Advances on the assessment of soil suitability for rammed earth

Construction and Building Materials 42 (2013) 40–47

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Advances on the assessment of soil suitability for rammed earth Daniela Ciancio a,⇑, Paul Jaquin b, Peter Walker c a

School of Civil and Resource Engineering, University of Western Australia, Australia Integral Engineering Design, Tollbridge Studios, Bath, United Kingdom c BRE Centre for Innovative Construction Materials, Department of Architecture and Civil Engineering, University of Bath, United Kingdom b

h i g h l i g h t s " Current assessment criteria of soil for rammed earth are vague and contradictive. " Soil grading curves alone should not be used to predict rammed earth performances. " Unstabilised rammed earth cylinders should not be oven-dried before being tested. " Performance requirements should reflect the specifics of the construction project.

a r t i c l e

i n f o

Article history: Received 27 September 2012 Received in revised form 19 December 2012 Accepted 23 December 2012 Available online 9 February 2013 Keywords: Rammed earth Particle size distribution Compressive strength Erosion Shrinkage

a b s t r a c t A soil grading curve is one of the most useful tools to assess the suitability of material for rammed earth construction. Different and sometimes contradictory proportions of clay, silt, sand and gravel are proposed for rammed earth soils. This paper investigates the reliability of current guideline values through comparative performance testing of rammed earth specimens. Ten artificial soil batches (five of them stabilised with cement and/or lime) deemed suitable for rammed earth according to the current guidelines were tested in terms of compressive strength, shrinkage and erosion. The investigation shows that complying with soil particle size distribution criteria does not alone necessarily mean suitability of a soil for rammed earth. Based on these results, this paper proposes recommendations and criteria to be implemented in the assessment of soil for rammed earth. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Rammed earth is a construction technique that offers social, economic and environmental benefits [10,15,18,27,30] especially when used in developing countries where material costs outweigh labour costs, where other construction materials and technologies might not be available [21], or in very isolated areas where transport costs can be prohibitive [8]. The environmental and financial impact of the construction is significantly reduced when the raw material (soil) is sourced on-site [20]. Due to the ease and simplicity of this building technique, a local unskilled labour force can be readily employed, supplying job opportunities to remote communities and eliminating the cost of accommodation and transport of labour brought from distance. A rammed earth building can have a significantly lower embodied energy and carbon footprint than an equivalent building made of more conventional materials such as concrete, steel or masonry [8,27].

⇑ Corresponding author. E-mail address: [email protected] (D. Ciancio). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.12.049

Rammed earth has been recently proposed as a suitable construction technique in the Aboriginal Housing program of the Department of Housing of the Government of Western Australia for the reasons discussed above. Rammed earth houses are to be built in communities of the remote Aboriginal areas of Western Australia and then assigned to Aboriginal families who cannot afford to pay the extremely high cost of a house in those locations. There are many communities involved in this project, and therefore many different construction sites characterised by different in situ soils. For each of those, the suitability for rammed earth must be checked. In Australia there is no official code that provides guidelines for the suitability of soil for rammed earth, however two handbooks, Bulletin 5: Earth-Wall Construction [19] (hereafter referred to as Bulletin 5) and The Australian Earth Building Handbook 195 [28] (hereafter referred to as HB195), are generally used as de facto standards. Additionally to these guidelines, a New Zealand standard for earthen construction exists [26] and further scientific studies in this area are available in the literature [1,4,6,7,13]. The majority of these documents recommend soil suitability criteria based on the Particle Size Distribution (PSD). Maximum and

D. Ciancio et al. / Construction and Building Materials 42 (2013) 40–47

minimum values of content of clay, silt, sand and gravel are given to aid selection, although in some cases are taken as ‘deemed to comply’ criteria. The majority of these recommendations are obtained from extrapolation or interpretation of experimental results on soils from different parts of the world. Because these soils are of different mineralogy, and the obtained results are correct for the specific tested soil, they cannot be generalised into universal rules for any type of soil. This is clearly shown by the fact that the recommendations available in literature are often in contradiction. This paper presents research which aims to show that the PSD alone is not sufficient to assess the suitability of a particular soil for rammed earth. Through an experimental program conducted on 10 artificial soil batches that generally comply with grading recommendations available in the literature, this study investigates the reliability of using grading curves for soil selection. The following section presents an overview of the Australian guidance, the New Zealand code and other relevant recommendations. Section 3 reports the characteristics of the artificial soils used in the experimental program. Section 4 describes the methods and procedures used in the laboratory to validate the suitability of the artificial batches. In Section 5 the results are discussed and Section 6 presents the major concluding remarks of this study.

2. Current recommendations It is not the aim of this paper to review all the current studies on soil suitability for rammed earth. An exhaustive evaluation can be found in the work of Jiménez and Cañas [17] and Walker et al. [29]. This paper reviews some relevant documents with the aim of clarifying the strengths and weaknesses of the current suitability assessment methods and of proposing more rational guidelines. Rammed earth can be made using either a natural or specially prepared (engineered) soil compacted at or near to its Optimum Moisture Content (OMC) for the method of compaction, to maximise its Dry Density. Traditionally rammed earth soils were often stabilised by the addition of lime, which improved strength and weathering resistance. Since WWII it has become common in some countries to add cement to rammed earth (cement stabilised rammed earth – CSRE). Soils without additional binders (unstabilised rammed earth – URE) purely rely on friction and particle interlock and presence of water to provide strength [16]. The authors note that engineering understanding and design of rammed earth is in an infant state, and well behind that of more common construction materials such and steel, masonry and concrete, and engineering descriptions of the constituent soils are not as useful as those developed for geotechnical engineering. However, the underlying material behaviour can be approximated using models developed for general structural materials, and therefore it is useful to determine mechanical properties as Young’s Modulus, Poissons ratio, Yield Strength and Ultimate Failure Strain. In this regard, certain concrete laboratory procedures to calculate material characteristics such as UCS [24], FS [22] and Young Modulus [23] are often adopted for cement-stabilised rammed earth. In selecting materials for rammed earth (and indeed concrete and masonry), it is most sensible to use the body of work developed for geotechnical engineering. Such testing allows the classification of different soils, and allows the grouping of those with similar properties according to the testing. The simplest measure of a soil is its particle size distribution, giving an indication of both the size of the particles and their relative proportions. Further tests such as those developed by Atterberg allow the soil to be further classified, and it is these classifications which are used to determine the suitability of a soil for use in rammed earth. The relatively small amount of rammed earth building (when compared to other

41

construction materials) means that while it is possible (i) to undertake the testing outlined above on soil samples, (ii) to construct rammed earth samples and (iii) to undertake structural testing on them, there exists a disconnect between the soil suitability and the performance of the completed rammed earth samples. Recent work [7,13] have developed methodologies for the screening of soils against certain acceptance criteria, but these have not yet gained widespread approval. The documents reviewed in the following section provide performance requirements for the completed rammed earth samples (Unconfined Compressive Strength UCS, Flexural Strength FS, Durability Index, Drying Shrinkage) and present recommendations for the material characteristics of the initial soil (PSD, Plasticity Index, cement content, etc). These recommendations assert that where the material characteristics are within the prescribed values, then the performance requirements will be met. This assumes that the rammed earth is compacted at the OMC, using sufficient compaction energy and that the workmanship is of sufficient quality. 2.1. Earth building guidelines in Australia and UK In Australia there are currently two established documents that provide directions for engineers and builders using rammed earth: Bulletin 5 [19] and HB195 [28]. Although neither is specifically written for stabilised rammed earth, cement stabilisation is mentioned in both references because it is a widespread practice in Australia. The two major tests recommended in Bulletin 5 to determine rammed earth suitability are the Unconfined Compressive Strength test and the Accelerated Erosion Test (AET). The suggested values are reported in Table 1. A recommended PSD is not given, but the clay content is suggested to be relatively low. The HB195 recommendations are in terms of grading and plasticity properties, as shown in Table 1. The maximum gravel size is not specified and appropriate clay types are not mentioned. HB195 describes in some detail the experimental procedures to measure strength and durability parameters, but leaves the proportion of cement stabilisation and suitable performance criteria to the user. New Zealand Standard 4298:1998 [26] outlines construction details for earth building in general, including rammed earth. It does not provide quantitative guidelines in terms of grading curves of suitable soils, but it focuses on testing procedures and performance requirements, some of which are presented in Table 1. Burroughs [6] conducted a study on 111 different soils sourced from around Australia in order to develop quantitative criteria for the selection and stabilisation of soils for rammed earth. Soils were classified using PSD, Plasticity Index and Linear Shrinkage tests. The suitability of soils for stabilisation was judged against an Unconfined Compressive Strength criterion of 2 MPa. The results are presented in terms of charts in which soils are deemed favourable or unfavourable for stabilisation; if favourable, another chart recommends the amount of cement and lime (between 1% and 5.5% of total mass) to be used. In the UK, Walker et al. [29] suggest soil gradings suitable for unstabilised rammed earth (URE) construction which are given in Table 1. It does however note that ‘‘influence of variation in grading on physical characteristics of rammed earth, including strength and durability, remains uncertain owing to lack of test data’’. 2.2. Some relevant soil mechanics research in soil stabilisation As previously mentioned, a review of the available studies on soil stabilisation is beyond the purpose of this paper. Here only the research by Akpokodje [1] and Bryan [4] are discussed. The work of Akpokodje stands as an exhaustive analysis in which the soil PSD is not used as the unique soil index. The soil

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D. Ciancio et al. / Construction and Building Materials 42 (2013) 40–47

Table 1 Summary of the material and performance requirements from Bulletin 5 [19], HB195 [28], Standards New Zealand [26] and Walker et al. [29]. Bulletin 5 [19]

HB195 [28]

NZS 4298:1998 [26]

Walker et al. [29]

Material requirements Sand and gravel (% by mass) Silt (% by mass) Clay (% by mass) Cement (% by mass) Liquid Limit Plasticity Index Linear Shrinkage

– – Relatively low content – – – –

45–75 10–30 5–20 4–12 <35–45 <10–30 –

– – – – – – –

45–80 10–30 5–20 4–12 <45 <2–30 <5

Performance requirements Unconfined Compressive Strength (>MPa) Flexural Strength (>MPa) Drying Shrinkage (%) Erosion Index (mm/min)

2 – – <1

1 0.5 – –

1.3 0.25 <0.05 <2

1 – <0.5 –

mineralogy, the different effects of cement and hydrated lime and other material parameters as strength, shrinkage and swelling are reviewed. The exhaustive results are conclusive for the investigated soil, i.e. the guidelines proposed by Akpokodje are not valid for any soil. With the aim of offering further guidance for the identification of soils to be stabilised with cement, Bryan highlights two main common trends: 1. there is a requirement for a small amount of clay for compaction purposes but an upper limit is also necessary to limit shrinkage to ensure effective stabilisation; 2. plasticity is required as a complementary measure to the limit of the clay content. Bryan produces a chart in which the suitability of soil stabilised with 7.5% cement (by mass) and compacted under 2 MPa is assessed only in terms of the PSD. For more active clay-mineralogy, further specific investigations are recommended. For unsuitable soils, it is considered doubtful that higher cement contents and compaction pressure can improve the tested soil parameters (density, strength, freeze thaw dilatation and absorption rate). It is crucial to state that the majority of the studies available in the literature are based on specific soil mixes, and hence the results are valid only for those mixes. Considering the endless variety of existing soils, drawing conclusions valid for any soil type from the results of a fraction of samples seems an impossible task. In this paper, although broad recommendations are considered useful for a preliminary judgment of the soil, rules based on PSD that can predict the mechanical performance of any soil mix are not considered suitable for the assessment of soil for rammed earth. 2.3. Assessment criteria: weakness and strength The standard geotechnical tests such as PSD, Atterberg Limits and Linear Shrinkage (LS) are useful material characteristics with which to describe a soil. The various guidelines presented before show that there are broad ranges of suitability of soils for use as rammed earth. However of paramount importance are the performance requirements of the finished product. These are usually dictated by the requirements of the project but recommended values are also given in the literature. The PSD is regarded as an indicative factor for soil suitability; a soil with a PSD which approximates to the Fuller and Thompson [12] grading rule is able to be compacted to maximum density where smaller particles fill the voids between the larger particles. Intuitively, a denser sample would be assumed to have a higher compressive strength due to an increased number of interactions between particles. Nevertheless, there is little clear scientific

evidences that correlate strength and density in both URE [13] and CSRE [9]. This might be explained by the fact that interlocking (strictly related to the particle shapes and the grading of the soil) is not the only source of strength in rammed earth and hence PSD is not the sole indicator of the suitability of soil for rammed earth when strength performance is assessed: clay type and content and water suction in pores are other important source of strength in URE. The important role of moisture content is often not discussed and the majority of the current guidelines do not explicitly take into account the role of suction [3,16], or the effect of the water–cement ratio. The cement hydration process produces a solid matrix within which the soil particles are bound. This is the major source of strength of CSRE. The clay/cement interaction, though, highly affects the quality of this matrix meaning that soils recommended for URE may not be suitable for CSRE. Geotechnical tests to characterise soil, such as the Plastic Limit (PL) and Liquid Limit (LL) – from which the Plastic Index (PI) is obtained – generally give an indication as to the presence of clay in a soil, and so have been proposed as a method for determining the suitability of soils for rammed earth [14]. However, determination of the PL and LL is usually performed on samples passing a 425 lm sieve and so the clay fraction of the sieved sample is artificially increased, meaning this may not be an accurate descriptor for rammed earth soil suitability. The Linear Shrinkage test might give valid information about the reactivity of the clay when its mineralogy is unknown. However, also this test is performed only on the fine particles. A Drying Shrinkage test of finished RE samples made of soil in its entire composition should provide more useful information to understand the shrinkage behaviour of a rammed earth structural member, which allows for the spacing of construction joints to be designed. The Unconfined Compressive Strength requirements presented in Table 1 provide restrictions useful for the structural design. However, if for instance the structural performance of a rammed earth wall is simplistically regarded as its ability to stand the vertical loads due to its own self weight and the weight of the roof, it is easy to show any compressive strength higher than 0.2 MPa would be sufficient to prevent the failure for crushing at the base of the wall. It is obvious that other failure mechanisms must also be analysed when designing a RE wall. However this simple calculation shows that for simple structures and load scenarios, the material UCS can be lower than the values in Table 1. 3. Materials and preliminary tests To investigate the reliability of current recommendations for soil suitability, 10 artificial soils comprised of different contents of kaolin clay, silica flour (also called ‘‘rock flour’’, an inert material used as silt), clean white sand and gravel (10 mm max size) were created. Five batches were unstablised, and five were stabilised with

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D. Ciancio et al. / Construction and Building Materials 42 (2013) 40–47 Table 2 PSD of the 10 artificial soils. Contents are in mass percentage. URE

CSRE

Batch

% Clay

% Silt

% Sand

% Gravel

Batch

% Clay

% Silt

% Sand

% Gravel

% Cement

% Lime

1 2 3 4 5

5 30 15 30 40

25 0 15 20 20

50 50 50 40 20

20 20 20 10 20

6 7 8 9 10

10 10 20 30 5

15 5 0 10 25

50 40 60 20 50

25 45 20 40 20

5 4.5 4 4 4.5

0 0 1 2 0

100

50 batch 1

90

Passing, %

batch 4

60

batch 5

50

batch 6

40

batch 7

30

batch 8

20

batch 9

10

batch 10

0.001

0.01

0.1

1

10

40

Plasticity Index (%)

batch 3

70

0 0.0001

A line unstabilised batches stabilised batches Walker (2005)

batch 2

80

30

20

10

100 0

particle size in mm

0

10

20

cement and lime. The grading of each soil is shown in Table 2 and Fig. 1, in terms of mass percentage. The cement and lime contents are calculated as percentage by mass of the total dry mass of soil excluding cement. The artificial batches 1–3–6– 7–8 and 10 have particle size distributions that make them suitable according to the grading limits presented in Section 2, Table 1. The batches 2–4–5 and 9 have a clay content that exceeds the maximum amount recommended by HB195 [28] and Walker et al. [29] but are in line with the recommendations from other studies [2,11]. The amount of cement satisfies the minimum requirements from [28,29]. The amount of lime was decided taking into account the recommendations from [7]. The Liquid Limit and the Plasticity Index of the soils were determined in accordance with AS 1289.1.1 [25] using only the fraction of soil passing a 425 lm sieve. The results are presented in Table 3 and in Fig. 2. The Liquid Limit and the Plasticity Index are all below the maximum recommended values reported in Table 1. Batches 2, 4, 5 and 9 do not satisfy the recommended maximum Linear Shrinkage strain value of 5% [29]. The OMC for each batch was determined using the Modified Proctor test [25].

4. Methods and procedures All samples were rammed adding to the artificial soils an amount of water equal to their OMC. It was then important to guarantee that the same compaction effort used in the Modified Proctor test was applied in the preparation of the moulded samples. The following procedure was used to achieve this goal. The energy used to compact a soil layer of the Modified Proctor Test sample is equal to:

EOMC ¼ nOMC ðm  g  dÞ

ð1Þ

30

40

50

60

70

80

Liquid Limit (%)

Fig. 1. Grading curves of the artificial soils used in the experimental program.

Fig. 2. Plasticity Index and Liquid Limit for the 10 artificial batches, compared to the limits (in dashed lines) given by Walker et al. [29].

being m the mass of the hammer in kg, g the gravitational acceleration in m/s2, d the drop height of the hammer in m and nOMC the number of times the hammer is dropped for that specific layer. The specific energy per volume is then equal to:

eOMC ¼

EOMC V OMC

ð2Þ

with VOMC being the volume of the layer of the sample. The moulded samples were rammed using a jackhammer. The energy per blow (j = Em/b) and the number of blows per minute (nb = b/tmin) of the jackhammer are characteristics that are given by the supplier. If Vm is the volume of the layer being rammed in the moulded sample, the specific energy em is defined similarly to Eq. (2), i.e. em = Em/Vm. By equating em and eOMC:

em ¼

Em jnb t min nOMC ðm  g  dÞ ¼ ¼ ¼ eOMC V OMC Vm Vm

ð3Þ

one can obtain the number of minutes needed to ram each layer of the moulded sample so that the compaction energies in both procedures are the same:

tmin ¼

V m nOMC ðm  g  dÞ j V OMC nb

ð4Þ

Table 3 Preliminary test results in terms of LL, PI, LS and OMC. URE

CSRE

Batch fines only

Liquid Limit (water content (%))

Plasticity Index

Linear Shrinkage (%)

Optimum Moisture Content (%)

Batch fines only

Liquid Limit (water content (%))

Plasticity Index

Shrinkage (%)

Optimum Moisture Content (%)

1 2 3 4 5

15.6 26.1 18.0 24.8 34.5

3.1 13.8 9.7 13.4 18.4

0.0 6.0 3.0 5.1 7.1

5.8 8.3 6.4 7.4 9.6

6 7 8 9 10

15.4 17.3 22.3 38.5 15.4

5.3 7.9 12.1 23.3 4.0

2.0 2.4 3.5 7.1 0.2

5.6 5.4 7.4 9.4 5.3

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D. Ciancio et al. / Construction and Building Materials 42 (2013) 40–47

It was planned that the formwork would be stripped after 1 day but as the atmosphere was humid due to persistent rain, the unstabilised samples were quite soft. They required careful handling as it was found that they were soft and prone to chipping. They were cured unwrapped in the formwork for 3 days then stripped and left to cure for a minimum of 28 days in ambient conditions. The formwork on the stabilised samples was stripped after 1 day and the samples were then wrapped in an impermeable membrane for 7 days to prevent water from evaporating and so allow the cement to fully hydrate. They were then left to cure at ambient conditions for at least 20 days. The samples containing lime were stripped from formwork after 1 day and wrapped for 21 days, as specified by Standards New Zealand [26]. When not wrapped, all samples were cured at ambient temperature and humidity conditions (mean monthly maximum temperature was 18°, mean monthly minimum temperature was 7° and mean relative humidity was 68%) in the UWA structures laboratory, protected from direct sunlight and rain. Ideally curing should take place under rigorously controlled conditions however this was not possible in this study due to insufficient availability of the laboratory facilities. All samples for Drying Shrinkage testing were cured in their formwork for at least 28 days, as specified by Standards New Zealand [26]. Following the guidelines discussed in Section 2, the Unconfined Compressive Strength and the Drying Shrinkage Strain were obtained experimentally for each of the 10 artificial soils; the Erosion Index was calculated only for the five unstabilised batches. 4.1. Dry Unconfined Compressive Strength For each batch, three cylindrical samples of 100 mm diameter and 200 mm height were moulded to obtain the dry UCS. All samples were at least 28-day old when tested. In preparation for testing, the cylinders were dried in oven at 100 °C for approximately 24 h, after which they were placed in a desiccator (to prevent further moisture gain) for approximately 5 h whilst cooling to room temperature as specified in Bulletin 5 [19]. Samples for UCS testing are required to have a smooth, level and even end surfaces so that the compression force is applied as a uniformly distributed load over a consistent area of the sample. If there are irregularities on the sample surface, the applied force could be concentrated through particular points of the sample which would affect the failure behaviour and possibly cause inaccuracies in the results. It was planned to cap any sample which had an uneven surface with a thin layer of dental plaster, as outlined by Walker et al. [29] and detailed in AS 1012.9–1999 [24]. Capping was attempted on one batch initially but proved difficult and damaged the weak unstabilised samples. It was also discovered that the capping broke away from the sample during oven drying so this method was deemed inappropriate and was abandoned for the remaining batches. Instead plywood packing was used on the top and bottom of samples, which is recommended in Bulletin 5 [19]. The plywood was easily compressed and deformed to the shape of the sample under compression. The minimum strength criteria for samples tested in this study was calculated according to the guidelines in Bulletin 5 [19] that suggest using the adjusted characteristic Unconfined Compressive Strength fc0 . To obtain fc0 for each batch, the UCS of each sample was initially calculated as UCS ¼ AF , F being the maximum compressive force and A the area of the surface where the load is applied. Each UCS value was adjusted to take into consideration the aspect ratio of the sample according to the formula:

UCS0 ¼ ka UCS

ð5Þ

in which ka was taken from Bulletin 5 [19]. Finally, for each batch, the average adjusted Unconfined Compressive Strength UCS0 was calculated and from it the characteristic value:

fc0 ¼ UCS0  1:65s0

ð6Þ

0

s being the standard deviation. 4.2. Accelerated Erosion Test The Accelerated Erosion Test (AET) involves subjecting a rammed earth block sample to a high pressure water spray for 60 min or until the sample has completely eroded through, whichever occurs first. Either the pitting depth after 60 min, or the time taken for water to penetrate the sample is recorded to give the erosion rate. The erosion rate of a sample (in mm/min) is determined by dividing the sample thickness by the time taken for the water to fully penetrate the sample [26]. From other studies [8] the erosion measured using the Accelerated Erosion Test (AET) on CSRE was negligible; for this reason, the AET was carried out only on the URE batches (1–5 in Table 2). One prismatic sample per unstabilised batch with dimensions of 200  200  300 mm [19] was prepared. The depth of the sample (300 mm) replicated the typical thickness of a rammed earth wall. 4.3. Drying Shrinkage One prismatic sample per batch with dimensions of 50  50  600 mm was rammed for the shrinkage test. Drying Shrinkage testing was conducted in accordance with the procedure outlined by Standards New Zealand [26]. While other shrinkage test procedures [29] specify screening out of large soil particles, Standards New Zealand [26] specifically state that ‘‘to test rammed earth shrinkage make up a sample of mix with the same percentage of material and at the same moisture content as would normally be placed in a wall’’. The formworks were made according to Standards New Zealand [26] specifications. The melamine formwork has a non-stick surface but the shrinkage moulds were also oiled to further reduce the friction between the mould and the sample. As the compacted sample remained in the formwork during curing, it was important that friction was minimised so that the sample was not constrained from shrinking. 5. Results and discussions 5.1. UCS The test results for all batches are presented in Table 4 in terms of dry fc0 and Dry Density. 5.1.1. URE samples For the unstabilised batches, the measured dry fc0 is far less than the expected minimum value of UCS in Table 1 equal to 1 MPa. All unstabilised batches satisfy the material but not the performance requirements. The low UCS values for the unstabilised batches might be explained by the fact that samples were oven dried before being tested. Water suction in partially saturated pores significantly contributes to the strength of URE [3,16]. Since all specimens were oven-dried at 100 °C and stored in a dessicator before being tested, the majority of the water could have been removed and thus the strength component due to water suction reduced. In this scenario, without the contribution of water suction, the only source of strength would be the particles interlock. Although this explanation requires further investigation to be generally proven, it may be reasonable to assume that unstabilised specimens tested at ambient conditions (therefore with a higher moisture content) may show UCS values higher (and definitively more realistic) than those obtained on oven dried specimens. Fig. 3 shows that there is little correlation between the adjusted UCS (UCS0 ) and the Dry Density of the samples and that there is a

45

D. Ciancio et al. / Construction and Building Materials 42 (2013) 40–47 Table 4 Results of the Unconfined Compressive Strength test. Batch No.

Dry Density (kg/m3)

UCS (MPa)

Aspect ratio (H/D)

ka

UCS0 (MPa)

UCS0 (MPa)

s0 (MPa)

fc0 (MPa)

1

1976.70 1924.84 2013.73

0.32 0.42 0.46

2.0 1.7 1.9

0.75 0.73 0.74

0.24 0.30 0.34

0.30

0.05

0.21

2

1992.62 1948.06 1967.16

0.92 0.42 0.83

2.0 2.0 2.0

0.75 0.75 0.75

0.71 0.32 0.64

0.56

0.20

0.22

3

2014.90 1941.69 2059.46

0.41 0.32 0.60

2.0 2.0 2.0

0.75 0.75 0.75

0.31 0.25 0.46

0.34

0.10

0.16

4

1795.27 1836.65 1741.16

0.39 0.43 0.84

2.0 2.0 2.0

0.75 0.75 0.75

0.30 0.33 0.63

0.42

0.18

0.12

5

1769.80 1766.62 1737.97

0.86 0.68 0.61

2.0 2.0 2.0

0.75 0.75 0.75

0.64 0.51 0.46

0.54

0.09

0.38

6

1963.97 2014.90 2005.35

4.36 7.75 5.76

2.0 2.0 2.0

0.75 0.75 0.75

3.38 5.85 4.46

4.56

1.23

2.52

7

2154.96 2164.51 2139.04

11.30 12.14 9.59

2.0 2.0 2.0

0.75 0.75 0.75

8.52 9.15 7.23

8.30

0.98

6.68

8

1932.14 1919.41 1967.16

4.65 4.93 5.85

2.0 2.0 2.0

0.75 0.75 0.75

3.51 3.72 4.41

3.88

0.47

3.10

9

1808.00 1788.90 1782.54

3.65 3.24 3.46

2.0 2.0 2.0

0.75 0.75 0.75

2.75 2.45 2.61

2.60

0.15

2.35

10

2005.35 2005.35 2011.72

8.12 7.21 8.04

2.0 2.0 2.0

0.75 0.75 0.75

6.12 5.43 6.06

5.87

0.38

5.25

Fig. 3. UCS0 vs. Dry Density of URE samples. Fig. 4. UCS0 vs. Dry Density of CSRE samples.

reasonably large spread in the results. This would indicate that the assertion of correlation between Dry Density and strength for unstabilised samples cannot be proven. 5.1.2. CSRE samples All stabilised batches satisfy the limit of 2 MPa proposed by Burroughs [6] and the requirements in Table 1. Fig. 4 shows that there may be some correlation between the adjusted Unconfined Compressive Strength and the Dry Density. The CSRE batch results are more closely clustered than the URE data and although further testing and analysis are required, it may be possible to use Dry Density as a proxy for Unconfined Compressive Strength. It is difficult, however, to draw definitive conclusions given the small number of specimens.

The basis for the lower bound compressive strength limits given in the guidance and standards is unclear. As mentioned in Section 2.3, the Appendix in this paper presents a rough calculation of the order of magnitude expected for compressive stresses acting at the foot of a bearing wall. It is shown to be far lower than the compressive strength prescribed in Table 1. Although the specification of the minimum material strength might provide some help for the assessment of basic material integrity, using lower bound limits should not be used to assess material suitability for specific structural applications. For instance, by limiting the assessment to Burroughs’ recommendations [7], batches 7 and 10 might be treated in the same way of batches 8 and 9; however, Fig. 4 shows that the former are far better than the latter.

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Fig. 5. Accelerated Erosion Test for batch 1 sample: (a) and (b) two stages soon after the beginning of the test; (c) and (d) two stages at the end of the test.

5.2. AET None of the unstabilised batches passed the AET. The water penetrated the depth of the samples in less than 1 h, as shown in Fig. 5 for the batch-1 sample. Even though it is often recommended as a suitable test to measure the durability of rammed earth in terms of erosion, the main criticism towards this test is that it creates conditions that are very dissimilar from the real environmental conditions to which a rammed earth wall will be exposed during its life time. Bui et al. [5] measured the real erosion of different rammed earth walls over 20 years, finding much less erosion than is created using the AET. Although the results are useful, more pertinent laboratory tests are still needed to assess the durability of rammed earth walls. 5.3. Drying Shrinkage According to Standards New Zealand [26], the maximum allowed shrinkage should be less than 0.3 mm (0.05% along a 600 mm long sample). For Walker et al. [29], the maximum shrinkage should not be higher than 30 mm (5%). It is interesting to notice the discrepancy between the two values of minimum

Fig. 6. 28 day-old rammed earth specimens used to measure the shrinkage strain. No volume reduction was visible at naked eyes.

Drying Shrinkage found in Standards New Zealand [26] and Walker et al. [29]. The former seems far too strict while the latter seems more realistic. All samples (shown in Fig. 6) did not show any evidence of shrinkage strain at 28 days visible to the naked eye.

6. Conclusions Laboratory testing and assessment of rammed earth materials can be an expensive and uncertain process, but where in situ unknown materials are to be used some form of assessment is necessary. Prescriptive rules based on material characteristics, such as PSD, are problematic and potentially restrictive due to the wide variation in natural soil deposits. Six of the 10 artificial soils created in this study were deemed suitable for rammed earth according to the PSD guidelines in HB195 [28] and Walker et al. [29] (reported in Table 1 in this paper). The remaining 4 presented a very high clay content that in certain studies [2,11] is also deemed suitable. The suitability of all of them has been tested in terms of PI, LL and Linear Shrinkage of the fine particles of the soil and characteristic Unconfined Compressive Strength, Erosion Index and Drying Shrinkage of the rammed earth samples. All soils satisfied the PI and LL limits proposed in HB195 [28] and Walker et al. [29]. Soils 2, 4, 5 and 9 (with the highest clay contents) did not comply with the maximum Linear Shrinkage limit of Table 1. None of the unstabilised batches complied with the minimum expected strength of 1 MPa; all cement-stabilised batches satisfied the material requirements and the strength limit of fc0 P 2 MPa proposed by Burroughs [6] with some of them (batches 7 and 10) significantly exceeding that strength limit (the criterion suggested by Burroughs would have been in this case quite conservative). All unstabilised batches failed to pass the AET according to the Standards New Zealand [26] and Bulletin 5 [19]. All batches (stabilised and unstabilised) could be deemed satisfactory in the Drying Shrinkage test according to Standards New Zealand [26] and Walker et al. [29].

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Not only do different guidelines give contradictory soil assessments, but many times the assessment itself might be questionable. The Linear Shrinkage might indicate if the clay is reactive or inert, so this test can be recommended when other mineralogy assessments are not available. However, this test does not seem to be a good indicator to predict the overall shrinkage needed to design movement joints: the results of the Drying Shrinkage test seem to not be affected by the clay content in each batch, making the Linear Shrinkage test unsuitable for this design purpose. The UCS cannot be a priori predicted or estimated from the PSD or other soil characteristics. It is advised that it should be experimentally assessed to check if it meets the designed structural requirements. The Accelerated Erosion Test does not seem to be a valid instrument to characterise in a realistic way the durability performance of the material. The recommendation to test the UCS of rammed earth samples in fully dry (oven-dried) or in fully saturated (soaked in water) conditions seems questionable for unstabilised samples. The latter condition is impossible to implement, and the former might give significantly under-estimated values of strength. Standards for rammed earth raw materials need to reflect the conclusions drawn in the paper. Performance testing of prototype rammed earth materials is the more appropriate approach, and performance limits should reflect the specific requirements of the construction project. Guidance on soil grading, plasticity and cement contents should be used in the assessment of material performance but should not be treated as prescriptive rules that should be adhered to. Therefore, rammed earth standards and guidelines should provide details of laboratory tests to obtain reliable and meaningful performance data, such as strength and durability. In many aspects, such as assessment of durability, realistic test procedures still require development. Acknowledgements The first author would like to thank the Australian Research Council (ARC) and the Department of Housing of the Government of Western Australia for their financial support. The authors also wish to acknowledge Miss Jane Agnew for her final year dissertation work.

Appendix A. Compressive stresses at the foot of a wall In this Appendix, the required compressive strength for a rammed earth bearing wall is calculated. The wall is a 2.4 m high panel of unit length of a one-storey house, with a thickness of 0.3 m (these are standard dimensions in Australia and in other parts of the worlds). Considering a density of 2000 kg/m3, the total vertical force acting at the base of the wall due to its self-weight is 14.4 kN. The additional force due to the weight of the roof is not usually higher than 3 kN/m. The total vertical force at the base of the wall turns out to be 17.4 kN that, when amplified by a load factor of 1.2, produces a uniform compressive stress state of 0.07 MPa. When applying a safety factor of 0.6 (proposed in several concrete design codes), the required compressive strength should be bigger

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