Quantitative pulse-phase-thermography for masonry and concrete structures: Fid-Bau Portal

Quantitative pulse-phase-thermography for masonry and concrete structures

Arndt, R. / Maierhofer, C. / Röllig, M.

An advanced quantitative approach of pulse-phase-thermography (PPT) for nondestructive testing in civil engineering is described in this contribution. The characteristic frequency of the maximum phase-contrast between defects and sound areas is used as a means for the characterization of its depth. The new approach is tested in the laboratory on concrete structures with defects of polystyrene. The surfaces of the structures were heated with IR-radiators for varying time periods. PPT is a suitable enhancement for qualitative and for quantitative IT-measurements in CE. The new inverse approach for quantitative PPT via characteristic frequency is very useful for the estimation of concrete cover of 3 to 10 cm for at least 10 min of heating and to 5 cm with 5 min of heating with an IR-radiator and 2 hours of observation. It can easily be programmed into post processing algorithms for thermography measurements. The methodology based on the maximum phase-contrast has one main advantage: fch is unproblematic to locate as the SNR for fch is higher then that for fb or fb' in all of the so far regarded cases of PPT in CE. The only known disadvantage is, that the frequency-resolution for deep-lying defects becomes more inaccurate and requires longer observation times and/or bigger frame-rates, to enhance the frequency-resolution, at the cost of computing power and/or time resolution. Another option to optimize frequency-resolution is versa zero-padding, but that again needs computing power, which is often not available, especially for the usually big thermography data-sets. By optimising the measurement parameters, as frame-rate and observation time, corresponding to the depth to be quantified, this should be possible, as documented by [6]. Taking all this into account the results are very promising. For a decided specification of heating times and conditions further-going investigations are momentarily carried out, that will be published elsewhere.