International Concrete Abstracts Portal

The three-dimensional fabric studied as reinforcement for concrete is a stereo-fabric made of fiber rovings, woven into three directions, and impregnated with epoxy resin. Fiber material, number of filaments, and distance between rovings can be varied easily. Efficient production is also possible, since three-dimensional weaving, resin impregnation, and hardening can all be done by an automatic weaving machine. The authors investigated the flexural and fire-resistance behaviors of three-dimensional fabric reinforced concrete (3D-FRC) toward applying the material to building panels. The fibers studied were carbon and aramid, and the matrix was vinylon short-fiber reinforced concrete. The results demonstrate that 3D-FRC panels have sufficient strength and rigidity to withstand design wind loads, and the fire resistance of 60 min was achieved. The 3D-FRC panels have been used for curtain walls, parapets, partition walls, louvers, etc., and installations amount to 7000 m 2.

Presents information on the development and use of carbon fiber reinforced plastic (CFRP) to strengthen reinforced concrete chimneys, bridge piers, and beams in Japan; bridge beams in Switzerland; and ongoing structural research and use of fiber reinforced plastic (FRP) composite materials to strengthen such structures in the U.S. The concept and equipment for strengthening existing reinforced concrete chimneys by wrapping them with carbon reinforced plastic materials began in Japan. The procedure permitted earthquake-damaged chimneys to be repaired without taking them out of service. Research in Switzerland has led to the use of adhesively bonded sheets of carbon reinforced plastic laminates to strengthen existing bridges. This concept is an extension of use of bonded steel plates to strengthen many types of structures throughout Europe. Research, development, and some use of these techniques has been done in the U.S.

FRP are new materials for structural engineers. Therefore, an overview on the important fiber properties, matrix resins, and composite elements becomes necessary to show the assets and drawbacks of FRP and to illustrate their potentials and limits. Besides several other fields, concrete prestressing seems to have become a promising field of application of FRP. In prestressed concrete construction, high strength and good corrosion resistance of FRP can be utilized optimally. In this field of application, FRP can compete with prestressing steel, especially in such cases in which the corrosion protection of prestressing steel becomes expensive or remains tarnished by residual risks. The post-tensioning of concrete structures requires anchorages with a high mechanical efficiency. The main avenues of development are discussed, and the necessary future research is outlined.

The question of whether it is right to bond tendons made of glass, aramid, or carbon fibers to concrete has not yet been addressed directly. Paper discusses the various issues involved and concludes that, in many cases, these tendons should remain unbonded. All the new materials that have a stiffness high enough and creep low enough show linear elastic response right up to failure. This contrasts with steel, even very high-tensile steel, which shows a considerable reduction in stiffness at high loads. In bonded beams, when cracks form on the tension face of the concrete, very high strains are generated across the crack. With a steel tendon, local yield must occur, with a consequent reduction in cross-sectional area, which leads to debonding of the bar on either side of the crack. This allows the strain at the crack to reduce below its theoretical maximum value. In calculation, average steel strains are used, which ignore any local increase at the crack positions, but there are some controversial code rules that limit the (average) steel strain to less than the material can actually sustain. When new materials are used, the local yielding mechanism is no longer available, and the concept of using average strains is no longer justified. In concrete reinforced with FRP, the entire strain capacity of the fibers is available, and it is unlikely that fiber failure will occur before the concrete strains become unacceptable. But in prestressed concrete, much of the fiber strain capacity is absorbed in the prestress, leaving a tendon very sensitive to high strains in the vicinity of cracks. There is a move to increase the ductility of beams reinforced or prestressed with FRP by using FRP cages in the compression zone. This will increase the chances of a bonded tendon snapping before concrete crushing occurs. These mechanisms are not present in unbonded tendons, where high local strains do not occur, and indeed the change in stress in the tendon is small. It has been argued that, for steel tendons, this is an economic disadvantage; however, for FRP tendons, it is shown here to be beneficial.

Because fibers made of such materials as glass, carbon, aramid, and vinylon have very high resistance to corrosion, more attempts are being made to utilize continuous fiber reinforcing materials (CFRM) in reinforced and prestressed concrete structures instead of ordinary steel. However, CFRM are composite materials composed of millions of fibers and binding material, and have little plastic behavior. The mechanical behavior of reinforced concrete using CFRM is quite different from conventional reinforced concrete. As of the present, there is no general agreement relating to the methodology to be adopted in design or testing methods of such fibers. Realizing this problem, the Concrete Committee of the Japan Society of Civil Engineers (JSCE) organized a subcommittee on CFRM in 1989. The following results have been published as the committee report in 1992: design concept for concrete members using CFRM; test methods for durability of CFRM; concept for durability of CFRM; and a state-of-the-art report on CFRM for concrete structures. Paper describes the design concept for concrete members using CFRM.