Design concept of the calibration blocks was from the calibration method C in clause 6.4 in
British Standard BS1881 Part 204 [1]. Figure 1 shows the design details of the calibration
blocks and Figure 2 shows photographs of the calibration blocks. Two calibration blocks
were made using plywood with N24 and N28 bars respectively (which were particularly
needed for calibration in a recent BCRC project). Four bars (either N24 or N28 for each
block) with 100mm spacing were used as the main bars for each block. These four bars were
in different depth (i.e. approximately 20mm, 30mm, 40mm and 50mm). Actual bar depths
were measured from the bar both ends using a digital calliper and average values were used
as the actual cover depth. Two secondary bars with the same diameter and 100mm spacing
were also installed under the main bars perpendicular to the first layer.
The “concrete cover” for each bar was the distance between the surface of the calibration
block and the top surface of the measuring bars which consists of plywood and air gap. Since
covermeter operation is based on measuring electromagnetic field, it can be reasonably
accepted as recommended in British Standard BS1881 Part 204.