Issue34

D. Scorza et alii, Frattura ed Integrità Strutturale, 34 (2015) 70-73; DOI: 10.3221/IGF-ESIS.34.06 70 In the present paper, the fracture behaviour of the red Verona marble is experimentally examined, focusing the attention on the effects caused by a thermal pre-treatment, consisting in freeze/thaw cycles, on peak failure load and critical stress- intensity factor. More precisely, the specimens employed in the experimental campaign are extracted from plates panelling the Auditorium of the University Campus in Parma. Such an investigation is here developed in order to understand the parting of a marble plate occurring from the Auditorium panelling. A possible cause seems to be the material ageing produced by temperature variations induced by seasonal range. (a) (b) Figure 1 : (a) Specimen geometry mean values and variations; (b) experimental set up of the Instron testing machine. The investigations found in the literature are mainly aimed at analysing the influence of thermal pre-treatments, carried out at constant temperature, on fracture failure and toughness of natural stones [3-7] but, to the best knowledge of the present authors, no experimental data regarding the effects of such pre-treatments on the red Verona marble fracture parameters are available. Recently, the Two-Parameter fracture Model, originally proposed for concrete, has efficiently been employed to determine the critical stress intensity factor of a no-thermally-pre-treated Carrara marble [8]. Therefore, also for the easiness in specimens preparation and the simplicity of loading configuration, such a model is herein used to evaluate the fracture behaviour of both thermal pre-treated and as-received red Verona marble specimens. S PECIMEN DESCRIPTION AND EXPERIMENTAL PROCEDURE or the experimental tests, we have used 14 red Verona marble specimens obtained from the same panel of stone and prepared in the same direction in order to minimize the errors generated due to the sampling. The prismatic specimens, having the geometry shown in Fig. 1a, have a notch in the middle cross-section with depth equal to about 9.54mm ± 8%. We consider two cases (as-received and thermal pre-treated specimens) and, therefore, the minimum number of samples according to the indications contained in the UNI EN 12371 [9] is 14, since at least 7 specimens are needed for each test condition. The specimens tested are as follows: 7 as-received specimens (signed as C0-X), and 7 specimens subjected to 32 thermal cycles (signed as C32-X). The thermal treatments, consisting in freeze/thaw cycles, are regulated by UNI 11186 [10]. The specimens are immersed in water at room temperature for 48 hours, in order to ensure a proper imbibition. Then, they are subjected to daily thermal cycles, each one to maintain the specimens at a temperature of -28°C for 6 hours and to spend the remaining 18 hours in water at room temperature. According to the UNI EN 12371 [9], periodic visual analysis of the specimen surface integrity are performed during the thermal cycles in order to assess possible damages caused by the thermal treatments. In the present study, this control has been made every 4 cycles and, on the scale from 0 (undamaged specimen) to 4 (specimen broken into two or more parts), the worst result obtained from our treated specimens is 2, i.e. small crack (<1mm) or detachment of small fragments (<30mm 2 ) have occurred. The specimens are subjected to three-point bending by using an Instrom testing machine (Fig. 1b). The tests are performed under crack mouth opening displacement (CMOD) control by employing a clip gauge with an average speed of 0.05 mm/h and, according to the Two-Parameter Model (TPM) [7], the specimen is unloaded when the load is about 95% of the peak load along the post-peak branch, and subsequently reloaded in order to determine the unloading compliance. F