Properties of Polyaramid Ropes and Implications for Their Use as External Prestressing Tendons

C.J. Burgoyne

Engineering Department, University of Cambridge, Cambridge, UK

The use of external tendons in prestressed concrete is an idea that has been tempting designers almost since the first use of prestressing. The biggest benefit is the saving of weight in the webs. They can be reduced to the thickness needed to carry the shear forces, without the necessity of providing cover for the tendons.

In the United Kingdom, as elsewhere, there have been problems with corrosion in external tendons. At Braidley Road viaduct, the external tendons had to be replaced and provided with additional corrosion protection after some of the tendons failed after only 12 months.

The conclusion that can be drawn is that new and existing structures prestressed with external steel tendons are very susceptible to corrosion. Many existing structures prestressed with internal steel tendons must also be very suspect, but how their condition can determined, without causing the corrosion that we wish to prevent, is a separate problem that is being investigated.

The paper describes the properties of parallel-lay ropes with a polyaramid (Kevlar 49) core, with particular reference to the long term properties which are of importance to the designers of prestresing systems. The anchorage and prestressing systems are described, and results are given for stress-strain, relaxation, creep, stress-rupture and fatigue behaviour. 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, which show 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 realisation of the full potential of externally prestressed concrete.