A method for the non-destructive testing of a reinforced / prestressed concrete building structure employs confocal magnetic tomography to obtain images of the magnetic distribution in planar fields at selected distances from the outer face of the structure. A magnetic sensor in a measurement plane (3) registers the combined effects of a magnetic source plane (2) incorporating the prestressed members (not shown) and a plane (1) nearer the concrete surface in which reinforcing rods (not shown) superimpose an unwanted distortion. Such distortion is suppressed by suitable filtration in order that faults/irregularities in the prestressing structure are reliably detected by a real-time Fourier transform programme.
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