Monument Future

Relevance of the calibration law

Before using the method at real scale, the relevance of the calibration law has been tested for several scenarios. One of them consisted in the desaturation survey of the limestone sample. IR thermography technic is carried out simultaneously with an electric technic and sample mass measurements.

Tests are carried out on cylindrical samples of 2 cm diameter and 2 to 4 cm height previously saturated using the 48 h porosity protocol. The electrical and infrared measurements are done inside a black chamber specially designed to attenuate the environmental infrared noise sources. During the electric and infrared measurements, it has been simultaneously measured the mass loss with a 1 mg accuracy. Multiple sensors are placed inside the black chamber allowing to record the chamber temperature, atmospheric pressure and relative humidity variations.

The monitoring of the desaturation process by the electrical method is done by analyzing the electrical resistivity variation using the two-point probes method. The electrical resistivity is calculated using the Ohms law. The voltage is measured between the two extremities of the sample for a fixed current.

The signal injected at the top of the sample is a square waveform of 12 V of amplitude and 137 Hz frequency. This signal is compared to the signal received at the bottom of the sample in order to calculate de variation of the voltage during the desaturation process. An Average Waveform Generator incorporated to a digital oscilloscope (TiePie Handyscope HS5) is used. The same oscilloscope is used to record the evolution of the signal with a resolution of 12 bits and a sampling frequency of 10 kHz.

The thermal scene has been recorded with the same camera previously presented. IR images of the thermal scene are recorded at a frequency of 0.01 Hz. From each IR Image a surface area has been selected to highlight the temperature evolution of the limestone sample and the reference (crumpled aluminum-foil).

Finally, a mass balance allows to measure the mass loss during the experience.

Results and Discussion

In order to determine the most convenient ROI used to calculate the thermal response versus the saturation degree, those histograms are used to determine the relevance of the ROI’s by analyzing their statistical moments. Evolution of the mean temperature values versus the water content is presented in the figure 4.

Figure 4: Mean temperature values and its standard deviations for each ROI with fit curve model.

255Functional mathematical relation is highlighted. In addition, with ROI 3 the skewness value is closed to zero. The third and fourth order moments values are consistent with a Laplace-Gauss distribution. As a consequence, the reduction of the surface analysis gives a better standard deviation to mean value ratio.

Finally, for a better accuracy of the calibration protocol, the reduction of the analysis surface helps to prevent against the edge effect and the risk of the non-homogenization of the water in the porous media.

In order to avoid edge effects, the calibration between the sample saturation and the IR response are defined by a fit function representing the evolution of the average temperature values of the ROI 3.

The second set of results corresponds to the experiments designed to survey the desaturation of the materials via electric and infrared imaging methods as well as by gravimetric measurements.

After data treatment the results are presented in the form of curves representing the evolution of electrical resistivity (deduced from the non-invasive electrodes) and temperature variation according to saturation, figure 5.

Figure 5: Thermal and electrical results of the desaturation experience showing the different zones related with each step of the desaturation process.

The results show that the saturation interval for which the IR and electric imaging method provides usable information is between 20 and 50 % and between 35 and 90 % respectively.

The interpretation of the results helps to define the phenomenological model of desaturation.

The results allow to identify six main steps corresponding to the desaturation dynamics model presented in the figure 5. The steps of the model correspond to the different zones reported in the figure.

ZONE 1: During this stage, there is a drastic temperature drop of the sample. This is due to the evaporation of the water covering the surface of the sample.

ZONE 2: During this stage, no significant temperature variation on the sample surface is detected. But in the same way, the water losses are detected by the electrical resistivity measurement. The water content decreasing (calculated from the mass measurement) is logically observed by the electrical resistivity increase. During this stage, the two used methods show the water redistribution in the volume probably through a stable hydrous transfer at the surface of the sample induced by the thermal equilibrium at the surface.

ZONE 3: During this stage, temperature and electrical resistivity increase quickly. This is due to the removal of the waterfront inwards of the sample and the gradual loss of the water volume.

ZONE 4: During this stage, only the temperature measurements seems to be able to characterize the sample saturation. A limit electrical resistivity is reached during the stage. This behavior is probably induced by the loss of electrical continuity in the volume (distribution of water in the form of clusters).

ZONE 5: Due to the gradual evaporation of the water from the unconnected clusters, a constant temperature is reached during this stage too.

ZONE 6: The sample is dry and as consequence non relevant signal is detected for both methods.

The results for the thermal response of the dynamic desaturation experiment allow us to analyze the validity of the calibration of IR responses based on the saturation calculated in a static frame.

In addition these results show that the combination of electric resistivity and IR imaging technics presents some advantages compared to others NDT as NMR or EFD (also known as Susi-R) that could be found in the literature (EN 16682, 2017, V. Di 256Tullio et al., 2010). The advantages and drawbacks of each method are presented in the table 2.

Table 2: Advantages and drawbacks of common NDT (EN 16682, 2017).

Indeed, due to the characteristics of the IR imaging method concerning its portability and ease of use as well as the quick diagnosis at metric scale, combined to the volumetric measures capabilities of the electric imaging method, the combination of those two imaging technics makes a highly appropriate and powerful proceeding for water content characterization on stone building materials.

Conclusions

The complementarity of the infrared thermography and electrical imaging methods is highlighted by showing the different relevant saturation ranges of both methods. This allows to establish the basis to the creation of a protocol which can be used to monitor the effects of changes of water content in the porous media. Illustration is given in the paper with limestones building material.

Calibration procedures are developed, and tests are carried out to highlight the degree of saturation by mean of electrical and IR thermography imaging. The identification of the material properties (like petrophysical properties) can be used to describe phenomenological patterns affecting the in-situ structure. Nevertheless, such point of view must be completed to highlight accurately the water content and its evolution to better understand the damages and evolution of the historical monuments.

The two combined methods seem to be well adapted to characterize the water transfer in the structures and for a better accuracy of the proposed diagnosis, the conclusions coming from another calibration scenario will be added.

Such a combination of NDT is very promising regarding other generally used methods.

References

D. Benavente, G. G.-H. Cultrone, The combined influence of mineralogical, hygric and thermal properties on the durability of porous building stones, Eur. J. Mineral. 20 (2008) 673–685.

ICOMOS ISCS, Illustrated glossary on stone deterioration patterns. Monuments and Sites XV, 2008.

EUROPEAN STANDARD EN 16682: Conservation of cultural heritage – Methods of measurement of moisture content in materials constituting immovable cultural heritage, European Committee for Standardization, Brussels, 2017.

V. Di Tullio, N. Proietti, M. Gobbino, D. Capitani, R. Olmi, S. Priori, C. Riminesi, E. Giani. Non-destructive mapping of dampness and salts in degraded wall paintings in hypogeous buildings: the case of St. Clement at mass fresco in St. Clement Basilica, Rome, Analytical and Bioanalytical Chemistry, vol. 396, no 5, p. 1885–1896, 2010.

E. Grinzato, N. Ludwig, G. Cadelano, M. Bertucci, M. Gargano, P. Bison, Infrared thermography for moisture detection: A laboratory study and in-situ test, Mater. Eval. 69 (2011) 97–104.

M. A. Hassine, K. Beck, X. Brunetaud, M. Al-Mukhtar, Use of electrical resistance measurement to assess the water saturation profile in porous limestones during capillary imbibition, Constr. Build. Mater. 165 (2018) 206–217.

D. Vermeersch, Le complexe religieux des Vaux-de-la-Celle à Genainville (95): nouvelle proposition de phasage du sanctuaire d’après les dernières fouilles, in: Etudier Les Lieux Culte Gaule Romaine, 2009.

BRGM, Carte géologique 1/50 000, feuille de: MANTES-LA-JOLIE, (1974).

257

EXPERIMENTAL CONSERVATION AND FIRST INVESTIGATIONS ON THE WEATHERING OF GEGHARD MONASTERY (ARMENIA)

Wanja Wedekind¹, Emma Harutyunyan², Nevenka Novakovic³, Siegfried Siegesmund¹

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 Geoscience Centre of the University of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany

2 National University of Architecture and Construction of Armenia, Teryan 105, 0009 Yerevan, Armenia

3 Cultural Heritage Preservation Institute of Belgrade, Kalemegdan Gornji grad 14, 11000 Belgrade, Serbia

Abstract

The mediaval monastery of Geghard is located in the Kotayk province of Armenia in the Azat River Gorge. Since 1986, the stone formations and the monastery are included in the UNESCO World Heritage List.

The main church was built in 1215, whereas the monastery complex was founded much earlier in the 4th century by Gregory the Illuminator. The church is constructed out of basalt ashlar. Parts of the monastery complex are carved from the rock formation of the Azat Valley.

Onsite investigations were carried out, and the petrophysical properties of the different building stones were investigated.

Further investigations show a high impact of hydric dilatation and a potential sensitivity to salt weathering on the rock parts of the monastery. Experimental conservation was carried out to reduce the hydric dilatation and to strengthen the poor hardness of the rock material.

Introduction

The Geghard Monastery is an outstanding example of the pinnacle of Armenian medieval architecture (Fig. 1a).

The complex of medieval buildings contain a number of churches and tombs, some of which are cut into the natural rock (Fig. 1b to d, 3a).

In 1679 the monastery was badly damaged by an earthquake. Restoring the monastery for the purpose of tourism started in the first half of the 20th century. This includes the relatively new roof made of basalt in the 1980s.

Geology

The Geghard Monastery is located some 40 kilometres east of Yerevan, 1650 m above sea level.

Armenia is located in the northeastern part of the Anatolian–Armenian–Iranian plateau (Meliksetian et al. 2014). During Armenia’s entire geological history, the country was subjected to volcanic activity. Most building stones are therefore tuffs and basaltic rocks.

Methods of investigation and experimental conservation

Petrographic analyses of the material were done on thin sections under a polarization microscope. Hydrostatic weighing on sample cubes of 65 mm edge length was carried out to acquire the particle and bulk density as well as the porosity (DIN 52102). The saturation degree S was determined by the 258quotient of unforced (atmospheric conditions) and forced (vacuum) water saturation. On sample cubes of 65 mm edge length, the capillary water adsorption (w value) was measured in a closed cabinet while weighing over time (DIN EN ISO 15148). Mercury intrusion porosimetry was used to acquire the pore radii distribution (Fig. 2g and h).

Figure 1: a) The Geghard Monastery complex, Upper Azat Valley. b) The prayer hall of the Kathoghike Church built from basalt. c) Rock cut tombs and cross stones. d) The Jamatoun of the Proshian cut into the rock.

The hydric expansion of the tuff rocks was measured on square samples (diameter 15 mm, length 100 mm) under conditions of complete immersion in demineralized water. A displacement transducer with a resolution of 0.1 µm measured the linear expansion as a function of time. Ultrasonic velocity was measured by using a pundipLap+ device (proceq). Surface hardness measurements were done in situ as well as on stone samples in the laboratory. For the measurements an Equotip 3 (proceq) portable testing device with an impact device D was used on dry and wet samples and at the onsite investiagtions.

Onsite investigations included damage mapping, surface hardness measurements, Karsten tube tests to detect the water uptake capacity, electrical capacity and conductivity. The investigations were done on the east façade of the Kathoghike Church (errected in 1215) and at the eastern wall of the Jamatoun of the Proshian (constructed in 1283). The latter consists of the rock cut structure (Fig. 1d, 3a).

Experimental conservation

Experimental conservation was done on the clast-free stone material of the rock by using a swelling inhibitor (Anti-Hygro, Remmers company) and consolidation with a silica sol and a silica acid ester (KSE 300, Remmers company). The samples were placed under conditions of complete immersion in the swelling inhibitor for one hour and dried afterwards. The same procedure was done in the case for consolidation. For consolidation the silica sol was diluted 1 : 1 with distilled water.

Results
Materials of construction and rock cut architecture

The basalt rock used for the construction of the architecture outside the rockface also shows 259varieties of different quality (Fig. 2a and 2b). It is noteworthy, that the foundation of the church building consists of a much harder basalt variety (BF) than the rising masonry (BW). The BF variety shows a macroporosity characterised by a distinctive lamination (Fig. 2a), while the basalt ashlars of the walls (BW) show a finely homogeneous porous structure (Fig. 2b).

The rock cut architecture is carved into the soft rock that can be divided into different varieties. A clear distinguishing feature is the clastic material. The sample material in this study distinguishes between a rock variety that is largely free of clasts (RF) and another that contains a high proportion of clasts (RC) shown in figure 2c and 2d. This clastic material may constitute up to 40 % of the rock and can display a diameter up to about 5 cm (Fig. 2c). The fine matrix has an ocher tone and shows a variety of brownish traces of oxidation (Fig. 1a, 1c, 2c and 3d).

It can be characterised as an ash-rich, welded tuff with a fine matrix containing a large percentage of microlites, more or less uniform in size from idiomorphic feldspar crystals (Fig. 2e and 2f). Some of these can be identified as plagioclase.

In this study two types of this tuff rock were investigated further: the clast-rich (RC) and a clast free material (RF), (Fig. 2c–2f).

Figure 2: Different building materials used for the monastery. a) The basalt of the foundation (BF) and b) the basalt of the walls (BW). c) The rock variety with clasts (RC) and d) the variety without clasts (RF). e) Thin section of RC and f) of RF under polarised light. The pore size distribution of RC (left) and RF variety (right) is shown below.

Onsite investigations

The results of the mapping of the eastern façade of the church building show that back-weathering, concentrated at the upper part of the façade is the main damage form reaching more than 5 square meters of ashlar (Fig. 3c and e). This can be partly associated with effloressences of salts. Using test stripes the high amount of sulfate (> 1,600 mg/l) and nitrate (500 mg/l) could be detected. The high values of sulfate may be due to the use of cement mortar, implemeted during restoration works on the roof. Moreover, rain water caused immense infiltration, which is noticable by the crust formation on the outer masonry shell (Fig. 1b). The basalt ashlar of the masonry (BW) shows a clear water absorbtion by Karsten tube, probably due to its remarkable porosity of 23.5 %. Its surface hardness with around 300 HLD is comparably low. The basalt ashlar of the foundation does not show any water uptake by Karsten tube tests. With around 700 HLD under dry conditions and 624 HLD under wet conditions a more than twice as high surface hardness than the ashlar of the walls could be measured for the BF variety. Cracks were only found in the lower part of the façade, probably due to seismic activities in the past (Fig. 3c).

In contrast to the basaltic ashlar (BF and BW), the natural rock material shows low values of surface hardness ranging between 275 and 300 HLD under dry and 261 HLD under wet conditions. A small 260reduction of 5 % at the lowest point. The clastic material (RC) also shows various values ranging from 130 to 450 HLD under dry conditions also depending on the rock clasts. The clasts show values between 250 and 450 HLD. The matrix reaches an average value of around 250 HLD under dry and 227 HLD under wet conditions, which is a reduction of 9 %. Sanding and back-weathering of the matrix is the main weathering form observed on the rock cut architecture (Figure 3d).

Figure 3: a) The floor plan of the monastery. Areas of investigation indicated. b) Architectonic drawing of the investigated church façade. c) Damage mapping. d) Crack formation or scaling of the rock material. d) Typical back-weathering of a porous basalt ashlar.

In some parts of the upper wall active water infiltration takes place. Electrical conductivity and capacity reaches critical values at various parts of the inner walls of the rock cut structure. The exposed rock also shows crust formation and cracks (Figure 3d). These cracks are partly closed by a restoration mortar to prevent water infiltration.

Water uptake by Karsten test pipes show high values for the rock. This also corresponds to the high porosity of the rock material investigated in the laboratory (Table 1).