Reinforced Concrete Column — Cross-Polarisation to Image the Far Face and Calibrate Permittivity

Description

Reinforced concrete column surveyed on all four faces, labelled A, B, C and D, by vertical scanning from bottom to top along the centre line of each face. Two configurations: a GSSI FLEX NX dual-channel antenna enabling cross-polarisation, inline and cross-line, then a single-channel NX25, both at 2.5 GHz. Eight files in total. The cross-section dimensions are known, distances A-C and B-D.

What makes this dataset useful is what cross-polarisation reveals: the concrete-air interface at the far face. In a conventional acquisition that back-wall reflection is often masked by reinforcement echoes and by attenuation. Under crossed polarisation the response from the steel weakens while the response from a planar interface persists, and the far face emerges.

That reflection has direct metrological value. The column width can be measured on site with a tape, so a known thickness (14,17 in) is paired with a measured two-way travel time. The ratio of the two gives the propagation velocity in that particular concrete, and therefore its permittivity — rather than a tabulated value somewhere between 6 and 8 depending on the reference consulted.

Once that constant is calibrated, the depth scale of the radargram is correct, and so is any cover measured on the stirrups. That is the difference between estimating cover and measuring it.

Three comparisons are available: between the four faces, between the two FLEX channels to assess what cross-polarisation adds to the back-wall reading, and between the two antennas at equal frequency.

Raw unprocessed files.