Monument Future - Siegfried Siegesmund

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       ACOUSTIC EMISSIONS – INSIGHTS INTO THE DECAY MECHANISMS OF THERMALLY TREATED MARBLES

      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.

       Abstract

      Since ancient times, marble has been the preferred material for monuments, sculptures, ornaments and architecture. Though the stone is often a chosen material, long-term exposure of marble results in cumulative deterioration of the rock fabric. The rate and extent of deterioration depends on the rock fabric and the climatic conditions. Besides the thermal vulnerability of marble, a combination of thermal and hygric fluctuation accelerates the deterioration process. The weathering sensitivity of marbles can be characterised by the irreversible length change of samples after heating under thermohygric conditions as residual strain. This residual strain is a non-reversible deterioration and caused by microcracking induced by a pronounced anisotropy of the thermal dilatation coefficient of calcite. In brittle materials like marble, cracking and crack growth or friction on crack surfaces are accompanied by release of acoustic waves. The analysis of these acoustic emissions can give a deeper insight into the deterioration mechanism of marble.

      In this study, acoustic emissions of thermohygric treated marble were analysed and correlated with ultrasonic velocities, thermal dilatation and residual strains. Therefore, different types of calcitic marble were cyclically heated from 20 °C to 90 °C and after equilibration of the samples cooled down again to 20 °C. While the first cycles were performed under dry conditions, the following were executed in a humid environment. The analysis of acoustic emissions enables one to determine when cracking occurs during the thermal treatment. It is also possible to differentiate microcracking from internal friction. Furthermore, the evolution of deterioration can be estimated based on ultrasonic velocities. The combination of acoustic methods and strain measurement gives an insight into the disintegration mechanism and supports the development of prevention strategies.

       Introduction

      Due to the exceptional microstructure of marble, thermal fluctuations initially cause an integral decohesion of the calcite crystal grains. This opens the intrinsically dense and tight structure to the ingress of water. Then, starting from the surface, ongoing weather-related temperature change and especially frost-thaw action accelerate the deterioration process. Conventional test methods as described in European standards are not adequate to evaluate the weathering resistance of the various types of marble. For a reliable assessment of the weathering resistance of marbles new methods are needed, covering the special deterioration mechanisms 186of marble. In this study, acoustic emissions of thermohygric treated marble were analysed and correlated with ultrasonic velocities, thermal dilatation and residual strains. The analysis of acoustic emissions enables one to determine when cracking occurs during the thermal treatment.

       Marble weathering

      The reason for the high sensitivity of calcitic marbles can be found by the pronounced anisotropy of the thermal expansion coefficient of the calcite single crystals (Fig. 1a). Temperature changes lead to anisotropic volume changes of the crystals, resulting in microstresses and microcracks. Even moderate repetitive temperature changes can cause significant deterioration. The extent of the local stresses and the resulting cracks are controlled by grain size, grain-to-grain misorientation as well as type and contact of grain boundaries (Rüdrich 2003, Shushakova 2013).

      Figure 1: Schematic sketch of the thermal dilatation of calcite (modified after Rüdrich 2003). a) Coefficients of thermal expansion in different directions. b) Sketch of a single calcite crystal in its initial state. Volume change due to c) heating and d) cooling.

       Marble varieties

      Four different calcitic marble varieties were analysed using acoustic emission (Figure 2).

      Figure 2: Macroscopic and microscopic image of all analysed marble varieties. Top left depicts the Blanco Macael, top right the Großkunzendorfer, bottom left, Lasa and bottom right the Gioia.

      The white spanish Blanco Macael (BM) shows a granoblastic grain fabric, with almost polygonal grain to grain contacts and a median grain size of 395 µm. The greyish Polish Großkunzendorfer (GK) shows a seriate to interlobate grain fabric, interlocking grain boundaries and a median grain size of about 545 µm. An equigranular-interlobate grain fabric was analysed for both Italian marble varieties. The white Lasa marble (LA) has a higher 187median grain size of 419 µm compared to the fine-grained Carrara Gioia (GI, 284 µm), a whitish variety decorated with greyish streaks. Textural analyses showed that, with the exception of the Lasa, all varieties have charateristics of a c-axis fibre type, with a single c-axis maximum for their lattice preferred orientation. They show moderate to slightly higher intensities between 1.6 mrd (multiple of random distribution) for the Großkunzendorfer and 2.9 mrd for the Blanco Macael. The Lasa marble shows a girdle-like c-axes distribution, surrounding the a-axis maxima.

       Thermal Expansion

      All four marble varieties have been analysed with regard to their thermal dilatation behaviour under dry and wet conditions (for details see Koch and Siegesmund, 2004). The irreversible length change (residual strain, RS), was determined for all samples (15 mm × 50 mm cylinders) after each heating-cooling cycle (20°–90°–20 °C).

       Acoustic Emission

      Cracking, crack growth and friction of fracture planes in brittle materials like marble generate short pulses of acoustic waves. These acoustic emissions (AE) can be detected by piezoelectric sensors on the surface of the material. The analysis of AE signal features can give a deeper insight into deterioration mechanisms (Tschegg, 2016).

      Figure 3 shows the experimental setup that was used to study the decay mechanism of thermally treated marbles. All parts are glued together with silicone, which is a good acoustic couplant between marble and sensors. The cylindrical marble samples

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