Civil Engineering Reference
In-Depth Information
Throughout the course of any investigation, colour photographs should
be taken of points of particular interest.
1.4.2 Covermeter survey
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: its magnetic permeability and its 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. 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 (or even scaffolding) within the range of
the primary field 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, to ensure that accurate results are obtained. Some covermeters
estimate bar size and can correct for size, so giving more accurate results if
lapped bars are encountered.
Some manufacturers claim that the size of the reinforcing bar may be
determined by the use of spacer blocks and some inbuilt mathematical
processing, or with some of the more sophisticated machines by internal
data processing only. Such methods work satisfactorily only where a single
bar is present within the range of the search head, but can be reasonably
accurate with some of the more modern devices. For best results, however, a
breakout is always more trustworthy.
British Standard 1881 : Part 204 : 1988 (BSI, 1988) requires that when
measuring cover to a single bar under laboratory conditions, the error in
indicated cover should be no more than ±5% or 2 mm, whichever is the
greater. For site conditions, an average accuracy of ±5 mm or 15% is
suggested as being realistic in the British Standard. Recent developments
in covermeters are now improving on this, with covermeters capable of
