Monument Future

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 have a diameter of 20 mm and a height of 50 mm.

In each experiment four marble samples and one reference sample of stainless steel were tested simultaneously in a climate chamber. In each cycle the temperature changed from 20 °C to 90 °C and back to 20 °C with a rate of 1 °K/min. The temperature was measured on the inner side of the acrylic glass cylinder with a thermocouple. Every temperature level was held for at least four hours. Ultrasonic velocity was measured automatically at 20 °C at the end of each cycle. For wet cycles the acrylic glass cylinders were filled with demineralized water before heating.

Figure 3: Experimental setup of the acoustic emission testing. Four specimens were tested at the same time in a climate chamber.

An AMSY5 AE-system with 10 AE-sensors type VS150MS was used to detect, process and to store the AE-data and to measure the ultrasonic velocity by active pulsing of the sensors. The bandpass filter was set to 25–850 kHz and the evaluation threshold was set to 36 dB_AE. For noise filtering the signals detected by the two sensors of each specimen were grouped to events by special time criteria. This type of filter is very effective and as expected, after filtering only a negligible number of events were detected on the reference specimen of stainless steel. In addition, the ultrasonic velocity measured on the reference specimen was constant throughout all tests.

Results and Discussion

The thermal expansion was analysed in two different directions. One according to the preferred c-axis orientation (z) and one perpendicular to z (y).

188In Figure 4, the irreversible length change (residual strain) is given in mm/m after each heating-cooling cycle (20°–90°–20 °C). After three dry cycles (red background) the samples were analysed under wet conditions (blue background). All four marble varieties show residual strain after the first dry cycle, with no or little increases in the following dry cycles. As the residual strain slows down, the adding of water again causes an increase of the residual strain. While the Lasa and Gioia marble shows no significant anisotropy for the residual strain, Großkunzendorfer and Blanco Macael show dependencies according to the analysed sample direction.

Figure 4: Residual strain [mm/m], after heating – cooling cycles under dry and wet conditions for a) Lasa, b) Großkunzendorfer, c) Gioia and d) Blanco Macael.

For these four marble varieties, the acoustic emission was analysed. Figure 5 shows the acoustic emission activity, namely events per time, detected during the first two dry cycles and during the first wet cycle on a Blanco Macael marble.

During the first heating a significant AE activity was detected from a temperature of approximatley 35 °C up to the maximum temperature of 90 °C. A significant part of these events with a cumulated energy of 78 × 10⁵ energy Units (eU) can be related to cracking and crack growth. The onset temperature of the acoustic emission activity of 35 °C indicates that the materials have not been heated up to higher temperatures before. The effect that acoustic emissions do not occur on loadings lower than before was first described by Kaiser 1950 for different mechanically loaded materials. For thermally loaded marbles this so-called Kaiser effect was verified in our own preliminary studies. In the following dwell time, when the thermal equilibrium is reached, the AE activity decreases to nearly 189zero. During the first cooling back to 20 °C, the number of events is comparable to the number of events during the first heating. Also, the cumulated acoustic energy of 56 × 10⁵ eU has the same order of magnitude like during heating. These events may be related to additional cracking, probably due to crack misfit.

Figure 5: Acoustic emission activity, the red graph shows the time-temperature course, while the blue peaks represent the number of events.

In the next cycles a noticeable AE activity was detected during cooling only. The acoustic emission energy in these cycles is significantly lower, nearly zero. This indicates that the main damage occurs during the first heating-cooling cycle. This is verified by the ultrasonic velocity as an indicator for the integrity of the marble (Köhler 1991). For the Blanco Macael marble the ultrasonic velocity measured in the y-direction decreases from 5,530 m/s to 4,450 m/s (Figure 6). In the following dry cycles no significant change of the ultrasonic velocity was detected. These results are in good correlation with the measurements of the thermal strains. No significant further increase of the residual strain was measured after the first cycle under dry conditions.

No AE activity was detected during heating or cooling in the wet cycles. A possible explanation is that no additional cracking occurs, and the water lubricates the grain boundaries and therefore no emissions from friction can be detected. A noticeable number of events was detected only in the dwell time at 90 °C, but the cumulative energy of these events is low. A part of the water evaporated during the dwell time and the upper part of the specimen was exposed to the ambient temperature of 90 °C, whereas the water temperature reached only 80 °C. The source of the events can be localized in the zone of the falling water surface. Therefore, the cause for this AE activity might be thermal or hygric strain in the water surface zone. The ultrasonic velocity of 3,940 m/s measured after drying indicates only a moderate additional degradation of the marble. This contrasts with the significant increase of residual strain in the wet cycles. Accepting that no further cracking occurs, the only possible explanation is an increased residual crack width. As shown in Table 1, the maximum expansions of Blanco Macael in wet cycles are equal or slightly lower than in the dry cycles. Thus, water remains in the pores and keeps them open after cooling. This hypothesis needs to be verified in further studies, e. g. by drying tests and microscopy.

The other tested marbles behave similarly. The highest AE activities and the main loss of ultrasonic velocity were measured during the first heating-cooling cycle.

As shown in Table 2, AE energy and change in ultrasonic velocity are in correlation with the residual strains in the first cycle. The seriate interlobate grain structure of the Grosskunzendorfer in combination with interlocking grain boundaries causes a high AE activity with high energies, indicating the formation of a large number of microcracks.

Figure 6: Acoustic emission activity during heating and cooling and ultrasonic velocity of Blanco Macael (BM), Gioia (GI), Lasa (LA), and Großkunzendorfer (GK) marble. The wet cycles are marked blue.

190Table 1: Maximum expansion [mm/m] for each thermal expansion cycle under dry and wet conditions.

Table 2: Residual strains, corresponding change of ultrasonic velocity (in the y-direction) and acoustic emission ernergy in the first dry and first wet cycle.

Blanco Macael, Lasa and Gioia have a comparable grain fabric and grain boundary geometry. According to their texture, the thermal anisotropy of Blanco Macael with a pronounced preferred orientation of the calcite crystals is more distinctive than the anisotropy of Lasa. This causes more intensive cracking and therefore a higher AE activity. The Gioia marble has a history of prior outdoor exposure. The low ultrasonic velocity before the first heating and the low AE activity can be explained by preexisting damage.

Summary

The deterioration mechanism of marble was studied by a combination of measurements of acoustic emission, ultrasonic velocity, thermal dilatation and residual strains. Four different types of marble were exposed to thermal fluctuations under dry and wet conditions. The results of the acoustic emission analysis show that the main damage arises during the first thermal treatment. This is confirmed by the development of residual strains and ultrasonic velocities in subsequent temperature cycles. Even under wet conditions acoustic methods detected no significant progression of deterioration. In contrast, under wet conditions a significant residual strain was observed. Apparently, the water prevents a complete closure of the cracks after cooling. This needs to be verified by further studies.

References

Kaiser, J., 1950. Untersuchungen über das Auftreten von Geräuschen beim Zugversuch. Dissertation. Technische Hochschule München

Köhler W., 1991. Untersuchungen zu Verwitterungsvorgängen an Carrara-Marmor in Potsdam-Sanssouci. – Berichte zu Forschung und Praxis der Denkmalpflege in Deutschland. Steinschäden – Steinkonservierung 2:50–53; (Hannover)

Koch, A., Siegesmund, S., 2004. The combined effect of moisture and temperature on the anomalous expansion behaviour of marble. Env Geol 46 (3–4).

Rüdrich, J. M., 2003. Gefügekontrollierte Verwitterung natürlicher und konservierter Marmore. Dissertation. Mathematisch-Naturwissenschaftliche Fakultäten der Georg-August-Universität zu Göttingen

Tschegg, E. K., 2016. Environmental influences on damage and destruction of the structure of marble. International Journal of Rock Mechanics and Mining Sciences 89, 250–258.

191

AUTOMATIC ESTIMATION OF THE P- AND S-WAVE ONSET-TIMES IN WEATHERED SANDSTONES BY SALT CRYSTALLISATION

David Benavente¹, Marli de Jongh², Juan J. Galiana-Merino³, Concepcion Pla⁴, Javier Martinez-Martinez⁵, Martin Lee², Maureen E. Young⁶

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 Department of Earth and Environmental Sciences, University of Alicante. 03690 Alicante, Spain

2 University of Glasgow, School of Geographical & Earth Sciences, University Avenue, Glasgow G12 8QQ, Scotland, UK

3 Department of Physics, Systems Engineering and Signal Theory, University of Alicante. 03690 Alicante, Spain

4 Department of Civil Engineering, University of Alicante. 03690 Alicante, Spain

5 Spanish Geological Survey. 28003 Madrid, Spain

6 Historic Environment Scotland, Forthside Way, Stirling FK8 1QZ, Scotland, UK

Abstract

In this investigation, we determined the onset of P- and S-waves considering signal pre-processing and the analysis of the recorded signals. For most stone conservation investigations, P-waves are easy to determine. However, the measurement of pure S-waves in fresh and weathered building materials presents critical experimental problems.

We recorded P- and S- waveforms in three sandstones used in the Scottish heritage. Doddington sandstone (D) is a quartz-arenite commonly used throughout Scotland and is currently utilised as a replacement stone at Jedburgh Abbey, Scotland. St. Bees sandstone (BC) is a dark-red lithic arkose and is currently used as a replacement stone at Arbroath Abbey. BC is quarried on the west coast of Cumbria, England and exhibits planar bedding. Forest of Dean (F) is a grey-green (sub)litharenite quarried in Gloucestershire and used throughout the UK, including in the restoration of Dunkeld Cathedral, Scotland. We obtained the P- and S-velocities as well as the calculation of wavelength of the P-waves in both fresh and salt weathered samples.

The recorded signals highlight that the microstructural components of rocks and their modification by salt crystallization affect the output signal. P-wave signals present a high signal/noise ratio and their arrival-times are clear. The wavelength increases with grain size and also the degree of salt crystallization. However, as grain size increases, the determination of S-wave arrival time becomes more problematic due to the contamination of S-waveforms by P-waves, a lower signal-to-noise ratio and an increase of wavelength. These difficulties are more prevalent in the weathered samples, where the manual picking of the onset-time becomes more difficult and time-consuming. In general, the automatic method obtains S-wave values slightly lower than the manual method and their discrepancies are, on average, lower than 5 %.

Keywords: Wavelet analysis, Building stones, Salt weathering, Primary wave velocity, Shear wave velocity Ultrasound, Non-destructive testing

192Introduction

These non-destructive methods based on elastic wave measurements are commonly used in stone conservation investigations in both laboratory and field experiments. The study of compressional and shear wave velocities is considered a reliable method of determining the elastic, physical-mechanical, and durability properties of studied samples. Elastic wave velocities are frequently investigated either individually or in combination. The compressional or primary (P) wave velocity, V_p, (also termed as ultrasonic pulse velocity) is a frequently studied parameter that is simple to measure in the laboratory. In addition, V_p is also used as an indicator of rock strength and degree of weathering. Obtained in combination with V_p, shear or secondary (S) elastic wave velocity, V_S, is used when calculating the Young and Poisson dynamic elastic moduli.

Modification of microstructural properties of rock caused by salt weathering, combined with the presence of crystallised salts and water, strongly affects the rocks elastic wave velocities. (Benavente et al., 2018). The determination of P-velocities in weathered stone samples could present difficulties due to waveform attenuation, which results in output signals showing a low signal-to-noise ratio (SNR). On the other hand, the generation and acquisition of pure S-waves in rocks is difficult (Wang et al., 2009). The most critical problems come from the contamination of S-waveforms by P-waves (P- wave appears before the S- wave arrivals), the reduction of waveform amplitude (lower SNR) and increase of wavelength.

As a result, picking of onset time of the S-wave, for both fresh and weathered samples, and the P-wave in highly weathered samples, becomes a complicated process. All of these difficulties limit the use of S-waves, particularly when calculating the dynamic elastic moduli, which considers a tabulated value of the Poisson’s ratio, yielding dubious estimations of elastic rock properties. Manual picking is a tedious and time-consuming process, which is subject to human error. Furthermore, manual operation relies on the attention of a trained technician, which can be a disadvantage when analysis of large volumes of data is required.

In this paper, we record P- and S-waveforms in three sandstones used in the Scottish heritage and obtain the P- and S-velocities as well as the calculation of wavelength of the P-waves in both fresh and salt weathered samples. We use an automatic methodology for the calculation of onset time that includes the signal pre-processing and the analysis, in the time-domain, of the first pulse symmetry, amplitude and duration criteria. This triple check provides greater confidence in the obtained results. This method limits human error, while improving the accuracy and reproducibility of P and S wave onset time estimations.

Materials and Methods
Stones

In this paper three sandstones used in the Scottish heritage are investigated:

Doddington sandstone (D) is a common building stone used throughout Scotland and is currently used as a replacement stone at Jedburgh Abbey, Scotland. The sandstone is quarried at Doddington quarry near Wooler in the Scottish Borders region; and forms part of the Fell formation, deposited during the early-mid Carboniferous. The sandstone is mineralogically mature and textural immature; with rounded-sub rounded grain shape and an average grain size of 0.25 mm. Mineralogy consists of: quartz (90 %); feldspar (mainly orthoclase) (10 %); and minor amounts of muscovite, lithic fragments and clay within the matrix. The sandstone is classified as a quartz-arenite.

St. Bees sandstone (BC) is currently used as a replacement stone at Arbroath Abbey and is quarried on the west coast of Cumbria, England. The red-dark red sandstone is part of the Chester sandstone formation, deposited during the early Triassic. The sandstone is texturally and mineralogically immature; with (sub)rounded – (sub)angular grain shape and an average grain size of 0.12 mm. The sandstone is classified as a lithic arkose. Mineralogy consists of: quartz (60 %); feldspar (orthoclase and plagioclase) (20 %); lithic fragments (10 %); mica (biotite and muscovite) (10 %); and clay-rich matrix. BC exhibits planar 193bedding on a scale of 1–2 mm. Therefore, during this study BC samples were measured in two directions: parallel and perpendicular to bedding, to determine the degree of anisotropy and potential effects on output signals.

Forest of Dean (F) is a distinctive grey-green sandstone used throughout the UK, for example in the restoration of Dunkeld Cathedral, Scotland. The sandstone forms part of the Pennant sandstone formation deposited during the mid-late Carboniferous; and is currently quarried at Barnhill quarry in the county of Gloucestershire, England. The sandstone is mineralogically and texturally immature; with (sub) angular grain shape and an average grain size of 0.19 mm. Mineralogy consists of: quartz (70 %); feldspar (orthoclase and plagioclase) (10 %); lithic fragments (15 %); muscovite (5 %); and a clay-rich matrix. The sandstone is classified as a (sub)litharenite.