Design of column strengthening with CF sheets - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 91

time of preparing the guideline there were no national technical approvals available for
CF sheets for strengthening reinforced concrete columns, but the industry representa-
tives on the DAfStb subcommittee responsible for drawing up the guideline wanted
expressly to include design concepts for column strengthening, the formulations in the
guideline have been deliberately kept general, which means that in many cases relatively
extensive calculations only have a marginal effect on the result. The verification concept
could be simplified at a later date for specific applications in connection with an approval
for a specific construction product.
7.2 Properties of CF sheets relevant to design
Numerous experimental studies involving concrete cylinders wrapped with CF sheets
have revealed that the tensile strength ascertained in tests on strips of material are not
achieved on the member. The ultimate strain ε Lu in tensile tests on commercially
available CF sheets is about 14-16mm/m. Much lower values in the region of approx.
2-4mm/m are recommended in the relevant design codes for the ultimate tensile stress,
or rather the corresponding ultimate strain. Such figures should be used unless more
accurate values are available. There are several reasons why the strain gure that can be
used in con ning reinforcement applications is much lower. One of these is the
transverse compression acting on the fibre-reinforced material perpendicular to the
direction of the fibres, which results from the longitudinal and transverse deformation of
the loaded confined concrete member and the rounding radius R c .
Several writers have reported that a breakdown in the concrete microstructure of
wrapped compression members leads to the formation of sharp-edged pieces that
eventually cause local failures of the wrapping material (see also section 3.4.2). The
specific behaviour of the confined concrete should therefore be seen as an additional
influencing factor. It is also known that the relative ultimate strain for a wrapped
reinforced concrete member is much lower than that of an unreinforced test specimen.
The reason for this is the additional transverse compression that is transferred from the
highly stressed longitudinal reinforcing bars to the confining reinforcement. Between
the supports provided by the links or helical reinforcement, local outward buckling of
the yielding longitudinal reinforcing bars is also prevented by the con ning
reinforcement. This problem has been investigated experimentally and analytically
by Tastani et al. [128]. In the DAfStb guideline [1, 2] this in uence is only taken into
account empirically by reducing the ultimate strain, as shown in Figure 7.2.
Beyond a certain rounding radius R c ,noin uence on the related ultimate strain has been
observed in either plain or reinforced concrete members, and this is taken into account in
the design model of the DAfStb guideline [1, 2] by the factor α r . If this limit value for the
rounding radius is taken as 60mm in all cases, then for column diameters relevant in
practice, D 120mm, a mean related ultimate strain amounting to 0.5 and a character-
istic value of 0.25 can be determined on the basis of numerous tests reported in the
literature. This latter value is taken into account in the DAfStb guideline [1, 2] by the
recommended system coef cient [ k 2 ].
When applied in several layers, the CF sheets are attached either in the form of single-
ply rings or a multi-ply winding. Confining reinforcement in the form of a CF sheet is