APPLICATION OF NON-DESTRUCTIVE ACOUSTIC TECHNIQUES FOR EVALUATING THE STATE OF CONSERVATION OF CARBONATE BUILDING STONES IN ARCHITECTURAL STRUCTURES

The application of non-destructive acoustic techniques is fundamental for damage diagnosis of the carbonate building stones in monuments and for evaluation of their conservation state. This experimental work focuses on the application of non-destructive acoustic techniques to evaluate the conservation state of carbonate building stones commonly employed as building materials in the monumental structures of the Mediterranean cities. The propagation of acoustic waves through natural stones commonly used as building materials in historical monuments is becoming of increasing importance to determine its dynamic properties hence its mechanical behaviour. In this work the acoustic techniques in the ultrasonic range 24-82 kHz were used to define quantitatively some dynamic properties of study rocks for evaluation of their conservation state and better understand their alteration processes. Measurements of compressional and shear wave velocities have been carried out in laboratory. The relationship between the above mentioned geophysical measurements and some physical properties of rocks (i.e. uniaxial compressive strength, Young modulus) have been carefully evaluated. The acoustic data were also corroborated by the study of other rock properties (i.e. density, porosity, textural features, mineralogy) to correlate these properties with the elastic ones. On the basis of the results of the laboratory measurements in situ ultrasonic investigations were carried out on significant monuments of historical downtown of Cagliari (Italy). The in situ application have been aimed to draw up compressional wave velocity maps on the investigated structural elements in order to check zones of weakness, to assess the weatherability of the carbonate stones and evaluate the effectiveness of the restoration works. In fact, alterations in the material cause a decrease in the ultrasonic velocity, which can be used as representative of its elastic status. As deduced from several tests carried out in different experimental conditions the ultrasonic velocity values must be interpreted as relative. Notwithstanding this, the velocity maps indicate variations of the elasto-mechanical behaviour of the materials and consequently of their integrity. From our experimental tests result that the percentage of error for the velocity measurements in the different experimental conditions is not more than 6%. Thus the acoustic techniques can be useful employed in the diagnostic analysis and in monitoring the health status of building materials. Clearly the time interval of the non invasive acoustic retesting depends on the results obtained by the laboratory and in situ ultrasonic measurements in conjunction with the results of the petrophysical study, moreover, on the local climatic factors as rainfalls, temperatures, freeze-thaw. Finally the integrated approach used in this study may represent a cost-efficient tool to define and quantify the degradation phenomena of monuments and monitoring their health status.