Civil Engineering Reference
In-Depth Information
movement is largest at the beginning of the ECE and decreases over the
duration of the treatment, whereas the portion of hydroxyl ion migration is
increasing at the same time.
Depending on the type of cement used for the concrete, some of the total
chloride may be bound - mainly by the C
3
A as Friedel's salt, but also by
C
3
S or C
4
AF. The bound chloride does not cause corrosion activity. With
the removal of free chloride from the pore area, bound chloride will be
dissolved and gets free, since there is a dynamic relationship between free
and bound chloride. The importance of this effect is related to the binding
capacity of a concrete and is under discussion.
Electrochemical reactions take place on the reinforcement surface: the
reduction of oxides, oxygen and water. All of them are related to the current
and electrical charge which is impressed into the reinforcement/cathode by
the ECE. Typical current densities (related to the reinforcement surface),
range between 0.5 and 2 A/m², but can be much higher during the first
hours/days of a treatment. Normally, the main process will be the reduction
of oxygen and water, and according equation (8.1a) it will generate hydroxyl
ions, which raises the alkalinity of the concrete in the reinforcement
vicinity and is the main target of a related method - the electrochemical re-
alkalisation:
