International Concrete Abstracts Portal

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.

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.

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.