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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.

Paper describes the properties of parallel-lay ropes with a polyaramid (Kelvar 49) core, with particular reference to the long-term properties that are important to the designers of prestressing systems. The anchorage and prestressing systems are described, and results are given for stress-strain, relaxation, creep, stress-rupture, and fatigue behavior. Durability and thermal response are also considered, and it is inferred that the lack of corrosion, in addition to the high strength and high stiffness, makes these materials ideal for use as prestressing tendons where the concrete cannot be used to provide corrosion protection to steel. Descriptions are given of tests on beams prestressed with external tendons, showing that a ductile response can be achieved in a beam made from two brittle materials. It is concluded that these materials will extend the range of structures that can be built with prestressed concrete, and will at last allow the realization of the full potential of externally prestressed concrete.

The main advantage of external prestressing is that it facilitates the placement and vibration of the web concrete and thus permits thinner webs. The main disadvantages of unbonded external prestressing are its reduced ultimate capacity and lack of contribution to crack control. The cost of bridges built with unbonded external tendons in Germany is currently higher than for those with bonded internal tendons, mainly because of code requirements for minimum web thickness and crack control, and higher costs for the external unbonded tendons. External prestressing has only limited application in Germany. For new concrete bridges, it has been used a few times since the 1950s for tendons bonded to the webs. The first experimental new bridge with unbonded external tendons in Germany is currently under construction. Special applications of external tendons are for incrementally launched bridges, the rehabilitation of bridges, and as longitudinal force couplers. In the future, unbonded external tendons will probably be used mainly for the rehabilitation of existing bridges in Germany.

Design considerations of externally prestressed large-span reinforced concrete girders with tendons completely outside the cross section are dealt with. The analyzed systems are two-chord structural systems. The lower, downward convex tensioned chord usually consists only of prestressing tendons, while the upper, compressed chord is a reinforced concrete straight-line or upward convex polygonal girder. The desired configuration of tendons is achieved by compressed elements interconnecting the two chords at suitable distances. In such a way, the rise of tendons can be several times larger than the height of the reinforced concrete section, thus greatly increasing their efficiency compared to the classical internally or externally prestressed girders. An important characteristic of such structural systems is that adding a very small prestressing force reduces the deformation. Therefore, the dead load deflection can be easily controlled by the suitable choice of prestressing force. The time-dependent deflection is not considerably greater than the elastic one, even for a very high creep and shrinkage, as it is also primarily governed by the shape and deformation of tendons. Because of such properties, these structural systems are exceptionally favorable for roof structures of medium and very large spans but can also be successfully used for highway bridges. Due to the significant reduction of the chords' cross-sectional areas and the bending stiffness of such structural systems, the design has to be done using the second-order theory. The criteria for cases when it is notnecessary are discussed. Besides the theoretical analysis, some experiences in design and construction of the new hangar at the Belgrade International Airport in Yugoslavia, whose 135.80 m (445 ft) span main roof reinforced concrete girders are externally prestressed with tendons free in space outside the concrete cross section, are also presented.

The behavior of two segmental concrete girders incorporating external tendons was compared to that of a similar girder with internal tendons. The girders were 31 ft long and consisted of 11 match-cast segments. Test variable was the location of the tendon ducts. In the first girder, the ducts were embedded in the girder cross section. The ducts of the second girder were external to the concrete cross section except at pier segments and intermediate deviation diaphragms. The third girder was similar to the second except that portions of the external ducts were embedded in a second-stage concrete cast. The segments included multiple shear keys and were dry jointed. All ducts were grouted. Each girder was simply supported over a 30-ft span and loaded statically to destruction under a two point load. The first and third girders attained their respective flexural strengths predicted by the classic bending theory for monolithic girders with bonded tendons. The second girder exceeded the flexural strength predicted by the provisions of the AASHTO specifications for members with unbonded tendons.