C.J. Burgoyne
Dept of Civil Engineering, Imperial College,
London, UK
Paper for inclusion in International Symposium
on External Prestressing in Bridges, ACI Convention, Houston, November 1988
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 addition,
the cables are accessible for inspection and, potentially, replacement.
The drawbacks lie in the lack of
the corrosion protection that is normally provided by the concrete. In the
absence of the passivating environment provided by the highly alkaline cement
matrix, the steel will corrode very rapidly unless extensive measures are taken
to prevent corrosion occurring.
In the United Kingdom, 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.
Even internal tendons can corrode.
A recent report on Ynys-y-G was bridge in Wales, which collapsed in 1985
under the action of dead load only, attributes failure to corrosion of the
prestressing tendons. This is despite the fact that the tendons are internal,
that most ducts were properly grouted and that the concrete was of adequate
quality. Furthermore, the bridge was regularly inspected and there were no
indications of anything awry before failure.
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