Civil Engineering Reference
In-Depth Information
Realkalisation
In equations (9.1) and (9.2) it can be seen how carbonic acid reacts with
calcium hydroxide to form calcium carbonate. This removes the hydroxyl
ions from solution, and the pH drops, so that the passive layer is no longer
maintained and corrosion can be initiated.
The cathodic reaction (9.4) showed that by applying electrons to the steel, it
is possible to generate new hydroxyl ions at the steel surface, regenerating the
alkalinity, and restoring the pH.
The realkalisation process has been patented, and uses the same cassette
shutter or sprayed cellulose system developed to apply chloride removal
(though not the same as the CITec system introduced in
Chapter 8).
In
addition to generating hydroxyl ions, the developers claim that by using a
sodium carbonate electrolyte they make the treatment more resistant to further
carbonation.
The patent claims that sodium carbonate will move into the concrete
under electro-osmotic pressure. A certain amount will then react with further
incoming carbon dioxide. The equilibrium is at 12.2% of l M sodium carbonate
under atmospheric conditions.
Na
2
CO
3
+ CO
2
+ H
2
O
→
2NaHCO
3
The sodium hydrogen carbonate so formed is still significantly alkaline, and
while sodium carbonate remains, can still buffer the pH so that it doesn't drop
disastrously. In laboratory tests it has been shown that it is very difficult, if not
impossible, for a treated specimen to carbonate again. Over 80 realkalisation
treatments (50,000 m
2
) have been undertaken on structures around Europe
over recent years. These include the Hoover Building on the A40 into
London and a number of large reinforced concrete hangars at RAF Wyton in
Cambridgeshire. The treatment is faster than chloride removal, only requiring
a few days of treatment. Work at Heriot Watt University (Al Khadimi and
Banfill, 1996) showed that realkalisation actually improved the properties of
concrete, reducing porosity and permeability, increasing strength and modulus
and not apparently causing any difficulties.
Figure 9.2
shows a schematic of a
realkalisation system.
Anode types
Anode types are the same as for chloride removal. Cassette shutters or
sprayed cellulose are used by the owners of the patented system, with a
steel or coated titanium mesh. The steel is more likely to be used here as
the treatment time is shorter and the steel is less likely to be completely
consumed.
Figure 9.3
shows a typical sprayed paper system and
Figure 9.4
shows a typical cassette shutter system.
