DAfStb guideline - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 7

example), timber components (see [33], for example), steel components (see [34], for
example) or composite components, nor does it cover combinations with other methods of
strengthening.
2.6.2 Strengthening systems
The DAfStb guideline [1, 2] covers strengthening systems incorporating adhesively
bonded reinforcement. German construction law requires that the strengthening system
being used must have been granted a national technical approval.
Externally bonded reinforcement assumes that an adhesive, based on an epoxy resin,
is used to attach reinforcing elements in the form of steel or carbon bre (CF)
materials to a concrete substrate from which all intrinsic substances (cement laitance)
and foreign matter (plaster, render, paint) have been removed using suitable methods.
An externally bonded strengthening solution therefore assumes that a compact tension
member, linear elastic in the area of the stress-strain curve considered, is attached to
the concrete with the help of a high-strength adhesive. Owing to this high-strength
adhesive and the compact form of the reinforcing element, a concrete failure is always
assumed in the case of the debonding of externally bonded reinforcement. For these
reasons, the design approaches cannot be directly transferred to other forms of
strengthening, e.g. upgrading with textile-reinforced concrete (see [35], for example).
Most of the experiments carried out and practical experience gained so far in Germany
has involved strengthening with steel plates and CFRP strips. Considerable experi-
ence has been gained with externally bonded CF sheets, too. However, there can be
much greater differences between different CF sheet products than is the case with
CFRP strips or steel plates. Therefore, the DAfStb guideline only specifies bond
values for CFRP strips or steel plates. Owing to the mechanics background to the
design equations, it is readily possible, however, to transfer the design approaches to
CF sheets as well by adapting or verifying the bond values given in the guideline.
These bond values are included in the national technical approvals for CF sheets.
When it comes to near-surface-mounted reinforcement (i.e. reinforcement bonded in
slots), it is assumed that reinforcing elements in the form of prefabricated CFRP strips
are bonded with an epoxy resin adhesive in slots sawn or milled in the concrete cover.
It is not possible to use steel elements instead of CFRP slots in such cases because
assuring adequate corrosion protection is awkward. Likewise, it is not possible to
apply the design approaches to other types of reinforcement, e.g. round bars, because
of the dissimilar bond behaviour.
Besides conventional bonding, it is also possible to use appropriate equipment to attach
prestressed CFRP strips (see [36-38], for example). However, owing to the numerous
unanswered questions about prestressed bonded reinforcement, the DAfStb guideline
[1, 2] does not include any design approaches for this form of strengthening.
2.6.3 Ambient conditions
As the acceptable environmental conditions depend heavily on the properties of the
strengthening system, the DAfStb guideline [1, 2] provides only general advice on