Monument Future. Siegfried Siegesmund

by DSCTGA analyses and indicated a temperature around 300 °C affecting the stone. Microstructural modifications were investigated by means of optical microscopy and UPV propagation test. A quantitative determination of physical parameters such as color, bulk density, porosity and water uptake was also performed. Implications of thermal stress with the mechanical properties were assessed through uniaxial compressive tests. Thermal effects on the stone microstructure were not microscopically evident. On the contrary, UPV decreases indicated a microfissuring, but this slightly affected porosity, water uptake and compressive strenght.

       Introduction

      Stone materials involved in fires may be affected by a variety of phenomena, which can change their original properties and related performance in buildings. Depending on the temperature, both chemical and mineralogical trasformations may occur and lead to stone colour changes (Kompaníková et al. 2014). In addition, volume variations due to phase transitions (Calia et al, 2015), different thermal expansion of adjacent minerals (Vázquez et al., 2015) or strongly anisotropic thermal properties of some minerals (e. g. calcite) (Siegesmund et al., 2000) may affect the stone microstruture and lead to microfissuring, which may increase the stone susceptivity to weathering and compromise its load-bearing performance (Sippel et al., 2007). Thermally induced effects on the microstructure strongly depend on the inherent stone characteristics and have a high incidence on low porosity materials, due to the dense packing of crystals and grains (Yavuz et al., 2010). Thermal behaviour has been studied for a variety of stones, which mainly include compact materials (Martinho et al., 2018), while poor literature deals with heating damage on porous stones (Gomez-Heras et al., 2006; Brotóns et al., 2013; 78Franzoni et al., 2013). In this paper we present the results of a case study where the effect of a fire on highly porous calcarenites were assessed by using integrated investigation techniques.

       Material

      The study was carried out on the stone materials used within the masonry of the ACAIT (Azienda Cooperativa Agricola Industriale del Capo di Leuca) industrial building (Fig. 1). The building was a factory for the processing of the tobacco, in the province of Lecce (Southern Italy). It was built in the early 1900 and dismissed at the end of the 1980s. The factory is a remarkable example of the industrial archaeological heritage relating to the tobacco manufacturing, a flourishing activity in which Puglia region had a leading role on a national basis in the past century.

      The original building developed on the ground floor only and it has undergone several enlargements over the years. Like all industrial buildings, it has a simple and modular layout, composed of large rooms with corner-vaulted (“volta a spigolo”) roof, typical of the local tradition, load-bearing pillars and masonry walls with regular local limestone ashlars.

      Figure 1: The building of the tobacco factory ACAIT affected by a partial collapse.

      Figure 2: Detail of the stone from the collapsed vaults, which shows a yellow-beige-color (Y) passing to a reddish one (R) across the thickness of the masonry unit.

      In 2018, after a strong rainstorm, part of the structure collapsed (Fig. 2), involving in a first time one vault and part of the two adjacent ones and in a second time (about 7 days later) the remaining part (five vaults) of the room. The collapse evidenced the inner structure of the vaults and external walls. The latter were two leafs walls without horizontal connections, filled with incoherent material (pieces of rocks and debris).

      The vault collapse revealed traces of an historic fire, which were hidden by the presence of plasters, in the form of fumes and a diffuse reddish color across the thickness of the masonry units up to some centimeters from the surface (Fig. 2).

      In the framework of a diagnostic activity supporting a restoration project in view of a building reuse, the study of the fire damage on the stone was undertaken.

       Methods

      Collapsed blocks measuring 21x20x50 cm were taken from the site and samples were obtained from both the unaltered and altered portions, having yellow-beige (Y) and reddish colour (R), respectively (Fig. 2).

      The following analyses and tests were performed.

      — Thin-section samples were observed in plane-polarised and cross-polarised transmitted light by means of an optical microscope (Eclipse LW100 Nikon) at magnifications of 50x e 100x.

      — X-Ray Diffraction analyses (XRD) were performed on both the whole rock and insoluble residue. The insoluble residue was separated by a chemical attack of the grinded stone with HCl-3N in order to remove the carbonates by dissolution. A Philips 1742 diffractometer (APD – 3.6j version) was used for the analyses (CuKα, 40 kV, 20 mA, 2ϑ step size of 0.02 °, counting time 1.25 s, scan interval between 3 ° and 60 °). The diffraction data 79were processed with a X’Pert software — Philips Analytical.

      — Simultaneous thermal analyses by Differential Scanning Calorimetry and Thermogravimetry (DSC-TG). A Netzsch STA 449 F3 Jupiter^® was used. Both samples of the whole rock and of the insoluble residue were analyzed. Approximately 25 mg of each powder sample were heated in air from the ambient temperature to 1,000 °C, at a heating rate of 10 °/min.

      — Colour changes were recorded by colorimetric measurements. These were taken by light absorption in diffuse reflection using a Konica Minolta CM700d spectrophotometer. They were carried out with a D65 illuminant and under a 10 ° standard observer. L*, a* and b* colour coordinates in the CIELab system were measured and the colour variation (ΔE*) was calculated.

      — Measurements of bulk density, porosity accessible to water and water absorption amounts of the stone samples were performed through saturation and buoyancy techniques, following the ISRM recommendation [ISRM, 1981].

      — Ultrasonic Pulse Velocities (UPVs) were measured on specimens (cubes 70 mm side obtained from masonry blocks coming from the site) after drying at 70 °C, according to ASTM D2845-05 (ASTM 2005). In particular, three visible unaltered (Y) and three colored specimens (R) were taken from the collapsed portion of the building. Velocities were measured by direct transmission method using a TDAS 16 (Boviar) instrument and probes with a frequency of 55 kHz. They were recorded in each direction (x, y, z) of the cubic specimens and expressed as mean values.

      — Compressive strength tests were performed according to UNI EN 772-1 (UNI 2011) on the same specimens used for UPV test, after drying at 70 °C. A universal testing machine (Metrocom Engineering spa), with a load capacity of 200 kN and a speed of 0.2 mm/min, was used for the test.

       Results and Discussion

      The petrographic characteristics, as observed by optical microscopy under polarized transmitted light (Fig. 3a), show that the investigated stone is a medium grainstone. It is almost exclusively made of calcareous fossil remains, which mainly consist of coralline algae and, at lower extents, of benthic foraminifera, echinoids, bivalves and bryozoans. The average dimensions of the bioclasts fall between 0.3 and 0.4 mm with a maximum size of 0.6 mm. The stone contains sporadic quartz and feldspar crystals. The micrite is nearly absent and the cement is made of calcite, with a texture varying from microsparitic to sparitic type. It is in poor amount and fills only partially the interparticle porosity, which results very high. At large extents the cement is in the form of a thin level surrounding the grain borders. In some areas it is present in larger spots and exhibits a well-developed sparitic texture.

      Figure 3: Microscopic features of the stone (N//). a: yellow-beige level; b: red level.

      No damage in the form of microfissuring affecting the stone microstructure