reinforced concrete beams retrofitted with CAA, NSM, and PAF FRP retrofit systems. Ten medium-scale beams were tested: six strengthened specimens, two per retrofit method, were tested under cyclic loading conditions, and four specimens, one per retrofit method and one control specimen, were tested monotonically to failure. The results of this study indicate that although all three methods of FRP application result in significant strength increases over the control specimen under monotonic loading conditions, the CAA method is outperformed by the other methods under cyclic conditions. A number of other relevant detailed conclusions with respect to performance and practical application issues are presented for each of the methods of retrofit examined in this study. Significantly, clear evidence of FRP debonding in the midspan region prior to specimen failure is presented.     

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

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

Concrete Slabs Reinforced with FRP Grids. I: One-Way Bending

Currently, considerable interest exists in the use of fiber-reinforced polymer (FRP) reinforcement for concrete structures. Due to the generally lower modulus of elasticity of FRP in comparison with steel and the linear behavior of FRP, certain aspects of the structural behavior of RC members reinforced with FRP may be substantially different from similar elements reinforced with steel reinforcement. In this two-part paper the use of different types of FRP grid reinforcement for concrete slabs is investigated, presenting detailed experimental and analytical work. In the first part, the structural behavior in one-way bending is considered. This paper shows which structural measures are needed to ensure acceptable serviceability behavior. The presented analysis and discussion of test results covers the ultimate state and the ultimate limit state for bending, serviceability limit states, ductility, deformability, and ultimate to service load ratio.     

Structural Retrofitting of Deteriorated Concrete Lighting Poles Using FRP Sheets in Wet Layup—Field Application

This paper investigates the structural retrofit of deteriorated reinforced concrete lighting poles using wet layup of fiber reinforced polymer (FRP) sheets. The study comprised of four full-scale Class B damaged poles and one full-scale Class C undamaged pole. Each pole was 14,000 mm long with a base diameter of 350 mm linearly tapered to a tip diameter of 165 mm. The damaged poles were reinforced with nine No. 10 steel bars each, whereas the undamaged pole had four prestressed steel strands of 3/8 in. (9.5 mm) diameter. Four different in situ applicable repair schemes, one for each damaged pole, involving the application of E-glass or carbon FRP lateral confining wraps only (unidirectional) or coupled with additional longitudinal flexure sheets (bidirectional) were investigated. The adequacy of the retrofit works was checked through full-scale bending test of the poles in accordance with specification of the Canadian Standard Association. All four repaired poles exceeded the CSA design transverse loading requirement for Class B poles. In addition, the bidirectional E-glass and carbon FRP repaired poles exceeded the benchmark performance of the undamaged Class C pole as a percentage of the required design load with the E-glass pole exhibiting the best performance.