Design of strengthening measures with externally bonded CFRP strips - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 36

stress ranges must be investigated, which, however, as with the fatigue of the concrete,
depend on the mean stress. This means that as the minimum load rises, so the fatigue
stress range diminishes.

3.6 Analyses for the serviceability limit state
The DAfStb guideline, supplementing DIN EN 1992-1-1 [20] together with its National
Annex [21], contains expressions for limiting stresses, crack widths and deformations
when assessing the serviceability limit state.
At the serviceability limit state, the strain in the strengthening system is limited for
reasons of the durability of the bond. The intention behind limiting strain to 2mm/m for
rare load combinations is to prevent signi cant damage to the bond of the externally
bonded reinforcement. Furthermore, yielding of the reinforcing steel under rare load
combinations is ruled out, likewise to prevent high bond stresses and irreversible
deformations of the structure.
The DAfStb guideline does not generally require veri cation that crack widths are not
excessive. However, adhesively bonded reinforcement can limit crack widths. To
exploit this effect, the guideline provides an accurate calculation method based on [91].
According to the DAfStb guideline, the limit values for member deformation to
DIN EN 1992-1-1 [20] in conjunction with its National Annex [21] may not be
exceeded, even after strengthening. When using the more accurate approach of
DIN EN 1992-1-1 [20] section 7.4.3, the bonded reinforcement can also be included
when calculating the deflection parameter. It is not possible to apply the simplified
method to calculate the deflection of strengthened members according to
DIN EN 1992-1-1 [20] section 7.4.2. However, aids (see [92]) may be used when
working with the more accurate method.

3.7 Detailing

3.7.1 Strip spacing
When using externally bonded CFRP strips, the centre-to-centre spacing a L of the
tension strips may not exceed the values according to Equation 3.87, which ensures that
the strips act uniformly over the width of the member:

<
:

0 effective span

5 slab depth
0

: 2

max a L (3.87)

: 4 length of cantilever

The values for this are included in all the former national technical approvals since
the first approval [93] for externally bonded steel plates and have proved to be
worthwhile ever since. Besides this requirement, it is also speci ed that the CFRP
strip closest to the edge of a member should be positioned no closer to that edge than a
distance equal to the concrete cover c nom to the internal reinforcement. The intention of
this is to guarantee that there is no spalling along the edge and that the strip forces are
transferred uniformly into the member in the region of the internal steel shear links.