Concrete beams with external prestressed
carbon FRP shear reinforcement
An increasing number of reinforced concrete (RC) structures are being assessed
as having inadequate shear capacity. Reasons for these strength deficiencies
include an ageing infrastructure, deterioration due to corrosive environments
and greater loads being applied to existing structures. There could
be significant economic consequences if these structures cannot be strengthened
as the only other options are weight restrictions or complete replacement.
One possible solution is to use external carbon fibre reinforced polymer
(CFRP) shear straps to strengthen existing RC beams. The CFRP material
is advantageous since it is non-corrosive and light weight. However,
the material is brittle and does not yield. The unbonded CFRP straps
are formed by wrapping layers of carbon tape around a beam. The straps
are installed at discrete locations within the shear span and prestressed.
In the design of a CFRP strap strengthening system, the number of tape
layers/stiffness, the strap spacing and the level of prestress must be specified.
The level of enhancement must be estimated numerically and the mode of failure
This thesis describes an investigation into the use of prestressed CFRP straps
to strengthen beams weak in shear. In the first instance, a series
of beam experiments were performed where the strap stiffness, spacing and
initial prestress were varied. The experiments gave insight into the
beam behaviour in terms of the peak load and the modes of failure. The
influence of the straps on diagonal shear cracks was also clearly identified.
The modelling of cracks in finite element (FE) formulations was studied since
the experimental results suggested a strong interaction between the straps
and the cracks. Two FE packages were used; DIANA a commercial
package, and CamFEA developed as part of the current work. This study
provided insight into the use of complicated and simplified crack models
and the possible effects of these models on the FE results in a full-scale
beam analysis. A detailed FE analysis was then performed on the experimental
beams. The FE results confirmed the observed behaviour and the peak
loads and modes of failure were predicted fairly accurately. The earlier
crack modelling predictions proved helpful in explaining the FE results.
The experimental results and FE analysis confirmed that the modes of failure
were primarily governed by the strap spacing, the stiffness and the initial
prestress level of the straps. A high initial prestress level is undesirable
as the reserve strain capacity of the strap becomes limited and strap failure
becomes imminent. However, a minimum prestress level is also
required in order for the strap to be effective. The strap spacing
interacts with the strap stiffness and initial prestress level to influence
the mode of failure. If the spacing is inadequate, then a shear failure
is bound to occur. While if the strap spacing is sufficient, then the stiffness
and prestress will affect the behaviour and subsequent mode of failure.
Two analytical models suitable for hand calculations were developed to predict
the level of resistance. Unlike many design methods which assume ductile
behaviour, these models rely on a compatibility condition which ensures that
the actual strain in the brittle elastic strap can be predicted and the possibility
of a strap failure captured. The two models differed in that one included
aggregate interblock whilst the second model ignored this component. In
general, the peak load predictions were fairly good and the models could
be used as a tool to aid practising engineers in estimating the required
number of layers (stiffness) and prestress levels required once a strap spacing
has been specified.
The project has shown that prestressed CFRP straps can be used to enhance
the shear strength of RC beams and change the brittle shear mode of failure
to a ductile flexural mode.
[Cambridge University | CUED | Structures
Group | Geotechnical
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(last update 9 August 2004)