In addition to providing priceless information as to the evolution dynamics of the structure, the stratigraphy readings also constitute a fundamental basis for the choice of homogeneous samples to analyze the characteristics of the materials. At the same time these surveys were carried out, an analysis campaign was organized that aimed at recognizing the stone materials present in the structure and at the characterization of the connecting grout used for the walls in the loggia.
This in-depth study is justified by the evident decay affecting the Torre's stone elements, especially the loggia, where decorations ean no longer be read and where entire sections of material have disappeared.
14 samples were taken from the four façades of the loggia to analyse the column, plinth and stone materials present in the masonry in the area below the loggia's floor.
Samples were analysed in a laboratory with polarized light microscopy (MO-LP) using traditional methods of mineralogieal analysis in "thin sections"
that entails the use of a very thin (about 0.03 mm) layer of stone glued between two slides. The material is observed through a polarizing microscope and the analysis is conducted under different types ofpolarized light in order to clearly observe all the characteristics of that particular sample. These characteristics, unique to each mineral, allow specialized technicians to recognize the materials on sight.
The results obtained confirm the visual analysis, in that they showed the great variety of the materials sampled, including: Pietra di Angera, Rosso di Verona, Trachite dei Colli Euganei, Pietra di Saltrio, Pietra Aurisina and Marmo Apuano.
The absence of consistent placement inside the building for each litote is moreover confirmed in the context of the elements themselves. The four columns analyzed are in fact one different from the other: the re-used stone materials originate from different urban "quarries".
Laboratory analyses continued, as previously mentioned, with the characterization of the mortar beds used in the loggia and were conducted with X-rays diffraction (XRD).
Diffraction allows the crystalline planes present in the samples analyzed to be recognised and, if compared with appropriate reference tables, they identify the elementary components of the material. In brief, the procedure uses X-rays that are directed towards a rotating sample and then diffracted with varying angles according to the materials that make up the sample. At the same time, a detector measures the width of the diffraction angles and records them on file for
subsequent analysis. This method provides qualitative and in some cases semi-quantitative results.
The results highlighted a rather uniform situation both in terms of the composition (high concentrations of quartz and calcite with a significant presence of potsherds) and of the state of conservation that is not good no matter the location and morphological aspect of the sample under study. In particular, nearly all the mortars are poorly consistent with pulverization in some sporadic cases, though, on the average they show a uniform and compact structure.
It is thus difficult to identify deterioration problems with respect to the exposition of the interior fronts as well as with respect to other factors external to the material. The results show a general state of conservation that is slightly worse as regards the mortars used to build the loggia parapet and those used for the masonry located over the arches and in the normal parameters relative to angle irons.
Analyses on the stone materials continued with ultrasound tests to control the compactness of the elements.
Ultrasound tests are based on the relationships that link the propagation speed of elastic waves in a given material to the elastic properties of the material itself. The propagation speed of these waves can be directly linked to the physical and mechanical characteristics of that material and, more to the point, a reduction of the propagation speed indicates irregular (cracks, empty spaces, detachments) or deteriorated material.
Ultrasound tests can be carried out in three ways (by transparency, on the surface and by radial transmission) according to the position of the transmitter and receiver on the surfaces under study.
These tests were performed on the small stone columns in the loggia and on the huge columns in cipolin marble at the Torre entrance. They revealed that the loggia elements are in good condition, despite the external surface attacked by corrosive elements and sulphatation, and that the material is compact. On the other hand, the columns at the entrance revealed low speed values and in some points the total absence of propagation of the ultrasound waves, indicating a situation that is worse than that seen by mere observation. The column on the left especially shows clear signs of chipping and lamination of the crystalline planes of the material with a progressive reduction of the resistant section.
The Torre has undergone a number of maintenance and restoration works over the past two centuries that implemented a series of measures aimed at improving the structure's static equilibrium.
The surveys conducted therefore focused on controlling the condition of these measures, and in particular the tension of the metal chains within the loggia that were not located inside the walls and the presence of steel strands within the masonry built (it is assumed) during the restoration works conducted in 1981.
The check on the chain tension was conducted through the dynamic analyses of free vibration.
Only the north and south chains (only two of the four chains in the loggia) were assessed, inasmuch as they were the only ones detached from the masonry. In fact, this circumstance greatly compromises the reliability of the analysis that was consequently not been conducted on the metal elements facing east and west.
The analysis was conducted by disturbing the chain, applying an impulse and recording the accelerations produced with an accelerometer placed at the center of the chain. The results acquired through a personal computer showed the poor tension of the metal elements that will undergo a tension enhancement procedure or will be replaced during the restoration works.
The last analysis consisted in checking, (using a magnetometer), the presence of metal consolidation strands which according to the projects that we possess were installed in 1981.
The theory that the documentation on paper referred to was confirmed by the presence of markers on the building's façades that were painted with colored paint which according to the other documents, represents the actual position in which the building firm that carried out the consolidation work had placed the strands.
Six readings were made on the external façades (on the north and east fronts) and two on the internal fronts (west façade), in order to verify the presence
of tie beams in correspondence with the paint traces that the positive outcome of the analysis had considered reliable with respect to the actual position of the structures.
The diagnostic analyses conducted (that constitute the necessary projects to know the static condition of the structure) were provided by the Regional Office of the Lombardy Region to the structural engineer who, on the basis of such data, will develop the security implementation plan for the Torre itself.
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