The theoretical gain in flexural strength of a reinforced concrete beam
strengthened with FRP plate is enormous, however researchers have observed
new types of failure that limit this gain. These observed failures
are most often brittle in nature and involve delamination, edge peeling and
interior debonding of the FRP plate. These occur at loads significantly
lower than the theoretical ultimate load of a strengthened beam. Thus,
an understanding of these failure mechanisms must be sought to safeguard
against their occurrence.
Many researchers have concentrated on the use of the strength and the
finite element methods to analyse these local failures of the concrete-FRP
bond. These methods require knowledge of the details of the crack tip and
the surface characteristics but these are not known in normal applications.
The present study uses the Hutchinson fracture model together with
the compatibility of the strains and changes in length along the unbond surfaces.
Based on the Hutchinson fracture model, the present study assumes that
flaws exist, and looks at whether these flaws can aid propagation of interior
debonding or edge peeling with a given energy release. The model also
is used to investigate whether debonding or edge peeling would occur first
and which would dictate the ultimate failure of plate bonded beams. Finally,
the unbonded length under any given loading conditions was determined.
The model was coded into a computer program using Matlab. For a
given load condition, the energy release rate, strains, stresses in various
components of the strengthened beam and the unbonded length were the output
of the program. A parametric study was conducted and the result revealed
that the thickness of the plate dictates, to some extent, the type of local
failure that would occur with a given load condition. It was concluded
that interior debonding failures that initiate from the constant moment region
are likely to occur before the other types of local failure since the energy
release rates are higher in that region.
[Cambridge University | CUED | Structures Group | Geotechnical Group]