C.J. Burgoyne and J.M. Lees
Engineering Department, University
of Cambridge, Cambridge, UK
In particular, the performance of
beams with prestressed aramid fibre reinforced plastic (AFRP) tendons can be enhanced
by the use of partially-bonded tendons. Beams prestressed with fibre reinforced plastic
(FRP) tendons are susceptible to two modes of failure, both of which degrade
the performance or economy of the material. lf the tendons are fully bonded to
the concrete then the strain in the tendon rises very quickly with increasing
curvature - the tendon snaps when it reaches its ultimate strength since it has
no ductility. The result is a high moment capacity (since the full strength of
the tendon is being used), but a low rotation capacity.
The crushing of concrete in
compression has traditionally been considered to be a brittle failure mode and,
in conventional design with steel reinforcement, beams are under-reinforced.
When using fibre reinforced plastic (FRP) tendon
materials the only way to avoid failure due to the tendons snapping is to use
an over-reinforced section where the capacity of the concrete is less than that
of the tendons. An over-reinforced beam is thus designed to fail due to the
concrete crushing and the behaviour of the compression zone becomes crucial. If
spiral or rectangular hoops of reinforcement are included in a concrete
compression specimen, the plastic capacity, and hence the duetility, of the
concrete increases markedly. Failure due to concrete crushing then becomes a
more attractive mode of failure.
There would be scope to combine the
concepts of concrete confinement and partial bonding into a single beam
specimen. In this manner it would be possible to achieve large rotations, a
high ultimate load capacity and some real ductility.
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