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• Bar Sizes
  • Metric sizes
  • Inch sizes
• Bar-sizing Methods
  • Spacer-Ratio
  • Orthogonal
  • Accuracy
• Bar size Comparisons
  • US bar-numbers to metric
  • Prestressing Tendons
  • ASTM "D-numbers"

Bar Sizes

Metric sizes

These are what we refer to as "Whole Sizes".

Other sizes also found in practice include the diameters 14, 18, 22 &28mm:
these are the "in between" values from an "E20" series (with twenty values per decade), which contains all the whole-sizes above, and which would also include the diameters 7, 9, 11, 36 & 44mm, although they are rarely (if ever) found in practice.
The increase between adjacent half-sizes varies between +10% and +16%, with an average of about +12% (or an 11% decrease from the next in the list).

These are what we refer to as "Half Sizes".

The CoverMaster^® CM52 cover meter supports the eleven bar diameters of:
5, 6, 7, 8, 10, 12, 16, 20, 25, 32 & 40 mm (the "whole" sizes, plus ø7 as found in mesh fabric).

The CoverMaster^® CM9 cover meter supports twenty-one bar diameters:
5, 5½, 6, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, 25, 28, 32, 36, 40, 44 &50mm
(the full E20 sequence of "half" sizes).

Inch sizes

The CoverMaster^® CM42 cover meter supports the eleven bar diameters of:
#2, #3, #4, #5, #6, #7, #8, #9, #10, #11 & #14.

The CoverMaster^® CM9 cover meter has sufficient metric diameters supported that there will always be a size available that is within ±1mm of the "soft metric" conversions of these inch-related sizes.

Tables comparing inch and metric conversions of bar diameters are given at the end of this Application Note.

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Bar-sizing methods

Spacer Ratio

This is a two-stage procedure:

In stage one, two measurements are taken, and a ratio calculated.
A graph is used to convert the value of that ratio into a distance, which is the distance from the surface of the concrete to the centre of the bar.
The "line" on the graph is broad, and indicates the probable error in the process up to that stage.

In stage two, all possible diameter settings of the cover meter are investigated, to see which diameter setting gives the best agreement between (distance-to-centre plus half-diameter) and indicated cover.
For the CM5 and CM52, the diameter settings are from the list of "whole sizes"; with care, and when measuring a single bar, an accuracy of ±1 size can be achieved (ie within one size either side of the true value).
Additional errors will be introduced if there are other bars in proximity to the bar being measured: in such cases, the estimated bar diameter will be too large, but the estimated cover will still be adequately accurate.

For the CM9, the same process is used, but is all performed automatically by the instrument. The possible diameter settings of the CM9 are those of the E20 list of "half sizes", but the accuracy of the result is still ±1 size (or ±2 half-sizes), and is similarly affected by nearby bars.

Orthogonal

Although two separate measurements are again required, the determination of diameter is a single-stage procedure.
A ratio is calculated, and a graph (or look-up table) used to convert this to a bar diameter.
Again, the "line" on the graph can be made broad to indicate the variation in experimental values for the ratios; and in practice (inside the CM9) a "staircase" line is used instead, so that the deduced diameter is "quantised" to the nearest standard value in the E20 "half-sizes" list.

With care, and on a single bar, the accuracy will be ±1 half-size.
If there are nearby bars affecting the readings, the indicated result will usually be too large, especially if transverse bars are present.

Accuracy

For the Spacer-Ratio and AutoSize methods, an accuracy of ±1 size can be achieved, which can also be quoted as ±25% of bar diameter.

For the Orthogonal method, an accuracy of ±½ size, or ±1 half-size, can be achieved; this could be quoted as ±15% of bar diameter.

Please note that accuracy, for both methods, using either instrument, is critically dependent upon precise positioning of the search head and upon correct zeroing before each measurement.

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Bar Size Comparisons

Table 1 lists the old US eighth-inch number sequence and the new ACI318M/ASTM "hard metric" diameters, together with the "soft metric" conversions and the recommended CM9 and CM52 equivalent settings.

Table 2 similarly lists the ASTM standard prestressing tendons.

Table 3 lists the "ASTM dimensional requirements for deformed steel wire for concrete reinforcements (SI units)".
This is a numbering system which is neither linear nor logarithmic in diameter, but is linear in cross-sectional area.

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