Design of column strengthening with CF sheets - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 93

Fig. 7.3 Creep rupture strength of CF sheets at laps

as an intrinsic stress state that reduces the load-carrying capacity. To consider this,
the strain in the CF sheet

is determined from the creep-induced longitudinal

deformation s ratio. The creep behaviour

of a reinforced concrete column with a wrapping of CF sheet is investigated in more
detail below. DIN EN 1992-1-1 [20, 21] speci es a Poisson's ratio ν = 0.2 for
uncracked concrete and according to the studies of Lanig [130] this represents an
approximation that lies on the safe side for the stresses in strengthened reinforced
concrete columns in the range of applications covered by the DAfStb guideline [1, 2].
The equation for determining the ultimate strain that may be assumed for a CF sheet is
therefore
ε ju α r ? α T ? α F ? α E ? α Z ? ε Lu ν ? ε cc

ε cc in simpli ed form for a constant Poisson '

(7.1)

7.3 Load-carrying capacity of cross-section
Many different approaches are possible when it comes to designing reinforced
concrete compression members with a wrapping of fibre-reinforced polymer material.
Some of those approaches have already been incorporated in national standards for the
subsequent strengthening of reinforced concrete cross-sections, e.g. [119, 123]. In
many instances the background to the approach is one of the models known from the
fundamental work on con ning compression members with reinforcing steel. How-
ever, as Figure 7.1 shows, these approaches are not entirely suitable for designing
strengthening with fibre-reinforced polymers and need some form of modi cation
at least.