Design of strengthening with near-surface-mounted CFRP strips - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 67

sufficient fatigue resistance for max. 2 10 6 load cycles. Design methods for numbers of
load cycles > 2 10 6 are not covered in the DAfStb guideline.
In this analysis, adequate resistance to fatigue for near-surface-mounted CFRP strips
may be assumed for up to 2 10 6 load cycles provided the end anchorage force for a

frequent cyclic action to DIN EN 1992-1-1 section 6.8.3 (3), and taking into account
the , does not exceed the value 0.6 F bLRd ( F bLRd to Equation 5.5) and the

strip stress range does not exceed a value given by Equation 5.10. The strip thickness
t L in mm should be used here so that the result is an admissible stress range in N/mm 2 .

' shift rule '

mm 2 500 N =

Δ σ L (5.10)

t L

5.6 Analyses for the serviceability limit state
The analyses for the serviceability limit state, which were described for externally
bonded CFRP strips in Section 3.6, also apply correspondingly for near-surface-
mounted CFRP strips.
It should be pointed out here that owing to their effective and relatively stiff bond
behaviour (see Figure 5.1), near-surface-mounted CFRP strips are ideal for retro tting
to control crack widths (see [109], for example). The method for allowing for the crack-
limiting effect of near-surface-mounted reinforcement is based on a method proposed
in [91], which assumes a bond-related interaction between the internal reinforcement
and the near-surface-mounted reinforcement. It is assumed here that the cracks are
closed or grouted at the time of strengthening and therefore no significant action effects
due to residual stresses and loads are present. In this method it is first necessary to
calculate the strip stress due to the load or restraint and assume a crack width. Owing to
the assumed crack width, Equation 5.11 can be used to calculate the slip of the internal
reinforcement and the near-surface-mounted reinforcement:

w k 2 ? s sr 2 ? s Lr (5.11)
With the help of the slip it is now possible to determine the mean bond stresses, the crack
spacing and the mean strains using the equations given in the DAfStb guideline. The
crack width can then be calculated with Equation 5.12:
w k s cr ; max ? ε Lm ε cm (5.12)

If the crack width from Equation 5.11 agrees with the assumption in Equation 5.12, this
is the crack width that will occur.

5.7 Detailing
Essentially, near-surface-mounted CFRP strips must comply with the same detailing
rules as those for externally bonded strips, which are described in Section 3.7. However,
when it comes to the strip spacing, near-surface-mounted CFRP strips must comply not
only with a maximum spacing, which is dealt with in the DAfStb guideline in the same
way as the externally bonded CFRP strips, but also with a minimum spacing. Further to