EXTERNALLY BONDED REINFORCED CONCRETE STRUCTURES MARCO DI LUDOVICO 1 , FRANCESCA CERONI 2 ,GIAN PIERO LIGNOLA 1 ,ANDREA PROTA 1 ,GAETANO MANFREDI 1 , AND EDOARDO COSENZA 1 1 University of Naples Federico II, Naples, Italy 2 University of Sannio, Benevento, Italy INTRODUCTION The use of Fiber-Reinforced Polymer (FRP) materials for repairing, strengthening, or retrofitting existing rein- forced concrete (RC) structures is becoming widely adopted around the world. Higher design loads, strength loss due to deterioration, design or construction deficiencies, damage caused by accidents and environmental conditions, and seismic capacity increase to satisfy current code require- ments, are typical situations in which a civil structure would require strengthening or retrofitting. FRP externally bonded reinforcement (EBR) has emerged as a sound technique alternative to conventional materials and construction systems such as externally bonded steel plates (beton plaqu´ e), steel or concrete jackets, and external post-tensioning. Steel plates epoxy bonded to the external concrete surface in the tension zone of beams and slabs are traditionally adopted as a simple and cost-effective solution to upgrade their flexural capacity. However, this technique suffers from several disadvantages: bond deterioration due to steel corrosion, difficulty in manipulating the heavy steel plates at the construction site, need for scaffolding, and limitation in available plate length. Steel or concrete jackets are mainly used to increase strength, stiffness, and ductility of RC members, but they result in invasive and difficult situations, from a constructability standpoint with a lengthy disruption of the function of the building and for its occupants. It increases the cross-sectional dimensions and dead loads of the structure resulting in potentially undesirable weight and stiffness increase (e.g., jacketing of columns may lead to an overturning moment increase and, thus, a foundation strengthening could be necessary). FRP fabrics or sheets applied in the tension zone with fibers parallel to the RC member longitudinal axis or wrapped around RC member with fibers perpendicular to its axis, provide satisfactory solutions to the problems described above both in terms of flexural strengthening or shear strength and ductility increase without an excessive stiffness change. They can provide economically viable alternatives to traditional systems because of their light weight, ease of installation, minimal labor, equipment costs and site constraints to install, high Wiley Encyclopedia of Composites, Second Edition. Edited by Luigi Nicolais and Assunta Borzacchiello.  2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc. strength-to-weight and stiffness-to-weight ratios, and durability [1]. The application of FRP composites in the field of strengthening started in the 1980s for providing addi- tional confinement to RC columns [2,3] or as flexural strengthening for RC bridges [4–6]; a sudden increase in the use of FRPs was observed in Japan after the 1995 Hyogen–Nanbu earthquake [7]. The applications have been rapidly growing worldwide in Europe, North Amer- ica, and Japan [8,9]. Standards, guidelines, and codes have been in development since the 1980s in Europe [10–12], Japan [13,14], Canada [15], and the United States [16] in order to safely allow the design of FRP reinforcement for concrete structures. Several special techniques related to the application of composites as EBR are now emerging as a promis- ing technique: prestressing of composite strips prior to the bonding procedure which results in a more econom- ical use of materials [17] but needs special clamping devices; near surface-mounted (NSM) systems, FRP sys- tems consisting of circular or rectangular bars or strips installed into grooves made in the concrete surface and filled by using a suitable cured in-place adhesive [18]; mechanically fastened FRP strips that can be rapidly attached to concrete beams for flexural strengthening using powder—actuated fasteners [19]; steel-reinforced polymer (SRP) materials consisting of cords formed by interwoven high strength steel wires embedded within a polymeric resin, which combine the advantages of steel and carbon FRP (CFRP): strength and Young’s modulus comparable to CFRP materials, ductile behavior of steel, low material cost, and the laminate remains quite flexible without the need of rounding corners in case of wrapping [20–22]. Finally, in order to overcome some drawbacks related to the use of organic epoxy resins (i.e., low performance under temperatures above the glass transition temper- ature and under direct fire, potential for emission of poisonous fumes under elevated temperatures, lack of vapor permeability, incompatibility of resins and substrate materials, and importance for quality control of chemical reactions), the use of composite systems based on the use of inorganic matrices (such as cement-based mortars) to replace organic resin are nowadays under investigation [23,24]. This would also require exploring new possibili- ties for reinforcing fibers (e.g., basalt fibers) that should be compatible with the alkaline environment generated by the cementitious binder. BOND BEHAVIOR In RC members externally strengthened with FRP materials, the stress transfer along the external reinforcement–concrete interface is a central topic in whatever the strengthening configuration. Actually, when the bond shear and normal interfacial stresses exceed the 1