Design of strengthening with near-surface-mounted CFRP strips - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 61

5 Design of strengthening with near-surface-mounted

CFRP strips

5.1 Principles
Besides strengthening options with externally bonded reinforcement, the DAfStb
guideline [1, 2] also includes a design concept for flexural strengthening using near-
surface-mounted CFRP strips.
In this form of strengthening the CFRP strips are fitted into slots sawn or milled in the
concrete and fixed with an epoxy resin adhesive. Bond tests have shown that placing the
strip in a slot and the associated distributed transfer of the tensile force into the
surrounding concrete results in a very favourable, robust bond behaviour. In contrast to
the situation with reinforcement bonded externally to the surface, a bond failure does not
take place in the layer of concrete near the surface, but rather in the high-strength
adhesive. So as it is not the moderate tensile strength of the concrete that determines the
loadbearing capacity, much higher bond stresses can be transferred. Apart from that,
high friction stresses can be transferred once the bond strength has been exceeded.
The comparison of the theoretical characteristic bond capacity at a single crack shown in
Figure 5.1, which presumes an approximately uniform effective bonded area, clearly
reveals the ef cient composite action of near-surface-mounted CFRP strips. In this
method the tensile strength of the strips can be reached over short transmission lengths.
Compared with externally bonded reinforcement, cracks in a strengthened member are
not a prerequisite for generating the tensile force (see [27]).
Owing to the very effective bond behaviour of near-surface-mounted CFRP strips,
which in terms of how they work is comparable with that of the reinforcing steel, the
known design approaches valid for conventional reinforced concrete can be applied here
with minor adjustments. The method for designing near-surface-mounted CFRP strips is
attributed to Blaschko [27] (q.v [99].) and has been incorporated in the DAfStb guideline
virtually unchanged from the earlier approvals (see [29], for example). This method only
applies within the limits given in section RV 3.8 of the guideline and cannot be applied
to other forms of reinforcement such as round bars.
Further background information on near-surface-mounted reinforcement can be found
in [27, 100, 101].

5.2 Veri cation of flexural strength
As with externally bonded CFRP strips, the analysis of the flexural strength can be
carried out in a similar way to that for a conventional reinforced concrete member by
investigating the cracked cross-section. The equations given in Section 3.2 can also be
used here. When using near-surface-mounted CFRP strips, it is necessary to determine
the effective structural depth d L of the CFRP strips as shown in Figure 5.2.

b L
2

d L h t s (5.1)