Design concept of the calibration blocks was from the calibration method C in clause 6.4 in British Standard BS1881 Part 204. 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.