The Effect of Relative Humidity and Temperature on the Unconfined Compressive Strength of Rammed Earth Christopher Beckett and Charles Augarde
Abstract. Rammed earth (RE) is an ancient construction technique now being considered for construction in a wide range of climatic conditions. As such, a new range of scientific investigations treating it as a highly unsaturated soil are being undertaken by several institutions in order to fully understand its properties and behaviour. This paper introduces laboratory work determining the effect of changing climatic conditions on the unconfined compressive strength of RE and offers comments on preliminary results; full results and their interpretations shall be discussed in a forthcoming paper. Keywords: Rammed earth, suction, climate change.
1 Introduction “Rammed earth” (RE) is a construction technique in which moist sandy-loam subsoil is compacted in layers between removable formwork, forming free-standing, monolithic structures. RE is still widely used in arid and semi-arid regions around the world and it is in such areas that surviving examples of ancient structures can be found. RE techniques were first developed by pre-Iron Age cultures in order to remain comfortable in otherwise hostile climates and the longevity of the RE technique is thanks to this comfort, its use of readily-available raw materials and the durability of resulting structures (Fitch and Branch, 1960; Taylor and Luther, 2004). As a result of these qualities, RE is now being considered for construction in remote areas in order to reduce the environmental impact of the transportation of materials Christopher Beckett School of Civil & Resource Engineering, University of Western Australia, Perth WA e-mail:
[email protected] Charles Augarde School of Engineering & Computing Sciences, Durham University, Durham, UK e-mail:
[email protected]
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over large distances, as well as becoming a topic for investigation to improve structural design in arid regions. Furthermore, RE’s natural appearance has also made it architecturally attractive, so that RE structures can also be found in affluent regions where other construction techniques, for example fired brick or concrete, are also common. Despite its heritage, RE has only recently been identified and investigated as a highly unsaturated soil (Jaquin et al., 2008, 2009). RE derives its considerable strength through drying; as the walls are free-standing, a significant proportion of their surface is exposed to the air so that the material achieves very low water contents, often around 1%, corresponding to very high internal suctions (>100 MPa) (Nowamooz and Chazallon, 2011). In an unstabilised state (“stabilisation” is the inclusion of cementitious materials for example cement or lime which will not be discussed in this paper), RE walls will absorb and release moisture depending on external conditions. As a result, the relative humidity within the structure remains roughly constant throughout the day (Allinson and Hall, 2010). However, prolonged exposure to high humidities, for example as might occur due to climate change, might subsequently cause a change in the material properties of RE due to a change in the internal water regime. Furthermore, the proliferation of the RE technique to regions not historically associated with RE construction might prove to be hazardous, as the use of techniques developed in a given region might not be suitable in another. Figure 1 shows the locations of RE structures built pre– and post the turn of the 19th century, clearly showing the significant increase in the range of climates in which RE is now expected to function; historically, most RE structures are confined to categories B and C (“arid” and “temperate” respectively), whereas structures built after this date are found across categories A (“tropical”) to D (“cold”) (Peel et al., 2007) (the reader is referred to this work for a more detailed explanation and derivation of the different climate categories). Therefore, it is necessary to determine the extent to which RE interacts with its environment and how that interaction affects the material properties if RE is to be successfully used in these new locations. Such knowledge would not only benefit the design of new structures but would also indicate how best to conserve heritage RE structures threatened by climate change. The aim of this paper is to introduce work investigating the effect of climate change, in terms of temperatures and humidities selected to be representative of those climates identified in Figure 1, on the unconfined compressive strengths (UCSs) of RE samples, as part of a larger investigation (encompassing several institutions) into the properties and behaviour of RE. This paper will be supported by a forthcoming paper in which laboratory results will be presented and discussed.
2 Experimental Procedure Figure 2 shows the principal experimental stages to be used during this investigation, where OWC refers to the material optimum water content and EC to the use of an environmental chamber.
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Fig. 1. Locations of RE sites compared to KGCC categories (after Peel et al. (2007)). Pre19th century sites are shown as light markers and post-19th century are shown as dark markers. Larger markers represent a greater number of RE sites within a given area.
Material used to produce RE samples is manufactured from known quantities of sand, gravel and silty-clay, following work conducted by Hall and Djerbib (2004), and is referred to as a “soil mix”, rather than a soil, as the latter suggests a natural material. This method allows for the careful control over the material particle grading, an important factor in RE construction due to the need to ensure adequate clay and “aggregate” (a term borrowed from concrete construction) contents (King, 1996; Walker et al., 2005; Easton, 2007). Soil mixes are referred to by their relative mix proportions; for example, mix “5-1-4” contains 50% sand, 10%gravel and 40% silty-clay by mass. Two soil mixes, namely 5-1-4 and 7-1-2, are to be used in this investigation in order to determine the effect of the presence of clay on the change in the material’s water content on changes in temperature and relative humidity. Dry silty-clay (‘Birtley Clay’, liquid limit 58.8%, plastic limit 25.7%, plasticity index 33.1%, predominantly kaolinite) is prepared by pulverising lumps of material which have been dried at 105◦C for 48 hours. Pulverised material is then passed through a 2.36 mm sieve in order to produce sufficiently small particles to mix uniformly with the remaining mix fractions. The sand fraction is sieved to pass 2.36 mm to remove gravel-sized particles. Gravel is sieved to pass 10 mm in order to prevent large particles interfering with the compressive strength testing (Hall and Djerbib, 2004). To improve the particle grading, sieved gravel is mixed in a 1:1 ratio by mass with those sand particles which did not pass 2.36 mm but which also all pass 10 mm.
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Fig. 2. Flow chart showing principal testing procedure stages and stage durations.
Soil mix OWC is determined via the light Proctor test following the work of Hall and Djerbib (2004). Wet soil mixes are prepared in sealable containers which are sealed and left to stand for 48 hours to allow for water equilibration. 100 mm cube samples are to be used for testing, as opposed to the traditional 200 mm × 100 mm diameter cylindrical samples, in order to produce a larger number of samples per amount of raw material and to be in keeping with current RE literature (Lilley and Robinson, 1995; Hall and Djerbib, 2004). The use of 100 mm cube samples is deemed acceptable given the use of particles ≤10 mm. Samples are manufactured in three layers, each compacted to the optimum density (as determined via the Proctor test) using an adjustable rammer. The sides of the mould are treated with form release oil in order to prevent the sample from being damaged on removal. A screed of mix material passing 1.18 mm is applied to the uppermost surface of the sample in order to provide a smooth surface for compressive testing; unlike in concrete testing, samples cannot be rotated to present a smooth surface due to material anisotropy (Bui and Morel, 2009). Samples are left to dry naturally on wire racks until achieving a constant mass (the Initial Equilibration Period (IEP) water content). They are then transferred to an EC to be held at a given temperature and relative humidity for 7 days, judged to be sufficient to ensure equilibration with the selected conditions (determined during previous testing). Temperatures and humidities used for testing, selected given climate data obtained for those sites shown in Figure 1 and the operational limits of the EC, are shown in Table 1. The equivalent suction for a given combination of temperature and relative humidity can be determined using Eqn 1 where ψ is the total suction, Ru is the universal gas constant (8.314 J/molK), T is the absolute temperature, RH is the relative humidity and Vm is the molar volume of water (18.016×10−6 m3 /mol); example suction values are shown in Table 1. Samples are then removed and their UCSs determined. The UCS is chosen to characterise the performance of the two RE mixes to remain in keeping with existing RE
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Table 1. Tested relative humidity and temperature values and equivalent suctions (at subscripted relative humidity and temperature values respectively using Eqn 1). Humidity (%)
90
70
50
30
Temperature (◦ C) 15 20 30 40 Equivalent suction (MPa) ψ90,15 = 14.0 ψ70,20 = 48.2 ψ50,30 = 96.9 ψ30,40 = 173.9
literature (Middleton and Schneider, 1992; Hall and Djerbib, 2004; Walker et al., 2005; Nowamooz and Chazallon, 2011). Sample material is then crushed and dried at 105◦C for 48 hours in order to determine the final sample water content (the ECEP water content), which can be compared to the IEP water content in order to determine any changes in the internal water (and so suction) regimes.
ψ =−
Ru T ln(RH) Vm
(1)
3 Preliminary Results and Observations Although full results are not yet available, several properties of the material are becoming apparent: • The UCS of RE is greatly affected by changes in the humidity and temperature, and so suction; • Results suggest that there are only very slight differences between wetting and drying suctions, due to the very low water contents present; • Results suggest that pore water remains in the capillary (bulk) regime despite 4γ equivalent pore diameters of roughly 2 nm, determined via d pore = − , where ψ d pore is the pore diameter and γ is the air-water surface tension, estimated via γ = 0.1171 − 0.0001516T (Edlefsen and Anderson, 1943). This is very close to the limit of capillarity of 1.4 nm given by Butt and Kappl (2009), further suggesting that the limit of capillarity is different for different clay types; • The water contents and relative adsorbed water contents of samples of mix 5-1-4 for a given applied suction are higher than those of 7-1-2 for the same applied suction due to the former’s higher clay content; • For a given applied suction, samples of mix 7-1-2 achieve greater UCSs than those of mix 5-1-4, suggestibly due to the former’s lower relative adsorbed water content (more water is available to contribute to capillarity). It is clear that further work is required in order to fully understand the processes present in those RE samples tested, however it is also clear that the suction regime under which RE is found greatly affects its strength. Therefore, a reduction in the
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suction present, say through the development of a cooler, wetter climate, is of significant concern to RE construction as structures could become in danger of collapse. Acknowledgements. This work was carried out whilst the first author was a PhD student at Durham University and was supported by a studentship awarded by the School of Engineering and Computing Sciences, Durham University.
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