Adequate cover to the steel
reinforcement in a structure is important to ensure that the steel
is maintained at a sufficient depth into the concrete so as to be
well away from the effects of carbonation or from aggressive
chemicals. However, excessively deep cover has its own problems;
crack widths may be increased and the lever-arm decreased.
All covermeters are
electromagnetic in operation. Electric currents in a coil winding in
the search head generate a magnetic field which propagates through
the concrete and will interact with any buried metal present, such
as reinforcing steel. The interaction will be due to either or both
of two physical properties of the steel: it's magnetic permeability
and it's electrical conductivity. The interaction causes a secondary
magnetic field to propagate back to the head where it is detected by
a second coil, or in some instruments by modifying the primary
field.
Hilti PS200 Ferroscan Covermeter and Typical
Reinforcement Scan
The signal received will increase
with increasing bar size and decrease with increasing bar distance
(cover). By making certain assumptions about the bar and
specifically by assuming that only one bar is present within the
primary magnetic field, the instrument can be calibrated to convert
signal strength to distance and hence to indicate the depth of
cover.
If there is more than one bar
within the range of the primary field (or even scaffolding!) the
instrument will receive a greater signal and indicate a shallower
cover than the true cover. The skilled operator will always
carefully map out the position and orientation of the steel,
breaking out some steel if necessary, to ensure that accurate
results are obtained.
Some manufacturers claim that the
size of the reinforcing bar may be determined by the use of spacer
blocks and some inbuilt mathematical processing. Such methods work
satisfactorily only where a single bar is present within the range
of the search head.
British Standard 1881 Part 204:
1988 requires that when measuring cover to a single bar under
laboratory conditions, the error in indicated cover should be no
more than plus or minus 5% or 2 mm whichever is the greater. For
site conditions, an average accuracy of plus or minus 5 mm or 15% is
suggested as being realistic in the British Standard. Recent
developments in covermeters have significantly improved accuracy,
with the Elcometer covermeters, for example, giving results within
better than 1% on average, over a wide range of bar sizes, lapped
bars and other difficult situations. Hilti's Ferroscan device now
enable an image of the underlying reinforcement to be plotted on a
screen after a bi-directional scan of the concrete surface (see
above).
The British Standard also lists a
number of extraneous factors which are potential sources of error.
Those concerned with magnetic effects from the aggregates or the
concrete matrix, and those due to variations in cross sectional
shape of the bars should not affect the modern covermeter, but care
must always be taken when dealing with multiple bars and the effects
of adjacent steel such as window frames or scaffolding as mentioned
earlier.
