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The development of external prestressing has been one of the major trends in construction during the last 10 years, along with the development of concrete cable-stayed bridges and the increasing use of high-strength concrete. After a historical review, the main principles of the design of externally prestressed bridges are presented. The paper then details the influence of the construction method on the external tendon organization: bridges built span by span, bridges built by the cantilever method or by methods which are mechanically equivalent, and bridges built by the incremental launching method. Some practical problems are presented, such as handling heavy jacks. A last chapter is devoted to composite structures, with concrete top and bottom slabs and steel webs, prestressed by external tendons. French experimental constructions of this type do not appear economically interesting, and prestressing classical composite structures is not yet considered as good a solution for the same reason. But external prestressing is now widely developed for concrete bridges in the United States, France and, more recently, in Belgium, Switzerland, Venezuela, Germany, and Czechoslovakia.

In the last decade, the use of external tendons for prestressing of concrete structures has been developed considerably for a variety of applications. This paper deals with the technical problems related to the use of external prestressing to emphasize the development and investigation of critical questions. The following aspects are discussed: behavior of concrete structures at he serviceability and ultimate limit states; the required minimum area of conventional (nonprestressed) reinforcement; general protection of the tendons; corrosion protection of the prestressing steel; design of the saddles--deviation zones--and the effect of tendon curvature on their strength and transfer of the prestress forces to the structure; and anchorage of the tendons and the zone of transfer of the prestressing force into the structure.

The state of Texas is involved in two projects that use precast concrete segmental erection methods with external post-tensioning tendons. Design and construction features of these projects, along with construction problems related to external tendons, are described. The future of segmental construction and use of external tendons in Texas are discussed.

The bridge that links Re Island and the mainland was completed in April, 1988. It crosses a 3000 m wide sea channel that separates the island from the town of La Rochelle, on the west coast of France. The deck of the bridge is a concrete box girder built by the balanced cantilever method with precast segments. The prestressing tendons are partly inside the concrete and partly external.

Over 600 bridges composed of adjacent prestressed concrete box beams were built in the early 1950s in Pennsylvania. The box beams were placed side by side and had an asphalt wearing course on top. Span lengths ranged form 30 to 60 ft. Their design was very conservative by today's standards. The concretes used in these bridges have high chloride contents; water leaks down through the joint between box units and the strands often have inadequate cover. Thus, it is not surprising that many of the box beams are deteriorating due to corrosion of their prestressing strands. This project was directed toward developing economical repair schemes for these bridges. The literature survey did not reveal any schemes specifically applicable to adjacent box beams. Two external reinforcement repair schemes were developed and trial installations were made on a bridge near York, Pa. Both schemes included the removal of deteriorated concrete, placement of external reinforcement beneath the beam, and application of shotcrete to the soffit of the beam. In Scheme 1, the external reinforcement consisted of epoxy-coated reinforcing bars. This repair method restored ultimate flexural capacity but did not restore lost prestress. It was the least costly of the two methods. In Scheme 2, post-tensioned, epoxy-coated strand was used. This restored the full ultimate flexural capacity and most of the lost prestress. Difficulties were encountered in installing anchors for the post-tensioned system, but its performance was good. The bridge was tested after repair. The external reinforcements were found to be fully composite with the original beams. The tests also revealed the lateral distribution of wheel loads. In spite of the poor condition of the bridge, the wheel loads were well distributed laterally, leading to a structure that was stronger and stiffer than expected.