thermoplastics and concrete is expected to be poor, and research on better bonding is necessary (e.g., pultrusion and surface patterning to improve mechanical interlocking and anchoring). In addition, Japanese research indicates that fiber-reinforced plastic composites show excessive deformation under load due to low elastic modulus and high creep (Henrichsen, 1996).

Numerous types of fibers with excellent chemical and environmental resistance (e.g., glass, aramid [Kevlar 49®], and carbon fibers) are currently available as the reinforcement phase in composites for various applications. All of the synthetic fibers have lower densities than steel, and some have greater strength. Aramid and carbon fibers also have a much higher specific modulus than steel. However, they are more expensive than steel and are also brittle. E-glass fiber provides high strength and a reasonable modulus and is available at low cost. Although it is not stable in the high pH environment of conventional concrete, E-glass can be used as a reinforcement material in a nonconventional concrete with a low pH matrix. Alternatively, an alkali resistant glass could be used.

For applications requiring a high-modulus reinforcement, there are much more cost-effective options than E-glass. High-strength carbon fiber is the most widely available and provides both high strength and high modulus. It is also moderately priced by composite standards but is much more expensive than carbon steel. High-modulus fiber can provide even higher modulus but is currently unattractive for high-volume applications due to its high cost by concrete standards. Aramid fiber also has good strength and modulus, but its main distinction is its exceptional toughness (Majumdar and Laws, 1991).

DISCONTINUOUS-FIBER REINFORCEMENT

Discontinuous-fiber reinforcement enhances the properties and performance of concrete in two ways: it increases the tensile properties of the material and reduces concrete shrinkage and the associated cracking. By definition, however, discontinuous-fiber reinforcement cannot be used as a prestressed reinforcement phase. In the current state of practice, discontinuous-fiber-reinforced concrete is typically made by adding a small fraction (usually 0.42 percent by volume) of short fibers (typically 5 to 500 mm [0.25 to 2 inches] long) to the concrete during mixing. Common types of fibers that have been used include steel, alkali-resistant glass (Majumdar and Laws, 1991), and



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