Civil Engineering Reference
In-Depth Information
8
Electrochemical chloride extraction
The target of electrochemical chloride extraction (ECE) is to reduce the
chloride content in reinforced concrete non-destructively down to a level
which is not critical for chloride induced corrosion activity. Within a short
time (usually 4 to 8 weeks for single treatments) corrosion affected structural
parts can be rehabilitated, and the corrosion protection of concrete for
the embedded reinforcement can be re-established. This is different
from cathodic protection (CP), which is meant to shift the reinforcement
permanently into an immune state, where it cannot corrode regardless of
environmental conditions. The feasibility and the efficiency of this treatment
depend on many factors and can vary over concrete surfaces. Thus, they
have to be evaluated thoroughly by an extensive condition survey and the
experience of the ECE designer prior to an application.
8.1 Work principle
The effect of chloride removal is caused by an electrical field between the
reinforcement and an external, non-permanent electrode (see
Figure 8.1).
This electrical field is controlled by the voltage between the electrodes, and
all ions dissolved in the pore solution are moved - negatively charged ions
such as chloride or hydroxyl ions towards the outside anode; positively
charged ions (mainly sodium in case of de-icing salt attack) towards the
reinforcement, which acts as cathode. The process requires wet concrete,
and since the number of water molecules around cations is larger than
around anions, more water will be moved into the concrete than out of it
during such a treatment. The higher the voltage that can be set, the more
intensive the chloride movement will be. Usually, 40 V is chosen for a good
chloride extraction progress under still-safe work conditions.
The migration of ions is a physical process which is forced along the field
lines between the electrodes: indeed it can happen only within the capillary
and shrinkage pores, which take other directions than the established
field lines. Due to the different size, specific movability (Elsener, 1990)
and concentration of the dissolved ions, they obtain varying percentages
of the total ion movement. Practically the portion of chloride in the anion
