Civil Engineering Reference
In-Depth Information
upper reinforcement. Usually the chloride ingress has reached greater depths.
Since the field lines get established from the anode mesh to the rebars, it is
normal to find a rather patchy chloride removal in case of a widely spaced
reinforcement layout; the desalination is concentrated then on the area of
the rebars. If there is a high reinforcement density in the upper layer, it can
shield the ECE effect from the second (deeper) reinforcement layer.
If either concrete cover or rebar spacing deviate within a treatment area,
corresponding anode zones have to be defined. They should stay within 10
to 15 m². So the individual circumstances of the structure strongly determine
how effective an ECE run will be and how much effort it needs.
Short circuits between reinforcement and anode - if the outside electrolyte
can be in direct contact with rebars - must be avoided; cracks or visible
reinforcement have to be sealed and tested before commissioning the ECE
process.
8.4 Concrete composition and structural defects
The concrete composition, mainly the permeability - determined by cement
type, cement content, w/c ratio, compaction, curing - also has a great effect
on the ECE, as it influences the chloride ingress before treatment. Great care
has to be taken with repair patches which can have much less permeability
than the original concrete and hence limit the chloride migration.
Although it is recommended to repair all concrete defects before installing
ECE, delaminated areas and cracks may be tolerated for the benefit of better
site management in some cases, as long as these defects do not interrupt the
chloride migration or result in short circuits. This has to be verified on a
case-by-case base. If concrete repair has to be done before an ECE treatment,
the repair concrete should have a similar quality in strength and permeability
as the original concrete; if the repair layer thickness is large enough (> 4
cm), regular, unmodified concrete should be used.
8.5 State of corrosion at the start of treatment
If the reinforcement surface is entirely covered by corrosion products -
which can be observed at high chloride concentrations in the reinforcement
vicinity - the major initial reaction will be the reduction of oxides, which
does not result necessarily in the formation of hydroxyl ions. Theoretically
it can require up to 500 Ah/m² to reduce tightly covering corrosion products
from the steel surface until oxygen reduction (and related hydroxyl ion
formation) can predominate. This should to be considered at the definition
of treatment targets.
This effect will be indicated by high initial currents and relatively low
potential shifts measured by reference electrodes (considering IR correction).
Investigations (Schneck, 1994) have provided the first indication of this effect
and practical applications have verified this in many cases. Consequently,
