Civil Engineering Reference
In-Depth Information
GGBS can also mitigate the problem) and structures with ASR-endangered
concrete are best kept dry. ECE can initiate or accelerate ASR because water
is moved into the concrete - it can raise the water content up to about 8%,
hydroxyl ions are generated by the oxygen and water reduction and sodium
is moved into the concrete if NaCl is the origin of the chloride ingress,
concentrating especially around the reinforcement used as the cathode.
Intensive research on this issue is available (e.g. Page and Yu, 1995),
showing that no easy dependencies could be observed between impressed
charge, time of application and effects on ASR, but it is advisable to pay
attention to the presence of alkali reactive aggregates in the concrete of a
structure subject to ECE, and if conditions seem to be critical, to run trial
applications beforehand.
8.7 Chlorine evolution and acidification of the concrete
surface
If no precautions are taken, the anodic processes according to equations
(8.2a), (8.2c) and (8.2d) will generate large quantities of chlorine and very
acidic conditions on the concrete surface. Chlorine is not only a health
and environment hazard, but can establish a very corrosive atmosphere for
neighbouring metallic parts (installation equipment, cars etc.), especially
because the ECE requires wet conditions that lead to an increased air
humidity adjacent to the application.
These effects can be limited by the use of DSA with high chlorine
over-voltage (requires a special surface coating), alkaline and/or buffering
electrolyte solutions or an anodic ion exchanger that binds chloride and
releases hydroxyl ions at the same time.
8.8 Bond strength
The possible reduction of bond strength as a result of ECE has been
investigated under various test conditions. Negative effects - the reduction
of bond strength of up to 50% - have been found especially at smooth,
corroded rebars after applying very high charges at high current densities,
although no definite dependencies between the test parameters could
be concluded. Vennesland et al. (1996) showed that an increase of bond
strength was observed at applied charges of about 10,000 Ah/m² as well as a
recovery of lost bond almost back to the initial value within one month after
the termination of treatment.
Broomfield (2007) suggests that an increase of bond strength as a pre-
stressing effect from corrosion products on the reinforcement has to be
considered, which can be removed during ECE, but that there is no reduction
of bond compared with the uncorroded state. Furthermore, if ribbed steels
are used, the main bond is provided by the shape of the interface, so the
possible slip on smooth surfaces cannot take place. Another influencing
