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Influence of particle grading on the hygromechanical properties of hypercompacted earth

Influence of particle grading on the hygromechanical properties of hypercompacted earth Civil engineering research is increasingly focusing on the development of sustainable and energy-efficient building materials. Among these materials, raw (unfired) earth constitutes a promising option for reducing the environmental impact of build- ings over their entire service life from construction to demolition. Raw earth has been used since old times but only recently has acquired prominence in mainstream building practice. This is mainly because of the development of novel methods to enhance the mechanical, hygroscopic and durability properties of compacted earth without increasing carbon and energy footprints. In this context, the present paper studies the dependency of the strength, stiffness, moisture capacity and water durability of compacted earth on particle grading. Results indicate that the particle size distribution is a key variable in defining the hygromechanical characteristics of compacted earth. The effect of the particle size distribution on the hygrome- chanical properties of compacted earth may be as important as that of dry density or stabilisation. This study suggests that a fine and well-graded earth mix exhibits higher levels of strength, stiffness, moisture capacity and water durability than a coarse and poorly-graded one. Keywords Raw earth material · Soil suitability · Hypercompaction · Durability 1 Introduction [1]. Unstabilised raw earth consists in a mix of clay, silt and sand, usually locally sourced, which is blended with water and The construction sector accounts for 30% of the worldwide compacted without further transformation [2]. The amount of carbon emissions and consumes more raw materials than energy required for the transportation and manufacturing of any other economic activity on the planet. It is therefore raw earth is relatively low compared to conventional construc- understandable that civil engineering research is currently tion materials. Similarly, the use of raw earth as a construc- focusing on the development of resource-effective construc- tion material facilitates the disposal or recycling of demolition tion materials that can reduce the environmental impact of waste at the end of service life. Raw earth also exhibits a strong buildings during construction, operation and demolition. ability to store or release ambient moisture while exchanging Raw (unfired) earth is a particularly attractive construction latent heat with the surrounding environment. This increases material that can cut down energy consumption and carbon the comfort of occupants and reduces the operational energy production over the entire lifetime of buildings, thus resulting required for conditioning indoor temperature and humidity [1, in lower levels of embodied, operational and end-of-life energy 3, 4]. Raw earth is not a novel material as it has been used for the construction of human dwellings since thousands of years. Only recently, however, new fabrication techniques have been * Alessia Cuccurullo proposed to enhance the strength, stiffness and durability of alessia.cuccurullo@univ-pau.fr compacted earth to the levels required by modern construc- Laboratoire SIAME, Fédération IPRA, E2S, Université de tion without significantly increasing the carbon and energy Pau et des Pays de l’Adour, Anglet, France footprints. Mechanical properties of raw earth are usually Department of Engineering, Durham University, Durham, improved by adding chemical stabilisers, such as cement and UK lime, and/or by densifying the material through compaction or School of Engineering, Newcastle University, vibration. An innovative “hypercompaction” method has been Newcastle upon Tyne, UK Vol.:(0123456789) 1 3 2 Page 2 of 9 Journal of Building Pathology and Rehabilitation (2020) 5:2 recently proposed by [5] whereby a large compaction effort plasticity properties of the base soil were measured on the of 100 MPa is applied to the earth producing a material with fine fraction, i.e. the fraction smaller than 0.400 mm, accord- a very low porosity of about 13%. As a term of comparison, ing to the norm AFNOR [7]. These measurements suggest natural sedimentary rocks exhibit similar levels of porosity. that the material is a low plasticity clay, which complies with While material stabilisation has attracted large research the requirements for the manufacture of compressed earth interest, the design of the base earth mix and, in particular, bricks according to the recommendations by Houben and the identification of the optimal plasticity and grading char - Guillaud [8] and AFNOR [9]; CRATerre-EAG [10]. acteristics have been rather overlooked. Fine soils retain more The particle size distribution of the base soil was instead water than coarse soils thus resulting in stronger hygroscopic determined by means of wet sieving and sedimentation in behaviour, which increases inter-particle capillary bonding compliance with the norms AFNOR [11] and AFNOR [12] and moisture buffering capacity. Nevertheless, an excessively while the specific gravity of the solid particles was meas- large fine fraction may weaken the mechanical behaviour and ured by means of pycnometer tests according to the norm undermine material durability. This means that not all soils are AFNOR [13]. Figure 1 shows the particle size distribution suitable for earth building or, at least, not all soils are suitable of the base soil together with the recommended limits sug- for all types of earth building. A comprehensive study of the gested by MOPT [14] and AFNOR [9]; CRATerre-EAG [10] optimal index properties of earthen materials was published for the manufacture compressed earth bricks. Inspection of by Delgado and Guerrero [6], who emphasized the importance Fig. 1 indicates that the base soil can be classified as a well- of developing technical guidelines to select appropriate earth graded silty clay, which lies close to the upper limit of cur- mixes for each building technique. rent recommendations. All index properties of the base soil The present paper contributes to overcome this gap of are summarized in Table 1. knowledge by investigating the influence of particle size Previous mineralogical studies of the base soil [15] distribution on the hygromechanical and durability charac- have also indicated a predominantly kaolinitic clay frac- teristics of compacted earth. In this study, different earth tion, which is suitable for earth construction because of the mixes with distinct particle gradings were hypercompacted low specific surface (10 m /g) and the consequently small at their respective optimum water contents. The stiffness swelling/shrinkage potential upon wetting/drying. The same and strength of these different materials were then meas- studies [15] have also characterized the hygromechanical ured by performing unconfined compression tests while properties of the material highlighting a reasonably good the hygroscopic properties were assessed by measuring the durability against water erosion. moisture buffering value (MBV). The durability of the mate- The base soil was blended with variable proportions of a rial against water erosion was also investigated by means of silica sand to obtain three distinct earth mixes with different immersion tests. clay fractions (i.e. with different fractions exhibiting particle Measurements indicate that particle size distribution and sizes smaller than 0.002 mm). Figure 2 shows the particle clay content have a marked influence on the mechanical, size distribution of the added silica sand, whose grading is hygroscopic and durability properties of hypercompacted monodisperse with almost all particles having a size com- earth. A fine and well-graded earth mix exhibit better prised between 0.06 and 2 mm. mechanical performance, larger hygroscopic capacity and greater water durability than a coarse and poorly-graded earth mix at similar dry density. The effect of particle grad- ing on material properties appears at least as significant as that of dry density. The study also identifies one important challenge ahead, which is the development of effective stabilisation tech- niques that can improve the water durability of raw earth without undermining the advantageous environmental char- acteristics of the material. 2 Materials and methods 2.1 Base soil and index properties Fig. 1 Particle size distribution of the base soil in relation to existing This study made use of a base soil provided by the Bouisset recommendations for the manufacture of compressed earth bricks by MOPT [14] and AFNOR [9]; CRATerre-EAG [10] brickwork factory from the region of Toulouse (France). The 1 3 Journal of Building Pathology and Rehabilitation (2020) 5:2 Page 3 of 9 2 Table 1 Index properties of the base soil Table 3 Physical composition of the different earth mixes Material Sand [%] Silt [%] Clay [%] Grain size distribution Gravel content (> 2 mm, %) 0 Earth mix 1 (base soil) ≈ 31 ≈ 35 ≈ 34 Sand content (≤ 2 mm, %) 31 Earth mix 2 ≈ 54 ≈ 23 ≈ 22 Silt content (≤ 63 μm, %) 35 Earth mix 3 ≈ 78 ≈ 11 ≈ 11 Clay content (≤ 2 μm, %) 34 Specific gravity 2.65 Atterberg limits Plastic limit (%) 18.7 Liquid limit (%) 29.0 Plasticity index (%) 10.3 Fig. 3 Particle size distribution of earth mixes in relation to existing recommendations for the manufacture of compressed earth bricks by MOPT [14] and AFNOR [9]; CRATerre-EAG [10] compressed earth bricks. Inspection of Fig. 3 indicates that the three earth mixes span the entire recommended range of Fig. 2 Particle size distribution of silica sand in relation to existing clay content, which is the fraction smaller than 0.002 mm. recommendations for the manufacture of compressed earth bricks by Earth mix 1 is well-graded while earth mixes 2 and 3 exhibit MOPT [14] and AFNOR [9]; CRATerre-EAG [10] a bimodal (gap-graded) particle size distribution. This is par- ticularly true for earth mix 3, whose particle size distribution Earth mix 1 does not contain any added sand and there- cuts across the entire recommended band from the upper to the lower limit. Nevertheless, in spite of these significant Table 2 Base soil and added sand percentages for the different earth mixes differences in grading, all earth mixes fall inside or close to the recommended band and are therefore compatible with Material Base soil percent- Added sand age [%] percentage existing guidelines. [%] 2.2 Hypercompaction of earth samples Earth mix 1 (base soil) 100 0 Earth mix 2 66 34 Compaction is an engineering technique to densify earth by Earth mix 3 32 68 packing particles close together and, hence, reducing the pore volume. In this work, each earth mix was compacted by applying a very high static pressure of 100 MPa to pro- fore coincides with the base soil while earth mixes 2 and duce extremely dense samples with porosities as low as 13%. 3 contain increasing percentages of added sand. Table  2 This heavy compaction technique was originally proposed shows, for each earth mix, the respective percentages of base by [5, 16], under the name of “hypercompaction”, to pro- soil and added sand while Table 3 summarises the resulting duce unstabilised earth bricks with levels of stiffness and composition of the three earth mixes in terms of sand, silt strength similar to those of conventional building materials. and clay contents. For example, the hypercompacted unstabilised earth bricks Figure 3 shows the particle size distributions of the three tested by Bruno et al. [17] exhibited values of compressive earth mixes together with the recommended bands suggested strength comparable to those of cement-stabilised and fired by MOPT [14] and AFNOR [9]; CRATerre-EAG [10] for earth bricks [18, 19] (Table 4). 1 3 2 Page 4 of 9 Journal of Building Pathology and Rehabilitation (2020) 5:2 Table 4 Compressive strength Material Compressive strength [MPa] of unstabilised, stabilised and fired earth bricks Hypercompacted unstabilised earth bricks (Bruno et al. [16]) 14.6 Compacted stabilised earth bricks (Guetlala and Guenfoud [18]) From 5.2 to 12.9 Standard fired earth bricks (ASTM C270 [19]) From 6.9 to 27.6 Table 5 Optimum water contents and corresponding maximum dry densities for the three hypercompacted earth mixes Material Optimum water Maximum dry content [%] density [g/cm ] Earth mix 1 (base soil) 4.9 2.31 Earth mix 2 4.7 2.30 Earth mix 3 6.5 2.12 4.9%. This value is markedly lower than the optimum water content of earth mix 3, which is about 6.5%. Table 5 sum- marizes the optimum water contents and the corresponding values of the maximum dry densities for the three hyper- Fig. 4 Hypercompaction curves of the three earth mixes subjected to a static pressure of 100 MPa compacted earth mixes. In this work, the dry soil was initially mixed with the chosen amount of water and subsequently placed inside three 3 Results plastic bags to prevent evaporation. The moist material was left to equalize for at least 1 day so that moisture could redis- A range of hygromechanical tests was performed to deter- tribute, before being placed inside a stiff cylindrical steel mine the strength, stiffness, moisture buffering capacity and mould with a diameter of 50 mm where it was vertically water durability of the three hypercompacted earth mixes. compacted under a pressure of 100 MPa by using a load- All tests were performed on cylindrical samples that were controlled press. Pressure was applied by two cylindrical hypercompacted under a static pressure of 100 MPa at their aluminium pistons acting on the top and bottom extremities respective optimum water contents (see Table 5). The cylin- of the sample. This double-piston compression reduces the drical samples had a diameter of 50 mm while the height was effect of friction between the inner mould surface and the either 100 mm or 50 mm depending on the type of test as sample sides, thus increasing stress uniformity inside the explained later. Cylindrical samples were preferred to bricks material. Eight fine longitudinal grooves were cut on the to avoid sharp corners that could induce stress concentration surfaces of the pistons to facilitate drainage of pore air, and during fabrication and testing. possibly pore water, during compaction. Additional details about the hypercompaction procedure are available in Bruno 3.1 Unconfined compression tests et al. [17]. Figure 4 presents the values of dry density, ρ measured Unconfined compression tests were conducted on cylindri- after hypercompaction of each earth mix at different water cal hypercompacted samples with a diameter of 50 mm and contents, w. Figure 4 also shows the equisaturation lines, a height of 100 mm. An aspect ratio of two was chosen to which converge towards the theoretical ‘‘no porosity’’ point limit the spurious radial confinement caused by the friction defined by a zero water content and a dry density equal to the between the sample extremities and the press plates during density of the solid particles. Inspection of Fig. 4 indicates axial compression. Before testing, all samples were equal- that the finer and better-graded earth mixes 1 and 2 exhibit ized inside a climatic chamber at a temperature of 25 °C and almost identical compaction curves with higher dry densities a relative humidity of 62%. This was considered necessary than the coarser and poorer-graded earth mix 3. Earth mixes to avoid the influence of potentially different ambient condi- 1 and 2 present an almost identical value of the optimum tions on the measured values of strength and stiffness. Dur - water content (i.e. the water content corresponding to the ing the equalization phase, the samples were weighted every highest dry density), which is comprised between 4.7 and 1 3 Journal of Building Pathology and Rehabilitation (2020) 5:2 Page 5 of 9 2 day until their mass changed less than 0.1% over a period of unloading, the Young’s modulus was determined from the at least 1 week, which generally took about 15 days. unloading branches of the five cycles. In particular, the A first series of tests was performed to measure the Young’s modulus was calculated as the average slope of the strength of the three hypercompacted earth mixes. Dur- five unloading branches in the stress-strain plane. Figure  6 ing these tests, the samples were compressed under a con- shows the measured values of Young’s modulus for the three stant axial displacement rate of 0.001 mm/s, which allowed different earth mixes, together with the respective values of recording also the post-peak part of the stress-strain curve. dry density (measured in g/cm ) in brackets. Similarly to The displacement rate was the slowest that could be applied compressive strength, the Young’s modulus increases with by the press and was chosen to obtain a regular stress-strain growing dry density but this increase is strongly not linear curve without instabilities [17]. Two samples were tested for due to the influence of the earth grading. each earth mix to confirm the repeatability of measurements Inspection of Figs. 5 and 6 indicates that significant dif- and to reduce errors. The final peak strength was then calcu- ferences of stiffness and strength exist between earth mixes lated as the average of these two measurements. 1 and 2 despite an almost identical value of dry density. Figure 5 shows the peak values of compressive strength An explanation of this result might be found in the differ - for each earth mix together with the corresponding values ent physical composition of the two mixes. Earth mix 2 is of dry density (measured in g/cm ) in brackets. Inspection a blend of silty clay and sand with a bimodal (gap-graded) of Fig. 5 indicates that, as expected, compressive strength particle size distribution while earth mix 1 is a well-graded increases with growing density, though this increase is far silty clay (Fig.  3). This indicates that dry density cannot from linear due to the influence of the  earth grading on be considered as the only factor governing the mechanical the material response. behaviour of hypercompacted earth but particle grading also A second series of unconfined compression tests was per - plays an important role. formed to compare the stiffness of the three hypercompacted earth mixes. The Young’s modulus was measured by per- 3.2 Moisture buffering value (MBV) tests forming five axial loading-unloading cycles, with a constant loading rate of 0.005 MPa/s, between one-ninth and one- The capacity of the hypercompacted earth to adsorb and third of the peak strength as measured from previous tests. release ambient humidity was experimentally assessed The axial strain was measured between two points located through the measurement of the moisture buffering value at a distance of 50 mm by means of extensometers mounted (MBV). The MBV “indicates the amount of water that is symmetrically with respect to the middle of the sample. The transported in or out of a material per open surface area, axial strain was the average of two measurements taken by during a certain period of time, when it is subjected to vari- two distinct extensometers (Model 3542-050 M-005-HT1— ations in relative humidity of the surrounding air” [20]. Epsilon Technology Corp.) placed on diametrically opposite Hypercompacted cylindrical samples with 50 mm diam- sides of the sample. eter and 100 mm height were exposed to step cycles of rela- Based on the assumption that material behaviour is tive humidity, between 75% and 53%, at a constant tempera- elasto-plastic during loading but essentially elastic during ture of 23 °C inside a climatic chamber (CLIMATS Type EX2221HA). Each humidity level was maintained for 12 h Fig. 5 Compressive strength: results of unconfined compression tests. Fig. 6 Young’s modulus: results of unconfined loading-unloading Values in parenthesis indicate dry density in g/cm cycles. Values in parenthesis indicate dry density in g/cm 1 3 2 Page 6 of 9 Journal of Building Pathology and Rehabilitation (2020) 5:2 while the sample mass was recorded every 2 h. This experi- mental procedure is consistent with the norm ISO 24353 [21] for the characterization of the hygrothermal behaviour of building materials exposed to cyclic variations of relative humidity over a daily (24 h) period of time. Each cylindrical sample was placed upright inside an aluminium foil pan so that only the top and lateral surfaces were directly exposed to the atmosphere inside the climatic chamber. The total area of the exposed surface was therefore about 0.018 m , which is higher than the minimum value of 0.010 m required by the norm ISO 24353 [21]. Three samples were tested for each earth mix to confirm the repeat- ability of measurements and to reduce errors, with the final MBV calculated as the average of the three measurements. Before the test, all samples were equalized at a tempera- ture of 23 °C and a relative humidity of 53%. Equalization was assumed complete when the mass of the sample changed Fig. 7 MBVs measured during moisture uptake and release stages of less than 0.1% over a period of at least 1 week (this took humidity cycles. Solid markers indicate MBV uptake while hollow markers indicate MBV release generally 2 weeks). After equalization, the samples were exposed to cyclic changes of relative humidity as previ- ously described and two different MBVs were calculated, Table 6 MBVs under steady state conditions corresponding to the uptake and release stages of each cycle, according to the following equation: Sample ID MBV [g/ m  %RH] Δm MBV = (1) SΔ%RH Earth mix 1 (base soil) 3.3 Earth mix 2 2.3 where ∆m is the absolute value of the sample mass varia- Earth mix 3 1.5 tion (in grams), S is the exposed surface (in square meters) and ∆%RH is the imposed relative humidity change (in per- centage). The value of ∆m measured at the end of the high moisture buffering capacity than the other two mixes. This humidity stage provides the “MBV uptake” while the value of ∆m measured at the end of the low humidity stage pro- is due to the larger fine fraction, and hence the greater water retention capacity, of earth mix 1 compared to the other two vides the “MBV release”. To take into account the small change of sample dimensions caused by the swelling/shrink- mixes. In particular, the MBV increases almost linearly with growing clay content (see Tables 3 and 6) being twice as age of the earth, the exposed surface was calculated as the average of three height measurements and three diameter large for earth mix 1 as for earth mix 3. Similar experimental observations were made for different materials by Jaquin measurements taken both at the beginning and end of each humidity stage. et al. [22] and Beckett and Augarde [23]. Results from MBV tests are often presented in the form Figure 7 shows that the MBV is larger during moisture uptake than during moisture release but this difference of moisture adsorption curves, which record the hygroscopic behaviour of the material throughout the cyclic variation reduces as the number of cycles increases and the material converges towards steady state. Steady state is convention- of relative humidity. The moisture adsorption is defined, at any given time, as the ratio between the variation of sample ally defined as the occurrence of three consecutive “sta- ble” cycles where moisture uptake at a humidity of 75% mass during a cycle (i.e. the difference between the current and initial mass of the sample) and the area of the exposed is approximately equal to moisture release at a humidity of 53%. In general, v fi e cycles were suc ffi ient to achieve steady surface. Figure  8 shows the moisture adsorption curves recorded for each earth mix during the last stable cycle when state. The final MBV of the material is conventionally meas- the hygroscopic behaviour is virtually reversible with the moisture uptake being approximately equal to the moisture ured under steady state conditions and it is calculated as the average of the uptake and release values of the last three release. Figure 9 presents the MBV of earth mix 1 measured dur- stable cycles. The final MBVs of the three hypercompacted earth mixes measured the present work are summarized ing the present work together with the MBV measured by Bruno et al. [17] on a different hypercompacted earth mix in Table 6, which shows that earth mix 1 exhibits a higher 1 3 Journal of Building Pathology and Rehabilitation (2020) 5:2 Page 7 of 9 2 3.3 Water durability tests Durability against water erosion was investigated by means of immersion tests on cylindrical hypercompacted samples of 50 mm diameter and 50 mm height according to the standard experimental protocol described in [13]. Before testing, all samples were equalized at the laboratory atmosphere, i.e. at a temperature of 25 °C and a relative humidity of 40% ± 5%, until the mass changed less than 0.1% over a period of at least 1 week (this took generally 3 weeks). After equalization, the sample was weighted to record its initial mass m and subsequently submerged in water for 10 min. The sample was then removed from water and equalized again to the laboratory atmosphere to attain the same moisture content as before immersion. Fig. 8 Moisture adsorption curves during the last stable cycle of each After equalization, the final sample mass m was recorded earth mix and introduced, together with the initial mass m , in the following equation to calculate the percentage mass loss and the MBVs measured by Rode et al. [20] on a variety of %Δm experienced by the sample during immersion: standard building materials. Note that the MBV reported by m − m i f Bruno et al. [17] was measured on earth bricks that were %Δm = × 100 (2) hypercompacted according to a similar manufacturing proce- dure to that adopted in the present work. Inspection of Fig. 9 Table  7 summarizes the results from all tests, which indicates that earth mix 1 exhibits an excellent hygroscopic confirms that the hydrophilic nature of the earth has a performance with a MBV which is slightly less than that negative impact on the water durability of the hypercom- measured by Bruno et al. [17] but about seven times higher pacted samples. All samples showed marked mass losses than that of traditional building materials, such as fired clay and exhibited numerous cracks after immersion. Neverthe- or concrete bricks, as reported by Rode et al. [20]. less, the finer and better-graded earth mix 1 experienced a relatively small mass loss of only 13% compared to 30% for earth mix 2 and a complete sample dissolution for earth Fig. 9 MBV of earth mix 1 compared to the MBVs of other building materials as reported by Bruno et al. [16] and Rode et al. [20] 1 3 2 Page 8 of 9 Journal of Building Pathology and Rehabilitation (2020) 5:2 Table 7 Percentage mass loss during immersion tests three distinct earth mixes characterised by significantly dif - ferent particle size distributions but compacted under the Sample ID %Δm [%] same pressure. This testing campaign included unconfined Earth mix 1 (base soil) ≈ 13 compression tests, moisture buffering tests and immersion Earth mix 2 ≈ 30 tests. Results suggest that the use of a fine and well-graded Earth mix 3 Complete dis- earth mix can significantly improve the strength, stiffness, solution after moisture capacity and water durability of the material com- 4′30′′ pared to a coarse and poorly-graded earth mix compacted at a similar density. Importantly, the enhancement of water durability, albeit insufficient for mainstream building, may reduce the extent of chemical stabilisation that is required to comply with current regulations. All three earth mixes tested in the present work are compatible with particle grading recommendations that have been published in the literature and may therefore be deemed suitable for construction. Nevertheless, the three mixes exhibited markedly different behaviour during tests, which raises questions about the validity of current recom- mendations and suggests the necessity of considering addi- Fig. 10 Hypercompacted earth mix 1 before (a) and after (b) immer- tional grading features such as, for example, the regularity sion in water of particle size distribution. Acknowledgements The authors wish to acknowledge the support of mix 3. Once again, these disparities might be attributed to the European Commission via the Marie Skłodowska-Curie Innovative the different dry densities of the tested samples but also to Training Networks (ITN-ETN) project TERRE ‘Training Engineers the distinct particle size distributions of the three mixes. and Researchers to Rethink geotechnical Engineering for a low carbon future’ (H2020-MSCA-ITN-2015-675762). Figure 10 shows two pictures of the sample of earth mix 1 taken before (a) and after (b) immersion in water. These pic- tures indicate that immersion in water produces a significant Compliance with ethical standards erosion of the sample surface even for the relatively dura- Conflict of interest On behalf of all authors, the corresponding author ble earth mix 1. This deterioration is expected to negatively states that there is no conflict of interest. affect also the strength and stiffness of the material, though this particular aspect has not been evaluated in the present Open Access This article is distributed under the terms of the Crea- work but will form part of future research. tive Commons Attribution 4.0 International License (http://creat iveco mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- tion, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the 4 Conclusions Creative Commons license, and indicate if changes were made. The utilization of raw (unfired) earth as a building material is attracting the interest of engineers and architects world- wide due to environmental and economic advantages but References also to the availability of novel fabrication techniques that can meet the demands of modern construction. 1. Pacheco-Torgal F, Jalali S (2012) Earth construction: lessons from Past research has indicated that densification of unsta- the past for future eco-efficient construction. Constr Build Mater 29:512–519 bilised earth by means of heavy compaction may improve 2. Jaquin PA, Augarde CE, Gallipoli D, Toll DG (2009) The strength and stiffness to levels that are comparable to those strength of unstabilised rammed earth materials. Géotechnique of traditional materials such as fired bricks, concrete blocks 59(5):487–490 and stabilised earth. This paper has shown that dry den- 3. Gallipoli D, Bruno AW, Perlot C, Mendes J (2017) A geotechnical perspective of raw earth building. Acta Geotech 12(3):463–478 sity is not, however, the only important factor governing 4. Morel JC, Mesbah A, Oggero M, Walker P (2001) Building houses the engineering properties of unstabilised earth but particle with local materials: means to drastically reduce the environmen- size distribution has also a significant influence on stiff- tal impact of construction. Build Environ 36(10):1119–1126 ness, strength, hygrothermal inertia and water durability. 5. Bruno AW, Gallipoli D, Perlot C, Mendes J (2017) Mechanical behaviour of hypercompacted earth for building construction. This conclusion is supported by a wide experimental cam- Mater Struct 50(2):160 paign that has been performed during the present work on 1 3 Journal of Building Pathology and Rehabilitation (2020) 5:2 Page 9 of 9 2 6. Delgado MCJ, Guerrero IC (2007) The selection of soils for unsta- 18. Guetlala A, Guenfoud M (1997) Béton de terre stabilisé: proprié- bilised earth building: a normative review. Constr Build Mater tés physicomécaniques et influence des types d’argiles. La Techn 21(2):237–251 Moderne 89(1–2):21–26 7. AFNOR (1993) NF P 94-051; Soils: Investigation and testing— 19. ASTM C270 (2014) Standard specification for mortar for unit Determination of Atterberg’s limits—Liquid limit test using Casa- masonry. In: American Society for Testing and Materials grande apparatus—Plastic limit test on rolled thread International 8. Houben H, Guillaud H (1994) Earth construction: a comprehen- 20. Rode C, Peuhkuri RH, Mortensen LH, Hansen KK, Time B, Gus- sive guide. Intermediate Technology Publications, London, p 362 tavsen A, Harderup LE (2005) Moisture buffering of building 9. AFNOR (2001) XP P13-901; Compressed earth blocks for walls materials. In: Report BYG∙DTU R-126, ISSN 1601-2917 (Vol. and partitions: definitions—Specifications—Test methods— 146). ISBN 87-7877-195. Technical University of Denmark, Delivery acceptance conditions Department of Civil Engineering, Denmark 10. CRATerre-EAG (1998) CDI, Compressed earth blocks: stand- 21. ISO 24353 (2008) Hygrothermal performance of building materi- ards—Technology series No. 11. Brussels: CDI als and products determination of moisture adsorption/desorption 11. AFNOR (1995) XP P 94-041. Soils: investigation and testing— properties in response to humidity variation. Geneva, Switzerland: Granulometric description—Wet sieving method International Organization for Standardization 12. AFNOR (1992) NF P 94-057. Soils: investigation and testing— 22. Jaquin PA, Augarde CE, Legrand L (2008) Unsaturated character- Granulometric analysis—Hydrometer method istics of rammed earth. First european conference on unsaturated 13. AFNOR (1991) NF P 94-054; Soils: investigation and testing— soils, Durham, England. Taylor & Francis, Abingdon, Oxon, pp Determination of particle density—Pycnometer method 417–422 14. MOPT (1992) Bases Para el Diseño y Construcción con Tapial. 23. Beckett CTS, Augarde CE (2012) The effect of humidity and Madrid, Spain: Centro de Publicaciones, Secretaría General Téc- temperature on the compressive strength of rammed earth. In: nica, Ministerio de Obras Públicas y Transportes Proceedings of 2nd European conference on unsaturated soils (pp. 15. (http://www .cerca d.fr/IMG/pdf/ter cr uso_r appo r t_final _lmdc2 013- 287–292). Springer, ISBN: 978-3-642-31342-4 04-r120.pdf), Tercruso (2013) Caractérisation des briques de terre crue de Midi-Pyrénées Publisher’s Note Springer Nature remains neutral with regard to 16. Bruno AW, Gallipoli D, Perlot C, Mendes J (2016) Effect of very jurisdictional claims in published maps and institutional affiliations. high compaction pressures on the physical and mechanical proper- ties of earthen materials. In: E3S Web of Conferences, p 9, 14004. EDP Sciences 17. Bruno AW (2016) Hygro-mechanical characterisation of hyper- compacted earth for sustainable construction. PhD Thesis, Uni- versité de Pau et des Pays de l’Adour, France 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Building Pathology and Rehabilitation Springer Journals

Influence of particle grading on the hygromechanical properties of hypercompacted earth

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Springer Journals
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Copyright © 2019 by The Author(s)
Subject
Engineering; Building Repair and Maintenance; Structural Materials; Engineering Thermodynamics, Heat and Mass Transfer; Energy Efficiency; Building Materials
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2365-3159
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2365-3167
DOI
10.1007/s41024-019-0066-4
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Abstract

Civil engineering research is increasingly focusing on the development of sustainable and energy-efficient building materials. Among these materials, raw (unfired) earth constitutes a promising option for reducing the environmental impact of build- ings over their entire service life from construction to demolition. Raw earth has been used since old times but only recently has acquired prominence in mainstream building practice. This is mainly because of the development of novel methods to enhance the mechanical, hygroscopic and durability properties of compacted earth without increasing carbon and energy footprints. In this context, the present paper studies the dependency of the strength, stiffness, moisture capacity and water durability of compacted earth on particle grading. Results indicate that the particle size distribution is a key variable in defining the hygromechanical characteristics of compacted earth. The effect of the particle size distribution on the hygrome- chanical properties of compacted earth may be as important as that of dry density or stabilisation. This study suggests that a fine and well-graded earth mix exhibits higher levels of strength, stiffness, moisture capacity and water durability than a coarse and poorly-graded one. Keywords Raw earth material · Soil suitability · Hypercompaction · Durability 1 Introduction [1]. Unstabilised raw earth consists in a mix of clay, silt and sand, usually locally sourced, which is blended with water and The construction sector accounts for 30% of the worldwide compacted without further transformation [2]. The amount of carbon emissions and consumes more raw materials than energy required for the transportation and manufacturing of any other economic activity on the planet. It is therefore raw earth is relatively low compared to conventional construc- understandable that civil engineering research is currently tion materials. Similarly, the use of raw earth as a construc- focusing on the development of resource-effective construc- tion material facilitates the disposal or recycling of demolition tion materials that can reduce the environmental impact of waste at the end of service life. Raw earth also exhibits a strong buildings during construction, operation and demolition. ability to store or release ambient moisture while exchanging Raw (unfired) earth is a particularly attractive construction latent heat with the surrounding environment. This increases material that can cut down energy consumption and carbon the comfort of occupants and reduces the operational energy production over the entire lifetime of buildings, thus resulting required for conditioning indoor temperature and humidity [1, in lower levels of embodied, operational and end-of-life energy 3, 4]. Raw earth is not a novel material as it has been used for the construction of human dwellings since thousands of years. Only recently, however, new fabrication techniques have been * Alessia Cuccurullo proposed to enhance the strength, stiffness and durability of alessia.cuccurullo@univ-pau.fr compacted earth to the levels required by modern construc- Laboratoire SIAME, Fédération IPRA, E2S, Université de tion without significantly increasing the carbon and energy Pau et des Pays de l’Adour, Anglet, France footprints. Mechanical properties of raw earth are usually Department of Engineering, Durham University, Durham, improved by adding chemical stabilisers, such as cement and UK lime, and/or by densifying the material through compaction or School of Engineering, Newcastle University, vibration. An innovative “hypercompaction” method has been Newcastle upon Tyne, UK Vol.:(0123456789) 1 3 2 Page 2 of 9 Journal of Building Pathology and Rehabilitation (2020) 5:2 recently proposed by [5] whereby a large compaction effort plasticity properties of the base soil were measured on the of 100 MPa is applied to the earth producing a material with fine fraction, i.e. the fraction smaller than 0.400 mm, accord- a very low porosity of about 13%. As a term of comparison, ing to the norm AFNOR [7]. These measurements suggest natural sedimentary rocks exhibit similar levels of porosity. that the material is a low plasticity clay, which complies with While material stabilisation has attracted large research the requirements for the manufacture of compressed earth interest, the design of the base earth mix and, in particular, bricks according to the recommendations by Houben and the identification of the optimal plasticity and grading char - Guillaud [8] and AFNOR [9]; CRATerre-EAG [10]. acteristics have been rather overlooked. Fine soils retain more The particle size distribution of the base soil was instead water than coarse soils thus resulting in stronger hygroscopic determined by means of wet sieving and sedimentation in behaviour, which increases inter-particle capillary bonding compliance with the norms AFNOR [11] and AFNOR [12] and moisture buffering capacity. Nevertheless, an excessively while the specific gravity of the solid particles was meas- large fine fraction may weaken the mechanical behaviour and ured by means of pycnometer tests according to the norm undermine material durability. This means that not all soils are AFNOR [13]. Figure 1 shows the particle size distribution suitable for earth building or, at least, not all soils are suitable of the base soil together with the recommended limits sug- for all types of earth building. A comprehensive study of the gested by MOPT [14] and AFNOR [9]; CRATerre-EAG [10] optimal index properties of earthen materials was published for the manufacture compressed earth bricks. Inspection of by Delgado and Guerrero [6], who emphasized the importance Fig. 1 indicates that the base soil can be classified as a well- of developing technical guidelines to select appropriate earth graded silty clay, which lies close to the upper limit of cur- mixes for each building technique. rent recommendations. All index properties of the base soil The present paper contributes to overcome this gap of are summarized in Table 1. knowledge by investigating the influence of particle size Previous mineralogical studies of the base soil [15] distribution on the hygromechanical and durability charac- have also indicated a predominantly kaolinitic clay frac- teristics of compacted earth. In this study, different earth tion, which is suitable for earth construction because of the mixes with distinct particle gradings were hypercompacted low specific surface (10 m /g) and the consequently small at their respective optimum water contents. The stiffness swelling/shrinkage potential upon wetting/drying. The same and strength of these different materials were then meas- studies [15] have also characterized the hygromechanical ured by performing unconfined compression tests while properties of the material highlighting a reasonably good the hygroscopic properties were assessed by measuring the durability against water erosion. moisture buffering value (MBV). The durability of the mate- The base soil was blended with variable proportions of a rial against water erosion was also investigated by means of silica sand to obtain three distinct earth mixes with different immersion tests. clay fractions (i.e. with different fractions exhibiting particle Measurements indicate that particle size distribution and sizes smaller than 0.002 mm). Figure 2 shows the particle clay content have a marked influence on the mechanical, size distribution of the added silica sand, whose grading is hygroscopic and durability properties of hypercompacted monodisperse with almost all particles having a size com- earth. A fine and well-graded earth mix exhibit better prised between 0.06 and 2 mm. mechanical performance, larger hygroscopic capacity and greater water durability than a coarse and poorly-graded earth mix at similar dry density. The effect of particle grad- ing on material properties appears at least as significant as that of dry density. The study also identifies one important challenge ahead, which is the development of effective stabilisation tech- niques that can improve the water durability of raw earth without undermining the advantageous environmental char- acteristics of the material. 2 Materials and methods 2.1 Base soil and index properties Fig. 1 Particle size distribution of the base soil in relation to existing This study made use of a base soil provided by the Bouisset recommendations for the manufacture of compressed earth bricks by MOPT [14] and AFNOR [9]; CRATerre-EAG [10] brickwork factory from the region of Toulouse (France). The 1 3 Journal of Building Pathology and Rehabilitation (2020) 5:2 Page 3 of 9 2 Table 1 Index properties of the base soil Table 3 Physical composition of the different earth mixes Material Sand [%] Silt [%] Clay [%] Grain size distribution Gravel content (> 2 mm, %) 0 Earth mix 1 (base soil) ≈ 31 ≈ 35 ≈ 34 Sand content (≤ 2 mm, %) 31 Earth mix 2 ≈ 54 ≈ 23 ≈ 22 Silt content (≤ 63 μm, %) 35 Earth mix 3 ≈ 78 ≈ 11 ≈ 11 Clay content (≤ 2 μm, %) 34 Specific gravity 2.65 Atterberg limits Plastic limit (%) 18.7 Liquid limit (%) 29.0 Plasticity index (%) 10.3 Fig. 3 Particle size distribution of earth mixes in relation to existing recommendations for the manufacture of compressed earth bricks by MOPT [14] and AFNOR [9]; CRATerre-EAG [10] compressed earth bricks. Inspection of Fig. 3 indicates that the three earth mixes span the entire recommended range of Fig. 2 Particle size distribution of silica sand in relation to existing clay content, which is the fraction smaller than 0.002 mm. recommendations for the manufacture of compressed earth bricks by Earth mix 1 is well-graded while earth mixes 2 and 3 exhibit MOPT [14] and AFNOR [9]; CRATerre-EAG [10] a bimodal (gap-graded) particle size distribution. This is par- ticularly true for earth mix 3, whose particle size distribution Earth mix 1 does not contain any added sand and there- cuts across the entire recommended band from the upper to the lower limit. Nevertheless, in spite of these significant Table 2 Base soil and added sand percentages for the different earth mixes differences in grading, all earth mixes fall inside or close to the recommended band and are therefore compatible with Material Base soil percent- Added sand age [%] percentage existing guidelines. [%] 2.2 Hypercompaction of earth samples Earth mix 1 (base soil) 100 0 Earth mix 2 66 34 Compaction is an engineering technique to densify earth by Earth mix 3 32 68 packing particles close together and, hence, reducing the pore volume. In this work, each earth mix was compacted by applying a very high static pressure of 100 MPa to pro- fore coincides with the base soil while earth mixes 2 and duce extremely dense samples with porosities as low as 13%. 3 contain increasing percentages of added sand. Table  2 This heavy compaction technique was originally proposed shows, for each earth mix, the respective percentages of base by [5, 16], under the name of “hypercompaction”, to pro- soil and added sand while Table 3 summarises the resulting duce unstabilised earth bricks with levels of stiffness and composition of the three earth mixes in terms of sand, silt strength similar to those of conventional building materials. and clay contents. For example, the hypercompacted unstabilised earth bricks Figure 3 shows the particle size distributions of the three tested by Bruno et al. [17] exhibited values of compressive earth mixes together with the recommended bands suggested strength comparable to those of cement-stabilised and fired by MOPT [14] and AFNOR [9]; CRATerre-EAG [10] for earth bricks [18, 19] (Table 4). 1 3 2 Page 4 of 9 Journal of Building Pathology and Rehabilitation (2020) 5:2 Table 4 Compressive strength Material Compressive strength [MPa] of unstabilised, stabilised and fired earth bricks Hypercompacted unstabilised earth bricks (Bruno et al. [16]) 14.6 Compacted stabilised earth bricks (Guetlala and Guenfoud [18]) From 5.2 to 12.9 Standard fired earth bricks (ASTM C270 [19]) From 6.9 to 27.6 Table 5 Optimum water contents and corresponding maximum dry densities for the three hypercompacted earth mixes Material Optimum water Maximum dry content [%] density [g/cm ] Earth mix 1 (base soil) 4.9 2.31 Earth mix 2 4.7 2.30 Earth mix 3 6.5 2.12 4.9%. This value is markedly lower than the optimum water content of earth mix 3, which is about 6.5%. Table 5 sum- marizes the optimum water contents and the corresponding values of the maximum dry densities for the three hyper- Fig. 4 Hypercompaction curves of the three earth mixes subjected to a static pressure of 100 MPa compacted earth mixes. In this work, the dry soil was initially mixed with the chosen amount of water and subsequently placed inside three 3 Results plastic bags to prevent evaporation. The moist material was left to equalize for at least 1 day so that moisture could redis- A range of hygromechanical tests was performed to deter- tribute, before being placed inside a stiff cylindrical steel mine the strength, stiffness, moisture buffering capacity and mould with a diameter of 50 mm where it was vertically water durability of the three hypercompacted earth mixes. compacted under a pressure of 100 MPa by using a load- All tests were performed on cylindrical samples that were controlled press. Pressure was applied by two cylindrical hypercompacted under a static pressure of 100 MPa at their aluminium pistons acting on the top and bottom extremities respective optimum water contents (see Table 5). The cylin- of the sample. This double-piston compression reduces the drical samples had a diameter of 50 mm while the height was effect of friction between the inner mould surface and the either 100 mm or 50 mm depending on the type of test as sample sides, thus increasing stress uniformity inside the explained later. Cylindrical samples were preferred to bricks material. Eight fine longitudinal grooves were cut on the to avoid sharp corners that could induce stress concentration surfaces of the pistons to facilitate drainage of pore air, and during fabrication and testing. possibly pore water, during compaction. Additional details about the hypercompaction procedure are available in Bruno 3.1 Unconfined compression tests et al. [17]. Figure 4 presents the values of dry density, ρ measured Unconfined compression tests were conducted on cylindri- after hypercompaction of each earth mix at different water cal hypercompacted samples with a diameter of 50 mm and contents, w. Figure 4 also shows the equisaturation lines, a height of 100 mm. An aspect ratio of two was chosen to which converge towards the theoretical ‘‘no porosity’’ point limit the spurious radial confinement caused by the friction defined by a zero water content and a dry density equal to the between the sample extremities and the press plates during density of the solid particles. Inspection of Fig. 4 indicates axial compression. Before testing, all samples were equal- that the finer and better-graded earth mixes 1 and 2 exhibit ized inside a climatic chamber at a temperature of 25 °C and almost identical compaction curves with higher dry densities a relative humidity of 62%. This was considered necessary than the coarser and poorer-graded earth mix 3. Earth mixes to avoid the influence of potentially different ambient condi- 1 and 2 present an almost identical value of the optimum tions on the measured values of strength and stiffness. Dur - water content (i.e. the water content corresponding to the ing the equalization phase, the samples were weighted every highest dry density), which is comprised between 4.7 and 1 3 Journal of Building Pathology and Rehabilitation (2020) 5:2 Page 5 of 9 2 day until their mass changed less than 0.1% over a period of unloading, the Young’s modulus was determined from the at least 1 week, which generally took about 15 days. unloading branches of the five cycles. In particular, the A first series of tests was performed to measure the Young’s modulus was calculated as the average slope of the strength of the three hypercompacted earth mixes. Dur- five unloading branches in the stress-strain plane. Figure  6 ing these tests, the samples were compressed under a con- shows the measured values of Young’s modulus for the three stant axial displacement rate of 0.001 mm/s, which allowed different earth mixes, together with the respective values of recording also the post-peak part of the stress-strain curve. dry density (measured in g/cm ) in brackets. Similarly to The displacement rate was the slowest that could be applied compressive strength, the Young’s modulus increases with by the press and was chosen to obtain a regular stress-strain growing dry density but this increase is strongly not linear curve without instabilities [17]. Two samples were tested for due to the influence of the earth grading. each earth mix to confirm the repeatability of measurements Inspection of Figs. 5 and 6 indicates that significant dif- and to reduce errors. The final peak strength was then calcu- ferences of stiffness and strength exist between earth mixes lated as the average of these two measurements. 1 and 2 despite an almost identical value of dry density. Figure 5 shows the peak values of compressive strength An explanation of this result might be found in the differ - for each earth mix together with the corresponding values ent physical composition of the two mixes. Earth mix 2 is of dry density (measured in g/cm ) in brackets. Inspection a blend of silty clay and sand with a bimodal (gap-graded) of Fig. 5 indicates that, as expected, compressive strength particle size distribution while earth mix 1 is a well-graded increases with growing density, though this increase is far silty clay (Fig.  3). This indicates that dry density cannot from linear due to the influence of the  earth grading on be considered as the only factor governing the mechanical the material response. behaviour of hypercompacted earth but particle grading also A second series of unconfined compression tests was per - plays an important role. formed to compare the stiffness of the three hypercompacted earth mixes. The Young’s modulus was measured by per- 3.2 Moisture buffering value (MBV) tests forming five axial loading-unloading cycles, with a constant loading rate of 0.005 MPa/s, between one-ninth and one- The capacity of the hypercompacted earth to adsorb and third of the peak strength as measured from previous tests. release ambient humidity was experimentally assessed The axial strain was measured between two points located through the measurement of the moisture buffering value at a distance of 50 mm by means of extensometers mounted (MBV). The MBV “indicates the amount of water that is symmetrically with respect to the middle of the sample. The transported in or out of a material per open surface area, axial strain was the average of two measurements taken by during a certain period of time, when it is subjected to vari- two distinct extensometers (Model 3542-050 M-005-HT1— ations in relative humidity of the surrounding air” [20]. Epsilon Technology Corp.) placed on diametrically opposite Hypercompacted cylindrical samples with 50 mm diam- sides of the sample. eter and 100 mm height were exposed to step cycles of rela- Based on the assumption that material behaviour is tive humidity, between 75% and 53%, at a constant tempera- elasto-plastic during loading but essentially elastic during ture of 23 °C inside a climatic chamber (CLIMATS Type EX2221HA). Each humidity level was maintained for 12 h Fig. 5 Compressive strength: results of unconfined compression tests. Fig. 6 Young’s modulus: results of unconfined loading-unloading Values in parenthesis indicate dry density in g/cm cycles. Values in parenthesis indicate dry density in g/cm 1 3 2 Page 6 of 9 Journal of Building Pathology and Rehabilitation (2020) 5:2 while the sample mass was recorded every 2 h. This experi- mental procedure is consistent with the norm ISO 24353 [21] for the characterization of the hygrothermal behaviour of building materials exposed to cyclic variations of relative humidity over a daily (24 h) period of time. Each cylindrical sample was placed upright inside an aluminium foil pan so that only the top and lateral surfaces were directly exposed to the atmosphere inside the climatic chamber. The total area of the exposed surface was therefore about 0.018 m , which is higher than the minimum value of 0.010 m required by the norm ISO 24353 [21]. Three samples were tested for each earth mix to confirm the repeat- ability of measurements and to reduce errors, with the final MBV calculated as the average of the three measurements. Before the test, all samples were equalized at a tempera- ture of 23 °C and a relative humidity of 53%. Equalization was assumed complete when the mass of the sample changed Fig. 7 MBVs measured during moisture uptake and release stages of less than 0.1% over a period of at least 1 week (this took humidity cycles. Solid markers indicate MBV uptake while hollow markers indicate MBV release generally 2 weeks). After equalization, the samples were exposed to cyclic changes of relative humidity as previ- ously described and two different MBVs were calculated, Table 6 MBVs under steady state conditions corresponding to the uptake and release stages of each cycle, according to the following equation: Sample ID MBV [g/ m  %RH] Δm MBV = (1) SΔ%RH Earth mix 1 (base soil) 3.3 Earth mix 2 2.3 where ∆m is the absolute value of the sample mass varia- Earth mix 3 1.5 tion (in grams), S is the exposed surface (in square meters) and ∆%RH is the imposed relative humidity change (in per- centage). The value of ∆m measured at the end of the high moisture buffering capacity than the other two mixes. This humidity stage provides the “MBV uptake” while the value of ∆m measured at the end of the low humidity stage pro- is due to the larger fine fraction, and hence the greater water retention capacity, of earth mix 1 compared to the other two vides the “MBV release”. To take into account the small change of sample dimensions caused by the swelling/shrink- mixes. In particular, the MBV increases almost linearly with growing clay content (see Tables 3 and 6) being twice as age of the earth, the exposed surface was calculated as the average of three height measurements and three diameter large for earth mix 1 as for earth mix 3. Similar experimental observations were made for different materials by Jaquin measurements taken both at the beginning and end of each humidity stage. et al. [22] and Beckett and Augarde [23]. Results from MBV tests are often presented in the form Figure 7 shows that the MBV is larger during moisture uptake than during moisture release but this difference of moisture adsorption curves, which record the hygroscopic behaviour of the material throughout the cyclic variation reduces as the number of cycles increases and the material converges towards steady state. Steady state is convention- of relative humidity. The moisture adsorption is defined, at any given time, as the ratio between the variation of sample ally defined as the occurrence of three consecutive “sta- ble” cycles where moisture uptake at a humidity of 75% mass during a cycle (i.e. the difference between the current and initial mass of the sample) and the area of the exposed is approximately equal to moisture release at a humidity of 53%. In general, v fi e cycles were suc ffi ient to achieve steady surface. Figure  8 shows the moisture adsorption curves recorded for each earth mix during the last stable cycle when state. The final MBV of the material is conventionally meas- the hygroscopic behaviour is virtually reversible with the moisture uptake being approximately equal to the moisture ured under steady state conditions and it is calculated as the average of the uptake and release values of the last three release. Figure 9 presents the MBV of earth mix 1 measured dur- stable cycles. The final MBVs of the three hypercompacted earth mixes measured the present work are summarized ing the present work together with the MBV measured by Bruno et al. [17] on a different hypercompacted earth mix in Table 6, which shows that earth mix 1 exhibits a higher 1 3 Journal of Building Pathology and Rehabilitation (2020) 5:2 Page 7 of 9 2 3.3 Water durability tests Durability against water erosion was investigated by means of immersion tests on cylindrical hypercompacted samples of 50 mm diameter and 50 mm height according to the standard experimental protocol described in [13]. Before testing, all samples were equalized at the laboratory atmosphere, i.e. at a temperature of 25 °C and a relative humidity of 40% ± 5%, until the mass changed less than 0.1% over a period of at least 1 week (this took generally 3 weeks). After equalization, the sample was weighted to record its initial mass m and subsequently submerged in water for 10 min. The sample was then removed from water and equalized again to the laboratory atmosphere to attain the same moisture content as before immersion. Fig. 8 Moisture adsorption curves during the last stable cycle of each After equalization, the final sample mass m was recorded earth mix and introduced, together with the initial mass m , in the following equation to calculate the percentage mass loss and the MBVs measured by Rode et al. [20] on a variety of %Δm experienced by the sample during immersion: standard building materials. Note that the MBV reported by m − m i f Bruno et al. [17] was measured on earth bricks that were %Δm = × 100 (2) hypercompacted according to a similar manufacturing proce- dure to that adopted in the present work. Inspection of Fig. 9 Table  7 summarizes the results from all tests, which indicates that earth mix 1 exhibits an excellent hygroscopic confirms that the hydrophilic nature of the earth has a performance with a MBV which is slightly less than that negative impact on the water durability of the hypercom- measured by Bruno et al. [17] but about seven times higher pacted samples. All samples showed marked mass losses than that of traditional building materials, such as fired clay and exhibited numerous cracks after immersion. Neverthe- or concrete bricks, as reported by Rode et al. [20]. less, the finer and better-graded earth mix 1 experienced a relatively small mass loss of only 13% compared to 30% for earth mix 2 and a complete sample dissolution for earth Fig. 9 MBV of earth mix 1 compared to the MBVs of other building materials as reported by Bruno et al. [16] and Rode et al. [20] 1 3 2 Page 8 of 9 Journal of Building Pathology and Rehabilitation (2020) 5:2 Table 7 Percentage mass loss during immersion tests three distinct earth mixes characterised by significantly dif - ferent particle size distributions but compacted under the Sample ID %Δm [%] same pressure. This testing campaign included unconfined Earth mix 1 (base soil) ≈ 13 compression tests, moisture buffering tests and immersion Earth mix 2 ≈ 30 tests. Results suggest that the use of a fine and well-graded Earth mix 3 Complete dis- earth mix can significantly improve the strength, stiffness, solution after moisture capacity and water durability of the material com- 4′30′′ pared to a coarse and poorly-graded earth mix compacted at a similar density. Importantly, the enhancement of water durability, albeit insufficient for mainstream building, may reduce the extent of chemical stabilisation that is required to comply with current regulations. All three earth mixes tested in the present work are compatible with particle grading recommendations that have been published in the literature and may therefore be deemed suitable for construction. Nevertheless, the three mixes exhibited markedly different behaviour during tests, which raises questions about the validity of current recom- mendations and suggests the necessity of considering addi- Fig. 10 Hypercompacted earth mix 1 before (a) and after (b) immer- tional grading features such as, for example, the regularity sion in water of particle size distribution. Acknowledgements The authors wish to acknowledge the support of mix 3. Once again, these disparities might be attributed to the European Commission via the Marie Skłodowska-Curie Innovative the different dry densities of the tested samples but also to Training Networks (ITN-ETN) project TERRE ‘Training Engineers the distinct particle size distributions of the three mixes. and Researchers to Rethink geotechnical Engineering for a low carbon future’ (H2020-MSCA-ITN-2015-675762). Figure 10 shows two pictures of the sample of earth mix 1 taken before (a) and after (b) immersion in water. These pic- tures indicate that immersion in water produces a significant Compliance with ethical standards erosion of the sample surface even for the relatively dura- Conflict of interest On behalf of all authors, the corresponding author ble earth mix 1. This deterioration is expected to negatively states that there is no conflict of interest. affect also the strength and stiffness of the material, though this particular aspect has not been evaluated in the present Open Access This article is distributed under the terms of the Crea- work but will form part of future research. tive Commons Attribution 4.0 International License (http://creat iveco mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- tion, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the 4 Conclusions Creative Commons license, and indicate if changes were made. The utilization of raw (unfired) earth as a building material is attracting the interest of engineers and architects world- wide due to environmental and economic advantages but References also to the availability of novel fabrication techniques that can meet the demands of modern construction. 1. 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Beckett CTS, Augarde CE (2012) The effect of humidity and Madrid, Spain: Centro de Publicaciones, Secretaría General Téc- temperature on the compressive strength of rammed earth. In: nica, Ministerio de Obras Públicas y Transportes Proceedings of 2nd European conference on unsaturated soils (pp. 15. (http://www .cerca d.fr/IMG/pdf/ter cr uso_r appo r t_final _lmdc2 013- 287–292). Springer, ISBN: 978-3-642-31342-4 04-r120.pdf), Tercruso (2013) Caractérisation des briques de terre crue de Midi-Pyrénées Publisher’s Note Springer Nature remains neutral with regard to 16. Bruno AW, Gallipoli D, Perlot C, Mendes J (2016) Effect of very jurisdictional claims in published maps and institutional affiliations. high compaction pressures on the physical and mechanical proper- ties of earthen materials. In: E3S Web of Conferences, p 9, 14004. EDP Sciences 17. Bruno AW (2016) Hygro-mechanical characterisation of hyper- compacted earth for sustainable construction. PhD Thesis, Uni- versité de Pau et des Pays de l’Adour, France 1 3

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Published: Dec 1, 2020

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