【正文】 inforccd 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 fiberreinforced polymer (GFRP) plates. Ritchie ct al. (1991) tested reinforced concrete beams strengthened for flexure using GFRP. carbon fibcrrcinforccd 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 pared to the nonstrgthcd 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 pared 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 lowstrain fibers such as ultrahighmodulus 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 pseudoductile 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 SIGNIFICANCEFRPs 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 pseudoductile FRP strengthening fabric. The fabric provides measurably higher yield loads for the strengthened beams and helps to avoid the loss of ductility that is mon with the use of currently available FRP.DEVELOPMENT OF HYBRID FABRICTo overe the drawbacks mentioned previously, a ductile FRP material with low yield strain value is needed.ACI Structural Journal, V. 99, No. 5, SeptemberOctober 2002.MS No. 01349 received October 23, 2001, 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. Pertinent discussion will be published in the JulyAugust 2003 ACl Structural Journal if received by March 1, 2003.ACI StructuralJournal/SeptemberOctober 2002ACI 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。 2) two layers of a uniaxial carbon fiber fabric with an ultimate load of kN/mm ( kips/in.) for the two layers bined: and 3) a pultrudcd carbon fiber plate with an ultimate load of kN/mm (16 kips/in.). The tested loadstrain diagrams for all these materials are shown in Fig. 5. Table 2 shows the properties of the strengthening materials, including the developed fabric.AdhesivesFor the hybrid fabric, an cpoxy resin (Epoxy A) was used to impregnate the fibers and as an adhesive between the fabric and the concrete surfacc. This epoxy had an ultimate strain of % to ensure that it would not fail before the failure of the fibers. For the beams strengthened with carbon fiber sheets, plates, and fabric, an cpoxy resin with an ultimate strain of % was used (Epoxy B). The mcchanical properties of the adhesives provided by their manufactures are shown in Table 3.StrengtheningThe beam bottom faces and sides were sandblasted to roughen the surface. The beams were then cleaned with acetone to remove din. Two strengthening configurations were used: 1) strengthening material on the bottom face of the beam only (Beam Group A)。 hence, it contributed to strengthening more effectively than the carbon fiber sheet before the steel yielded。 to current carbon fiber strengthening materials, the devel