Design of strengthening with near-surface-mounted CFRP strips - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 63

where Δ c tool is the tolerance specific to the tool used to measure the concrete cover
according to [98] or the manufacturer's instructions and should be at least 1mm. The
allowance for the slot depth Δ c slot takes into account the construction tolerances when
cutting the slot; 2mm should be selected as a minimum. In addition, Δ c member is required

to take into account how the concrete cover varies over the member; Δ c member = 0mm

may be selected for slabs, but c member should be at least 2mm for all other types of

member. Where the accuracy of measuring the concrete cover is improved by employ-
ing suitable measures, e.g. random checks of the cover by exposing the reinforcement
locally,

Δ

c member can be neglected.
In the DAfStb guideline the assumed ultimate strain is reduced by the factor

Δ

0.8
according to [27] when verifying the flexural strength in order to guarantee a residual
minimum deformability upon reaching the theoretical bending capacity.
ε LRd ; max κ ε ? ε Lud

κ ϵ =

(5.4)

5.3 Bond analysis
Essentially, the concept proposed by Blaschko [27] (q.v [99]) is used for verifying the
bond. In this concept it is assumed that the CFRP strip makes a full contribution and
there is good composite action between strip and concrete, with the strength of the
adhesive usually governing this composite action. As the composite action is very
effective, the full tensile strength of the CFRP strip can be anchored within a very short
length - similar to conventional steel reinforcing bars. It is therefore suf cient to check
the end anchorage at the point at which the strip is no longer required for the load-
carrying capacity, very similar to anchoring steel reinforcing bars. This concept to
describe the way in which strips in slots work has proved worthwhile over the past 10
years in the former national technical approvals and was therefore included in the
DAfStb guideline.
As with conventional reinforced concrete construction with curtailed reinforcing
bars, checking the bond according to the DAfStb guideline [1, 2] requires verification
of the curtailment taking into account the end anchorage of the CFRP strip.
This involves verifying that the design value of the member resistance is greater
than the design value of the acting internal forces in the strengthened condition
for every cross-section of the strengthened member. The partial tensile forces assigned
to the lines of reinforcement can be determined in a simpliedwaybyassuminga
planar strain distribution. Figure 5.3 provides an overview of the curtailment
veri cation.
As is apparent in Figure 5.3, verifying the anchorage of a near-surface-mounted CFRP
strip can therefore be carried out at the point at which the CFRP strip is first required for
loadbearing purposes (point A). The anchorage length l bL of the strip in this analysis is
the distance between point A and the end of the strip. To verify the anchorage, the
resulting design value of the bond capacity per CFRP strip depends on this anchorage
length l bL and the distance of the longitudinal axis of the strip from the free edge of the
member a r , which may not be > 150mm, according to Equation 5.5: