C. J. Burgoyne
Lecturer, Engineering Department, University of Cambridge,
Cambridge, UK
Presented at FIP 93 Symposium, Kyoto, Japan,
October 1993.
The advent of new materials such as
glass, carbon and aramid fibres has ushered in a new range of prestressing
systems. These offer potential benefits, including most importantly, lack of
corrosion. Most of these systems are made from resin based pultrusions which
are (or at least can be) bonded to the concrete, by analogy with steel tendons.
But an alternative route for development is from ropes; one such system is
described in this paper. The system was developed with a view to maximizing the
strength and stiffness of fibre ropes for mooring offshore facilities. Once the
technology was developed, however, it was clear that the ropes had applications
in the structural engineering field, especially in the form of prestressing
tendons.
Aramid Fibres and Parallel-Lay Ropes
Aramid fibres are today made by a number
of companies, under a variety of trade names. The first such fibre was Kevlar
(developed by Du Pont), which has been followed by Twaron (developed by Akzo),
and Technora (developed by Teijin). Kevlar and Twaron are virtually identical,
while Technora is a slightly more complex polymer but with very similar
properties.
Aramid fibres are essentially made
up from long straight chains of benzene rings, with extra carbon and nitrogen
atoms between the rings. These atoms support oxygen and hydrogen atoms respectively
which form multiple (but weak) hydrogen bonds between adjacent chains. The
aramid molecules are liquid crystals when in solution, which leads to the
formation of long parallel chains which achieve their strength by the
cross-linking formed by the hydrogen bonds. In what follows, ropes made from
Kevlar yarns will be described, but it must be expected that most of the
results would be the same for ropes made from any of the aramid fibres.
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