After 10 or so years of research and practical application, the external prestressing of concrete is now becoming a normal procedure, soon to be codified in some countries. Among the technical solutions applied to a large number of works, in the U.S.A. and France particularly, some emerge as being the best examples both from the point of view of performance and economy. This is the case where normal strands in HDPE ducts are used with an injected grout. This technique can be used equally well with removable external prestressing and with external prestressing that is partially bonded. In difficult cases, such as very long structures (bridges with lengths exceeding 200 m or 600 ft) or structures with high curvature (tanks and various vessels) or where it is difficult to bring large jacks up to the anchorages, an external cable, formed of projected strands, gathered together in the same HDPE duct and isolated from one another, is a very effective and elegant solution with an unequalled degree of protection and with the opportunity to check the prestressing force throughout the entire life of the structure.

The use of tendons placed outside the concrete for the longitudinal prestressing of bridge decks is particularly well suited to the triangulated trusses that have now proven effective. Experience acquired first on the Tehran Stadium roofing (internal tendons) and then on the externally prestressed Bubiyan Bridge in Kuwait has highlighted the numerous advantages to be derived from the latter system. The technology has, therefore, been used again, with external tendons, in the construction of the Sylans and Glacieres viaducts located along the Macon-Geneva-Mont Blanc highway in France, on behalf of the Societe des Autoroutes Paris Rhin-Rhone. The two viaducts have a total length of 1500 m. Each consists of two parallel decks, 10.75 m wide. Typical spans are 60 m long. The deck is precast in forms as a series of 4.66 m long elements and erected by sequential cantilevering using a launching girder.

A simple methodology for the solution of beams prestressed or partially prestressed with external or unbonded tendons in the linear elastic cracked and uncracked range of behavior is described. It leads to equations allowing the computation of stresses in the concrete section, the tensile reinforcing steel, the compression reinforcing steel, and the prestressing steel. In particular, it is shown that the stress in unbonded tendons is a function of the applied loading, the steel profile, and the ratio of the crack width (or crack band width) to the span. These factors can all be accounted for through the use of a strain reduction coefficient ê for the uncracked range of behavior and a similar coefficient êc for the cracked range of behavior. It is shown that, when the strain reduction coefficients ê and êc are taken equal to unity, the solutions developed here revert to the solutions developed earlier for partially prestressed beams with bonded tendons.

A research project is currently underway at the Swiss Federal Institute of Technology, Zurich, to establish the feasibility of an alternative structural system for short-span highway bridges. Concerns over the long-term durability of structural systems currently used in the 25 to 40 m span range provided the primary motivation for the study. The proposed system consists of a solid concrete slab that is externally prestressed. The external tendons are deviated at the third points of each span using struts. A 1:3-scale model bridge has been constructed and is currently being tested to verify the behavior of the bridge under permanent, service, and ultimate static loads, as well as dynamic and fatigue loads. The favorable results obtained thus far have confirmed the feasibility of the proposed structural system, and will serve as a basis for extending the concept to spans greater than 40 m in length.

A laboratory investigation was performed to study deviation saddles, a type of tendon deviator used in externally post-tensioned precast segmental box girder bridges. Ten reduced-scale models of deviation saddles were fabricated and loaded to ultimate using a specially designed testing apparatus that applied load to each deviator just as would be applied to a deviator in a bridge. The objectives of the study were to: experimentally investigate the strength and ductility of deviators; evaluate deviator details in light of observed performance; define behavioral models for deviators; determine the effects of using epoxy-coated reinforcement; and establish design criteria. Data from the test series are presented, two analysis techniques are formulated, and design recommendations are made for design of tendon deviators.