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INTRODUCTION

Fiber-reinforced polymer (FRP) composites have been proposed for use as prestressing tendons in concrete structures. The promise of FRP materials lies in their high-strength, lightweight, noncorrosive, nonconducting, and nonmagnetic properties. In addition, FRP manufacturing, using various cross-sectional shapes and material combinations, offers unique opportunities for the development of shapes and forms that would be difficult or impossible with conventional steel materials. Lighter-weight materials and preassembly of complex shapes can boost constructibility and efficiency of construction. At present, the higher cost of FRP materials suggests that FRP use will be confined to applications where the unique characteristics of the material are most appropriate. Efficiencies in construction and reduction in fabrication costs will expand their potential market. FRP reinforcement is available in the form of bars, grids, plates, and tendons. This document examines both internal and external prestressed reinforcement in the form of tendons.

One of the principal advantages of FRP tendons for prestressing is the ability to configure the reinforcement to meet specific performance and design objectives. FRP tendons may be configured as rods, bars, and strands as shown in Fip. I . 1 . The surface texture of FRP tendons may vary, resulting in bond with the surrounding concrete that varies from one tendon configuration to another. Unlike conventional steel reinforcement, there are no standardized shapes, surface configurations, fiber orientation, constituent materials, and proportions for the final products. Similarly, there is no standarbation of the methods of production, such as pultrusion, braiding, filament winding, or FRP preparation for a specific application. Thus, FRP materials require considerable engineering effort to use properly. Bakis (1 993) has outlined manufacturing processes.

FRP tendons are typically made from one of three basic fibers. These fibers are aramid, carbon, and glass. Aramid fibers consist of a semicrystalline polymer known as aromatic polyamide. Carbon fibers are based on the layered graphene (hexagonal) networks present in graphite, while glass generally uses either E-glass or S-glass fibers. E-glass is a low-cost calcium-aluminoborosilicate glass used where strength, low conductivity, and acid resistance are important. S-glass is a magnesium-aluminosilicate glass that has higher strength, stiffness, and ultimate strain than E-glass. S-glass costs more than E-glass, and both are susceptible to degradation in alkaline environments. Table I. 1 gives properties of typical fibers.

The selection of the fiber is primarily based on consideration of cost, strength, stifhess, and long-term stability. Within these fiber groups, different performance and material characteristics may be achieved. For example, aramids may come in low, high, and very high modulus configurations. Carbon fibers are also available with moduli ranging from below that of steel to several multiples of that of steel. Of the several fiber types, glass-based FRP reinforcement is least expensive and generally uses either E-glass or S-glass fibers.

The resins used for fiber impregnation are usually thermosetting and may be polyester, vinylester, epoxy, phenolic, or polyurethane. The formulation, grade, and physical-chemical characteristics of resins are practically limitless. The possible combinations of fibers, resins, additives, and fillers make generalization of the properties of FRP tendons very difficult. Additionally, FRP composites are heterogeneous and anisotropic. Final characteristics of an FRP tendon are dependent on fiber and resin properties, as well as the manufacturing process. Specific details of a particular tendon should be obtained from the manufacturer of the tendon.