Design of strengthening with near-surface-mounted CFRP strips - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 65

strength f Gtk and the compressive strength f Gck . However, to adjust the values from the
bond tests, a system coefficient k sys specific to the product was incorporated in the
equation. The strength of the adhesive and this system coefficient can be found in the
national technical approvals for the systems and depend on the internal monitoring on
the building site. If the tensile and compressive strengths are checked as part of this
internal monitoring, then according to the national technical approvals for the systems,
values between 21 and 28 N/mm 2 can be assumed for f Gtk and between 75 and 85 N/mm 2
for f Gck . However, these characteristic values must also be obtained in the internal
monitoring according to part 3 of the DAfStb guideline following a statistical evalua-
tion. The product-speci c system coef cient k sys lies between 0.6 and 1.0 depending on
the system.
The concrete can fail in the case of a very low concrete strength and therefore the bond
strength of the concrete according to Equation 5.8 governs:

p

f cm

τ bck k bck ? (5.8)

In a similar way to the bond of reinforcing steel (see [102 105], for example), this bond

strength is calculated from the square root of the concrete compressive strength and a
calibration factor k bck . The system coefficient for the bond failure of the concrete can be
taken from the national technical approval for the system. Tests carried out at the
Technische Universität München established a characteristic value k bck =

-

4.5.

The factors α bG and α bC were introduced into Equation 5.6 to take account of the long-

term durability behaviour of the materials involved. As these are also coef cients
speci c to particular products, they can again be obtained from the national technical
approvals. Many studies of the long-term durability behaviour of concrete have been
carried out, and this behaviour is covered by DIN EN 1992-1-1 [20] together with its
associated National Annex [21]. Therefore, the long-term effect coef cient α bC for a
bond failure in the concrete should lie between 0.85 and 1.0. However, adhesives can
exhibit a much lower long term strength in some cases (see [100, 106-108], for
example). Depending on the adhesive and the ambient conditions of the application,
the long-term effect coef cient α bG for a concrete bond failure lies between 0.50
and 0.85.

5.4 Shear Force Analyses
When analysing the shear capacity, the same requirements apply for near-surface-
mounted CFRP strips as for externally bonded strips. This means that as described in
Section 3.4.1, verifying the shear capacity should be carried out according to DIN EN
1992-1-1 [20] together with its associated National Annex [21]. As with externally
bonded CFRP strips, the area of a near-surface-mounted strip may not be counted as part
of the tension reinforcement A sl in Eq. (6.2a) of DIN EN 1992-1-1 [20]. Counting the
CFRP strip as part of this reinforcement is not carried out in the DAfStb guideline
because only a few shear tests have been carried out on strengthened members without
shear reinforcement and so it is dif cult to predict the effect of this. If the shear capacity
analysis is not satis ed, shear strengthening for near-surface-mounted CFRP strips can
be provided as described in Section 3.4.2.