Digital Processing of IR Thermography and Ultrasonic Signals in the Diagnostics of Carbonate Building Materials

Silvana FAIS^1,2 / Concetta FERRARA³ / Paolo FRONGIA³ / Paola LIGAS¹ / Giuseppe PIRAS³
(¹Dipartimento di Geoingegneria e Tecnologie Ambientali, University, Cagliari / ²Consorzio Interuniversitario Nazionale per l’Ingegneria delle Georisorse (CI.NI.Geo), Roma / ³Dipartimento di Scienze della Terra, University of Cagliari Italy)

Abstract: Non destructive diagnostic techniques have been undertaken using IR Thermography (IRT) and ultrasonic methods (UM) in order to assess the state of the decay of carbonate building materials of the important monument of the Tower of San Pancrazio in the historical town centre of Cagliari (Italy).

IR thermography is one of the most widely applied technologies in the diagnostics of monumental building materials. Thanks to the method, especially if carried out with instruments characterised by a high resolution and thermal sensitivity, it is possible to visualise and analyze, based on a calibration curve, the thermal profile of a structure starting from measured surface temperatures closely linked to the emissivity of the structure itself, which depends on its composition.

Therefore a thermographic investigation can provide a large amount of fundamental data for a precise and detailed diagnosis of the shallow parts of the investigated structures.

Ultrasonic methods based on the analysis of the propagation of elastic waves through a material are widely used to detect the elastic characteristics of carbonate building materials and thus their mechanical behaviour. UM are non-destructive and effective both for site and laboratory tests, though it should be pointed out that many features of the material (moisture, heterogeneity, porosity and other physical properties) can affect the propagation of the elastic waves.  In this study the IR thermography survey was performed by the Thermo Tracer TH9260 System, while the ultrasonic technique was performed by PUNDIT (CNS Electronics, London).

In this experimental application the analysis of the IR thermography and ultrasonic signals acquired in a sector of the masonry wall of the Tower of San Pancrazio was enhanced by the digital processing of the data while analysing them in the frequency domain. The Maximum Entropy Method (MEM) and Fast Fourier Transform (FFT) programs were used to the power spectrum estimation. The frequency spectra, interpreted also considering the petrophysical properties of the investigated rocks measured in the laboratory, have provided useful and objective information of quantitative type on the quality of the carbonate rocks. The knowledge of the spectral composition of the signals increases the quality of the diagnostic process compared to that performable from measurements of velocity or thermal response alone.

Keywords: IR Thermography, ultrasonic measurements, digital signal processing, FFT, MEM