Design of strengthening measures with externally bonded CFRP strips - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 11

3 Design of strengthening measures with externally

bonded CFRP strips

3.1 Principles
The design of a strengthening measure must be carried out in such a way that the
potential modes of failure are avoided. The various failure modes and their critical loads
must be known in order to carry out the design with a margin corresponding to the level
of safety for the critical load of the respective governing mode of failure. Figure 3.1
provides an overview of the failure modes that can occur. First of all, modes of failure
related to the function of the CFRP strip can be added to those familiar from con-
ventional reinforced concrete:
- Failure of concrete in compression zone
- Yielding of internal reinforcement followed by concrete failure
-

Yielding of internal reinforcement followed by failure of the adhesively bonded strip

- Shear failure

- Yielding of externally bonded steel plate.

Besides these modes of failure well known from conventional reinforced concrete and
relatively easy to describe, there are other modes specific to strengthening measures with
externally bonded reinforcement. The first of these that should be mentioned is concrete
cover separation failure, where the concrete cover becomes detached at the end of a strip.
This occurs due to the additional, vertical offset between shear link and strip because the
tensile stresses from the strip cannot be tied back to the compression zone of the beam.
This mode of failure therefore corresponds to a horizontal shear failure in the area
between the externally bonded reinforcement and the internal reinforcement.
The bond between the adhesive and the concrete often fails when using externally
bonded reinforcement. In such a bond failure the layers of concrete near the surface
break away once the tensile strength of the concrete has been exceeded. Owing to the
only moderate tensile strength of the layers of concrete near the surface, following local
debonding of externally bonded reinforcement, the result is mostly a total failure of the
bond between the externally bonded reinforcement and the concrete as the load rises
further because the forces involved cannot normally be carried by any remaining areas
of intact bonding (unzipping effect). This behaviour means it is necessary to consider the
bond of externally bonded reinforcement very carefully.
As flexural strengthening with externally bonded CFRP strips represents the most
common form of strengthening and considerable research into this form of strengthen-
ing has been carried out in recent years, a staged analysis concept is available. We can
choose between elaborate and simple analyses depending on requirements regarding
accuracy or economics. Figure 3.2 provides an overview of this staged concept.
Basically, designing flexural strengthening with CFRP strips always requires the
designer to carry out a flexural analysis, in which failure of the concrete in compression
or the reinforcement in tension is ruled out, a bond analysis and a shear analysis, and also
to check for the risk of concrete cover separation failure.