Civil Engineering Reference
In-Depth Information
will lead to durability problems and coating failure. In line with good
practice, integrity would benefit by the use of a fairing or scrape coat.
Realkalisation.html
accessed 06/12/2010
Allowing the concrete to dry thoroughly after treatment and scrupulous
cleaning is therefore another approach, with an appropriately formulated
fairing coat perhaps able to hold back any crystalline salts which might
disrupt coatings.
Typically the process uses an applied voltage of 10-40 V DC and a
current of 1-2 A/m
2
of steel reinforcement. The voltage is adjusted to give
the required current density. In such conditions, realkalisation treatment
is usually complete in 5-7 days, or even faster if the concrete is very
permeable.
It is easy to determine when the process is complete: a simple measurement
of carbonation depth will show when it has been reduced to zero. Measurement
of the ratio of voltage to current (effectively the resistance) is monitored and
once it reaches a steady value, the process is deemed complete. Also, an on-
site method for measuring sodium content of dust samples using a rapid ion
selective electrode method, has been developed by the author (unpublished),
where sodium carbonate is used.
As a smaller charge density is applied, the risks of damage are lower
than for chloride removal. As mentioned above, ASR is a risk if sodium or
potassium carbonate is used as the electrolyte.
Sodium carbonate can also cause short-term efflorescence, and the high
alkalinity after treatment can attack some coatings. Sodium carbonate will
attack oil-based paints, varnishes and natural wood finishes, but see the
comments above on mitigating these problems.
9.2 Case histories
Well over 100 structures have been treated with realkalisation around
Europe. Two cases are summarised above. Work has been carried out on a
1500 m
2
roof area of Walthamstow Magistrates Court. Carbonation depths
ranged from 5 to 25 mm. After treatment a polymer modified mortar was
applied to the surface and an elastomeric decorative finish was applied. The
advantage to the Client in this case was the lack of noise, so the Court could
remain in session.
What are the advantages of realkalisation over patch repair?
Realkalisation is a simpler and shorter treatment than chloride removal.
However, the alternative of patching and coating with an anti-carbonation
coating is much more effective than patching and coating for chloride attack.
The extent to which carbonation has reached the rebar, and the requirements
