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Assessing the environmental performance of stabilised rammed earth walls using a climatic simulation chamber
ARTICLE IN PRESS
Building and Environment 42 (2007) 139–145 www.elsevier.com/locate/buildenv
Assessing the environmental performance of stabilised rammed earth walls using a climatic simulation chamber Matthew R. Hall School of the Built Environment, University of Nottingham, University Park, Nottingham NG7 2RD, UK Received 30 June 2005; accepted 16 August 2005
Abstract The SHU climatic simulation chamber is a novel piece of apparatus that allows testing of full-sized walls with realistic inner and outer wall climatic conditions. Four SRE test walls were successfully constructed and tested over four separate regimes to measure physical properties such as pressure-driven moisture ingress, rate of moisture penetration, and internal/interstitial condensation. The walls far exceeded a series of cyclic pressure-driven rainfall penetration tests based on BS 4315-2. After 5 days of exposure to static pressure-driven moisture ingress there was no evidence of moisture penetration or erosion. The embedded sensor array detected no significant increase in the relative humidity or liquid moisture content inside the test walls, throughout a range of temperature differentials with high levels of humidity, indicating a negligible risk of internal or interstitial condensation. r 2005 Elsevier Ltd. All rights reserved. Keywords: Stabilised rammed earth; Environmental performance; Climatic simulation
1. Introduction Stabilised rammed earth (SRE) is an eco-friendly masonry wall material made from a carefully controlled mix of aggregates such as graded sub-soil, quarry waste or recycled crushed demolition waste. The aggregates are stabilised using Portland cement and then dynamically compacted inside removable shuttering. The walls are immediately load bearing and can either be solid or insulated-cavity construction. The aggregates must be carefully graded to produce a particle-size distribution that is both suitable for compaction whilst providing minimal linear shrinkage [1]. SRE offers a rapid rate of production at typically 10–15 m2 of 300-mm-thick solid wall per day and has a uniquely attractive, layered appearance that closely resembles natural sandstone [2]. A recent SRE project is the Juvenile Justice Detention Corresponding author. Tel: +44 0 115 846 7873; fax: +44 0 115 951 3159. E-mail address: [email protected].
0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2005.08.017
Centre at Dubbo in south eastern Australia (see Fig. 1) which was made using locally available soils. Dampness in buildings affects several million people throughout the UK and so the design and construction of modern masonry walls demands careful attention [3]. SRE represents a new breed of environmentally friendly wall construction that has succeeded in many developed countries throughout the world but has yet to make a significant impact in the UK despite the availability of experienced contractors. Under the Building Regulations for England & Wales there are currently no recognised standards or specifications for SRE wall construction [4]. This factor, combined with the need for further research, has been instrumental in facilitating a conservative approach towards the suitability of SRE walls in the damp UK climate. Laboratory tests have shown that both capillary and pressure-driven moisture ingress in SRE materials has been observed to vary greatly depending upon soil type, and that the performance of the material can be greatly increased through optimisation of the soil grading [5].
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Fig. 1. Juvenile Justice Detention Centre in New South Wales, Australia (photo: Earth Structures Pty Ltd.).
Fig. 2. Schematic diagram of the SHU climatic simulation chamber (photo: courtesy [6])
2. SHU climatic simulation chamber Sheffield Hallam University’s climatic simulator was originally designed and built by Alan Taylor-Firth and David Flatt to investigate the performance of building materials and full-sized building elements (e.g. walls) under a wide range of simulated climatic conditions [6]. The main advantage of the climate chamber is that it represents the elusive ‘middle-ground’ between naturally exposed outdoor test walls and less representative small-scale laboratory tests. Full-sized test walls can be constructed inside the climate chamber following normal trade practices, and realistic climatic effects of weather and exposure can be accurately simulated and monitored under laboratory conditions. The simulator itself is composed of two separate chambers called the ‘design’ side and the ‘climate’ side. The dimensions of each chamber are 4 m long, 3 m wide and 2.6 m in height. The normal operating temperature range is +20 to 15 1C (75 1C) depending on the internal conditions specified [7]. A schematic diagram illustrating the basic operation of the climatic simulation chamber is shown in Fig. 2. The design side of the simulation chamber represents the interior conditions of a building. It has the capacity to maintain standard indoor room conditions of 20 1C711 and 40% RH 75%. The climate side of the simulation chamber creates realistic sequences of different external weather conditions that can run in a fixed mode, sequence mode, or cyclic mode. The advantage of this is that the start and finish temperatures can be specified, as well as the rates of change of other weather components such as relative humidity and/or rainfall. 3. SRE test walls Four 300-mm-thick SRE test walls were constructed inside the SHU climatic simulation chamber on top of a concrete plinth (see Fig. 3). Houben and Guillaud [8] of The International Centre for Earth Construction—School
Fig. 3. Construction detail at the base of the SRE test walls.
of Architecture, Grenoble (CRATerre-EAG) recommend parameters for a suitable rammed earth particle-size distribution. These parameters have been superimposed onto the British Standard BS1377 [9] particle-size distribution chart illustrated in Fig. 4. A standard methodology for blending graded quarry material to produce 100 mm SRE cube samples has been published by the author [1]. This enables the specification and consistent production of rammed earth mix designs of known composition (see Fig. 4). The mix designs are reproducible and, for the purposes of testing, allow accurate control of parameters such as particle-size distribution. Each of the three mix designs was stabilised with 6% (by dry mass) ordinary Portland cement. Prior to compaction the soils were mixed to their optimum moisture content, previously determined in accordance with BS 1377: 4 using the well-known ‘Proctor method’. The earth was placed in layers 150 mm thick inside oiled proprietary formwork and compacted using an Atlas Copco RAM30 pneumatic tamper.
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4. Test run 1: low-velocity rainfall simulation
Fig. 4. Particle-size distribution chart showing the grading parameters for SRE mix designs.
Fig. 5. Completed SRE test walls viewed from the climate side of the simulation chamber.
The four test walls were separated from one another by a 100 mm thick timber-framed air cavity that incorporated the sealed end boards from the formwork. The end boards thus became ‘sacrificial’ and were retained as a part of the experimental design. Following their completion, the mini test walls were allowed to cure for a minimum period of 28 days in laboratory conditions at 22 1C711 and 40% RH75%. The wall edges were then overlapped and sealed with phenolic resin-coated plywood and caulked around the entire perimeter with silicone sealant. The top half of the climate chamber (the remaining height above the test walls) was completed with timber stud walling and ply wood sheets. These were then covered with 1200 g damp proof membrane and fastened around the edges with cloth tape to pressure-seal the climate side of the chamber. The resultant dimensions for the exposed test face of each wall were 500 mm wide by 900 mm high (area ¼ 0.45 m2). The result of the finishing & sealing work described here can be seen from the picture in Fig. 5.
The test methodologies and apparatus described in the following section are based on the specifications provided in BS 4315-2: 1970 Methods of test for resistance to air and water penetration—permeable walling constructions (water penetration) [10]. Test run 1 employed a sparge pipe delivery system with a low-velocity constant head supply of water. The 15 mm diameter copper sparge pipes (1 per wall) each had an array of eight holes at 0.7 mm diameter in order to deliver the rainfall ‘trickle’. Each sparge pipe had a skirting attachment connecting the flow of water with the wall in such a way that the run-off was distributed more evenly across the face of the test wall. The water supply was pumped from a reservoir into a separate header tank that was positioned on the roof of the chamber. This ensured a greater degree of accuracy in maintaining consistent water flow rates. Between the supply of water from the header tank and each of the four sparge pipes there was an in-line gate valve to enable the water supply to be shut off from within the chamber. Also fitted in-line was a 6.7 mm needle valve to regulate flow through a range between 0.2 and 2.0 L/min. The overall configuration of this rainfall delivery system is illustrated in Fig. 6. The rate of water delivery specified in BS 4315-2:1970 is 0.5 L/m2 min [10]. Since the surface area of the rammed earth test walls is 0.45 m2 the required rate of water delivery equates to 0.225 L/min per wall. The process of calibration includes covering the wall face with a removable plastic sheet such that the total amount of water delivery can be measured before being applied to the porous wall face. Due to the small pressure required to achieve this very low flow rate the delivery pattern of the rainfall was observed to trickle down the wall in discreet channels. The outlet for each decanting pipe was positioned 6 mm above the base of the guttering. This ensured that the guttering acted as a silt trap for any solid particles that washed away from the wall surface. The ‘silt trap’ first had to be primed (prior to the test commencing) by filling it with water until the decanting pipe overflowed the excess amount of water. It was intended that the total
Fig. 6. Schematic diagram of the low-velocity sparge pipe rainfall system.
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dry mass of the trapped particles would be indicative of the level of erosion that occurred due to rainfall. The test run lasted for a total of 5 days followed by a 4 week drying period. To start the test, the protective plastic sheets (used during the calibration phase) were removed, and the climate side of the chamber was pressurised to 250 N/m2. This is the figure recommended by BS 4315-2: 1970; it is a static equivalent to strong wind pressure and has the effect of exacerbating moisture penetration. The rainfall and pressure differential was constantly maintained for a cycle of 6 h, after which the pressure and rain are turned off for an 18 h drying cycle. This first test cycle represents day 1, where the entire test programme runs for a total of 5 days. The water run-off was collected at the following intervals after the start of each 6 h pressurised rainfall cycle: 10 min, 20, 30, 40, 50, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330, and 360. That is to say, the readings are taken every 10 min for the first hour and then every 30 min for the remaining 5 h. Since the rate of flow is known, the mass of run-off water collected over a 1 min period could be calculated as an effective rate of absorption in ml/m2 min for each of the four test walls. Theoretically, the initially dry test wall(s) should absorb a relatively large amount of water during the first hour of exposure, with decreasing amounts of water absorption as the remainder of the test period proceeds. It was anticipated that factors such as the total amounts of water absorption, rate of penetration, and the rate of decreasing absorption over time would be significantly affected by the soil type in each of the different rammed earth test walls. All four test walls ‘passed’ the test with distinction because no signs of water penetration or leakage occurred during the entire 5-day test run. In comparison, a similar study was recently performed in the climate chamber on a series of 300 mm thick granite block walls with hydraulic lime mortar, some with the additional application of dense lime render. Most of the stone test walls began to leak during the first 6 h of exposure to these conditions, and even those protected by render began to permit water penetration to occur after only a couple of days. On the SRE test walls, the calculated absorption rates appeared to be masked by small, intermittent fluctuations in the rate of water delivery. As a direct result of these issues, the calculated data for water absorption in the test walls is intangible. It was concluded that this particular methodology of water delivery was perhaps only suitable for a qualitative analysis of the time taken for full penetration/ leakage to occur, as specified in BS 4315-2, rather than attempt to measure the rate of water absorption over time.
drop nozzles produced a 1201 flat, fan-shaped spray pattern. This gave an efficient distribution of water and subsequent run-off across the full width of the test wall face. The spray nozzles were supplied by a high-pressure potable water supply delivered via a large header tank at a pressure of 9 bar (130 PSI). The in-line gate valves remained connected to act as an emergency shut off valve operated from inside the climate side of the chamber. The in-line needle valves were removed, however, and a single needle valve was used to regulate the mains water supply that fed a common rail connecting all four spray nozzles. The design for this high-pressure rainfall delivery system is illustrated in the diagram shown in Fig. 7. The 5-day test methodology was identical to that described in test run 1. It was observed that a flow rate of 0.65 L/min was the minimum rate at which a very good spray pattern could be achieved using this apparatus. The average variation in the delivery rate was 70.04 L/min per wall. During the 6 h daily rainfall period the water runoff from each test wall was collected and measured at regular intervals. The water run-off collection data from each of the 5 days was plotted against elapsed time on a graph. Even though the level of variation in water delivery had been much improved over that of test Run 1, its effects still appeared to have a significant influence. As one would expect, the recorded mass of collected water run-off for each of the test walls was smaller during the early stages of the test and then began to increase back towards the original delivery rate. To look at this data in isolation, however, would be misleading since the delivery rates between test walls can actually vary by up to 70.04 L/min. For each test wall, one can calculate the mean absorption rate by deducting the collected amount of water run-off, at each time interval, from the set delivery rate. The mean absorption in each SRE test wall was very low and typically less than 80 ml/ min per wall. Since the typical dry mass for one of these 300 mm thick test walls is 280 kg, the calculated mean water absorption only represents around 0.01%. It appeared that only a thin outer layer of each wall became saturated and in doing so preventing subsequent moisture ingress, i.e. the ‘overcoat’ effect (Fig. 8).
5. Test run 2: high-velocity rainfall simulation Test run #2 utilised high-pressure water supplying highvelocity raindrop spray nozzles. The purpose of this testing was to quantify moisture absorption, erosion and rate of pressure-driven moisture ingress. The high-pressure rain-
Fig. 7. Schematic diagram of the high-velocity spray nozzle rainfall system.
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The test methodology provides the effects of static pressure differential combined with a known quantity of surface absorption; therefore attempts were made to calculate the rate of water absorption per unit inflow surface area, i.e. the initial surface absorption (ISA). The findings were compared with the results of previous experimental work published by the author [11] involving 100 mm SRE cube samples made from the same soil mix recipes (433, 613, and 703). The samples had the same material dry density and were stabilised with the same percentage of cement (6% by mass). The graphs in Figs. 9, 10 and 11 show the comparison between the ISA of rammed earth cube samples for a given mix recipe, with the ISA observed in the corresponding climate chamber test wall. All of the test walls appear to have similar ISA values to one another regardless of the type of soil mix recipe or the day on which it was tested. However, the differences in ISA values for the cube samples vary greatly according to the type of soil mix recipe. There is virtually no disparity between the cube sample and test wall ISA values for the 433-mix recipe. The test wall ISA values for the 613-mix recipe are significantly lower than the cube sample values, whilst the test wall ISA values for the 703-mix recipe are considerably lower than their equivalent cube sample values. It was hypothesised that the insignificant level of moisture ingress that occurred in the climate chamber test
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Fig. 9. A comparison between the ISA values for SRE test cubes and the SRE test wall using the 433 mix design.
Fig. 10. A comparison between the ISA values for SRE test cubes and the SRE test walls using the 613 mix design.
Fig. 11. A comparison between the ISA values for SRE test cubes and the SRE test wall using the 703 mix design.
Fig. 8. Saturated fac- ade of an SRE test wall during high-velocity rainfall simulation.
walls may result from the moisture source being dynamic and not static (as in the standard ISA test). The forces of capillarity and applied pressure differential, which are working to incise absorption of the surface run-off, are perhaps working against the kinetic energy of the moving water. The clayey-silt content of each test wall was kept constant, and yet the granular particle-size distribution was varied considerably by using different mix recipes. We can conclude that, for full-scale test walls under simulated rainfall conditions, the initial surface absorption of pressure-driven water run-off appears to be independent of the granular particle-size distribution in a soil. In addition, it would appear that at this scale the level of clay content in a soil might be the principal factor controlling the level of moisture ingress and migration. Summarily, an
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approximate clay content (CC) of 0.1 and a silt content of 0.2 [5] can be used to produce stabilised rammed earth walls (with 6% cement) that have a very low absorption when exposed to pressure-driven water run-off. 6. Test run 3: embedded sensor arrays After the first and second test runs had been completed, an array of electronic sensors was retrofitted to each of the four rammed earth test walls. Three different types of sensors were installed such that the properties of temperature, relative humidity (RH), and liquid moisture content could be monitored throughout subsequent test regimes. The sensors were embedded by drilling a small hole from the inside of the wall to the required depth and then carefully inserting the sensor using a metal rod marked at different depths. Once in place the sensor hole was capped and sealed using silicone caulking. In each wall, six type-T copper/constantan thermocouples (protected by hollow plastic tubing) were installed to depths of 25, 50, 75, 100, 125, and 150 mm. In addition, armoured thermocouples were mounted onto both the internal and external wall faces. A single Rotronic RH sensor (encased in a protective PTFE membrane) was embedded to a depth of 150 mm, whilst six gold-plated resistance probes (for moisture content) were embedded to the same depths as the thermocouples. The sensors were energised, where appropriate, using a 5 V DC stabilised supply and logged readings every 10 min for the entire 5-day period of test run 3. The same methodology for pressure-driven rainfall that was used in run 2 was also applied in run 3 using highvelocity spray nozzles. The target rainfall delivery rate remained at 0.65 L/min per wall and the applied static pressure differential had a force equivalent to 250 N/m2. No changes in liquid moisture content or relative humidity were detected between the depths of 150 and 300 mm (inside face) from the exposed external face. From this we may deduce that, under these conditions, the test walls did not appear to allow the pressure-driven moisture to penetrate as far as 150 mm into the wall. The relative humidity recorded at the centre point of each test wall typically remained constant with the values observed prior to testing, and ranged between 93% and 94%. The observed cooling effect of the surface run-off water (approx. 15 1C) upon the wall surface was significant, and it affected the wall temperature all the way to the interior surface. The thermal behaviour for each of the test walls was observed to be very similar with no significant differences occurring between soil mix recipes. The initial temperature of both the internal and external wall faces was approximately 22 1C, and the temperature of the exposed faces were lowered by the cooling effect of the water to around 18 1C. The core temperature of each test wall gradually dropped by up to 1.5 1 (approx. 20.5 1C) below the interior wall face temperature during the 6 h period of exposure to rainfall. We can observe from
Fig. 12. A typical temperature depth profile analysis of an SRE test wall during a cyclic pressure-driven rainfall/drying regime.
Fig. 12, the cooling effects of the rainwater run-off were cumulative resulting in a gradual reduction in wall core temperature over the 5-day period. The implications are that potential may arise for interior surface condensation or the accumulation of interstitial condensation within an SRE wall. This would strongly depend upon the vapour permeability of the material, the determination of which would require further research at this stage. 7. Test run 4: static climate differentials The effect of temperature differential has been explored further by creating various fixed temperature and humidity gradients across the test walls. This was achieved by maintaining a difference in temperature/humidity between the design side and the climate side of the chamber for a 24 h period. All of this testing was performed in the absence of any rainfall or pressure differential. The indoor (design side) conditions were maintained at 20 1C71 1 and 40% RH 75% to represent a comfortable indoor living environment. The outdoor conditions (climate side) were kept at a constantly high level of relative humidity: 75% 75%. This represents a damp outdoor environment that is typical of the inclement British weather. Test run 4 was performed in order to provide additional data on the interesting effect of temperature depth profile analysis. The test run lasted for a total period of 4 days and included three different outdoor temperature levels. On days 1 and 2 the ‘outdoor’ conditions were continuously maintained at 8 1C 75% RH, on day 3 they were maintained at 0 1C 75% RH, and on day 4 they were maintained at 8 1C 75% RH, during which time snowfall began to occur. During all 4 days of test run 4 the sensor array was used for recording the performance of the test walls. The temperature depth profiles appeared to be remarkably similar to one another and independent of soil type. The typical thermal performance of a test wall can be represented by the example shown in Fig. 13. No significant changes were observed in the relative humidity or moisture content resistance probes during run 4. This
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independently of particle-size distribution. The embedded sensor array detected no significant increase in the relative humidity or liquid moisture content inside the test walls from a minimum depth of 150 mm away from the exposed face. This observation applied throughout a range of temperature differentials and high levels of humidity indicating a negligible risk of internal or interstitial condensation. Acknowledgements Fig. 13. A typical temperature depth profile analysis of an SRE test wall during different static temperature and humidity gradients.
suggests that no interstitial condensation could be generated within the confines of this test regime. 8. Summary The SHU climatic simulation chamber is a unique and well-established base for experimental work involving fullscale building elements. A series of stabilised rammed earth test walls have successfully been constructed inside the climate chamber in a manner that conforms to established procedures and parameters for this apparatus. Test methodologies for investigating rainfall penetration with static pressure-differential, based upon BS 4315-2: 1970, are suitable for assessing the environmental performance of SRE walls. All of the cement-stabilised rammed earth test walls exceeded the SHU climatic simulation chamber rainfall penetration tests based upon BS4315-2: 1970. After 5 days of exposure to static pressure-driven moisture ingress there was no evidence of moisture penetration or erosion. Under these test conditions, the ISA is a comparatively low value that is similar for each test wall and appears to be independent of differences in the particle-size distribution between mix recipes. This indicates that the kinetic energy of the dynamic moisture source (i.e. run-off water) is sufficient to exceed the capillary potential of the wall face. However, since the clayey-silt content of each mix recipe was constant it may also indicate that the proportion of cohesive fines in the mix determines the ISA of a wall
The author wishes to acknowledge the advice and assistance of laboratory technician Stephen Hetherington (Sheffield Hallam University) for the experimental work detailed in this paper. References [1] Hall M, Djerbib Y. Rammed earth sample production: context, recommendations and consistency. Construction and Building Materials 2004;18(4):281–6. [2] asEg. New earth structures, [brochure] available, Affiliated Stabilised Earth Group, North Fremantle, Australia, 2003. [3] Hall M, Djerbib Y. Moisture ingress in rammed earth: part 1—the effect of particle-size distribution on the rate of capillary suction, Construction and Building Materials 2004;18(4):269–80. [4] Hall M, Damms P, Djerbib Y. Stabilised rammed earth (SRE) and the building regulations (2000): part A—structural stability, Building Engineer 2004;79(6):18–21. [5] Hall M, The mechanics of moisture ingress & migration in rammed earth walls. PhD. Thesis, Sheffield Hallam University, UK, 2004. [6] Taylor-Firth A, Flatt DE. Climatic simulation and environmental monitoring—a facility for realistic assessment of construction materials in-service performance. Construction and Building Materials 2001;5(1):3–7. [7] Laycock EA, Hetherington S, Hall M. Damp towers—understanding and controlling the ingress of driven rain through exposed solid masonry wall structures. London: Confidential report for English Heritage; 2002. [8] Houben H, Guillaud H. Earth construction: a comprehensive guide. 2nd ed. London: Intermediate Technology Publications; 1996. [9] BSI. BS 1377-2: 1990—soils for civil engineering purposes—part 2: classification tests. London: British Standards Institute; 1990. [10] BSI. BS 4315-2: 1970—methods of test for resistance to air and water penetration: part 2—permeable walling constructions (water penetration). London: British Standards Institute; 1970. [11] Hall M, Djerbib Y. Moisture ingress in rammed earth: part 2—the effect of particle-size distribution on the absorption of static pressuredriven water. Construction and Building Materials 2005, in press.
Building and Environment 42 (2007) 139–145 www.elsevier.com/locate/buildenv
Assessing the environmental performance of stabilised rammed earth walls using a climatic simulation chamber Matthew R. Hall School of the Built Environment, University of Nottingham, University Park, Nottingham NG7 2RD, UK Received 30 June 2005; accepted 16 August 2005
Abstract The SHU climatic simulation chamber is a novel piece of apparatus that allows testing of full-sized walls with realistic inner and outer wall climatic conditions. Four SRE test walls were successfully constructed and tested over four separate regimes to measure physical properties such as pressure-driven moisture ingress, rate of moisture penetration, and internal/interstitial condensation. The walls far exceeded a series of cyclic pressure-driven rainfall penetration tests based on BS 4315-2. After 5 days of exposure to static pressure-driven moisture ingress there was no evidence of moisture penetration or erosion. The embedded sensor array detected no significant increase in the relative humidity or liquid moisture content inside the test walls, throughout a range of temperature differentials with high levels of humidity, indicating a negligible risk of internal or interstitial condensation. r 2005 Elsevier Ltd. All rights reserved. Keywords: Stabilised rammed earth; Environmental performance; Climatic simulation
1. Introduction Stabilised rammed earth (SRE) is an eco-friendly masonry wall material made from a carefully controlled mix of aggregates such as graded sub-soil, quarry waste or recycled crushed demolition waste. The aggregates are stabilised using Portland cement and then dynamically compacted inside removable shuttering. The walls are immediately load bearing and can either be solid or insulated-cavity construction. The aggregates must be carefully graded to produce a particle-size distribution that is both suitable for compaction whilst providing minimal linear shrinkage [1]. SRE offers a rapid rate of production at typically 10–15 m2 of 300-mm-thick solid wall per day and has a uniquely attractive, layered appearance that closely resembles natural sandstone [2]. A recent SRE project is the Juvenile Justice Detention Corresponding author. Tel: +44 0 115 846 7873; fax: +44 0 115 951 3159. E-mail address: [email protected].
0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2005.08.017
Centre at Dubbo in south eastern Australia (see Fig. 1) which was made using locally available soils. Dampness in buildings affects several million people throughout the UK and so the design and construction of modern masonry walls demands careful attention [3]. SRE represents a new breed of environmentally friendly wall construction that has succeeded in many developed countries throughout the world but has yet to make a significant impact in the UK despite the availability of experienced contractors. Under the Building Regulations for England & Wales there are currently no recognised standards or specifications for SRE wall construction [4]. This factor, combined with the need for further research, has been instrumental in facilitating a conservative approach towards the suitability of SRE walls in the damp UK climate. Laboratory tests have shown that both capillary and pressure-driven moisture ingress in SRE materials has been observed to vary greatly depending upon soil type, and that the performance of the material can be greatly increased through optimisation of the soil grading [5].
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Fig. 1. Juvenile Justice Detention Centre in New South Wales, Australia (photo: Earth Structures Pty Ltd.).
Fig. 2. Schematic diagram of the SHU climatic simulation chamber (photo: courtesy [6])
2. SHU climatic simulation chamber Sheffield Hallam University’s climatic simulator was originally designed and built by Alan Taylor-Firth and David Flatt to investigate the performance of building materials and full-sized building elements (e.g. walls) under a wide range of simulated climatic conditions [6]. The main advantage of the climate chamber is that it represents the elusive ‘middle-ground’ between naturally exposed outdoor test walls and less representative small-scale laboratory tests. Full-sized test walls can be constructed inside the climate chamber following normal trade practices, and realistic climatic effects of weather and exposure can be accurately simulated and monitored under laboratory conditions. The simulator itself is composed of two separate chambers called the ‘design’ side and the ‘climate’ side. The dimensions of each chamber are 4 m long, 3 m wide and 2.6 m in height. The normal operating temperature range is +20 to 15 1C (75 1C) depending on the internal conditions specified [7]. A schematic diagram illustrating the basic operation of the climatic simulation chamber is shown in Fig. 2. The design side of the simulation chamber represents the interior conditions of a building. It has the capacity to maintain standard indoor room conditions of 20 1C711 and 40% RH 75%. The climate side of the simulation chamber creates realistic sequences of different external weather conditions that can run in a fixed mode, sequence mode, or cyclic mode. The advantage of this is that the start and finish temperatures can be specified, as well as the rates of change of other weather components such as relative humidity and/or rainfall. 3. SRE test walls Four 300-mm-thick SRE test walls were constructed inside the SHU climatic simulation chamber on top of a concrete plinth (see Fig. 3). Houben and Guillaud [8] of The International Centre for Earth Construction—School
Fig. 3. Construction detail at the base of the SRE test walls.
of Architecture, Grenoble (CRATerre-EAG) recommend parameters for a suitable rammed earth particle-size distribution. These parameters have been superimposed onto the British Standard BS1377 [9] particle-size distribution chart illustrated in Fig. 4. A standard methodology for blending graded quarry material to produce 100 mm SRE cube samples has been published by the author [1]. This enables the specification and consistent production of rammed earth mix designs of known composition (see Fig. 4). The mix designs are reproducible and, for the purposes of testing, allow accurate control of parameters such as particle-size distribution. Each of the three mix designs was stabilised with 6% (by dry mass) ordinary Portland cement. Prior to compaction the soils were mixed to their optimum moisture content, previously determined in accordance with BS 1377: 4 using the well-known ‘Proctor method’. The earth was placed in layers 150 mm thick inside oiled proprietary formwork and compacted using an Atlas Copco RAM30 pneumatic tamper.
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4. Test run 1: low-velocity rainfall simulation
Fig. 4. Particle-size distribution chart showing the grading parameters for SRE mix designs.
Fig. 5. Completed SRE test walls viewed from the climate side of the simulation chamber.
The four test walls were separated from one another by a 100 mm thick timber-framed air cavity that incorporated the sealed end boards from the formwork. The end boards thus became ‘sacrificial’ and were retained as a part of the experimental design. Following their completion, the mini test walls were allowed to cure for a minimum period of 28 days in laboratory conditions at 22 1C711 and 40% RH75%. The wall edges were then overlapped and sealed with phenolic resin-coated plywood and caulked around the entire perimeter with silicone sealant. The top half of the climate chamber (the remaining height above the test walls) was completed with timber stud walling and ply wood sheets. These were then covered with 1200 g damp proof membrane and fastened around the edges with cloth tape to pressure-seal the climate side of the chamber. The resultant dimensions for the exposed test face of each wall were 500 mm wide by 900 mm high (area ¼ 0.45 m2). The result of the finishing & sealing work described here can be seen from the picture in Fig. 5.
The test methodologies and apparatus described in the following section are based on the specifications provided in BS 4315-2: 1970 Methods of test for resistance to air and water penetration—permeable walling constructions (water penetration) [10]. Test run 1 employed a sparge pipe delivery system with a low-velocity constant head supply of water. The 15 mm diameter copper sparge pipes (1 per wall) each had an array of eight holes at 0.7 mm diameter in order to deliver the rainfall ‘trickle’. Each sparge pipe had a skirting attachment connecting the flow of water with the wall in such a way that the run-off was distributed more evenly across the face of the test wall. The water supply was pumped from a reservoir into a separate header tank that was positioned on the roof of the chamber. This ensured a greater degree of accuracy in maintaining consistent water flow rates. Between the supply of water from the header tank and each of the four sparge pipes there was an in-line gate valve to enable the water supply to be shut off from within the chamber. Also fitted in-line was a 6.7 mm needle valve to regulate flow through a range between 0.2 and 2.0 L/min. The overall configuration of this rainfall delivery system is illustrated in Fig. 6. The rate of water delivery specified in BS 4315-2:1970 is 0.5 L/m2 min [10]. Since the surface area of the rammed earth test walls is 0.45 m2 the required rate of water delivery equates to 0.225 L/min per wall. The process of calibration includes covering the wall face with a removable plastic sheet such that the total amount of water delivery can be measured before being applied to the porous wall face. Due to the small pressure required to achieve this very low flow rate the delivery pattern of the rainfall was observed to trickle down the wall in discreet channels. The outlet for each decanting pipe was positioned 6 mm above the base of the guttering. This ensured that the guttering acted as a silt trap for any solid particles that washed away from the wall surface. The ‘silt trap’ first had to be primed (prior to the test commencing) by filling it with water until the decanting pipe overflowed the excess amount of water. It was intended that the total
Fig. 6. Schematic diagram of the low-velocity sparge pipe rainfall system.
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dry mass of the trapped particles would be indicative of the level of erosion that occurred due to rainfall. The test run lasted for a total of 5 days followed by a 4 week drying period. To start the test, the protective plastic sheets (used during the calibration phase) were removed, and the climate side of the chamber was pressurised to 250 N/m2. This is the figure recommended by BS 4315-2: 1970; it is a static equivalent to strong wind pressure and has the effect of exacerbating moisture penetration. The rainfall and pressure differential was constantly maintained for a cycle of 6 h, after which the pressure and rain are turned off for an 18 h drying cycle. This first test cycle represents day 1, where the entire test programme runs for a total of 5 days. The water run-off was collected at the following intervals after the start of each 6 h pressurised rainfall cycle: 10 min, 20, 30, 40, 50, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330, and 360. That is to say, the readings are taken every 10 min for the first hour and then every 30 min for the remaining 5 h. Since the rate of flow is known, the mass of run-off water collected over a 1 min period could be calculated as an effective rate of absorption in ml/m2 min for each of the four test walls. Theoretically, the initially dry test wall(s) should absorb a relatively large amount of water during the first hour of exposure, with decreasing amounts of water absorption as the remainder of the test period proceeds. It was anticipated that factors such as the total amounts of water absorption, rate of penetration, and the rate of decreasing absorption over time would be significantly affected by the soil type in each of the different rammed earth test walls. All four test walls ‘passed’ the test with distinction because no signs of water penetration or leakage occurred during the entire 5-day test run. In comparison, a similar study was recently performed in the climate chamber on a series of 300 mm thick granite block walls with hydraulic lime mortar, some with the additional application of dense lime render. Most of the stone test walls began to leak during the first 6 h of exposure to these conditions, and even those protected by render began to permit water penetration to occur after only a couple of days. On the SRE test walls, the calculated absorption rates appeared to be masked by small, intermittent fluctuations in the rate of water delivery. As a direct result of these issues, the calculated data for water absorption in the test walls is intangible. It was concluded that this particular methodology of water delivery was perhaps only suitable for a qualitative analysis of the time taken for full penetration/ leakage to occur, as specified in BS 4315-2, rather than attempt to measure the rate of water absorption over time.
drop nozzles produced a 1201 flat, fan-shaped spray pattern. This gave an efficient distribution of water and subsequent run-off across the full width of the test wall face. The spray nozzles were supplied by a high-pressure potable water supply delivered via a large header tank at a pressure of 9 bar (130 PSI). The in-line gate valves remained connected to act as an emergency shut off valve operated from inside the climate side of the chamber. The in-line needle valves were removed, however, and a single needle valve was used to regulate the mains water supply that fed a common rail connecting all four spray nozzles. The design for this high-pressure rainfall delivery system is illustrated in the diagram shown in Fig. 7. The 5-day test methodology was identical to that described in test run 1. It was observed that a flow rate of 0.65 L/min was the minimum rate at which a very good spray pattern could be achieved using this apparatus. The average variation in the delivery rate was 70.04 L/min per wall. During the 6 h daily rainfall period the water runoff from each test wall was collected and measured at regular intervals. The water run-off collection data from each of the 5 days was plotted against elapsed time on a graph. Even though the level of variation in water delivery had been much improved over that of test Run 1, its effects still appeared to have a significant influence. As one would expect, the recorded mass of collected water run-off for each of the test walls was smaller during the early stages of the test and then began to increase back towards the original delivery rate. To look at this data in isolation, however, would be misleading since the delivery rates between test walls can actually vary by up to 70.04 L/min. For each test wall, one can calculate the mean absorption rate by deducting the collected amount of water run-off, at each time interval, from the set delivery rate. The mean absorption in each SRE test wall was very low and typically less than 80 ml/ min per wall. Since the typical dry mass for one of these 300 mm thick test walls is 280 kg, the calculated mean water absorption only represents around 0.01%. It appeared that only a thin outer layer of each wall became saturated and in doing so preventing subsequent moisture ingress, i.e. the ‘overcoat’ effect (Fig. 8).
5. Test run 2: high-velocity rainfall simulation Test run #2 utilised high-pressure water supplying highvelocity raindrop spray nozzles. The purpose of this testing was to quantify moisture absorption, erosion and rate of pressure-driven moisture ingress. The high-pressure rain-
Fig. 7. Schematic diagram of the high-velocity spray nozzle rainfall system.
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The test methodology provides the effects of static pressure differential combined with a known quantity of surface absorption; therefore attempts were made to calculate the rate of water absorption per unit inflow surface area, i.e. the initial surface absorption (ISA). The findings were compared with the results of previous experimental work published by the author [11] involving 100 mm SRE cube samples made from the same soil mix recipes (433, 613, and 703). The samples had the same material dry density and were stabilised with the same percentage of cement (6% by mass). The graphs in Figs. 9, 10 and 11 show the comparison between the ISA of rammed earth cube samples for a given mix recipe, with the ISA observed in the corresponding climate chamber test wall. All of the test walls appear to have similar ISA values to one another regardless of the type of soil mix recipe or the day on which it was tested. However, the differences in ISA values for the cube samples vary greatly according to the type of soil mix recipe. There is virtually no disparity between the cube sample and test wall ISA values for the 433-mix recipe. The test wall ISA values for the 613-mix recipe are significantly lower than the cube sample values, whilst the test wall ISA values for the 703-mix recipe are considerably lower than their equivalent cube sample values. It was hypothesised that the insignificant level of moisture ingress that occurred in the climate chamber test
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Fig. 9. A comparison between the ISA values for SRE test cubes and the SRE test wall using the 433 mix design.
Fig. 10. A comparison between the ISA values for SRE test cubes and the SRE test walls using the 613 mix design.
Fig. 11. A comparison between the ISA values for SRE test cubes and the SRE test wall using the 703 mix design.
Fig. 8. Saturated fac- ade of an SRE test wall during high-velocity rainfall simulation.
walls may result from the moisture source being dynamic and not static (as in the standard ISA test). The forces of capillarity and applied pressure differential, which are working to incise absorption of the surface run-off, are perhaps working against the kinetic energy of the moving water. The clayey-silt content of each test wall was kept constant, and yet the granular particle-size distribution was varied considerably by using different mix recipes. We can conclude that, for full-scale test walls under simulated rainfall conditions, the initial surface absorption of pressure-driven water run-off appears to be independent of the granular particle-size distribution in a soil. In addition, it would appear that at this scale the level of clay content in a soil might be the principal factor controlling the level of moisture ingress and migration. Summarily, an
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approximate clay content (CC) of 0.1 and a silt content of 0.2 [5] can be used to produce stabilised rammed earth walls (with 6% cement) that have a very low absorption when exposed to pressure-driven water run-off. 6. Test run 3: embedded sensor arrays After the first and second test runs had been completed, an array of electronic sensors was retrofitted to each of the four rammed earth test walls. Three different types of sensors were installed such that the properties of temperature, relative humidity (RH), and liquid moisture content could be monitored throughout subsequent test regimes. The sensors were embedded by drilling a small hole from the inside of the wall to the required depth and then carefully inserting the sensor using a metal rod marked at different depths. Once in place the sensor hole was capped and sealed using silicone caulking. In each wall, six type-T copper/constantan thermocouples (protected by hollow plastic tubing) were installed to depths of 25, 50, 75, 100, 125, and 150 mm. In addition, armoured thermocouples were mounted onto both the internal and external wall faces. A single Rotronic RH sensor (encased in a protective PTFE membrane) was embedded to a depth of 150 mm, whilst six gold-plated resistance probes (for moisture content) were embedded to the same depths as the thermocouples. The sensors were energised, where appropriate, using a 5 V DC stabilised supply and logged readings every 10 min for the entire 5-day period of test run 3. The same methodology for pressure-driven rainfall that was used in run 2 was also applied in run 3 using highvelocity spray nozzles. The target rainfall delivery rate remained at 0.65 L/min per wall and the applied static pressure differential had a force equivalent to 250 N/m2. No changes in liquid moisture content or relative humidity were detected between the depths of 150 and 300 mm (inside face) from the exposed external face. From this we may deduce that, under these conditions, the test walls did not appear to allow the pressure-driven moisture to penetrate as far as 150 mm into the wall. The relative humidity recorded at the centre point of each test wall typically remained constant with the values observed prior to testing, and ranged between 93% and 94%. The observed cooling effect of the surface run-off water (approx. 15 1C) upon the wall surface was significant, and it affected the wall temperature all the way to the interior surface. The thermal behaviour for each of the test walls was observed to be very similar with no significant differences occurring between soil mix recipes. The initial temperature of both the internal and external wall faces was approximately 22 1C, and the temperature of the exposed faces were lowered by the cooling effect of the water to around 18 1C. The core temperature of each test wall gradually dropped by up to 1.5 1 (approx. 20.5 1C) below the interior wall face temperature during the 6 h period of exposure to rainfall. We can observe from
Fig. 12. A typical temperature depth profile analysis of an SRE test wall during a cyclic pressure-driven rainfall/drying regime.
Fig. 12, the cooling effects of the rainwater run-off were cumulative resulting in a gradual reduction in wall core temperature over the 5-day period. The implications are that potential may arise for interior surface condensation or the accumulation of interstitial condensation within an SRE wall. This would strongly depend upon the vapour permeability of the material, the determination of which would require further research at this stage. 7. Test run 4: static climate differentials The effect of temperature differential has been explored further by creating various fixed temperature and humidity gradients across the test walls. This was achieved by maintaining a difference in temperature/humidity between the design side and the climate side of the chamber for a 24 h period. All of this testing was performed in the absence of any rainfall or pressure differential. The indoor (design side) conditions were maintained at 20 1C71 1 and 40% RH 75% to represent a comfortable indoor living environment. The outdoor conditions (climate side) were kept at a constantly high level of relative humidity: 75% 75%. This represents a damp outdoor environment that is typical of the inclement British weather. Test run 4 was performed in order to provide additional data on the interesting effect of temperature depth profile analysis. The test run lasted for a total period of 4 days and included three different outdoor temperature levels. On days 1 and 2 the ‘outdoor’ conditions were continuously maintained at 8 1C 75% RH, on day 3 they were maintained at 0 1C 75% RH, and on day 4 they were maintained at 8 1C 75% RH, during which time snowfall began to occur. During all 4 days of test run 4 the sensor array was used for recording the performance of the test walls. The temperature depth profiles appeared to be remarkably similar to one another and independent of soil type. The typical thermal performance of a test wall can be represented by the example shown in Fig. 13. No significant changes were observed in the relative humidity or moisture content resistance probes during run 4. This
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independently of particle-size distribution. The embedded sensor array detected no significant increase in the relative humidity or liquid moisture content inside the test walls from a minimum depth of 150 mm away from the exposed face. This observation applied throughout a range of temperature differentials and high levels of humidity indicating a negligible risk of internal or interstitial condensation. Acknowledgements Fig. 13. A typical temperature depth profile analysis of an SRE test wall during different static temperature and humidity gradients.
suggests that no interstitial condensation could be generated within the confines of this test regime. 8. Summary The SHU climatic simulation chamber is a unique and well-established base for experimental work involving fullscale building elements. A series of stabilised rammed earth test walls have successfully been constructed inside the climate chamber in a manner that conforms to established procedures and parameters for this apparatus. Test methodologies for investigating rainfall penetration with static pressure-differential, based upon BS 4315-2: 1970, are suitable for assessing the environmental performance of SRE walls. All of the cement-stabilised rammed earth test walls exceeded the SHU climatic simulation chamber rainfall penetration tests based upon BS4315-2: 1970. After 5 days of exposure to static pressure-driven moisture ingress there was no evidence of moisture penetration or erosion. Under these test conditions, the ISA is a comparatively low value that is similar for each test wall and appears to be independent of differences in the particle-size distribution between mix recipes. This indicates that the kinetic energy of the dynamic moisture source (i.e. run-off water) is sufficient to exceed the capillary potential of the wall face. However, since the clayey-silt content of each mix recipe was constant it may also indicate that the proportion of cohesive fines in the mix determines the ISA of a wall
The author wishes to acknowledge the advice and assistance of laboratory technician Stephen Hetherington (Sheffield Hallam University) for the experimental work detailed in this paper. References [1] Hall M, Djerbib Y. Rammed earth sample production: context, recommendations and consistency. Construction and Building Materials 2004;18(4):281–6. [2] asEg. New earth structures, [brochure] available, Affiliated Stabilised Earth Group, North Fremantle, Australia, 2003. [3] Hall M, Djerbib Y. Moisture ingress in rammed earth: part 1—the effect of particle-size distribution on the rate of capillary suction, Construction and Building Materials 2004;18(4):269–80. [4] Hall M, Damms P, Djerbib Y. Stabilised rammed earth (SRE) and the building regulations (2000): part A—structural stability, Building Engineer 2004;79(6):18–21. [5] Hall M, The mechanics of moisture ingress & migration in rammed earth walls. PhD. Thesis, Sheffield Hallam University, UK, 2004. [6] Taylor-Firth A, Flatt DE. Climatic simulation and environmental monitoring—a facility for realistic assessment of construction materials in-service performance. Construction and Building Materials 2001;5(1):3–7. [7] Laycock EA, Hetherington S, Hall M. Damp towers—understanding and controlling the ingress of driven rain through exposed solid masonry wall structures. London: Confidential report for English Heritage; 2002. [8] Houben H, Guillaud H. Earth construction: a comprehensive guide. 2nd ed. London: Intermediate Technology Publications; 1996. [9] BSI. BS 1377-2: 1990—soils for civil engineering purposes—part 2: classification tests. London: British Standards Institute; 1990. [10] BSI. BS 4315-2: 1970—methods of test for resistance to air and water penetration: part 2—permeable walling constructions (water penetration). London: British Standards Institute; 1970. [11] Hall M, Djerbib Y. Moisture ingress in rammed earth: part 2—the effect of particle-size distribution on the absorption of static pressuredriven water. Construction and Building Materials 2005, in press.