Monument Future

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IN-SITU INVESTIGATION OF STONE CONSOLIDATION EFFECTS WITH IMMERSED ULTRASONIC DOUBLE-PROBE

Miloš Drdácký¹, Marek Eisler², Rolf Krompholz³

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, drdacky@itam.cas.cz

2 Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, Department of Diagnostics and Conservation of Monuments, Prosecká 76, 190 00 Praha 9, Czech Republic, eisler@itam.cas.cz

3 GEOTRON-ELEKTRONIK, Leite 2, D - 01796 Pirna, Germany, service@geotron.de

Abstract

This paper introduces a portable ultrasonic double-hole probe capable of recording changes in material properties along a depth profile and assessing consolidant penetration depth. Application of the device requires drilling two 20 mm holes of arbitrary distance in the surface layer of the masonry. The holes can be drilled directly into a continuous stone block or advantageously in the mortar joints between blocks, provided that appropriate contacts are achieved between the probe transmitter and receiver and the continuous stone. This moderately destructive method is useful mainly for conducting measurements on stone masonry façades or structures in which drilling holes, with subsequent repair, is acceptable. Consolidation assessment is demonstrated on quartz sandstone blocks in a laboratory as well as on a masonry wall made of the same ashlar blocks. The probe is fully compatible with ultrasonic laboratory equipment, e. g. UKS 12 or UKS 14 Geotron Elektronik.

Introduction

Controlled structural impregnation and consolidation of porous materials require a reliable technique that enables the assessment of consolidation effects, especially the penetration depth of consolidating agents.

Consolidation effects are typically tested on specimens extracted from the treated bodies in the form of drilled cylindrical cores or cuboids cut from the object. The specimens are further cut into thin slices beginning at the treated surface and continuing along the depth. The slices are then tested using destructive methods – typically as discs or short beams loaded in bending (Drdácký & Slížková 2008). The determined strengths indicate changes in mechanical characteristics of the treated material along the depth.

The individual specimens are also suitable for non-destructive laboratory investigation of the penetration depth of consolidants using ultrasonic measurement in the transmission mode (Sasse & Snethlage 1996).

Such monitoring should ideally be performed during the consolidation process, i. e. after each impregnation cycle. It is practically impossible, however, to cut samples from an object after each treatment cycle.

A semi-destructive method that exploits the measurement of resistance to drilling is also used for in-situ testing of stone, but the specific character of the method prevents repeated application at identical points, and the monitored data is thus 234unreliable due to the heterogeneity of the tested material. Ultrasonic measurement, on the other hand, can provide integral information across a larger domain of treated stone objects, and the monitored data is less sensitive to small-scale heterogeneity within the material.

This paper introduces a new portable ultrasonic double-probe for recording changes in material properties along a depth profile and assessing consolidant penetration depth.

Double-probe ultrasonic device

For quality control of stone-masonry conservation, an innovative ultrasonic device has been developed jointly with Rolf Krompholz (GEOTRON-ELEKTRONIK) within the EK FP7 Stonecore project.

The device consists of two probes: an US transmitter and receiver. They were basically designed to be inserted into holes of 20 mm in diameter drilled into the investigated surface layer at a distance of up to 100 mm from their centres and a depth of up to 60 mm. Certain design features (a flat base and adjustable rods for the transmitter and the receiver) allow for reproducible insertion of the device into prepared holes in order to acquire a reliable series of measurements for the investigation of changes in material characteristics. The device is portable and fully compatible with ultrasonic laboratory equipment, e. g. UKS 12 or UKS 14 Geotron Elektronik.

The device is robust and well-engineered for problematic outdoor measurement conditions. It is possible, for the first time, to measure the properties of materials in situ with an acceptable impact. The main advantage over the standard drilling technique is that it is possible to monitor the conservation effect during the intervention process because changes can be measured at an identical place and over the same volume of material after individual impregnation steps. This allows restorers to continue or terminate impregnation repetitions in response to the measured impact, i. e. consolidation depth and expected strengthening effects. In such cases, optimum control of conservation and reasonably low material consumption are desirable.

The device can be adapted for measurements across the dimensions of entire stones with holes drilled in joints, as this may be more acceptable on historic stone facades. During such measurements, the probes are fixed in a special rail rig allowing for longer distances between them and providing their firm fixture in the holes.

Figure 1: The double probe scissors set up with the UKS 14 ultrasonic testing system.

Figure 2: US measurement across the whole length of an ashlar block with probes inserted into holes drilled in the masonry joints.

Impact of measurement conditions

The uncertainty of the measurements depends on several factors. The application presented here only requires comparable data for an assessment of the attained consolidation effect. Therefore, consistent sets of relative values are adequate, and the absolute values of the US velocities are not necessary.

235When using the rail arrangement for measurement of probe distances above those possible with the standard scissors double-probe device, it is necessary to slightly adjust the oscilloscope parameters, which in turn impacts the determination of US velocities. This effect, however, does not hinder the reliable determination of relative material characteristics.

Another important consideration is that measurement is sensitive to faulty contacts between the tested material and the transmitting and receiving probes. This is crucial when drilling holes for the probes. Sandstone is heterogeneous – it does not have a uniform density throughout its volume – and parts of the stone with higher densities respond differently to drilling than those of lower density. The resulting hole may therefore be irregular in shape, i. e. neither perfectly cylindrical nor perpendicular to the surface.

Variations in the determined US velocities for Hořice quartz sandstone are presented in Table 1. The uncertainty characteristics were calculated from 20 sets of 10 measurements done on an ashlar block, with probe distances of around 50 mm for the scissors and 170 mm for the rail arrangement.

Table 1: Statistics of the wave propagation velocities.

Furthermore, the presence of water in the pores of the stone affects the propagation velocity of the sound wave. The wetting of stone causes either an increase or a decrease in the speed of sound wave propagation, up to a point when saturation of the stone reaches its maximum value. The character of the change in sound propagation due to water saturation is dependent on the properties of the stone as well as the wave propagation direction with respect to the sedimentation layers of the sandstone. Typical values for the Hořice quartz sandstone are shown in Table 2.

Table 2: Change of wave propagation velocities in m/s.

The data in Table 2 was acquired on specimens of dimensions 300 mm × 50 mm × 50 mm. The axis of the beam corresponds to the longitudinal direction of the wave propagation.

Graphical representation of the impact of water, as well as reproducibility of acquired data, is shown in Figure 3.

It is apparent that differences arising from repeated measurements are sufficiently lower than the difference resulting from water saturation.

The velocity of the sound wave propagation in homogeneous materials generally depends on their modulus of elasticity in tension, density, and porosity. In heterogeneous materials, the propagation speed varies, resulting in different values when measured over various distances. The impact of probe distance was checked in the laboratory as well as on a stone wall.

The results presented in Figure 3 and Figure 5 indicate that on a practical scale the measured velocities are influenced less by distance than by other factors, namely the contact between the probe and the stone surface. Also, in real situations, differences are more the result of moisture content than variations in material quality. The surface layers of the stones in the masonry wall may be dryer or wetter than the inner layers of the wall, which can cause apparent variations in US velocity along the depth. However, the difference is only of the order of 3,2 % to 4,3 % in the tested Hořice sandstone wall.

Impact of consolidation treatment

The in-situ application of the method described above was tested on a quartz sandstone (Hořice quarry) wall after a demonstrative impregnation with a tetraethyl orthosilicate agent. Due to their chemical similarities to sandstones in particular, and their simple application, silicic acid esters 236(SAE) are one of the most commonly and most successfully applied materials in stone conservation, and therefore they were selected for the pilot tests. The average bulk density value of Hořice sandstone is 1,810 kg.m-3, its porosity is 25.84 %, and the diameter of the highest occurring pore is 40.22 mm. The measured US velocities correspond to the references in literature for sandstones of similar bulk density and porosity, e. g. S. Garia et al. or Freund (1992).

Figure 3: Comparison of US velocities measured with double-probe variants on dry and saturated sandstone.

Figure 4: Testing of the influence of the probe distance and the contact effects.

Figure 5: Measured US velocities in stones of different average travel length – 310 mm in stone E; 295 mm in stone I; 375 mm in stone O – compared to velocities for a stone block measured in the laboratory with the scissors device over a travel length of 50 mm and the rail device over a length of 170 mm.

Figure 6: Drilled holes in the treated areas, and the orientations of the transmission waves.

A Hořice quartz sandstone masonry wall was treated, in limited and well-defined areas, with three types of agent: the Remmers KSE 300 – without solvent, with 30 % gel deposition potential; the Remmers KSE 510 – without solvent, with 45 % 237gel deposition potential; the Porosil Z – containing solvent (ethanol), acid catalyst, with 30 % gel deposition potential. The consolidants were applied to the sandstone blocks by brushing. Then a set of holes was drilled.

The impact of the consolidation effect is clearly visible from the obtained data in Figure 7, as are the gradient of its distribution along the depth profile starting from the surface and the attained penetration depth.

Figure 7: US velocities in stones treated with Porosil, KSE 300, and KSE 510 – all applied by brushing – and an untreated stone.

Conclusions

This method provides restorers with a unique capability to monitor the progress and results of consolidation. Measurement is quick and reliable, though problems may arise involving hidden material irregularities, unknown moisture distribution, or gross imperfections along the hole surfaces preventing the necessary probe contact.

Acknowledgement

This paper is based on the results of research supported by the institutional project RVO 68378297.

References

Drdácký, M. F., Slížková, Z. Performance of glauconitic sandstone treated with ethyl silicate consolidation agents, In Proc. of the 11th Int. Congr. on deter. and cons. of stone – J. W.Łukaszewicz, P.Niemcewicz (eds.), Vol.2. Toruń: Nicolaus Copernicus University Press; 2008. pp.1205–1212.

Sasse, H. R., Snethlage, R. Evaluation of stone consolidation treatments. Science and Technology for Cultural Heritage. 1996;5(1):85–92.

S. Garia, A. K. Pal, K. Ravi, A. M. Nair: A comprehensive analysis on the relationships between elastic wave velocities and petrophysical properties of sedimentary rocks based on laboratory measurements. Journal of Petroleum Exploration and Production Technology. 2019, Volume 9, Issue 3, pp. 1869–1881.

Freund D.: Ultrasonic compression and shear velocities in dry clastic rocks as a function of porosity, clay content and confining pressure. Geophys. J. Int. (1992), 108, 125–135.

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239

A NEW SYSTEM FOR FAST ULTRASOUND-TOMOGRAPHY AT MARBLE SCULPTURES

Frank Tiefensee, Christoph Degel, Peter Weber, Wolfgang Bost, Manfred Moses, Marc Schmieger

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.

Fraunhofer-Institute for Biomedical Engineering (IBMT), St. Ingbert, Germany

Abstract

The authors present a new ultrasonic system for the automatic determination of tomography on stone. The tomography can be determined with up to 32 ultrasonic transducers in a variable length holding system. The positions of the sensors are determined with a magnetic field tracking or optically with Aruco markers. The signal control is done by a single channel electronic with 32x multiplexer. In this way the determination of a tomography from 32 measuring positions could be reduced to approx. 1 hour.

Introduction

For about 50 years, various ultrasound-based methods have been developed for the examination of stone in the field of cultural heritage and building technology [1]. The review article by Chiesura et al. [2] and the book Architecture in Stone by S. Siegesmund and R. Snethlage [3] provide an informative overview. The speed of sound is the most widely used parameter in the investigation of marble, particularly the speed of sound of longitudinal waves used in 70 % of the articles sighted. The speed of ultrasound depends on the mineralogical, physical and mechanical properties [4], the degree of saturation of water [4, 5] and the degree of aging. The speed of sound in marble varies between 6000 m/s for freshly quarried material and 1500 m/s for heavily weathered material. Therefore, the determination of the speed of sound provides valuable information about the properties and condition of the marble. The most widespread method is the measurement of the sound velocity of longitudinal waves in transmission. Two ultrasonic transducers are mounted opposite each other on the stone to be examined and the sound velocity between them is determined. The transmission method can be modified in various ways by varying the arrangement of the transmitter and receiver in order to adapt them to different measurement tasks. There are, for example, the so-called radial transmission [6], the semi direct transmission [7] or refraction methods with surface waves [7]. A very meaningful technique is ultrasound tomography. It allows the representation of velocity distributions in cross-sections of marble objects and thus the assessment of the course of weathering [8] and the success of conservation measures [9]. Up to now, the measured values for the calculation of a tomography have been recorded manually with (digital) calipers and single-channel electronics with two ultrasonic transducers between 46 kHz and 250 kHz. First, the transmitter transducer is in any measuring position and the receiver transducer 240successively takes up the other measuring positions in the same plane. For each measuring position, the velocity of sound between the transmitter and receiver is determined. The amount of sound velocities that can be assigned to a transmitter position is called projection. After the data of the first projection has been acquired, the transmitter is moved to the next measuring position and the data set of the second projection is measured. A tomography of 32 measuring positions consists of 32 projections, i. e. 31 × 32 = 992 individual measurements. It is easy to understand that recording the data of an ultrasound tomography is very time-consuming and can take more than one working day. The ultrasound system presented here for the first time reduces the required time for a tomography with 32 measuring points to approximately 1 hour.