Design of column strengthening with CF sheets - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 90

This restriction agrees with the recommendations of Menne [113] for reinforced
concrete columns with helical reinforcement. His tests on members demonstrated that
Müller's proposal [124] to limit the eccentricity to one-eighth of the core diameter,
and which was included in the 1972 edition of DIN 1045 [114], was too conservative.
Further, the range of applications covered by the DAfStb guideline [1, 2] is limited to
compression members with a circular cross-section. In principle, confinement can also
increase the load-carrying capacity of members with a square or rectangular cross-
section. However, the con nement can induce a suf ciently high transverse compres-
sive stress in certain areas of the cross-section only. This fact has been veri ed by Sheikh
and Uzumeri [125] for reinforced concrete columns. They showed that only the area
lying inside parabolic arcs can be assumed to be effectively con ned, where the arcs
spring from the corners of the links. In square columns with typical dimensions, the ratio
of the effective con ned area to the area of the concrete is already < 60%. As the cross-
section deviates more and more from a square section, there is a rapid decrease in the
effective con ned area.
In addition, the high transverse compression acting on the fibre-reinforced material in
the region of the rounded corner leads to a lower tensile stress that can be accommodated
by the con ning reinforcement and hence to a further drop in the effectiveness of the
strengthening measures for rectangular cross-sections. On the whole it can be said that
confinement in the form of fibre-reinforced materials for compression members with a
rectangular cross-section does not represent a reasonable method of strengthening in
most instances. Therefore, modifications to the method to allow for this type of cross-
section have already been investigated in order to increase the efficiency. These
modifications entail building up the cross-section into a circular or elliptical form
(see [126], for example), or including expanding elements to prestress the wrapping
between the rounded corners [127]. The latter function as additional supports for the arcs
and hence lead to an increase in the effective confined area.
As the history of DIN 1045 has meant that high-strength concretes only began to be
widely used in Germany quite recently, members made from normal-strength concrete
represent the standard case for strengthening projects at the moment. The applications
covered by the DAfStb guideline [1, 2] are therefore limited to the subsequent
strengthening of existing members made from normal-strength concrete.
There are various ways of arranging externally bonded fibre-reinforced materials. The
DAfStb guideline [1, 2] only deals with wrapping applied over the entire surface, which
compared with wrapping applied in strips or in a spiral does not involve any reduction in
the con ning effect in the longitudinal direction of the column.
The range of applications covered by the DAfStb guideline [1, 2] is limited to carbon
fibre (CF) sheets, which when used as confining reinforcement must have a national
technical approval. The wet lay-up method is used to ensure that all fibres are fully
soaked with resin.
The following sections explain the background to and sources of the provisions
contained in the DAfStb guideline [1, 2]. A more detailed description of the underlying
models can be found in [56] and the references given in the following sections. As at the