Effects of temperature and humidity excursions and wind exposure on the arch of Augustus in Aosta

Journal of Cultural Heritage 13 (2012) 462–468

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Case study

Effects of temperature and humidity excursions and wind exposure on the arch of Augustus in Aosta Denise Ponziani a,∗ , Enrico Ferrero b , Lorenzo Appolonia c , Simonetta Migliorini a a b c

Laboratorio di Analisi Scientifiche, Direzione Ricerca e Progetti Cofinanziati, Regione Autonoma Valle d’Aosta, Piazza Narbonne 1, 11100 Aosta, Italy Dipartimento di Scienze e Tecnologie Avanzate, Università del Piemonte Orientale “Amedeo Avogadro”, viale Teresa Michel 11, 15121 Alessandria, Italy Direzione Ricerca e Progetti Cofinanziati, Soprintendenza per i Beni e le Attività Culturali, Regione, Autonoma Valle d’Aosta, Piazza Narbonne 1, 11100 Aosta, Italy

a r t i c l e

i n f o

Article history: Received 13 September 2011 Accepted 9 January 2012 Available online 17 February 2012 Keywords: Microclimate Preservation Monitoring Thermal stress Condensation cycles Freezing cycles Pudding-stone

a b s t r a c t This study presents the results of the microclimate monitoring of the Arch of Augustus. This is a monument from the Roman era, situated in an urban area in the Western Alpine region of Aosta Valley, Italy. Measurements were carried out on different monument positions, corresponding to the four faces and below the vault. The measurements refer to air and surface temperature, air relative humidity, wind speed and direction. The environmental conditions are described in order to underline the differences among the four faces of the monument and to explain the nature of the decay observed on the monument. The damage risk, caused by the occurrence of phenomena like freezing-thawing cycles, thermal stress and water condensation, is estimated by relating microclimatic conditions to the stone damage processes. The results are compared to the decay map and the correlation between damage and microclimate are finally discussed. © 2012 Elsevier Masson SAS. All rights reserved.

1. Research aims The decay of historical monuments exposed to the outdoors is caused by the interaction of numerous factors, natural and anthropic. Particularly, weather conditions are significant in the material’s deterioration processes: they can induce stress (derived from frost growth and changes of temperature), but also facilitate the deposition of pollution, salt dissolution and recrystallisation, the creation of black crust, the development of biological organisms, and all physical and chemical processes responsible for change in materials [1]. Studies about the outdoor microclimate, in which the monument degradation is correlated to environment condition in large urban areas, are available in literature. In works [2–4], the relevance in stone deterioration of temperature changes, freezing-thawing and condensation cycles, together with wind and precipitations, is evaluated; the studies [4–6] explain the complex interaction between monument and environmental conditions, and relate the dissimilarity in stone decay with the exposure orientation of different sides of the same building. This work considers the environmental conditions in the Western Italian Alps and, specifically, in the urban area where the Arch of Augustus, an ancient roman arch, is situated. The climate

∗ Corresponding author. Tel.: +39 0165 771700; fax: +39 0165 765813. E-mail address: [email protected] (D. Ponziani). 1296-2074/$ – see front matter © 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2012.01.005

parameters examined are air and stone’ surface temperature, air relative humidity, wind speed and direction. Some periodical damaging events were considered, such as stress due to diurnal temperature variations, condensation and water freezing on the walls, the role of wind on dust deposition as well as rain washing. This paper focuses on the differences of microclimatic conditions due to the wall exposure [9]. A quantitative assessment of the decay of the Arch’s walls is calculated through the application of damage functions available in literature [7,8]. Finally, the results of the analysis are compared with the decay map and consistency or dissimilarities are pointed out and discussed.

2. Introduction The subject of this study is an honorary Arch from the Roman era, built in the year 25 B.C., when the city Augusta Praetoria (presently Aosta) was founded. Originally situated at the entrance of the city, the Arch is today included in the urban area, in the middle of a roundabout. The city of Aosta is situated in a large glacial Alpine valley, at about 600 m above sea level. The climate is characteristic of a mountain area, which includes cold winters, dry summers and high daily temperature variations. The arch is built with Dora Baltea pudding-stone, a local stone created by sedimentation and cementation of fluviatile deposits. The pudding-stone is a conglomerate of pebbles, different in size and lithological characteristics, embedded in a matrix of sand and silt and bound together by carbonate cement

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[10]. The stone blocks, forming the monument, vary in granulometry, texture and percentage of sand matrix and carbonate cement. The monument was restored in 1913 and has not been cleaned or maintained after this date. The surfaces of the monument are covered by dust and biological colonisation; the blocks of stone are eroded and there are cracks and gaps. When the monument’s state of decay was mapped in 2006, before beginning the measurements, all types of decay observed were traced on the Arch’s ortho-pictures. The main types of decay observed are deposit (dust) and black crust, internal detachment, weathering, cracking, chromatic alteration, coating and encrustation, biological colonisation. The incoherent deposit affect all surfaces; however, it is more intense on the south side and under the vault; black crusts are rare but appear more developed on the south side of the vault. The presence of internal detachments inside many of the stone blocks has been observed; moreover, weathering, cracking and loose of material have also been observed; these forms of decay are more frequent on the south side of monument and on the semicolons. Biological colonisation affects just the lower part of the monument, on the north, east and west sides. It can be concluded that some differences in degradation are related not only to the stone block patterns, but also to the exposure of the monument’s walls.

3. Materials and methods The study is based on data collected from a comprehensive set of measurement instruments situated directly on the monument, in contact with the stone, or close to the surface. The measurements were taken on the four faces and below the arch vault in order to evaluate the differences due to faces exposure and orientation: the seven sets of probes (Fig. 1) allowed us to obtain a detailed description of the microclimate situation all around the monument. Data have been collected for about three years, from December 2007 to July 2010. Therefore, the analysis can be considered representative of local conditions including season variability. Air temperature was measured with platinum thermoresistances (accuracy 0.3 ◦ C, repeatability 0.1 ◦ C); Relative humidity was measured with capacitive sensors (accuracy 1.5%, repeatability 0.5%); Surface temperature was measured with thermistors (resolution 0.1 ◦ C, linearity 0.15 ◦ C). Probes for the measurement of air temperature and humidity were placed close to the monument’s surface, on the four external sides and under the Arch’s vault, as shown in Fig. 1. Thermistors have been placed on the stone surface, close to thermo-hygrometric probes; they have been sealed after assembly with a waterproof and heat-resistant mastic. In order to investigate prevailing winds, a cup anemometer and a wind vane (accuracy 0.5 m/s and 5◦ ) have been installed on the top of the monument’s roof. The data were validated and processed to obtain daily and seasonal cycles of temperature and humidity. The analysis of daily cycles shows the range of temperature and humidity variability, as well as the frequency of some risky events for all the different wall exposures. We consider the daily range of surface temperature to evaluate the thermal stress [7]; moreover, the surface temperature is used to evaluate the frequency of freezing-thawing cycles [11,12]. The relative humidity cycles are considered to estimate the frequency of condensation events [13,14]. The dependence of freezing and condensation of water on the pore size is studied on the basis of a pudding-stone samples investigation. The analysis of data from the anemometer on the top of the Arch describes the wind speed in different seasons of the year as well as the wind prevalent directions. The same analysis is performed for rainy days to evaluate the rain-wash-out of surfaces.

Fig. 1. The Arch of Augustus: a scheme of position of probes under the vault, on east, south, west and north side.

4. Result and discussion 4.1. Thermal stress Thermal stress is supposed to be an important cause of microfractures between the mineral grains of a rock, granular decohesion of the surface material, and variation in pore size distribution [17]. Short thermal fluctuations determine the anisotropic thermal expansion of crystals. Temperature differences between various minerals in rock surface, and differences in temperature of surface and substrate are sources of thermal stress [15,16]. Moreover, thermal variations affecting mechanisms, like salt crystallisation, may indirectly induce damages. Thermal cycles are more significant for surfaces exposed to direct solar radiation. For example, in the case of the Arch of Augustus, the daily variation of the South side’s surface temperature varies between 0 and 30 ◦ C and more in February-March or September-October, while the daily variation of the North surface’s temperature is less than 14 ◦ C, or under 5 ◦ C during winter. The South side appears more damaged than the other sides of the monument and thermal cycles may reasonably be an important deterioration factor. Mathematical models to evaluate the thermal stress [7] and describe the behaviour of rocks subjected to thermal

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Table 1 Percentile of thermal stress distribution calculated for different walls of the Arch percentile.

0 1 5 25 50 75 95 99 100

N

S

E

W

VS

VA

VN

0 0 0.50 0.92 1.28 1.66 2.05 2.25 2.85

0 0 0.49 2.13 3.62 4.44 5.30 5.63 5.92

0 0 0.48 0.96 1.59 3.00 4.03 4.24 4.52

0 0 0.54 1.74 3.59 4.51 5.44 5.87 6.72

0 0 0.22 0.81 1.43 1.87 2.50 2.77 2.93

0 0.16 0.30 0.77 1.20 1.55 2.05 2.21 2.35

0 0.14 0.28 0.67 0.99 1.37 1.85 2.09 2.25

cycles [8] are available in literature. Bonazza et al. [7] used the Maximum Thermal Stress (MTS) function, which takes into account the daily range of surface temperature, to predict the risk of thermal stress in Europe: T =

E · ˛ · T (maximum thermal stress) 1−

where E is the Young’s modulus (MPa), ␣ the thermal expansion coefficient (K−1 ),  the Poisson’s ratio and T the daily surface temperature variation. The parameters E, ˛ and  depend on the particular rock. Since laboratory test on Dora Baltea pudding-stone are not available, we refer to the mechanical properties calculated for similar rocks [18–21]. Considering the values of Lombardy conglomerates, we assume that E = 18 MPa GPa, ˛ = 8·10−6 K−1 and  = 0.3. Results for the MTS obtained using these parameters are summarized in Table 1. MTS was calculated for each monument’ side and its distributions corresponding to 3 years of measurements was obtained; Table 1 describes the main percentile values of these MTS distributions. The 100th percentile corresponds to the maximum value of MTS calculated for the 3-year period and the 0th percentile corresponds to the minimum. The comparison between the MTS and maximum sustainable load (MSL) gives an estimation of damage risk for the monument. MSL is obtained dividing material’s compressive strength by a safety factor. For conglomerates, a safety factor of 3 is generally assumed. The compressive strength of a conglomerate of Lombardy varies between 12 and 20 MPa, thus a compressive strength of 16 MPa gives MSL = 5.33 MPa. MTS is calculated for the different Arch’s sides. As it can be see in Table 1, it is great than MSL, for South and West walls, for about 5% of days per years, while others sides do not exceed this limit. 4.2. Freezing-thawing cycles The freezing-thawing cycles weathering effects on the rocks have been observed and investigated in numerous works [2,3,22,23]; frost damage is the consequence of mechanical stress resulting from an increase in the volume of water possibly present on and within the rock. Mutluturk et al. [8] proposed a descriptive model of the rocks behaviour exposed to recurrent freezing-thawing cycles. According to this model, the integrity of the stone after N freezing-thawing cycles, is given by the simple equation: I = I0 .e−N where I is the integrity of the rock. Experimental results by Mutluturk’s work [8] seem to confirm this model. In that work, the decay constant  was estimated for each different kind of rock, and the rock integrity was found to be reduced to half of its initial value after about 100 or 200 freezing-thawing cycles. However, in real cases, it is reasonable to suppose that not all the freezing-thawing cycles have a damaging effect, because a stone

is not always saturated with water like it is in laboratory tests. For this reason, in this paper, the number of freezing-thawing cycle is considered just as an index of damage risk, and not a value to calculate the life span of a rock, as suggested in [24]. The hourly distributions and the percentiles of the surface temperature values have been analyzed for a 2-year period, from December 2006 to November 2008. Fig. 2 shows the air temperature daily percentile trends measured at North and South sides of the Arch during the autumn-winter and spring-summer periods respectively. Observing the distance among the percentile lines, for each hour of the day, we can deduce the shape of hourly distribution of temperature. It can be observed that in autumn and winter, the temperatures are right-skewed, while in spring and summer, in the central part of the day, they are left-skewed. The harsh winter temperatures are sometimes raised by Foehn wind events, which cause mild temperatures during the night. On the other side, during spring and summer, cold air and snow on the top of the surrounding mountains can suddenly reduce temperatures in the valley. The percentile curves have been analyzed to evaluate the number of freezing-thawing cycles: the nth percentile, corresponding to the curves that have a minimum value of 0 ◦ C, represents the frequency of days with at least one temperature value below zero 0 ◦ C. The frequency of days with a temperature constantly below zero is very low for the North and East faces (respectively less than 0.1 and 0.3%) and zero for the other faces. Thus, it can be reasonably assumed that the nth percentile with minimum value of 0 ◦ C correctly represents the percentage of days with a freezing-thawing cycle of the water that may be present on the wall surfaces. However, the temperature at which the freezing occur, changes appreciably with the rock porosity [25]. Porous materials have a lower water freezing point, with respect to the flat surface, because of the Kelvin’s law [11,12]. The water freezing point value (TFP ) depends on the pore radius of the stone and can be calculated with the following equation: 2

TFP (r) = −273. rps Lsl (◦ C) f

(1)

where r is the pore radius,  sl is the surface tension at the solidliquid interface, s is the ice density and Lf is the latent heat fusion. The freezing temperature calculated with the above equation has been compared with the monument surface temperature. As mentioned above, the annual frequency of days with temperature below the freezing point has been calculated with the nth percentile method. This frequency is representative of the freezing and thawing cycles number. In Table 2, the annual frequencies of freezing-thawing cycle are presented as a function of the pore radius for the different wall of the Arch. The freezing temperature decreases for shorter pore radii and, as a consequence, the frequency of freezing is reduced. Obviously, it is smaller for Arch’ sides more exposed to direct sun radiation. The probability of water freezing is zero for pore radii less than 0.007 ␮m. It can be observed that the frequency grows with the pore radius till about 0.05 ␮m radius, while for larger values, it attains an almost constant values, which depends on the orientation of the monument side. These results have been compared to the porosity analysis made on three pudding-stone samples, of local origin, possessing characteristics similar to those of the Arch’s stones. The porosity analysis has been performed with a mercury porosimeter. This instrument measures the pore size distribution of the sample, for pore’s radii between 0.002 and 25 ␮m. The analysis was achieved in collaboration with the Politecnico of Milano (Chemical Laboratory of Department of Diagnostics, Monitoring and Research on Building Materials and Cultural Heritage, of LPM, Structural Engineering Department). The mercury porosimeter instrument is based on the capillary law governing liquid penetration into small

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Fig. 2. Autumn-winter (a, e, c, g) and spring-summer (b, f, d, h) daily percentile trends for temperature (a, b, e, f,) and relative humidity (c, d, g, h) at the north side (a, b, c, d) and south side (e, f, g, h).

pores, which links the pore diameter to the applied pressure on the liquid. The volume of mercury penetrating the pores is measured directly as a function of applied pressure. The results show that the Arch of August’ stone is mainly constituted by big size pores, with radii larger than 10 ␮m (macropores), and by pores with radii between 0.03 and 0.1 ␮m (micropores) (Fig. 3).

Consequently, it is possible to associate two freezing probability values to both pores classes: the macro- and the micropores. Using Kelvin’s law, the macropores probability can be considered comparable to that of the flat surfaces. The probability for the second class is the mean value of frequencies, calculated on the micropores in the range 0.03–0.1 ␮m. Table 3 resumes the freezing

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Table 2 Day cycle annual frequencies are presented for different pore radius. Pore radius (␮m)

Freezing point TFP (◦ C)

Flat surface 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.009 0.008 0.007 0.006 0.005

0.00 −0.29 −0.32 −0.36 −0.41 −0.48 −0.58 −0.72 −0.97 −1.45 −2.90 −3.22 −3.62 −4.14 −4.83 −5.80

Percentage of day per year in which value of temperature is lower than TFP N (%)

S (%)

W (%)

E (%)

VN(%)

VA (%)

VS (%)

7.3 6.5 6.4 6.4 6.3 6.1 5.8 5.7 5.6 4.5 1.6 0.8 0.4 0 0 0

0.6 0.5 0.4 0.4 0.4 0.4 0.3 0.3 0.2 0.1 0 0 0 0 0 0

4 3.1 3.1 3 2.7 2.5 2.4 2.3 1.7 0.5 0 0 0 0 0 0

8.6 7.6 7.6 7.6 7.5 7.5 7.1 6.8 6.2 5.4 2.8 2.1 1.2 0.4 0 0

8 7.2 7.2 7.1 7.1 6.8 6.6 6.2 5.5 4.5 1.7 0.9 0.4 0.3 0 0

11.6 11 10.8 10.5 10.3 9.9 9.4 8.5 7.7 5.6 2.3 1.5 0.9 0.4 0.3 0

12.7 11.8 11.7 11.4 11.4 11.1 10.4 9.8 8.9 7.3 2.6 2 1.2 0.7 0.3 0

humidity sensors. When the DDP value is below zero, the water vapor in the air condenses over the material’ surface. The result of the analysis shows that the frequency of condensation events is very low or null, depending on the face exposition. However, the effect of surface tension of porous materials can induce condensation inside pores even when temperature is higher than the DP [11,13,14]. Referring to the Kelvin equation, condensation happens inside pores even if the relative humidity is below 100%, and the effect is more relevant in presence of pores with a very small radius. The following equation has been used for the calculation of critical relative humidity values (RHC ): RHC = 100 · e(2V/rRT ) (%)

(2)

where  is the surface tension of water, V is the molar volume of water, R is the gas constant, r is the pore radius and T (◦ C) is the temperature of thermodynamic system in equilibrium. A preliminary data analysis has showed that the RHC variation with temperature, in the range of the values considered, is negligible compared to the sensor accuracy. Thus, a constant temperature value (T = 20 ◦ C) has been assumed. To assess the frequency of condensation, the value of RHC should be compared with the RHC of the air layer in contact with the stone surface. The values of relative humidity and temperature of the surrounding air are measured at about 5 cm from the surface. It has been decided to estimate the relative humidity inside the pores by assuming that the pores have the same temperature of the stone, and that the specific humidity inside the pores was the same as the specific humidity of the surrounding air. Thus, the following equation has been used to compute the relative humidity of the pores.

Fig. 3. Mercury porosimetry analysis of pudding-stone samples. The shapes show pores size distribution.

event probability for macro- and micropores and for the different monument walls. The freezing event occurrence is appreciable, in particular for North and East sides (respectively 6 and 7.1%) and for the vault walls (6.7, 9.7 and 10.8%). 4.3. Condensation The natural atmospheric moisture condensation is considered an important decay factor for historical buildings. The presence of liquid water facilitates chemical reactions of the pollutants, deposited on the surfaces during the dry phase. They may react with the surface itself producing crusts, or change the pH of the water and facilitate the dissolution of soluble salts. Moreover, the presence of liquid water aids the growth of biological organisms on the stone [2,3,26,27]. As it is well known, the atmospheric moisture condensation on a surface depends on the relative humidity and the surface temperature. A general overview of relative humidity the percentile trend is reported in Fig. 2. In order to assess the condensation events frequency, we resorted to the Distance to Dew Point (DDP) index [11]. The DDP equals the difference between the stone surface temperature and the dew point air temperature (DP). The DP is calculated from data by air temperature and

aT

aTstone

RHpores = RH.10 b+T − b+Tstone ,

(3)

b = 237.3 ◦ C

[11]. where a = 7.5 and Table 4 shows the results of the comparison between RHpores and RHC as a function of pore radius. The percentage of the days per year in which RHpores > RHC is reported. The frequencies of the condensation events are divided for each of the monument sides. It can be observed that the minimum pore radius with a frequency higher than 1% is 0.02 ␮m in correspondence of the south side of

Table 3 Freezing probability associated to monument’s walls.

Probability of freezing events in macropores (%) Probability of freezing events in micropores (mean) (%)

N

S

W

E

VN

VA

VS

7.3 6.0

0.6 0.3

4.0 2.4

8.6 7.1

8 6.7

11.6 9.7

12.7 10.8

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Table 4 Percentage of day per year in which relative humidity is higher than RHC . Pore radius(␮m)

RHC (%)

N (%)

S (%)

W (%)

E (%)

VN (%)

VA (%)

VS (%)

Flat surface 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.009 0.008 0.007 0.006 0.005

100 98.939 98.821 98.675 98.487 98.237 97.889 97.368 96.506 94.805 89.88 88.821 87.514 85.863 83.709 80.784

0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.4 0.5 1.5 1.9 3.2 4.5 6.6 10

0 0 0 0 0 0 0 0 0 0.1 0.3 0.5 0.6 1.1 1.8 3.3

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.7 0.9 1.4 2.1 4 6.7

0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.3 0.4 0.7 1.7 2.5 3 4.7 6.6 9.7

0 0 0 0 0 0 0 0 0 0 0.3 0.6 0.7 1.3 2.1 4.4

0 0 0.1 0.1 0.1 0.2 0.3 0.4 0.5 1 3.3 4.2 4.6 6.2 7.7 11.5

0 0.2 0.3 0.3 0.4 0.4 0.5 0.7 1.2 1.5 3.7 4.5 5.2 6.8 8.3 11

Fig. 4. Frequency of wind speed class in rainy days and Hydrometeor Roses (a) and a detail of west and south walls of the Arch (b).

the vault. On this wall, a 0.005 ␮m pore radius has a condensation frequency above 11%. The frequency of condensation is considerable only for very small pores, which does not constitute the majority of pudding-stone pores according to the porosimetry analysis. Therefore, we can reasonably conclude that, in the case of Arch of Augustus, condensation is not one of the main causes of decay. 4.4. Wind speed and direction The action of wind may be related to many phenomena connected to monument decay. Turbulences, generated by the impact of wind on surfaces, may favour the dry deposition of pollutants, the evaporation of water by stone surface and the disjointed grains fall. Moreover, during rain events, wind speed and direction determines the washing degree of each wall [28,29]. It worth to mention that the Arch is situated in the centre of a roundabout in the urban area. Thus, the pollutants concentration is not negligible, according to the report of the Regional Agency for the Environmental Protection (ARPA) [30]. The annual mean of fine particles (PM10) concentration in the urban area is about 30–35 ␮g/m3 , and the daily mean pass the limit of 50 ␮g/m3 for more than 35 days per year. Dust deposition on the Arch stone and the consequently crusts formation is a matter of risk. Anemometer data, measured from an anemometer placed above the arch roof, were analysed to investigate the contribution of the wind on the deposition. Since data inside the vault differ from the surrounding environment, we limited our conclusion about wind effects to the external walls.

Data have been subdivided by month to investigate seasonal wind characteristics of wind speed and wind direction. During the cold season (from October to February), wind speed is low (about 90% of wind speed data are smaller than 2 m/s) and more frequent wind directions are from West and North-West. On the contrary, in spring and summer, wind speed is frequently higher than 4 m/s and more recurrent directions are from North-West and East. To evaluate the rain-wash action, wind speed and direction have been examined during rainy days. Data about precipitations are available from the ARPA meteorological station located in a square at about 400 m from the Arch’s site. The results are reported in Fig. 4a. It was found that more than 50% of the rain events are associated with wind velocity larger than 0.5m/s, and with North-West and East prevailing wind directions. The result is consistent with the damage observed on the monument; in fact, dust deposition is more intense on the south wall compared to the others sides of the Arch, as can be observed in Fig. 4b. Pollutant deposition is favored on the south wall by the turbulence generated in the wake due to the interaction of the monument and the wind blowing from the North; moreover, the downwind position of this wall during rainy days reduces its wash-out. 5. Conclusions The environmental data, collected in the Arch of Augustus’ area, are analysed to estimate the risk of damage caused by thermal stress, freezing-thawing cycles, water condensation, dust deposition and rain-washing.

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The risk of thermal stress is estimated by the frequency of day per year with a value of thermal stress higher than the MSL. The mechanical properties of the material determine the value of thermal stress and of the MSL. In this work, the characteristics of the Dora Baltea pudding-stone are considered similar to those of the conglomerate of Lombardy. It is found that the thermal stress overcomes the MSL for about 5% of the days per year, at the south and west sides, while for the other sides this effect is negligible. Furthermore, the daily thermal variation is particularly high in February–March and September–October. Therefore, these months can be considered those with the higher thermal stress risk. Thermal stress may be related to internal detachments or cracking; these kind of damages are effectively more frequent on the south wall respect to the others sides of monument; since the risk of thermal stress is the same for south and west side, but the decay is more intense on the south, it can be concluded that on the south wall thermal stress is not the only cause of decay. The evaluation of the freezing and condensation events risk is performed, firstly through the analysis of the temperature and humidity daily percentiles variation, and then deepened by estimating the freezing and condensation frequency as a function of the pore radius. The results have been correlated to the porosimetry analysis performed on pudding-stone samples. Two pores classes have been individuated for the pudding-stone: macropores (radius > 10 ␮m) and micropores (radii between 0.03 and 0.1 ␮m). The freezing-thawing cycle frequency is found to be more than 6% for both the pore classes, on the north and east sides, and on the vault walls. On the contrary, the risk on the south and west monument sides can be considered negligible. The risk of condensation is low for the entire monument and it is reasonable to assume that condensation is not a cause of Arch damage. Because of material decay, the size of pore could grow, consequently the risk of condensation will decrease and the risk of freezing will increase: thus the decay induced by freezing may accelerate while the probability of condensation events remains low. The biological colonization is present only in the lower part of north and east walls, thus far from the point of measurement. Furthermore, the presence of grass soil is likely to increase the humidity in the lower part of the monument. For these reasons, we cannot relate the microclimatic conditions to the biological colonisation. The air velocity, which was measured from a unique anemometer placed above the arch roof, was considered to evaluate the effect to the external walls only. In fact, this measurement cannot be considered representative of wind speed and direction under the vault. The analysis of wind speed and direction, in relation with the orientation of the monument and the observation of its decay, explains the different degrees of dust deposition on monument’s walls. The north, east and west sides of the Arch appear less blackened than the south side and the vault. The analysis confirms that the prevailing wind blows from North-West and East, thus inducing pollutant deposition and reducing the washout on the opposite side. The results of this study will be considered for planning the future interventions of restoration of the Arch of Augustus and to choose adequate restoration materials. The knowledge of environmental conditions, as well as of the magnitude and frequency of phenomena affecting deterioration processes of materials, will then be employed to plan maintenance and other measures for preventive conservation. The planning of such activities is a good practice, as it may reduce and optimize the number of interventions, avoid excessively invasive restorations as well as cut down costs.

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