C. J. Burgoyne
Department of Engineering, University of Cambridge,
Cambridge, UK
The question of whether it is right
to bond tendons made of glass, aramid or carbon fibres to concrete has not yet
been directly addressed. This paper discusses the various issues involved, and
concludes that in many cases, these tendons should remain unbonded.
All the new materials which a high
enough stiffness and a low enough creep show a linear elastic response right up
to failure, with little or no ductility. This contrast with steel, even very
high tensile steel, which shows a considerable reduction in stiffness at high
loads. In a bonded beam, when cracks form on the tension face of the concrete,
very high strains are generated across the crack. This allows the strain at the
crack to reduce below its theoretical maximum value. In calculation, average
steel strains are used, which ignore any local increase at the crack positions,
but there are some controversial code rules which limit the (average) steel
strain to less than the material can actually sustain.
When new materials are used, the
local yielding mechanism is no longer available, and the concept of using
average strains is no longer justified. In concrete reinforced with FRP, the
whole strain capacity of the fibres is available, and it is unlikely that fibre
failure will occur before the concrete strains become unacceptable. But in
prestressed concrete, much of the fibre strain capacity is absorbed in the
prestress, leaving a tendon very sensitive to high strains in the vicinity of
cracks.
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