DAfStb guideline - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 8

this aspect. The permissible ambient conditions, such as exposure classes and other
environmental influences, are - like the ensuing protective measures - regulated in the
national technical approvals for the strengthening systems.
Generally, without additional protective measures, externally bonded reinforcement
may only be used for exposure classes X0, XC1 or XC3 to DIN EN 1992-1-1 Table 4.1.
In addition, the members in the region of the bonded reinforcement may not be exposed
to strong UV radiation (direct sunshine or indirect sunshine reflected off snow or water)
or alternating or permanent saturation.
One special aspect of bonded strengthening systems is their sensitivity to elevated
temperatures. Cold-curing epoxy resin adhesives are normally used for retro tted
strengthening measures. These thermosetting polymers are amorphous and very stable
below a certain temperature. However, at higher temperatures the crystalline phase
gradually breaks up and the adhesive loses its strength over the glass transition range. The
guideline therefore speci es that no loads may be allocated to the externally bonded
reinforcement once the start of the glass-liquid transition (minus a safety margin) has been
reached. This temperature is denoted T f and owing to its dependence on the particular
product is noted in the associated national technical approval. Without heat treatment, this
figure lies between 40 and 60 ° C for the epoxy resin adhesives currently on the market.
According to the current state of knowledge, it is also known that the glass transition
temperature essentially depends on the temperature during bonding and curing and during
any further hardening involving intensive heating. It was observed in [39], for example,
that a static glass transition temperature is not acceptable as a thermal serviceability limit
because it is heavily dependent on the curing conditions. Therefore, care should be taken
to ensure that when bonding at the bottom end of the service temperature range, no abrupt
rise in temperature to the top end of the glass transition range can take place.
2.6.4 Fire protection
As described in the preceding section, adhesively bonded reinforcement is especially
sensitive to elevated temperatures. Fire protection should therefore be paid special
attention. Basically, the options are either carrying out a structural fire analysis without
taking into account the bonded reinforcement or protecting the bonded reinforcement
against heat by applying a suitable protective system. Examples of structural re
analyses for members with bonded reinforcement can be found in [40], for example.
A structural fire analysis, ignoring the adhesively bonded reinforcement, can be carried
out according to DIN EN 1992-1-2 [41] in conjunction with its National Annex [42].
However, this is generally associated with the degree of strengthening being limited to
some extent. In addition, through using an approved fire protection system to protect the
internal reinforcement it is often possible to verify that the member has an adequate
load-carrying capacity in the event of a fire even in the case of failure of the bonded
reinforcement.
The other possibility is to protect the adhesively bonded reinforcement with a fire
protection system that is approved for that particular reinforcement. At the time of
drawing up the DAfStb guideline, however, no systems for protecting bonded