REVIEW OF TECHNOLOGY

This section is a brief overview of technical topics, with special emphasis on needs, current lacunae and relevance to practice in the United States.

As detailed in Chapter 3, a large number of reinforcement and cable systems consisting of carbon or aramid fibers impregnated with a resin system have been developed in geometrical forms ranging from round bars and spirally deformed bars to twisted strands and flat bars. The successful implementation of these systems depends on the anchorage. Except for the development of a few composite anchorage systems (Chapter 3), for the most part conventional anchorages are used, including the die-cast wedge, multiple wedges and bonding anchors. Although a fairly detailed set of draft "Design and Construction Guidelines for "Prestressed Concrete Highway Bridges using FRP Tendons" was issued by the PWRI in March 1994, there is still a lack of (a) appropriate anchorage systems capable of sustaining high levels of prestress under design loads, (b) long-term durability and failure data, and (c) cost data related to both installation and life-cycle costs. Nippon Steel is working on the development of models for the latter. On the other hand, there is considerable data from laboratory testing coming from field implementation relevant to use in the United States concerning performance issues and behavioral characteristics. If used as a basis for further development, this data could provide a means for speeding R&D efforts here. Design philosophy, actual construction procedures, and codes are likely to differ, however, so direct transfer of practice may be limited.

As detailed in Chapter 4, there has been considerable work in the area of external strengthening/rehabilitation of concrete structures in Japan using fiber tows and/or fabric. The use of unidirectional carbon fiber sheet forms for the strengthening of bridge decks and girders appears to be largely motivated by deficiencies in deck punching shear capacity and the need for increasing live load capacities. In a number of cases, retrofit appears to be primarily to stop crack widening and further cracking, which could be due to a number of reasons listed in Chapter 4. The use of unidirectional material also supplements the nominally provided longitudinal deck reinforcement. The systems have been used for strengthening slabs in parking garages and buildings as well, but the motivation appears to be more to prevent further exfoliation of concrete, or deterioration of the structural element, than to increase load levels. These applications and technologies are of direct relevance to U.S. practice as long as codes and specification criteria in terms of fire and overall durability can be met.

In the area of seismic retrofit and strengthening of columns, there are a number of philosophical differences in design approach that must be considered. In Japan, the primary emphasis in seismic retrofit is on strengthening rather than in enhancing deformation capacity or ductility. This emphasis is due to both an overall philosophy of replacement, and to technical reasons. Technically, in general, columns have cut-offs of main column longitudinal steel reinforcement along the height based on linear elastic demands of the first mode, or as directly derived from cantilever response. Flexural hinges are formed at these cut-off locations and longitudinal fiber reinforcement is generally applied in addition to hoop constraining reinforcement to overcome the flexural strength deficiency. The vertically aligned fibers significantly increase column shear strength, however, which could make them more vulnerable to seismic events than columns retrofitted with only hoop reinforcing fibers that enhance ductility. Most of the retrofitted columns on highways are square or rectangular, and of large dimensions. Consequently, full constraint action by the circumferential fibers may not be attainable.

Considering that field applications and demonstrations of the tow winding systems date back a decade and those relating to the tow sheet system date from 1986, while applications with the Replark system date to the early 1990s, there is an enormous amount of "real world" data that if collected and presented appropriately could be of significant use. Since some applications have been standing for years, they could serve as test monitors to provide continuous guidelines for suitability and overall use. However, a number of projects appear to have used the sheet type form to cover minor cracks as a protective coating or as an aesthetic covering, rather than for strengthening. The data and technology in these areas is clearly relevant and should be made use of as much as possible in U.S. developments. The large number of different applications already demonstrated also presents a good basis for accelerating future work.

Although a considerable number of tests have been conducted on the durability of these materials, the tests have often been conducted in an isolated fashion and without standardization. There is still a critical need for the development of test methods and test data on long- and short-term effects of environment and aging. This is especially true at the interfacial level between the composite and concrete. Available data can definitely form the basis for focused efforts in the United States in areas related to environmental durability, creep, relaxation, and fire.

Some good draft design guidelines already exist in Japan. But since design criteria, philosophy and procedures will need to be established by the U.S. civil engineering community, direct transfer will not be possible. Design guidelines need to be developed based on well-defined performance criteria, rather than on specifics of each materials system under consideration.


Published: November 1998; WTEC Hyper-Librarian