849

INFLUENCE OF ASPECT UPON SANDSTONE WEATHERING: THE ROLE OF CLIMATIC CYCLES IN FLAKING AND SCALING.

HALSEY, D. P., DEWS, S. J., MITCHELL, D. J. and HARRIS, F. C.

School of Construction, Engineering and Technology, Univ. of Wolverhampton, Wulfruna Street, Wolverhampton. WV1 158. U. K

SUMMARY

To examine the factors responsible for the flaking and scaling of sandstone, a quantitative survey of the weathering fonTis affecting thirty sandstone churches was carried out in the West Midlands, England. Along with other factors it was found that the aspect of the stone influences the fonTI of flaking and scaling. The influence of aspect upon the microclimate of the stone was investigated with a data logger controlled monitoring system. Analysis of the data has enabled the calculation of rates of temperature change, rates of moisture change, heating-cooling cycles and wetting-drying cycles. These variables differ greatly between aspects and aid the explanation of the influence of aspect upon the occurrence of flaking and scaling. Further to this, they give an important insight into some of the mechanisms responsible for the creation of different forms of flaking and scaling.

1. INTRODUCTION

The West Midlands, England contains a range of historic sandstone buildings, which experience considerable deterioration. The flaking and scaling of the stone is an especially important form of deterioration. To gain a greater insight into the factors responsible for flaking and scaling, a quantitative survey of the weathering fonTis affecting sandstone buildings was conducted. In addition to regional variations in stone decay (Halsey et al., 1996) it was found that the aspect of the stone influences the form of flaking and scaling. To investigate this influence of aspect stone temperatures and moisture contents were measured at a culturally important building in the study area.

2. METHODOLOGY

The quantitative survey of weathering forms affecting sandstone buildings involved thirty sandstone churches in the West Midlands. Churches were chosen for the survey as altars have traditionally been located in the east of the buildings, consequently churches have walls facing north, east, south and west. Also, many were built of ferruginous sandstone in the 19tli century in a similar architectural style. Cleaned or heavily restored buildings were excluded from the survey. To record the type and amount of weathering a system to classify the different weathering forms was devised. An existing classification with over 40 different types of weathering (Fitzner et al., 1992), was considered and reduced to 18 forms of sandstone weathering by removing those inappropriate to sandstone and generalising on the less common fonTis of deterioration. In this paper the influence of aspect upon six forms of flaking and

scaling will be considered (Table 1).

Black flaking

Case hardened flaking

Autotrophic flaking

Black scaling

Case hardened scaling

Autotrophic scaling

850

-Detachment of a blackened surface layer where the thickness of the

stone lost is less than 3 mm.

-Detachment of a case hardened (i.e. the surface is visibly enriched by salts or re deposited cement from within the stone) surface layer

where the thickness of the stone lost is less than

3mm.

-Detachment of a surface layer covered with algae, moss or lichen where the thickness of the stone lost is less than 3 mm.

-Detachment of a blackened stone layer where the stone lost is greater than 3 mm thick.

-Detachment of a case hardened surface layer where the thickness of the stone lost is greater than 3 mm thick.

-Detachment of a surface layer covered with algae, moss or lichen where the thickness of the stone lost is greater than 3 mm thick.

Table 1 Classification of different forms of flaking and scaling.

For each building the amount of stone affected by each weathering form was recorded as percentage cover to the nearest five per cent. This was done by analysing one metre quadrats of sandstone, between the heights of 0.2 and 1.2 metres, and 1.2 and 2.2 metres. These heights were selected so that the lower one represented stone influenced by the capillary rise of ground water, while the upper one was less influenced. The survey concentrated on vertical walls, but the weathering of horizontal or sloped surfaces, if present, were recorded separately and are not considered in this investigation.

To examine differences in the temperature and moisture conditions for different aspects, monitoring equipment was installed 37 metres above ground level, on the Central Tower of Lichfield Cathedral, Staffordshire, England. The equipment consisted of a 'Unidata' data logger, four steel sheathed thermistor probes and four 'Vaisala' relative humidity probes. One thermistor and one relative humidity (RH) probe were positioned in each wall of the tower in January 1994 and June 1994, respectively. This gave four highly exposed positions with the orientations north, east, south and west. In each case, the thermistors (4 mm diameter) were inserted into the stone parallel to the exposed face, but 5 mm from the surface (Figure 1). The thermistors were held in place with thermally conductive adhesive, which ensured a good thermal contact between the sensor and the stone. The RH probes (25 mm diameter) were inserted by drilling a 25 mm deep hole in the exposed face of the stone. The sensing part of the probe (the last 25 mm) was inserted and an air tight seal created with silicon sealant (Figure 1). Therefore, the humidity reading represents the RH of a pocket of air trapped beneath the surface of the stone to a depth of 25 mm. The data logger was programmed to take raw readings every fifteen minutes. In addition to mean monthly stone temperatures and relative humidities, several factors considered to be important to weathering were calculated from the raw data using Microsoft Excel (Table 2).

851

----------------------------------------PLAN VIEW

Thermistor

To Data Logger

\

1-I

19

FRONT VIEW 100

10 40

SCALE 1 :5 APPROXIMATELY. ALL DIMENSIONS IN mm.

Sandstone

Relative Humidity Sensor

SIDE VIEW 70

~s4 Adhesive tape

Supporting bracket

25 210

~--------------------------------------- J Figure 1 Diagrammatic representation of the positioning of a relative humidity probe and thermistor at Lichfield Cathedral.

Factor

Rates of temperature change (monthly mean)

Heating-cooling cycles (number month-1)

Rates of relative humidity change (monthly mean)

Wetting-drying cycles (number month-1)

852

Definition Temperature changes per fifteen minutes.

Cycles consist of temperatures increasing and then decreasing. When temperatures start to rise again a new cycle begins. For

an increase or decrease to be registered the mean rate of temperature change must be >1°c hr-1

Relative humidity changes per fifteen minutes.

Cycles consist of relative humidity increasing and then

decreasing. When relative humidity starts to rise again a new cycle begins. For an increase or decrease to be registered the mean rate of change must be >1% hr-1 .

Table 2 Factors calculated using temperature and relative humidity data. and their definitions.

3. RESULTS AND DISCUSSION Monitoring at Lichfield Cathedral shows that aspect exerts an influence upon the temperature conditions experienced by the stone (Table 3). Mean rates of temperature change are greatest for the west and south aspects. Warke and Smith (1994) point out that disagreement exists over the importance of changes in stone temperature causing deterioration of the stone. Young and Young (1992) consider that the breakdown of sandstone by thermal stresses is significant. Fluctuations in temperature cause expansion and contraction movements of the sandstone. However. rates of expansion and contraction differ with depth, due to thermal gradients, and between minerals as each mineral expands at different rates and rates of expansion differ along crystallographic axes. The stress produced by expansion and contraction is thought to be important if rapid changes in temperature occur or if repeated heating-cooling causes fatigue (Warke and Smith, 1994).

As well as receiving the greatest rates of temperature change the south and west aspects also experienced the greatest occurrence of heating-cooling cycles (Figure 2). Intermediate values were obtained for the east, and the lowest values for the north. The number of heating-cooling cycles, suggest that in summer months, diurnal cycles are very active and a small number of additional cycles also occur for the south and west aspects. This is demonstrated by the number of cycles per month exceeding thirty (Figure 2). This observation is confirmed when examining temperature fluctuations for single days (Figure 3). For example, on 8 June 1995 all aspects show the expected diurnal cycle, but shorter cycles underlay this main cycle, such as experienced between 16:30 and 17:45 for the west aspect (Figure 3). The diurnal heating-cooling cycles often demonstrate rapid heating in the morning, but slow cooling at the end of the day. In contrast, the smaller heating-cooling cycles often demonstrate both rapid heating and cooling (Figure 3). During heating tensile stresses develop between the surface and the subsurface of the stone, as the surface is expanding quicker than the subsurface. When cooling conditions prevail surface temperatures drop causing the subsurface to be warmer than the surface. This creates compressive stress as the surface contracts. This reversal in the direction of the stress, experienced by the stone may in time cause fatigue. Fatigue occurs where variations in the applied stress cause the stone to fail at a stress level, the fatigue limit, far below the strength of the stone, as determined by conventional testing. Haimson (1974) showed that cycles of tension and compression resulted in a fatigue limit of about 30% of the compressive strength.

853

1994 - 1995 A s 0 N D J F M A M J J Mean

Temp. N 15.3 12.1 9.3 9.6 5.6 3.9 5.8 4.9 9.2 12.6 15.0 20.1 10.3 {°C) E 16.8 13.1 9.8 9.9 5.7 4.2 6.4 6.4 10.3 14.2 16.0 21.3 11 .2

s 17.2 13.5 10.7 10.2 6.4 4.6 6 .9 7.3 11.2 14.4 16.5 21.1 11 .7 w 16.4 13.0 10.7 10.3 6.4 4.6 6.5 6.2 10.8 13.4 16.2 20.9 11 .3

Temp. N 0.21 0.15 0.13 0.08 0.10 0.10 0.11 0.16 0.20 0.27 0.27 0.32 0.18 change E 0.28 0.20 0.15 0.08 0.09 0.10 0.15 0.24 0.25 0.34 0.27 0.33 0.21 (OC) s 0.37 0.28 0.26 0.14 0.19 0.18 0.23 0.36 0.36 0.39 0.31 0.37 0.29

w 0.41 0.34 0.36 0.17 0.21 0.19 0.24 0.38 0.43 0.44 0.41 0.48 0.34

RH N 80.6 93.4 96.0 98.9 98.2 99.4 99.3 98.5 90.9 77.1 71.7 61.3 88.8 (%) E 65.4 90.3 94.0 96.1 95.7 95.5 96.9 97.1 N/D N/D N/D N/D 91.4t

s N/D 75.2 78.0 96.8 96.6 94.5 98.1 91 .7 74.4 49.7 50.0 43.7 77.2; w 73.1 91 .4 90.0 97.2 98.4 99.5 98.8 94.S 78.4 71.1 71 .0 64.1 85.6

RH N 0.23 0.20 0.16 0.08 0.13 0.11 0.12 0.15 0.19 0.26 0.29 0.31 0.19 change E 0.24 0.14 0.18 0.10 0.13 0.14 0.12 0.17 N/D N/D N/D N/D 0.15t (%) s N/D 0.18 0.19 0.06 0.07 0.10 0.05 0.12 0.33 0.34 0.31 0.33 0.19;

w 0.38 0.23 0.27 0.13 0.13 0.10 0.19 0.40 0.47 0.43 0.43 0.56 0.31

Table 3 Mean monthly sandstone temperature (OC), temperature (OC) changes per fifteen minutes, relative humidity (%) and relative humidity (%) changes per fifteen minutes. t Data missing for April to July 1995. ; Data missing for August 1994.

Further cycles of expansion and contraction will be caused by changes in the moisture content of the stone. Weathering of sandstone containing swelling clays, due to repeated wetting and drying has been reported by Young and Young (1992). In a similar way expansion of organic matter, such as algae and fungi, due to wetting-drying may cause weathering (Ortega-Calvo et al., 1992). Rates of relative humidity change are greatest for the west aspect (Table 3). Aspect influences the number of wetting-drying cycles, with the west and to a lesser extent the south aspects experiencing a greater number of cycles than the north and east (Figure 4). Schaffer (1932) points out that precipitation causes rapid wetting of the outer layer of the stone and therefore, stress is produced between the outer layer and the core of the stone. A rapid increase in moisture content, due to rainfall , may cause the sandstone temperature to drop, causing expansion due to moisture uptake, but contraction due to a decrease in temperature. It is possible that these stresses may counteract each other, however, at

the mineral scale differential expansion and contraction is likely to occur.

From the above discussion it is apparent that aspects experiencing the greatest rates of temperature

and humidity change, and the highest frequency of temperature and moisture cycles will experience the greatest stress and fatigue. If these mechanisms are involved in flaking and scaling it would be expected that the south and west aspects would experience a greater amount of flaking and scaling

than the north and east aspects.

40

35

30 25 20 15

10 5 0

40 35 30 25 20

15 10 5 0

40 35 30 25 20 15 10 5 0

40

35

30

25

20

15

10

5

0

854

North

East

South

West

J F M A M J J A S 0 N D J F M A M J J

Figure 2 Number of sandstone heating-cooling cycles per month at Lichfield Cathedral. 1994-5. (ND - no data)

25

20

15

lO r----~-----~-5

855

North

o.._.__~ ............. ++-+-+__,__._, ............. ___ ~-+-+-+-~~~H-+-+-++-+-+~~~~~-+-+-+-++-+-+~~H-+-+----~

25

20

15

lO r-----/ 5

East

O +-+--~++-+-+___,._,H-+-+-~H-+-+-~~~~H-+-+-~~++-+-+~H-+-+-~~~~H-+-+-~~+-+++~~

25

20

15

lO r---~------5

South

o -.-.~~~H-+-+-~~~~~~H-+-+-~~~~---.-~~~~---.-~~~~~

25

20

15

lO r-----------5

West

O +-+-++-+++-<-+-+-+-~~-+++-<-+-+-+-~+-+++ ........... ~-+-+-+----........... ~-+-+-+-,._._,_.__._ ........... -r+-+-<-+-+-+-+-++++-+++ ........... -++++-<

00:00 03 :00 06:00 09:00 12:00 15:00 18:00 21 :00

Figure 3 Sandstone temperatures recorded at fifteen minute intervals at Lichfield Cathedral on the 8 June 1995.

30

25

20

15

10

5

0

30

25

20

15

10

5

0

30

25

20

15

10

5

0

30

25

20

15

10

5

0

ND ND

J J A s 0

856

North

East

ND ND ND ND

South

West

N D J F M A M J J

Figure 4 Number of sandstone wetting-drying cycles per month at Lichfield Cathedral. 1994-5. (ND - no data).

857

Results from the building survey show that aspect has a limited effect on the occurrence of total flaking and scaling (Figure 5). Therefore, it appears that temperature and moisture cycles are not important in the formation of flaking and scaling. However, if individual forms of flaking and scaling are examined, aspect can be seen to exert an influence. Black flaking and black scaling occur in greatest quantities on the south and west aspects (Figure 6). Blackening of the sandstone may result from the deposition of soots and fly ash or biological activity (Halsey et al., 1995a). Blackening of the stone affects the thermal and moisture properties of the stone. The albedo of the stone is lowered, increasing absorption of incident solar radiation. This may increase the occurrence of heating-cooling and wetting-drying cycles, and rates of temperature and relative humidity change. Previous research has commented on the possible damaging effects of black soiling trapping moisture behind the soiled outer layer of the stone (Bluck and Porter, 1991). If black soiling totally covers sandstone, water absorption can be reduced by a factor of ten (Halsey et al., 1995b). Consequently, any drying occurs at the most permeable area, enhancing localised stone breakdown by wetting-drying cycles and the action of salts. Additionally, during low temperatures, freezing may occur, forcing water to expand in a confined space behind the black soiling. This generates further stresses that may enhance deterioration and indeed rapid stone breakdown has been reported following a severe frost (Smith et al., 1994).

Case hardened flaking and case hardened scaling are influenced by aspect, with the percentage cover being highest for the north and to some extent the east aspects (Figure 6). Case hardening occurs when soluble components of the stone are dissolved by pore water and deposited on the surface of the stone to form a hard, cemented layer. For this to occur, water must have sufficient time to percolate through the stone, dissolving material and then evaporating at the surface of the stone. Therefore, the occurrence of case hardening is greatest where high, stable moisture contents exist and low, stable temperatures support slow drying at the surface of the stone. These are the conditions experienced by the north and to a lesser extent the east aspect (Table 3). Furthermore, as the prevailing wind is south westerly the north and east aspects experience lower inputs of wind­driven rain. Therefore, runoff is less common than for other aspects, so the case hardening is rarely washed away. Due to lower amounts of wind-driven rain, water drawn up from the ground by capillary rise is more important as a water source than for the south and west aspects. This water may contain dissolved salts, and passes slowly through the interior of the stone, dissolving soluble material. This may result in a relatively saturated solution, which dries slowly at the surface of the stone giving an enriched surface layer, with its case hardened appearance. The process of case hardening may increase salt concentrations in the outer layer of the stone. The action of these salts may increase the effectiveness of cycles of wetting-drying and heating-cooling as weathering mechanisms.

Autotrophic flaking and scaling show a similar trend as case hardened flaking and scaling, with the greatest occurrence on the north and to some extent the east aspects (Figure 6). This is expected, as autotrophic growth is supported by a regular supply of moisture (Caneva et al., 1992). Autotrophic organisms may increase the stresses created by wetting-drying cycles, as they expand and contract, due to hydration and dehydration. Due to these mechanisms this layer may experience greater expansion and contraction than the interior of the stone leading to flaking and scaling of the stone. The results show that autotrophic scaling is much more prevalent than autotrophic flaking (Figure 6). This is the opposite to blackened stone and case hardened stone, which experienced greater flaking than scaling. The processes that cause the blackening and case hardening of stone suggest that only the outer few millimetres of stone may be altered. However, autotrophic growth may penetrate further into the stone. This greater depth of alteration may be responsible for autotrophic scaling to

dominate over flaking.

858

• Total Flaking D Total Scaling

North East South West

Figure 5 The effects of orientation upon the occurrence of total flaking and total scaling.

4. CONCLUSIONS

Monitoring stone temperature and relative humidity has shown that aspect has a marked influence upon the rates of temperature and relative humidity change, and the occurrence of heating-cooling and wetting-drying cycles. South and west aspects experience a greater occurrence of these cycles than north and east aspects. These cycles cause stresses to the stone which may cause deterioration. Quantitative measurement of the occurrence of flaking and scaling shows that the total amount of stone effected by flaking and scaling is not influenced by aspect. However, individual forms of flaking and scaling are influenced by aspect in different ways. Black flaking and scaling occur in greatest quantities on the south and west aspect. Blackening of the stone may increase rates of temperature change, and the occurrence of heating-cooling and wetting-drying cycles. Therefore, these factors may be involved in the flaking and scaling of blackened stone. Autotrophic flaking and scaling, and case hardened flaking and scaling show the opposite trend. Case hardening and autotrophic growth are supported by more stable, higher moisture contents and lower, more stable temperatures. It is these conditions that are responsible for the initial surf ace alterations, however, these alterations may increase the effectiveness of heating-cooling and wetting-drying cycles as weathering mechanisms.

6

~ 4

u ~ 0 2

0

6

'"' 4 Q)

> 0 u ~ 0 2

0

6

'"' 4 ~ u ~ 0 2

0

859

• Black Flaking D Black Scaling

North East South West

• Case Hardened Flaking D Case Hardened Scaling

North East South West

• Autotrophic Flaking D Autotrophic Scaling

North East South West

Figure 6 The effects of orientation upon the occurrence of different types of flaking and

scaling.

860

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CANEVA, G. et al. (1992) Incident rainfall in Rome and its relation to biodeterioration of buildings.

Atmospheric Environment. 26B (2), 255-259.

FITZNER, 8. et al. (1992) Classification and Mapping of Weathering Forms. IN RODRIGUES, J. et al.

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HAIMSON, B. (1974) Mechanical behaviour of rock under cyclic loading. IN. Proceedings of the 3rd congress of the International Association for Rock Mechanics, Volume 2. Denver, National Academy

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