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Rebar Location and Cover Measurement to Aid Corrosion Potential Surveys
John C Alldred MA MSc, Protovale (Oxford) Ltd
email: protovale@argonet.co.uk
Web: http://www.argonet.co.uk/users/protovale
Abstract
The measurement of half-cell potential at the surface of concrete
is one common and standard method of estimating the degree of
corrosion in reinforced concrete. Although this technique can
be used on its own, there are advantages in combining it with
a survey to locate the reinforcing bars and measure their cover.
The first advantage arises from the fact that it is necessary
to expose and access one bar in order to connect the reference
electrode. By being able to accurately locate a suitable bar,
it is possible to minimize both the time taken in breaking out
the bar, and also the amount of reinstatement work necessary after
completion of the survey. The second advantage is in the interpretation
of the half-cell potential readings. Although guidelines exist
suggesting limits for estimating the likelihood of corrosion,
it is also known that absolute values of potential alone are not
sufficiently reliable as indicators. A survey of concrete cover
to the reinforcing steel can be used in conjunction with the survey
of potentials to assist in the interpretation of the potential
readings and to improve the assessment of likelihood of corrosion.
This paper presents some practical applications of this dual approach.
Introduction
Throughout Europe, several countries are currently undertaking
a program of bridge-deck inspections, to assess the degree of
corrosion of the reinforcing steel, and hence forecast projected
lifetime.
In the United Kingdom, the test measurements are made according
to the Department of Transport Advice Note BA 35/90. This describes
how the surface of the structure under investigation is mapped
out into test areas of one meter by two meter rectangles, with
measurements being taken at 500mm intervals, yielding 15 measurement
points per test area.
Three separate measurements are made at each point:
- Cover: the concrete cover is measured to the reinforcement;
in this instance, measurements are not necessarily made at a grid
point, but directly over a bar near to a grid point;
- Chloride: total chloride ion content is measured by chemical
analysis of dust samples obtained by drilling at each test point;
- Half-Cell Potential: measurements are taken at each of the
grid points;
- Resistivity: measurements are made at each grid point, and
may be used to assist interpretation of the potentials recorded
above.
Practicalities
Although determination of concrete cover is only one of four different
measurements made, the choice of a suitable instrument can greatly
assist the other measurements: Chloride: BA 35/90 requires that
for prestressed concrete, all tendons and wires must be located
prior to drilling, to avoid damage to those members. It is also
necessary to locate the reinforcement so that samples can be obtained
from a representative depth yet avoid damage to the drill. Potential:
These measurements are made relative to a reference electrode
which is in direct contact with the reinforcement; it is therefore
necessary to locate a suitable place to break out the concrete,
which will give adequate access to a reinforcing bar yet minimise
the amount of subsequent reinstatement work required.
The CoverMaster CM52 and CM9 cover meters from Protovale allow
rapid location of reinforcing steel and determination of bar orientation;
and also yield accurate values for concrete cover, even to bars
of unknown diameter, as required for the cover measurements.
Interpretation
The simplest appraisal of half-cell potential readings is that
in ASTM C 876, where values more positive than -200mV (assuming
copper/copper-sulfate) are assumed to have a low probability of
corrosion, and values more negative than -350mV are assumed to
have a high probability of corrosion.
However, it has become apparent over the last few years that absolute
values of potential alone are not enough for reliable determination
of corrosion probability, as the absolute values observed can
vary significantly with moisture content (and also some lesser
factors, such as temperature).
The Resistivity measurements were introduced into the DOT procedure
so that, where potential readings are borderline (-350mV to -400mV),
the assessment of corrosion probability is based on resistivity
values (greater than 12000 ohm cm being low probability, values
from 12000 down to 5000 ohm cm representing increasing probability
of corrosion).
Unfortunately, resistivity is itself determined by a number of
factors, including moisture and temperature, and so some form
of confidence appraisal is desirable. In practice, since the largest
single factor in protecting against corrosion is the provision
of adequate concrete cover (but not significantly more than necessary,
otherwise the tensile strength of the ferro-concrete may be compromised),
there is usually observed to be a correlation between corrosion
of steel and shallowness of cover.
Hence, if areas of high deduced corrosion probability correspond
to shallow covers, and areas of greater cover correspond to lower
deduced corrosion, then the engineer will have improved confidence
in those estimated corrosion probabilities; whereas if there is
little or no correlation between cover measurements and the potential
results, there will be grounds for appreciating that other factors
may need to be investigated and taken into consideration.
Cover Accuracy
The only National standard for cover meters is the British Standard
BS1881:204. This requires a cover meter to be accurate to plus/minus
2mm or 5% when measuring the cover to a single bar of known diameter
under ideal laboratory conditions, but concedes that errors may
increase to plus/minus 5mm or 15% under site conditions. Advice
Note BA 35/90 requires the cover meter to be accurate to plus/minus
5mm or 15% when measuring cover to bars of diameters from 10mm
to 32mm; and assumes that corrections may be necessary for special
cements, heavy or lightweight concretes, or proximity of overhead
power lines.
The CoverMaster CM52 and CM9 cover meters from Protovale are both
accurate to within plus/minus 2mm or 5% when measuring covers
to bars of known diameter under all realistic site conditions.
The CM52 is accurate to better than plus/minus 11% when measuring
cover to bars of unknown size in the range 10mm to 32mm diameter,
and can be used to estimate bar size to further improve accuracy.
The CM9 measures the cover to bars of unknown size to full accuracy;
and automatically determines bar diameter in all but the most
congested of situations. Both instruments are completely unaffected
by cement weight, moisture content, changes in ambient temperature,
and the proximity of power lines.
Summary
The two Protovale CoverMaster instruments have been used by test
consultants as a vital tool in the assessment of corrosion in
bridge decks, and have not only met or exceeded all the stipulated
requirements for cover measurement, but have also: rapidly located
a suitable bar for the half-cell potential reference electrode;
been able to locate all reinforcing bars, and all prestressing
tendons within drilling depth, prior to drilling or coring for
chloride-analysis samples, thereby eliminating damage to both
drills and structural steel; assisted in the interpretation of
both half-cell potential measurements and concrete resistivitymeasurements and thereby increased the confidence in the corrosion
probabilities deduced.
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