is reported in the literature (Yavuz et al. 2010, Andriani 2014), although at entities depending on the temperature and stone structure, as well. It mainly relates to a thermal micro fissuring, which causes the reduction of the propagation velocities.
The microfissuring detected by the UPV test slightly affected the above mentioned physical parameters measured by stauration and buoyancy techniques. This finding suggests that the recorded decreases of the wave velocities may be relevant to the generation of a microporosity which has no effect on the water penetration, as reported in previous studies (Franzoni et al., 2013; Freire-Lista et al., 2016). Microfissuring recorded by UPV had a negligible effect also on the mechanical performance of the stone. Very close values of the compressive strength were measured in both yellow and red levels, corresponding to a strenght loss of 4 % in the discoloured level (Table 1). Similar entities of decrease have been recorded for porous limestones by Franzoni et al., 2013.
Conclusions
Macroscopic evidences of a fire in the calcarenites employed in an historic building were confirmed by mineralogical changes, which reflects 82on strong color changes. In particular, the change from yellow-beige to reddish color of the stone is consistent with the thermally induced transformation of goethite to hematite. This transition phase indicates that temperatures around 300 °C were reached in the red stone levels during the fire. Effects on the stone microstructure were not visible under optical microscope. Nor the measurement of physical properties showed meaningful variations in this regard. On the contrary, UPV detected a decrease of the propagation velocities, which probably denotes a stone microfissuring. Nonetheless, its entity did not compromise the mechanical resistance of the stone, which remained almost unchanged. High porosity may account for a slight microstructural damage recorded for the investigated calcarenite, where pores likely behave as free spaces for expansion of calcite grain preventing in this way an extensive damage.
Table 1: Color change (ΔE), bulk density (γb), porosity accessible to water (P), water absorption (WA), ultrasonic pulse velocity (UPV) and uniaxial compressive strength (UCS) measured for the stone from the yellow-beige and red levels.
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VARIATIONS OF CHARACTERISTICS OF SANDSTONE SUBJECTED TO WEATHERING AND CONSERVATION INTERVENTIONS
Miloš Drdácký1, Dita Frankeová2, Zuzana Slížková2
IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.
– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –
VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.
1 Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, Department of Heritage Science, Prosecká 76, 190 00 Praha 9, Czech Republic, [email protected]
2 Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, Department of Material Research, Prosecká 76, 190 00 Praha 9, Czech Republic
Abstract
The paper presents selected results of a comprehensive study of characteristics and behavior of seven typical sandstone types used for historic buildings in the Czech Republic, mostly in Prague. Stone characteristics were studied on materials affected by various historic environmental impacts and conditions generated by previous interventions. From seven types of sandstone were prepared nine series of test specimens which included chemically deteriorated surface layers (crust), cleaned
