Corrosion inhibitors - Concrete Repair - page 179

10.2.2 Inhibitors applied as surface treatments
Restoring alkalinity by surface treatments
Surface treatments for restoring alkalinity have been used, in which the
treatment is applied to the surface of the concrete and hydroxyl ions penetrate
the concrete surface by a combination of capillary action and diffusion. In
practice, such treatments are likely to only be effective on rather permeable
concretes, if the reinforcement is at any significant depth. Page et al. (2000)
cites German and Finnish research into this method, and concludes that to
have any chance of effectiveness, the coating would have to be quite thick,
probably greater than 10 mm, to provide the required buffer of hydroxyl
ions. Humidity would also be necessary to drive the diffusion of the hydroxyl
ions into the concrete. In practice, this type of restoration of hydroxyl ions
in the concrete is best done by electrochemical realkalisation, which has
been proved to be effective (see Chapter 9).
Penetrating corrosion inhibitors
S ODIUM MONOFLUOROPHOSPHATE
The American Strategic Highways Research Programme (SHRP) funded
investigations of monofluorophosphate (MFP), and it was concluded that
the material, when applied by ponding concrete with a 0.1 M solution, was
not effectively transported into chloride-contaminated concrete and was
thus ineffective (Page et al., 2000).
European reports on this product have been variable, with some workers
finding that the material was adequately transported to normal cover depths
after several applications by roller or spray application at 10-25% by mass or
in solutions of approx 1-2 M. Other trials failed to show adequate penetration
by the inhibitor, whether the concrete was pre-dried or not.
Schiegg and co-workers (Schiegg et al., 2000) carried out a three-year
field trial involving both MFP fluoride and Sika Ferroguard inhibitors and
concluded that neither material showed any significant effect on corrosion
behaviour. Figure 10.1 presents three cumulative frequency graphs showing
the behaviour of a reference panel and panels treated in 1997 with the two
types of corrosion inhibitor, with a second line showing their behaviour two
years later. The differences are insignificant.
On the other hand, Raharinaivo and Malric (1998) showed that it performed
quite successfully in carbonated concrete.
C ALCIUM NITRITE
While more normally used as an admixed inhibitor, some commercial patch
repair products have included calcium nitrite, presumably in the hope that
some of the nitrite will diffuse out of the repair into the surrounding concrete