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Reprofiling of altered building sandstones : on-sitemeasurement of the environmental conditions and theirevolution in the stone

Author(s): Demoulin, Thibault; Girardet, Fred; Flatt, Robert J.

Publication Date: 2014

Permanent Link: https://doi.org/10.3929/ethz-a-010286799

Rights / License: In Copyright - Non-Commercial Use Permitted

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Reprofiling of altered building sandstones :on-site measurement of the environmental condi-tions and their evolution in the stone

Thibault Demoulin1 , Fred Girardet2 and Robert J. Flatt1

1 ETH Zurich - Institute for Building Materials - Stefano-Franscini-Platz 3, CH-8093Zurich. Corresponding author : tdemoulin@ethz.ch

2 RINO Sàrl - Ruelle Belle Maison 14, CH-1807 Blonay

RÉSUMÉ. Des pierres artificielles produites à partir de poudre de pierre et de résine acryliqueont été utilisées lors de la restauration de l’Eglise Catholique Notre-Dame de Vevey (Suisse)pour le reprofilage de molasse, un grès largement utilisé dans les monuments du patrimoine.Une campagne de mesure sur ce site a débuté en juillet 2013 pour quantifier les paramètresauxquelles sont exposés les pierres artificielles et naturelles. Des mesures de température etd’humidité relative sont effectuées depuis la surface jusqu’à 6.7 cm dans le mur, sur une façadeouest exposée à des cycles de mouillage et séchage. La quantité de pluie battante, ruisselanteet absorbée dans la pierre est également relevée. Cette communication présente le dispositif demesure utilisé et les premiers résultats obtenus pendant les mois d’été et d’hiver 2013. Leurimplication en terme de durabilité dans les pierres naturelle et artificielle est considérée.

ABSTRACT. Artificial stones produced from stone powder and acrylic resin have been used duringthe restoration of the catholic church Notre-Dame de Vevey (Switzerland) for the reprofiling ofmolasse, a sandstone widely used in the built heritage. A measurement campaign began in July2013 to quantify the environmental conditions that affect the artificial and natural stones. Thetemperature and relative humidity are measured from the surface to 6.7 cm in the wall, on aWest façade exposed to wetting and drying cycles. The amounts of wind-driven rain, run-offand absorbed water are quantified. This communication presents the instrumentation and thefirst results obtained from July 2013 to January 2014. Their implication in terms of durabilityin the natural and artificial stones is considered.

MOTS-CLÉS : grès, reprofilage, mesure sur site, pierre artificielle, résine acrylique, compatibilité

KEYWORDS: sandstone, reprofiling, on-site measurement, artificial stone, acrylic resin, compati-bility

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1. Introduction

In historical buildings, reprofiling refers to the insertion of a small patch of repairmaterial that aims to replace an altered part of a natural stone. It is an interesting ap-proach when the preservation of the original material is the priority since it avoids itsfull replacement and extends its lifetime. The present work is part of a project dedi-cated to the study of a particular artificial stone based on acrylic resin that had beendeveloped in the late seventies in the Ecole Polytechnique Fédérale de Lausanne inSwitzerland and applied on the townhall as a reprofiling material. After more thanthirty years, the durability of this kind of mortar has relaunched the interest of stonecarvers and it has been applied during the restoration of the catholic church of Notre-Dame de Vevey (Canton of Vaud, Switzerland) in 2011. However, a series of questionsarise about possible incompatibilities between the neighbouring natural and artificialstones after restorations are completed. In particular, questions of how the differentialthermal, hydric and hygric properties affect the durability of the repair and, more im-portantly the state of the underlying stone have not yet been examined systematically.Hygric properties refer here to the water content of the stone related to the ambientrelative humidity. The coupling of these effects are indeed known to govern the de-gradation of building stones [ALO 14B][ALO 14C]. These questions are especiallyrelevant due to the characteristics of the reprofiling, namely a direct insertion withoutjoint, and the nature of the repaired stone considered in this study, a Swiss sandstonecalled molasse. It is a soft sandstone mainly composed of quartz and felspars cementedby calcite and clays [KUN 97], sensitive to wetting and drying cycles, that commonlylead to spalling of flakes of 0.5 to 3 cm [FUR 83][JIM 08]. These alterations are for-med parallel to the outer surface and are independant of the stone bedding orientation.However, their depth does vary between stones. This suggests that a combination oftransport properties and environmental conditions causes the stress of a degradationprocess to reach critical levels only at a certain depth. Deeper within, the stone is ho-wever found to remain in good conditions. An edifying illustration can be seen in thecore drill of Figure 1 taken from the outer side of a wall in Vevey. The core showsa characteristic spalling crack parallel to the outer surface (2 cm deep in this case),and perpendicular to the bedding, which is on the longitudinal axis. With the aim ofstudying the conditions that can lead to these alterations and the impact of the reprofi-ling on the stone, a campaign of measurement has started in July 2013 in Notre-Damede Vevey. The temperature and relative humidity are monitored in two locations in awall, at the surface of a repaired and an unrepaired stone, until a depth of 6.7 cm. Therainfall and the subsequent water absorbed by the natural and the artificial stone atthese two locations are also measured. The apparatus used and the data obtained fromJuly 2013 to January 2014 are presented.

2. Materials and methods

The best location to position the sensors, based on its potential of receiving rainwater and sun was identified to be on a buttress of an apse, at approximately six meters

Reprofiling of altered building sandstones : on-site measurement. 3

Figure 1. View of a core drilled out of an altered sandstone, compared with the sensorsbelow it (grey discs).

from the ground. It is shown in the top view of the Figure 2, oriented to the North.The selected location is facing West and thus receives the warm sun of the afternoonduring summer. An important factor in the selection of this area was to have largeenough surfaces to collect enough run-off water during raining event. Therefore, theareas shown in Figure 2 are much larger than typical reprofiling patches. Due to thesurrounding buildings and to the low position of the sun during automn and winter, it isshielded from the sun during these seasons. The area is subjected to harsh conditionsas indicated by the recent restorations, where the old molasse sandstone has beenreprofiled in some parts with the artificial stone or fully replaced by Ostermundigenblue sandstone. The measurements are done on the molasse sandstone and on the sameadjacent stone reprofiled with a layer of repair material. Figure 2 allows to apprehendthe proximity of the measurements and the dimensions of the repair layer (12 cmwidth for 2 cm thickness), represented as a grey rectangle delimited by dotted linein the scheme. The availability on the market of sensors of small size, robust andautonomous allow to develop an instrumentation relatively non invasive which can beabandoned in a wall during several weeks. Similar project has shown the feasabilityof such investigation related to the degradation of building stones. [ALO 14A].

2.1. Temperature and relative humidity

In order to monitor the temperature and humidity at different depths in the wall,several loggers are put together at different distances from each other and separated byexpanded PVC, a material with a low thermal conductivity (0.06 W/m.K, against 2.30W/m.K for the molasse). In Figure 1, are shown the loggers (grey discs) mounted to-gether with the expanded PVC (white material), before the threading of the wires andthe insertion of the whole in a protective shrinking tube. As seen in the same figure,the region of interest for monitoring the temperature and relative humidity extendsfrom the surface to the first centimeters of the stone ; in our case, these parameters are

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Figure 2. Top view of the church (Google Earth) and location of the sensors of tempe-rature and humidity. Note in the picture the artificial stone at the right of the naturalstone.

monitored in the air, at the surface, at 0.8, 2.5, 4.1 and 6.7 cm in the stone. The sensorlocated in the artificial stone thus record the data in the repair material (at the surfaceand at 0.8 cm) and in the natural stone it covers (at 2.5, 4.1 and 6.7 cm).The sensors applied at the surface of the materials are encapsulated in expanded PVC,and the thermal contact is ensured by a metallic washer. In order to protect the volumeof air limited by the washer from the intrusion of contaminants like salts, that couldperturb the measurement of humidity, the washer is surrounded by a filter of PTFE.The sensor measuring the temperature and humidity in the air is positioned at fewcentimeters of the wall ; hence, it records the direct environment of the building. It isprotected from the direct sun radiations by a white screen.The sensors are capacitive Dallas Hygrochron buttons that measure both temperatureand relative humidity, and function through the 1-Wire protocol. It allows to have asmany wires as sensors plus one for the ground, hence keeping as low as possible thephysical impact of the measurement that could be deteriorated by water infiltration orthermal heating of the wires. Moreover, each sensor being autonomous, the need for acentral acquisition system is avoided, thus reducing the invasive nature of the installa-tion.The sensors measure the temperature in a range of -20 to 85 ◦C with a resolution of0.06 ◦C and an accuracy of 0.5 ◦C between -10 to 65 ◦C, that well cover the condi-tions observed during the campaign. The relative humidity is measured from 0 to100%, with a resolution of 0.04%, and with an accuracy of 2% in a safe operatingzone, namely in a temperature window between 0 and 50 ◦C, that were the conditionsencountered from July to end of November. Measurements of relative humidities hi-gher than 90% are however subjected to more inaccuracy.

Reprofiling of altered building sandstones : on-site measurement. 5

2.2. Run-off and absorbed water

During a rain event, the rain driven to the stone by the wind can be either absorbedby the stone or run off its surface. Since the water transport is of utmost importance inthe degradation of stone, the behaviour of both artificial and natural stones concerningrainwater has to be evaluated and the importance of the phenomenon, quantified. Tothis end, the horizontal rainfall, the wind-driven rain and the run-off water are mea-sured. The difference between the two last quantities give the water absorbed by thestone.The apparatus used for the measurements is shown in Figure 3. The measurementsof the run-off water are achieved through the collection of the water flowing over anarea delimited by a frame, at the surface of the natural and the artificial stone. Thewind-driven rain is quantified using the same equipment, but the surface of the stoneis covered by a fully non-absorbing material. The frames delimit an area of 0.069 m2.The water is collected in glass bottles and weighed ; the sampling rate is thus deter-mined by the frequency of the weighing done by an operator. An automation of thesystem is in progress, but was not available at the time of the measurements reportedin this study.

Figure 3. View of the complete installation with the temperature and humidity sensors,and the frames for the measurement of the run-off water.

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3. Results and discussion

3.1. Temperature

The overall variation of the temperature at the surface of the stone follows thesame trend as the temperature of the air, namely daily and seasonally but with a hi-gher amplitude due to the direct radiative heat of the sun (Figure 4, left). From Julyto January the temperature of the air dropped from 35.2 to −2.3 ◦C in the vicinity ofthe church while the surface of the stone varied from 47.2 to −2 ◦C. But the highestamplitudes are recorded in July and August as illustrated in Figure 4 (right), wherethe temperature fluctuated daily with an amplitude average of 20.3 ◦C ; the maximalheating rate has been recorded in August 1st, where the temperature at the surfaceof the stone increased at a rate of 0.2 ◦C/min, leading to a temperature difference of13.5 ◦C in the first 6.7 cm of the stone. High temperatures over 40 ◦C at the surfaceare not uncommon since they occurred 25 times during this period.On the other hand, temperatures below 0 ◦C at the surface of the stone happened 16times during the months of December and January. On these occasions, the tempera-ture at 6.7 cm in the stone fell below 0 ◦C 14 times. The winter 2013 is not consideredas a cold winter in Switzerland.

Figure 4. Evolution of the temperature at the surface of the natural stone during themeasuring period (left) and zoom on the temperature at the surface of the two mate-rials during the warmest summer month (right).

3.2. Wind-driven rain and absorbed water

Despite the small distance between the frames, the particular location of the mea-surement on a buttress close to the apse favour wind turbulences and provoke differentrain exposure. In an attempt to quantify this difference, non-absorbing material hasbeen positioned to measure the wind-driven rain at the three locations during six rain

Reprofiling of altered building sandstones : on-site measurement. 7

events, from October 2nd to 28th. A scaling factor and error interval for each posi-tions have been calculated which, despite the random nature of precipitations, helpthe comparison between the values measured with the different frames. Compared tothe fully non-absorbing material, the natural stone received during these rain events1.62 ± 0.35 times more water and the artificial stone 1.21 ± 0.15 times more. Thesefactors are used in the calculations that led to the following results.During the studied period, 407.9 kg/m2 of water fell on the ground, during 14 distinctrain events, and 14.8 kg/m2, thus 3.6%, was driven by the wind towards our wall. Ofthis amount, 41.8% of the wind-driven was absorbed by the natural stone, while only2.4% was absorbed by the artificial stone. Even though these figures are highly depen-dant on the random nature of precipitations, it is still possible to say that the artificialstone absorbs on average 17 times less water than the natural one, despite its muchhigher porosity (35% against 17% for the Villarlod and Ostermundigen molasse). Thedifferent absorption behaviour is explained by the hydrophobic aspect of the acrylicresin. As a consequence, a large part of the rain water that is driven by the wind to-wards the surface of the artificial stone is run-off water, and re-distributed downwardsthe wall. It should however be noted that in general the areas covered by the artificialstones are very small compared to the surfaces of the natural stone, and that not allthe water will be immediately absorbed by the natural stone below the artificial one.The redistribution of this extra run-off water will depend on the precipitation charac-teristics (rain intensity, wind) and its impact needs to be evaluated in a global contexttaking into account the respective surfaces of the natural and the artificial stones.

Figure 5. Evolution of the relative humidity during the measuring period (left) andclose-up view of the relative humidity at a depth of 2.5 cm (right).

3.3. Relative humidity

The amplitude of the variations of the relative humidity at the surface of the na-tural stone is overall less pronounced than in the air, but still shows high variationsduring summer, as shown in Figure 5 (left). It can be seen that the amplitude of thesevariations greatly reduce in the depth of the stones. They are relatively large in the

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first centimeter, but fade very rapidly and are almost completely damped by the fourthcentimeter. From October on, for the two materials, the relative humidity at a depthcomprised between 1 and 6.7 cm remains at 100%. The measurement of such highrelative humidities is subjected to an increased inaccuracy [ALO 14B], and an overes-timation of the humidity cannot be excluded ; even though a direct look at the surfaceof the stone tends to show that it remains wet in winter.The regular variations brought by the alternance of night and day are of the order of20% in the summer months. The high dimensional change (up to 2.34 mm/m) measu-red on the Villarlod molasse when immersed in water [FEL 95] leads one to assumethat such repeated variations in the humidity of the air may also fragilize the stone byfatigue.It can be noted that the largest variations of relative humidity occurred in July andAugust. Therefore, we have represented in Figure 5 (right) a zoom on this period,comparing the relative humidity in the natural stone, covered or not by the artificialstone. It is interesting to note that these variations are on average more pronouncedin the first centimeters of the natural stone than on the ones covered with the repairmaterial.The lowest relative humidity recorded at the surface of the two materials was 32%in July, while the fastest variation happened at the end of the same month, when itabruptly rose from 54.5 to 100% due to a sudden rain.

Naturalstone Arficialstoneandbehind

Figure 6. Comparison of the dew point at different depths, in the natural stone (left)and in the repair material and the natural stone it covers (right).

3.4. Dew point

Comparison of the dew points at different locations in the stone at a particulartime gives useful hints on the direction of the water vapour transfer. The dew point,calculated by the Magnus formula from the values of temperatures and relative hu-midities [LAW 05], is directly related to the water vapour pressure. An interestingfact, shown in Figure 6, can be noticed on the vapour migration at a depth of 2.5

Reprofiling of altered building sandstones : on-site measurement. 9

to 4 cm below the surface, after a period of warm weather without precipitations atthe end of July 2013. When the nocturnal decrease in temperature occurs, the partialwater vapour pressure inside the stone is higher than the one on the surface and inthe air. The surface can dry and a vapour migration can take place from the inner tothe outer part of the stone. However, during the diurnal warming up, the dew pointis constantly higher at a depth between 2.5 to 4 cm. It implies that vapour migrationtakes place towards the surface but also towards the inner side of the stone. This factis noticeable in both locations, suggesting that the artificial stone does not hinder thevapour migration, which is consistent with its high porosity. The occurrence of a zoneof higher humidity at some depth within the stone is consistant with modelling resultsreported by Snethlage and Wendler [SNE 97] showing this for liquid water. This situa-tion can conceptually explain the development of stresses in clay bearing stones [SCH05] [WAN 08] as this molasse and also be a possible explanation for the formationof scales previously mentioned for these stones. To the best of our knowledge directexperimental measurements for this have not yet been reported and this is thereforesomething that must be seriously followed-up.

4. Conclusion

These first results obtained from July 2013 to January 2014 show that the tempera-ture during the summer months can vary regularly due to nocturnal convective coolingand diurnal radiative heat transfer with a high amplitude (average of 20.2 ◦C) and ahigh heating rate (0.2 ◦C/min) at the surface of the stone. The same holds for the rela-tive humidity which can vary from 32 to 100%. These periodic variations associatedto the relatively high dimensional changes in both hygric and hydric conditions of themolasse may be invoked in its degradation by fatigue. In particular the developmentof a humid zone at some depth within the stone may in part explain the formation ofscales in the type of stones studied. This has to be followed-up in the coming measu-rements.The measurements of the absorbed and run-off water evidence the dissimilar proper-ties of the two materials when exposed to liquid water : the artificial stone absorbs 17times less water than the natural one. However, the relative humidity behind the arti-ficial stone shows sligthly more stable conditions which may be an advantage concer-ning the risk of damage due to the swelling of the molasse.Because of the complexity of stone degradation in-situ, it is important to consider spe-cific exposure conditions when studying the durability of restoration material. There-fore the data gathered during on-site measurements serve to define on a case-by-casebasis the conditions to use in accelerated but nevertheless relevant aging tests.

5. Bibliography

[ALO 14a] AL-OMARI A., BRUNETAUD X., BECK K., AL-MUKHTAR M.« Effect of thermalstress, condensation and freezing-thawing action on the degradation of stones on the Castle

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Of Chambord, France »,Environmental Earth Sciences, vol. 71, no 9, 2014, p. 3977-3989.

[ALO 14b] AL-OMARI A., BRUNETAUD X., BECK K., AL-MUKHTAR M.« Hygrothermalstress and damage risk in the stones of the Castle Of Chambord, France », InternationalJournal of Civil and Structural Engineering, vol. 4, no 3, 2014, p. 402-418.

[ALO 14c] AL-OMARI A., BRUNETAUD X., BECK K., AL-MUKHTAR M.« Coupledthermal-hygric characterisation of elastic behaviour for soft and porous limestone »,Construction and Building Materials, vol. 62, 2014, p. 28-37. In progress.

[FEL 95] FÉLIX C. « Choix de grès tendres du Plateau Suisse pour les travaux de conserva-tion », Conservation et Restauration des Biens Culturels, R. Pancella, Ecole Polytech. Fed.Lausanne, Montreux, 1995, p. 45-71.

[FUR 83] FURLAN V., GIRARDET F. « Pollution atmosphérique et dégradation de la mo-lasse », Chantiers/Suisse, vol. 14, 1983, p. 989-994.

[JIM 08] JIMÉNEZ-GONZÁLEZ I., RODRÍGUEZ-NAVARRO C., SCHERER G. « Role of clayminerals in the physicomechanical deterioration of sandstone », 2008, Journal of Geophy-sical Research, vol. 113.

[KUN 97] KÜNDIG R. Die Mineralischen Rohstoffe der Schweiz, Schweizerische Geotech-nische Kommission, ETH-Zentrum, Zürich, 1997.

[LAW 05] LAWRENCE M. « The Relationship between Relative Humidity and the DewpointTemperature in Moist Air : A Simple Conversion and Applications », Bulletin of the Ame-rican Meteorological Society, vol. 86, no 2, 2005, p. 225-233.

[SNE 97] SNETHLAGE, R., WENDLER E.« Moisture cycles and sandstone degradation »,1997, Saving our cultural heritage : the conservation of historic stone structures. Dah-lem workshop reports.

[SCH 05] SCHERER G., JIMÉNEZ-GONZÁLEZ I.« Characterization of swelling in clay-bearing stone », 2005, Geological Society of America Special Papers, no 390, p. 51-61.

[WAN 08] WANGLER T., SCHERER G.« Clay swelling mechanism in clay-bearing sand-stones », 2008, Environ Geol, no 56, p. 529-534.