Strengthening of Concrete Beams Using Innovative Ductile Fiber-Reinforced Polymer  Fabric abstract An innovative, uniaxial ductile fiber-reinforced polymer (FRP) fabric has been researched, developed, and manufactured (in the Structural Testing Center at Lawrence Technological University) for strengthening structures. The fabric is a hybrid of two types of carbon fibers and one type of glass fiber, and has been designed to provide a pseudo-ductile behavior with a low yield-equivalent strain value in tension. The effectiveness and ductility of the developed fabric has been investigated by strengthening and testing eight concrete beams under flexural load. Similar beams strengthened with currently available uniaxial carbon fiber sheets, fabrics, and plates were also tested to compare their behavior with those strengthened with the developed fabric. The fabric has been designed so that it has the potential to yield simultaneously with the steel reinforcement of strengthened beams and hence, a ductile plateau similar to that for the nonstrengthened beams can be achieved. The beams strengthened with the developed fabric exhibited higher yield loads and achieved higher ductility indexes than those strengthened with the currently available carbon fiber strengthening systems. The developed fabric shows a more effective contribution to the strengthening mechanism. keyword：Concrete, ductility, textile fiber reinforcement, distortion 57335

INTRODUCTION 

The use of externally bonded fibcr-rcinforccd polymer (FRP) sheets and strips has recently been established as an effective tool for rehabilitating and strengthening reinforced concrete structures. Several experimental investigations have been reported on the behavior of concrete beams strengthened for flexure using externally bonded FRP plates, sheets, or fabrics. Saadatmancsh and Ehsani (1991) examined the behavior of concrete beams strengthened for flexure using glass fiber-reinforced polymer (GFRP) plates. Ritchie ct al. (1991) tested reinforced concrete beams strengthened for flexure using GFRP. carbon fibcr-rcinforccd polymer (CFRP). and G/CFRP plates. Grace et al. (1999) and Trian- tafillou (1992) studied the behavior of reinforced concrete beams strengthened for flexure using CFRP sheets. Norris. Saadatmancsh. and Fhsani (1997) investigated the behavior of concrete beams strengthened using CFRP unidirectional sheets and CFRP woven fabrics. In all of these investigations, the strengthened beams showed higher ultimate loads compared to the nonstrcngthcncd ones. One of the drawbacks experienced by most of these strengthened beams was a considerable loss in beam ductility. An examination of the load- deflection behavior of the beams, however, showed that the majority of the gained increase in load was experienced after the yield of the steel reinforcement. In other words, a significant increase in ultimate load was experienced without much increase in yield load. Hence, a significant increase in service level loads could hardly be gained. Apart from the condition of the concrete element before strengthening, the steel reinforcement contributes significantly to the flcxural response of the strengthened beam. Unfortunately, available FRP strengthening materials have a behavior that is different from steel. Although FRP materials have high strengths, most of them stretch to relatively high strain values before providing their full strength. Because steel has a relatively low yield strain value when compared with the ultimate strains of most of the FRP materials, the contribution of both the steel and the strengthening FRP materials differ with the deformation of the strengthened element. As a result, steel reinforcement may yield before the strengthened element gains any measurable load increase. Some designers place a greater FRP cross section, which generally increases the cost of the strengthening, to provide a measurable contribution. even when deformations arc limited (before the yield of steel). Debonding of the strengthening material from the surface of the concrete, however, is more likely to happen in these cases due to higher stress concentrations. Debonding is one of the nondesired brittle failures involved with this technique of strengthening. Although using some special low-strain fibers such as ultra-high-modulus carbon fibers may appear to be a solution, it would result in brittle failures due to the failure of fibers. The objective of this paper is to introduce a new pseudo-ductile FRP fabric that has a low strain at yield so that it has the potential to yield simultaneously with the steel reinforcement, yet provide the desired strengthening level. RESEARCH SIGNIFICANCE  FRPs have been increasingly used as materials for rehabilitating and strengthening reinforced concrete structures. Currently available FRP materials, however, lack the ductility and have dissimilar behaviors to steel reinforcement. As a result, the strengthened beams may exhibit a reduced ductility, lack the desired strengthening level, or both. This study presents an innovative pseudo-ductile FRP strengthening fabric. The fabric provides measurably higher yield loads for the strengthened beams and helps to avoid the loss of ductility that is common with the use of currently available FRP.    DEVELOPMENT OF HYBRID FABRIC To overcome the drawbacks mentioned previously, a ductile FRP material with low yield strain value is needed. and reviewed under Institute publication policies. Copyright €) 2002, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. member Nabil F. Grace is a professor and Chair of the Structural Testing Center, Department of Civil Engineering, Lawrence Technological University, Southfield, Mich. He is a member of ACI Committee 440, Fiber Reinforced Polymer Reinforcement; and Joint ACI-ASCE Committee 343, Concrete Bridge Design. His research interests include the use of fiber-reinforced polymer in reinforced and pre stressed concrete structures.  George Abdel-Sayed is Professor Emeritus in  the Department of Civil and Environmental Engineering, University of Windsor, Windsor, Ontario, Canada. His research interests include soil-structure interaction.  Wael F. Ragheb is a research assistant in the  Department of Civil Engineering at Lawrence Technological University. He is a PhD candidate in the Department of Civil and Environmental Engineering, University of Windsor, Windsor, Ontario, Canada. Table 1—Mechanical properties of composite fibers*  Fiber  material Description  Modul us of elasticity. GPa(Msi)  Tensile strength, MPa (ksi) Failure  strai n, %  Carbon No. I Ullra-high-modulus carbon fibers  379 (55) 1324(1 92) 0.35 Carbon No. 2 High-modulus carbon fibers 231 (33.5) 2413(3 50)  0.9  to  1.0  Gl ass  E-glass fibers 48 (7)  1034(1 50)  2.1 * Composite properties are based on 60% fiber volume fraction.