Design of strengthening measures with externally bonded CFRP strips - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 34

Fig. 3.12 Cause of transverse tensile stresses at end of strip

This is known as a concrete cover separation failure. The additional, vertical offset
between internal shear links and external strip leads to the build-up of tensile forces
between the internal and the externally bonded reinforcement. This problem is similar
to the situation with dapped supports, which are frequently encountered in precast
concrete construction. The acting forces can be determined approximately with a truss
model, as shown in Figure 3.12. To do this, the shear force is distributed in line with
the truss according to DIN EN 1992-1-1 [20] over the strut in the concrete and the tie
in the form of the internal shear link for the unstrengthened cross-section. A similar
truss is now set up for the component of the tensile force in the externally bonded
reinforcement and the two systems are superimposed to create one total system. A
concrete cover separation failure occurs when the tie in the concrete, which results
from the force in the strip, can no longer be carried by the tensile strength of the
concrete.
In order to prevent this type of failure, a shear strap must be positioned at the end of
the strip according to Equation 3.84 when the acting shear force at the end support, or
end of the beam, is greater than the shear capacity. In Equation 3.84, V Rd,c is
determined according to DIN EN 1992-1-1 [20] in conjunction with its National
Annex [21]. The variable a L is the distance of the strip from the end support, or end of
beam, in mm.

!

0 : 15

100 ρ s1

V Rd ; c ; LE 0 : 75 ? 1 19 : 6 ?

? V Rd ; c (3.84)

a 0 : 36
L

The critical value of the shear stress comes from a conversion of the model by
Jansze [87] to the model of DIN EN 1992-1-1 [20] in conjunction with its National
Annex [21]. This conversion and a validation can be found in [11].