mccabe 2007

1874 S. McCabe, B. J. Smith and P. A. Warke

Copyright 2007 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 32, 1874 1883 (2007)DOI: 10.1002/esp

Earth Surface Processes and LandformsEarth Surf. Process. Landforms 32, 18741883 (2007)Published online 15 March 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/esp.1503

Sandstone response to salt weathering followingsimulated fire damage: a comparison of the effectsof furnace heating and fireS. McCabe,* B. J. Smith and P. A. WarkeSchool of Geography, Archaeology and Palaeoecology, Queens University Belfast, UK

AbstractFire has long been recognized as an agent of rock weathering. Our understanding of theimpact of fire on stone comes either from early anecdotal evidence, or from more recentlaboratory simulation studies, using furnaces to simulate the effects of fire. This papersuggests that knowledge derived from simulated heating experiments is based on the pre-conceptions of the experiment designer when using a furnace to simulate fire, the operatordecides on the maximum temperature and the duration of the experiment. These are keyfactors in determining the response of the stone to fire, and if these are removed from real-world observations then knowledge based on these simulations must be questioned.

To explore the differences between heating sandstone in a furnace and a real fire, sampleblocks of Peakmoor Sandstone were subjected to different stress histories in combination(lime rendering and removal, furnace heating or fire, frost and salt weathering). Block responseto furnace heating and fire is discussed, with emphasis placed on the non-uniformity of the fireand of block response to fire in contrast to the uniform response to surface heating in a furnace.

Subsequent response to salt weathering (by a 10% solution of sodium chloride and mag-nesium sulphate) was then monitored by weight loss. Blocks that had experienced fire showeda more unpredictable response to salt weathering than those that had undergone furnaceheating spalling of corners and rapid catastrophic weight loss were evidenced in blocksthat had been subjected to fire, after periods of relative quiescence. An important physicalside-effect of the fire was soot accumulation, which created a waxy, relatively impermeablelayer on some blocks. This layer repelled water and hindered salt ingress, but eventuallydetached when salt, able to enter the substrate through more permeable areas, concentratedand crystallized behind it, resulting in rapid weight loss and accelerated decay. Copyright 2007 John Wiley & Sons, Ltd.

Keywords: sandstone; fire; furnace; simulation; stress

*Correspondence to: S. McCabe,School of Geography, Archaeologyand Palaeoecology, QueensUniversity Belfast, BT71NN, UK. E-mail:[email protected]

Introduction

Fire has long been recognized as an agent of change in the natural environment and as an agent of rock weathering(Blackwelder, 1926; Emery, 1944; Scotter, 1970; Dorn, 2003). Early research on fire consisted mainly of anecdotalevidence, while more recent studies have sought to better understand how fire impacts stone through laboratorysimulation experiments (Goudie et al., 1992; Allison and Goudie, 1994; Allison and Bristow, 1999; Gomez-Heraset al., 2006). Understanding in this area relies heavily on these laboratory experiments, which often employ furnaceheating to simulate the extreme heat of a fire.

Fire has been placed in the context of heritage (McCabe et al., in press a), raising the issue of the potential long-term impact of historical fire on the performance of masonry. Fire is a major threat to cultural heritage, with estima-tions of one historic structure being lost every day in the EU (Gomez-Heras et al., 2006; COST C17, 2001). It is likelythat over a long lifespan a historic structure will experience fire (sometimes multiple fires) at some stage (Obojeset al., 2006). The importance of research into the impact of fire on stone has also been highlighted in the area of rock

Received 25 October 2006;Revised 5 January 2007;Accepted 11 January 2007

Sandstone response to salt weathering following simulated fire damage 1875


art (Tratebas et al., 2004), where culturally priceless works of ancient art are lost to fire every year. With this threat tocultural heritage ever present, practical research is needed to better understand the complex problem of fire damage tostone, and how the stress legacy left by fire can be exploited by subsequent background environmental weatheringprocesses.

Fire has implications for both physical and chemical stone decay processes. Mechanical strain in stone can bebrought about because of the sudden extreme temperature changes caused by fire. Internal stress gradients are rapidlyestablished, and can cause spalling or splitting of rock. This is a common response to thermal shock change/stressthat is beyond the ability of the stone to absorb. Thermal shock occurs when temperatures rise or fall very suddenly,generating steep surface/substrate temperature gradients. It is the mechanical failure of stone brought about inresponse to a single event (in contrast to fatigue, which describes the mechanical failure of stone in response tocumulative thermal stresses) (Gomez-Heras, 2006). When temperatures increase suddenly, as in the case of a fire,failure of the stone surface can occur in the form of spalling this is the result of compressive surface stress and theshear stress induced by it (Yatsu, 1988). Past studies have suggested that quartz sandstones can be particularlysusceptible to extreme heat (Goudie et al., 1992), and it is common for quartz grains to fracture when temperaturesexceed 573 C (Chakrabarti et al., 1996). Chemically, the extreme heat caused by fire can trigger changes in themineral matrix or cement of a sandstone. As is often the case in stone decay, chemical processes can weaken theintegrity of the stone, leaving it susceptible to physical decay processes and mechanisms.

A major question that needs to be addressed is whether heating in a furnace provides a true picture of the impact offire on stone, and on its subsequent response to background environmental factors (salt weathering and temperaturecycling). Furnace heating replicates only one very controlled component of the complex environmental conditions andconsequences experienced and produced in a fire (that is, it provides constant heat). Furthermore, furnace heating isbased on the pre-conceptions of the experiment designer the operator decides what temperature blocks shouldexperience and for how long they should be kept in the furnace. Yet it is these factors that are the essential controls onthe impact of fire on masonry (Goudie et al., 1992). The use of furnace heating to study fire damage may reflect abelief that the only significant factor is temperature something that this paper seeks to challenge. While furnaceexperiments may provide a better understanding of how stone responds to extreme heat, they may not necessarilyimprove understanding of the impact of fire on stone or the response to of stone to background environmentalfactors after a fire event. For example, the impact of historical fire on masonry is not likely to be uniform each blockin a faade may experience different temperatures for different durations, leaving the structure with a wide range ofinheritance effects. Thus, some stone may have escaped the fire and have no memory of the event. Other blocks mayhave lost the memory of historical fire through the detachment of material. Perhaps most importantly for conserva-tors, some blocks may still hold the memory of fire, and conceal stresses and weaknesses that are yet to be exploited(McCabe et al., in press a). Temperature changes in a natural fire are not consistent spatially or temporally they canrise extremely rapidly at the surface (creating stress gradients), but are not stable, fluctuating continually through time.Furthermore, the gases produced in a real wood fire are complex, and the surfaces of stone blocks are likely toaccumulate deposits of combustion particles and residues (for example, soot and oils) that may have an impact on theblocks subsequent response to its environment.

Methods

To explore the differences between furnace heating and a real fire, blocks of Peakmoor Sandstone (10 cm 10 cm 10 cm), quarried from Matlock in Derbyshire, were divided into three main groups. The characteristics of thissandstone can be seen in Table I. This sandstone is commonly used as a building stone in the UK, and is oftendeployed in conservation and restoration work in historic structures. Table II illustrates the different combinations ofstress histories experienced by the three groups and the order in which the pre-treatments were carried out (from leftto right). This order is based on an actual event timeline of a real historic sandstone structure, Bonamargy Friary onthe north Antrim coast, Northern Ireland construction, lime rendering, fire, frost (likely to be enhanced during theLittle Ice Age, which many historic structures will have experienced) and salt (McCabe et al., in press a). The groupswere then further divided into subsets (a and b), where subset a would undergo furnace heating and subset bwould experience a real wood fire.

Three groups were placed in a furnace at 500 C for 30 minutes. One of these groups had experienced a lime renderpre-treatment before going into the furnace, while the other two groups were fresh, untreated, sandstone. Limerendering, and its removal over time, is a common event in the history of many historic structures (McCabe et al., inpress a, 2006). Blocks were lime rendered (to a depth of approximately 1 cm) in a test wall, which was left to dry fora month before being dismantled and the render chipped and scraped off. A thin residue of lime render was left on



Table I. Characteristics of Peakmoor Sandstone

Colour BuffPorosity 1646%Ave. permeability 3167 mDSaturation coefficient 068Absorption 507% (by wt)Compressive strength 725 MPaBulk specific gravity 2210 kg/m3

Sodium sulphate crystallization test 107% mean wt loss

Table II. Different stress histories experienced by blocks (groups 1a 3b) to explore the differences between furnace heating andreal fire

Lime render Furnace Fire Frost Salt Weight loss (as % of block weight in grams)

Group 1a i i 129Group 1b i i 242Group 2a i i i 147Group 2b i i i 211Group 3a i i i i 020Group 3b i i i i 593

blocks as a result. The furnace was pre-heated to 500 C so that blocks would experience a rapid extreme temperaturechange. 500 C was chosen as the temperature because this is thought to be the average temperature experienced by astructure during burning fuelled by wood (Gomez-Heras, 2006). After being removed from the furnace, blocks wereallowed to cool naturally (it is unlikely that historic fires were put out with the same efficiency as in the present day).

The other groups of blocks were burned in a real wood fire (Figure 1). Again, one of these groups had beenpreviously lime rendered (as with the furnace experiment). Blocks were placed in an empty oil drum and surroundedby wood. The fire was lit and was not interfered with it was allowed to take its natural course. Every minute thetemperature of the fire was measured in three places (flame temperature, block surface temperature and temperature atthe base of the fire) with an infrared thermometer. The block surface that was monitored was located at the edge of the

Figure 1. A photograph showing the real wood fire experiment.



Figure 2. The temperature regime experienced by blocks during salt weathering cycles.

fire and facing outwards, so it is likely that this gives a conservative value of the temperatures experienced by blocksduring the experiment. The base temperatures are likely to give a more accurate representation of temperaturesexperienced at the centre of the fire; however, base temperature could not be monitored for the duration of the firebecause it was too hot and beyond the range of the thermometer until c. 45 minutes had passed.

As illustrated in Table II, after the furnace/fire pre-treatment, groups 2 and 3 were subjected to freezethaw cycles.As well as experiencing environmental cycles over daily and seasonal scales, historic masonry may also experience achange in local climate during its lifetime (McCabe et al., 2006). For example, many historic structures will haveexperienced the Little Ice Age, when freezethaw conditions are likely to have been more frequent and intense. Theconsideration of extreme frost events should not be limited to the past, however, as they may still occur as highmagnitude/low frequency events in the present day. To simulate the effects of freezethaw, groups 2 and 3 underwent50 freezethaw cycles (with temperatures cycling between 10 and 10 C twice daily). At the beginning of eachalternate cycle, blocks were immersed in de-ionized water briefly, to provide moisture this simulated the periodicwetting of blocks experienced in temperate environments.

To further investigate the differences in response of Peakmoor Sandstone to the stress legacies of furnace heatingand a real fire, blocks were subsequently subjected to salt weathering cycles under controlled laboratory conditions (ina climate cabinet). The salt weathering and temperature cycles were designed to simulate background environmentalfactors that structures experience on a daily basis. It can be hypothesized that background environmental factors suchas temperature cycling and salt weathering can exploit weaknesses in stone brought about by the effects of the variousstress history combinations lime rendering, fire and freezethaw weathering. The temperature regime experienced bythe blocks during the two-day experimental cycle is shown in Figure 2. This temperature regime is based on observa-tions from the North Antrim Coast, Northern Ireland, during the month of May. Temperature data (which informedlaboratory simulation) were collected in the field by embedding bead thermistors within blocks at different distancesfrom the block surface. The top temperature experienced in the laboratory simulation is commensurate with sandstonesurface temperatures achieved on clear, sunny days on the North Antrim Coastline. At the beginning of each two-daycycle, blocks were immersed for approximately 10 seconds in a 10% salt solution (equal parts NaCl and MgSO4) andthe debris released from blocks during this immersion was collected, dried and weighed. This immersion againsimulated the periodic wetting (and subsequent drying) of building sandstones in temperature environments. A 10%salt solution was used because this strength provided the breakdown of blocks without being too aggressive, allowinglessons to be drawn from the decay the study sought to investigate the slow, and realistic, breakdown of PeakmoorSandstone, monitoring subtleties in their decay pathways, rather than causing rapid deterioration of blocks.



Results

Furnace HeatingBlocks that had experienced heating in the furnace had a fairly uniform response, congruent with the uniform heatingat 500 C. Blocks underwent a colour change that can be attributed to the oxidation of iron in the stone (Figure 3).This colour change (from very pale brown, Munsell 10YR 8/3, to pink, Munsell 75YR 7/4) was consistent throughoutthe blocks that were subjected to furnace heating, although some blocks showed a slight surfacesubsurface gradientof change (presumably due to block-specific variations in stone characteristics). The furnace experiment is character-ized by uniformity.

Real FireIn contrast, the wood fire experiment is characterized by the non-uniform response of Peakmoor blocks (consistentwith the non-uniformity of the fire itself). Figure 4 shows temperatures recorded during the experiment. Flametemperatures were recorded until the flames died down, after approximately 50 minutes. Around this time it waspossible to begin measuring the base temperature, which was around 700 C. Block surface temperatures actuallyreached their peak (320 C) after the flames had died down. It took the blocks approximately 220 minutes to cooldown from the start of the fire. This experiment was non-uniform both spatially and temporally. Across the area of thefire, and even across the face of a single block, different temperatures were experienced. This is illustrated by the factthat some sides of blocks showed the effects of fire (both blackening from soot and reddening from the oxidation ofiron), while others did not, depending on their position in the fire. Temperatures fluctuated significantly during theduration of the fire, although the cooling of the blocks from 150 minutes to 220 minutes was relatively uniform.Figure 5 shows blocks that experienced the real wood fire.

From a scientific point of view, there are obvious limitations to this experiment. It is difficult to control/monitor andto some extent unrepeatable. However, these are two defining characteristics of real fires, and even a one-off experi-ment such as this can give important insights into the impact of fire on sandstone.

One very important difference between the furnace and the real fire is the presence of soot. The soot often createda black, waxy layer on the surface of the stone. Infrared spectroscopy spectra for this humic-like substance indicatethat it is made up of hydroxyl and amino compounds, likely to be derived from the burning wood (Dinar et al., 2006).Areas of the stone surface affected by soot showed a drop in average permeability from 3167 mD (on fresh PeakmoorSandstone, from a range of 988 mD) to 2820 mD (from a range of 860 mD). This soot layer is likely to have animpact on the subsequent performance of the block, for example in stone response to weathering processes such as salt

Figure 3. A photograph showing the change in colour experienced by blocks that had been in the furnace at 500 C for30 minutes.



and frost, which are in turn controlled, in part, by surface moisture ingress and egress. The soot layer formed a partialbarrier to fluxes of heat and moisture at the stone surface. Soot may also act as a store for salts that may subsequentlybe mobilized and precipitated in the substrate (Schaffer, 1932).

Response to Subsequent Salt Weathering Results and Discussion

The discussion in this paper focuses on the differences seen in the response of the sandstones to salt weathering afterexposure to furnace heating and a wood fire. The implications of variable response in relation to the other stresshistory factors (lime rendering and frost) are investigated in ongoing research (McCabe et al., in press b). The

Figure 4. Temperatures (flame, stone surface, and at the base of the fire) recorded during the real wood fire experiment.

Figure 5. A photograph showing non-uniform change in blocks from the fire.



cumulative weight loss graphs for representative blocks in each of the groups are shown in Figure 6. Generally, it canbe seen that, in each case, blocks that had been through the real wood fire yielded more debris (usually in rapidcatastrophic decay) than blocks that had experienced furnace heating. The blocks that had been in the fire behaveddifferently to the furnace blocks in response to salt weathering they had a much more episodic/unpredictableresponse, likely to be related to the microfractures created by the multiple and complex non-uniform stresses producedin the fire. Specific observations and implications related to the response of different stress history groups arediscussed below.

Group 1: blocks 1a (furnace and salt) and 1b (fire and salt). Blocks in group 1a and 1b followed a similar patternof weight loss until approximately cycle 56. At this stage, the weight loss for the block that had experienced the firejumped considerably. This increase in debris release was related to the spalling off of a corner during the fire, amicrofracture had developed (picture), and this was subsequently exploited by salt weathering the decreasingstone strength threshold was crossed by the cumulative stress applied by salt weathering cycles. After the spallingevent (approximately cycle 56), the weight loss shows a concave shape, reflecting a period of relative quiescence indebris release. The corner that spalled was not associated with further granular disaggregation, but was, rather, aclean break.

Group 2: blocks 2a (furnace, frost and salt) and 2b (fire, frost and salt). Both subsets again followed a similardecay path until approximately cycle 60. At this stage weight loss in block 2a tailed off, while weight loss in block2b continued to accelerate. While block 2b shows no major jump in weight loss related to, for example, the spallingoff of a corner (as other subset b blocks do), the continued accelerated weight loss is clearly related to the sootlayer deposited on the block during the fire. It is in response to repeated wetting and drying with salt solution thatthe importance of soot is highlighted. The waxy impermeable soot layer has a water-repellent quality the saltsolution simply beaded on the surface of the block and slowly crystallized, or ran quickly off the block surface.Similar hydrophobic behaviour has been reported in soil response to forest fires (Mataix-Solera and Doerr, 2004).It is documented that humic-like substances, and especially amino acids, can have hydrophobic characteristics(Meirovitch et al., 1980). However, the relatively impermeable soot layer did not cover blocks completely and sosome salt was penetrating into the substrate through the more permeable areas of the block surface. After around60 cycles, the less permeable sooty crust began to flake off (Figure 7), presumably due to the concentrationand crystallization of salts behind the soot layer. The exposed surface of the stone under the soot layer showeddiscoloration due to the fire, and also had a much higher average permeability than fresh Peakmoor Sandstone(133 mD from a range of 78169 mD).

Group 3: blocks 3a (lime render/removal, furnace, frost and salt) and 3b (lime render/removal, fire, frost and salt).The most divergent response was seen in blocks 3a and 3b. In block 3a, decay appears to be suppressed within the

Figure 6. Weight loss of blocks from different stress history groups in response to salt weathering cycles.



Figure 7. A photograph showing the response of a soot-covered block to salt weathering.

period of the experimental run. Over 75 cycles, block 3a released the least amount of debris (020% of its totalweight). The reason for this suppressed response is likely to be the order in which the pre-treatments (lime render,fire, frost) were applied. After lime rendering and removal, blocks experienced furnace heating. This appears tohave baked and hardened the remnant surface lime render layer (as well as the lime water that had infiltrated theblocks). Ostensibly, this has led to an increase in the durability of block 3a. However, this is likely to be a false,short-term, durability, as salts have been shown to concentrate at depth in previously lime rendered blocks (McCabeet al., 2006). These salts may have implications for future decay a short-term increase in durability does notpreclude the future rapid catastrophic decay of block 3a. In sharp contrast, block 3b followed a similar suppressedpath of weight loss to begin with (cycles 064), but by the end of the experimental run it had yielded the mostdebris of the different stress history groups (593% of its total weight). This can be explained, once again, by thenon-uniform stressing in the real fire causing more microfracturing. Debris released appears to have been sup-pressed initially, again because of the baking of the remnant lime layer. At approximately 65 cycles, however, acorner spalled off block 3b (Figure 8), resulting in a jump in weight loss. A microfracture resulting from the fire hadbeen developed and exploited by the salt weathering cycles. After the major spalling event (cycle 65), the weightloss graph shows a convex shape, reflecting continuing exaggerated granular disaggregation associated with theextreme weight loss event. It may be surmised that the fire has left an inherited weakness in this area of the block,related to the spalled corner. It is likely that the continued granular disaggregation occurred because debris that hadbeen held in place by the baked lime render layer was released when the hard layer was breached by the spalling ofthe corner. Furthermore, the corner that was lost from block 3b was significantly bigger than that from 1b, and islikely to have left a greater legacy of stress.

As the above discussion suggests, Peakmoor Sandstone response to salt weathering after furnace and fire pre-treat-ments can be variable. While heating blocks in a furnace provides some insight into the impact of extreme heat onsandstone performance, it cannot recreate the complexity of a real fire fluctuating temperatures producing multiplecomplex thermal stress gradients, and the by-product of soot, which has had a significant role to play in the subse-quent response of Peakmoor Sandstone to salt weathering.



Figure 8. A photograph showing a block that experienced the spalling of a corner as salt weathering exploited fracturespropagated during the fire.

Conclusions

The use of furnaces to investigate fire damage may be a reflection of the belief that temperature is the onlysignificant factor this may not be the case, as soot cover and the irregular and complex stress legacy left by fireplay an important role in subsequent block decay.

Knowledge obtained by heating stone in a furnace is based on the preconceptions of the experiment designer, whichmay mean that results are not a true representation of reality.

Furnace experiments were characterized by the consistent (and predictable) sandstone response of Peakmoor Sand-stone, producing uniform colour change.

Real fire experiments were characterized by non-uniform sandstone response (reflecting the nature of fire) someblocks showed the effects of fire more obviously than others. Effects included blackening from soot, reddeningfrom iron oxidation and fracturing (in extreme cases).

During subsequent salt weathering blocks that had experienced the real fire showed a more unpredictable response,with rapid weight loss due to the spalling of corners this response is likely to be due to the multiple microfracturenetworks produced by complex stressing in the fire.

Soot is an important side-effect of a fire, with a hydrophobic impermeable layer hindering the ingress of salt insome blocks. The eventual detachment of the soot layer produced rapid debris release and accelerated decay.

AcknowledgementsThis research was funded by the Department of Employment and Learning, Northern Ireland. Thanks to the Environment andHeritage Service for providing the sandstone for laboratory work. Thanks to Gill Alexander (QUB cartographic unit) for thepreparation of figures, and to Dr Jennifer McKinley and Dr Alastair Ruffell for help with permeability measurements. The com-ments of two anonymous reviewers are gratefully acknowledged.



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