| |
| (a) |
to enhance concrete strength and deformation
capacities, |
| (b) |
to provide lateral support to the longitudinal
reinforcement and |
| (c) |
to prevent the concrete cover from spalling. |
In case of circular columns, these goals can be achieved by applying
external FRP jackets, either continuously all over the surface or
discontinuously as strips. In the case of rectangular columns, the
confinement can be provided with rectangular-shaped reinforcement,
with corners rounded before application. Note that rectangular confining
reinforcement is less effective (but still possible) as the confinement
action is mostly located at the corners and a significant jacket
thickness needs to be used between corners to restrain lateral dilation
and rebar buckling.
The stress-strain response of FRP-confined concrete is illustrated
schematically in the following picture:
The figure displays a nearly bilinear response with a sharp softening
and a transition zone at a stress level that is near the strength
of unconfined concrete, fco. After this stress the tangent
stiffness changes a little, until the concrete reaches its ultimate
strength fcc when the jacket reaches tensile failure
at a stress ff,e and a corresponding strain  fu,e,
which is, in general, less than the uni-axial tensile strength fu.
This reduction is attributed to several reasons, including:
| (a) |
(a) the tri-axial state of stress in the FRP (due
to axial loading and confining action, but also due to bending,
e.g. at corners of low radius); and |
| (b) |
(b) the quality of execution (potential local
ineffectiveness of some fibres due to misalignment, and over-stressing
of others; damaged fibres at sharp corners or local protrusions
etc). |
Input:
- Mean strength after strengthening fcc: This
is the value of the strength of FRP-confined concrete corresponding
to the FRP jacket of thickness tf.
- Ultimate axial strain after strengthening cu:
This is the value of the ultimate axial strain of FRP-confined
concrete corresponding to the FRP jacket of thickness tf.
The total required thickness tf of the FRP jacket is
provided. Upon completion of the solution process, the "Results"
window provides the following:
After the input of the effective thickness of the
FRP - layer, the number of layers and the effective reinforcing
capacity can be calculated.
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