274-TCE : Testing and characterisation of earth-based building materials and elements
Technical Committee 274-TCE
Deputy Chair: Dr. Antonin FABBRI
Definition of the TC subject and of the studied material
The goal of this TC is to define dedicated testing procedure for stabilized and unstabilized earth as a building construction material (crude earth). A good understanding of the earthen constructions requires taking into account their large variability. A first reason of this variability is that the local soils are used as building materials. The local soils are variable depending on the pedology of the site, and the construction of earth buildings cannot be totally included in an industrial process. Then partly to adapt the building technique to the different soils, several constructions technics have been invented, which is the second reason of the variability. Among these technics the three most common ones are adobe blocks, cob and rammed earth (Hall, et al., 2012). Adobe blocks are produced or manufactured using wet earth based mixture molded which is removed before drying. Cob consists in stacking clods, made of a mix of plastic earth and fibres, in order to build a monolithic wall. Finally, rammed earth is formed by compacting raw earth (ideally with a water content which insures its highest density) within a framework and layered down in successive beds with a rammer. These definitions are however quite general and the exact protocols which are used strongly vary in function of the texture of the earth, the period and weather, the location and the mason. In addition, other construction technics like Compacted Earth Blocks, Extruded Earth Blocks, and Prefabricated Rammed Earth are gaining more and more interest.
On the other side, whatever the construction technics and the material used, some similarities exist. At first, earth walls are quite thick (30 to 50 cm) and are separated from the ground by a basement which is designed to prevent capillary rise. Earth materials are composed of clays, silts, sands and possibly gravels.
The continuous clayey matrix plays the role of the binder between all these constituents and thus provides cohesion and strength. But the connections between the constituents are not perfect and some small voids, called pores, are embedded within the solid material. The network formed by the connection of these voids, called the porous network, enables fluids (either liquid or gas) to flow through the material. Due to this property, earthen material is a porous medium with a quite low resistance to the vapor and liquid transfers. In addition, clays and fibers are by themselves porous media. Their high affinity with water molecules allows capillary condensations or adsorptions phenomena. Thanks to the combination of these two processes (vapor transport and in-pore water sorption/desorption), earthen materials are hygroscopic. In other words, raw earth has a great potential to regulate indoor relative humidity levels. The latent heat associate to the sorption/desorption phenomena can also strongly modify the thermal behavior of the material. Finally, this hygroscopic behavior impacts the mechanical characteristics of the material. For example the interactions of clay particles and/or fibers with the water molecules can be responsible for complexities in mechanical behavior, such as swelling and shrinkage (Van Damme, 2002; Lei, et al., 2014). In addition, the mechanical resistance of a soil decreases with its water content (i.e. mass of water on mass of dry solid) which is well known in soil mechanics but it also applies to earth buildings. It seems therefore that the water content in raw earth materials is crucial to predict its behavior.
In view of these complexities in behavior and these large variabilities, two main questions arise. The first one concerns the characteristics that must have an earth mixture to be used as a construction material. First answers have been provided by recommendation guides and standards. For example, BS1377 standards define a range of admissible values for the particle size distribution of earth. Along the same line, a proportion of 15-30% of clays, 10-30% of silts and 50-75% of sand is commonly recommended for the construction of adobe blocks (Garrison & Ruffner, 1983). In general, the clay content proportion should thus be sufficient to ensure a good material stiffness and strength, but the proportion of expansive clay must remain limited (lower than 50% of the total clay amount) in order to avoid cracking.
Therefore, the study of existing construction indicates that the problem is not so simple, and that the particle size distribution cannot be used as a discriminating parameter (Aubert, et al., 2014; Gomes et al., 2014; Champiré et al, 2016). In consequence, up to now, no clear objective rule exists to discriminate the ability of an earth to be used as a building material solely on the analysis of its granular and mineralogical properties.
The second main question concerns the evaluation of the performances (sustainability, strength, seismic resistance, thermal, moisture buffering, …) of the material. For the moment, the performance are estimated through the some common existing experimental methods: unconfined compressive strength to assess the mechanical, sorption/desorption curves and dry/wet cups for vapor adsorption and diffusion, hot wire or hot place for thermal conductivity, etc… These studies underline an important variability of the measured parameters. Numerous reasons of these disparities can be intuited like the lack of hygrometry and temperature control during the tests, the differences in samples manufacture, geometry, and in test protocol … As an example, in function of the experimental protocol used, an unconfined compressive test can lead to compressive strength up to 40MPa for earthen material (Aubert, et al. 2013), which is not realistic.
In addition, the knowledge of these common parameters is found to be insufficient to properly model the complex behavior of earthen walls (Soudani et al. 2016, Nowamooz and Chazallon, 2011, Bui et al., 2014).
In particular, the complexity in mechanical behavior of the material cannot be well described by the sole knowledge of a compressive strength and the apparent Young’s modulus of the first loading (Champiré et al, 2016), while its thermal insulation potential may be significantly underestimated if it is only based on its thermal conductivity and heat capacity (Soudani et al., 2016).
A first step in order to take into account material hygroscopicity in moisture buffering capabilities has been realized within the framework of the Nordtest project (Rode et al., 2005). It leads to the definition of test to determine the Moisture Buffering Value (based on the water mass uptake/release by the material when it is submitted to a daily cycle of relative humidity between 33%RH and 75%RH, which is representative of the average indoor RH daily variations in Northern European Countries. Although the principle of this test is interesting, it remains quite limited because of its lack of repeatability. In addition, its aim is to be applied to all hygroscopic materials, which is not necessarily a good option since the phenomena of sorption and desorption which occur within them can be strongly different. Finally, the applicability of this test in warmer European countries like Portugal or Italy, where the cycles of indoor relative humidity would rather be between 60%RH and 85%RH, is questionable.
In this context, the goal of this technical committee is to give an objective answer of these two questions by the definition of performance-oriented tests. These tests should depend on the use that will be made of the material, and their clear definition requires a good understanding of the specificities of earth, as well as the identification of the key parameters which drive its behavior.
Level of investigation
The objective is to define the minimal number of laboratory tests in order to provide an accurate assessment of the mechanical, thermal and hygroscopic performances of the material. To that aim, whenever possible, existing experienced tests will be used. However, due to the particularities of the material, development of new experimental protocols and/or the adaptation of existing ones may be necessary. These tests procedures and the way to analyse their results will be defined in the view of recent theoretical developments published in literature and/or developed elsewhere by the members of the TC. The behaviour of the material is strongly impacted by the external conditions (temperature and humidity variations, saline environment, etc…). In consequence, the definition of dedicated tests to assess the performance of the material in some specific environmental conditions may be necessary.
A main importance will be given to the repeatability and the robustness of the tests. Round robin tests will be performed, and, in function of the obtained results, the experimental protocols and even the nature of the test may be modified. The accuracy of the tests will be studied by comparing, at least qualitatively, their results to the on-site behaviour of earthen constructions. To allow this kind of confrontations, all the tested earths will come from existing constructions which are properly instrumented.
Limit of the scope
There is several ways to use earth in constructions (load-bearing walls, filling materials, plasters,…). This TC will be limited to load-bearing walls. However, several construction technics (rammed earth, CEB, adobe block, cob), made with unstabilized or lime/cement stabilized earths and with or without fibers will be studied. Whenever possible, the developed testing procedure would be such that they can be used for any of these constructions technics.
No standardisation activity, in its strict sense, will be made during the lifetime of this TC. However, all the experimental developments should be able to serve as a basis for the development of international/European standards on earth testing procedures
Finally, only experimental studies will be performed within the framework of this TC. All the theoretical developments required for this study have to be extracted from a bibliographic survey and/or realized elsewhere.
Terms of reference
This TC is mainly based on experimental researches, which are quite time consuming. Thus, an estimation of 5 years should be quite realistic.
The TC should be mainly composed by academics (researchers, research engineers) and engineers. But strong interactions with architects and masons will be favoured by the realisation of dedicated workshops. The best number should be around 20 persons from at least 10 different laboratories.
Detailed working programme
The TC will be divided in two main working groups (WG). The first one will be focused on the thermal/hygrothermal characterisations and the second one the (hydro)-mechanical performances and seismic resistance. Each WG will follow the same general program as described above.
1. Bibliographic study and initial set-up of key parameters
The first action of the TC will be the identification of the key parameters which must be measured in order to correctly assess the performance of the material.
To reach this goal, even before any additional development, an important action of this TC would be to bring together international recognized experts on earthen constructions. Then, the required theoretical knowledge on earthen materials will be provided by the TC’s members, coupled with an up-dated bibliographic survey.
This study will be formalised by the submission of a review paper in Materials and Structures and/or the redaction of a public report (D1).
In addition, an international conference on earth construction will be organised. The idea is to have a clear picture, at that stage, of all the newest development (both experimental and theoretical) on earthen constructions. In consequence, the proceedings of this conference (D2), which will be the second deliverable of the TC, will be completely in line with the objective of this first part.
2. Initial set-up of laboratory tests
From the knowledge of the key parameters, a first set of experimental tests will be defined. Whenever possible, the use will be made of existing tests (like uniaxial compression test, hot plate test, etc…), but some new tests could be developed in order to take into account the specificity of the material (notably its strong interaction with water). Let us note that, the more rapid and cheaper tests will be preferred.
In addition, some dedicated tests for specifics environmental conditions will be developed. The definition of the specific conditions which are worth to be considered will be made on the basis of the knowledge of the TC’s members.
The results of this task will lead to the redaction of a confidential report (D3).
3. Final set-up of laboratory tests
The accuracy of this first set of experimental tests will be tested in two ways.
The first one consists in a confrontation with on-site measurement. The idea is to verify that the performance of the material which is predicted from the laboratory tests is in concordance with the behaviour of constructions made with this material. To do so, all the tested material will come from existing constructions. Consequently, one of the first tasks of this TC will be to identify these constructions. This confrontation will be made on the bases of the temperature / hygrometry measurements inside/outside the construction, on the monitoring of energy consumption (when possible) and on the identification of possible pathologies.
The second one consists in assessing the robustness and repeatability of the tests. To do that, the idea is to realise round robin tests between all the involved laboratories. In function to the obtained results, some tests procedures will be modified.
In addition to scientific publications, the results of these tests will be formalised by a report defining the dedicated testing procedures for earth materials (D4).
A closure symposium will ultimately be organised in order to present the results of the TC to the scientific and the practitioner’s communities (D5).
This TC is clearly in the continuity of the TC 164-EBM, which have been dedicated to the realisation of the Compacted Earth Blocks. In addition, this TC will beneficiate from the results of the TC 236-BBM on the bio-based materials. Indeed, even if in some ways the materials studied in the two TC are strongly different, they share some characteristics as their affinity with the water molecule. The link between the two TC will be insured by the presence of members involved in the two TCs
Finally, this TC would be linked to local associations.
In France, it will be linked to the association TERA. This association is supported by Rhône-Alpes region. The steering committee is composed by FFB, CAPEB, CINOV, CROARA, UNTEC and DRAC. The goal of this association, composed by professional of earthen constructions (mason, architects, engineers, economists), is to promote knowledge, to establish and promote the best practices for the restoration of earthen constructions, to enhance the visibility for local publics institutions and, at end, to contribute to the development of earthen construction by a regional structuration of the sector. It will be also linked to the “Collectif Terreur Armoricain” which will allow a direct link between the TC and the French practitioners of cob.
In Portugal the TC will be linked to Centro da Terra association, which gather professionals with diversified backgrounds - from researchers, engineers, architects, producers of low embodied energy materials, builders, and masons – connected with earth-based construction. Finally, the TC will be linked to Earth Building UK & Ireland (EBUKI).
In accordance with the RILEM’s main mission, this TC will promote an international cooperation bringing together the leading word experts on earth buildings. Its main goal is to advance scientific knowledge related to the performance assessment of earth as a building material and to encourage the transfer and application of these advances to practitioners through the edition of guidance and the organisation of dedicated workshops. If the material is used in a proper way, which implies a good assessment of its performances and of its limitations, the earth-based constructions are sustainable, environmental friendly, and safe.
Because this TC focuses on material characterisation we think that an assignment to the Cluster A should be the more appropriate.
As detailed previously, a first result of this TC will be to produce a state-of-art report based on the knowledge of the members of the TC combined with an up-to-date bibliographic study. However, its final goal is to provide harmonised tests methods for earthen materials when they are used to build load bearing walls. The direct benefits of these results are explained more in detail in the §8.
Two symposiums on earthen constructions will be organised during the lifetime of this TC. The first one will occurs at the end of the first year and will be dedicated to scientists. Its goal is to have a global picture of the recent advances of the research in the field of earthen construction. The second one will occurs at the end of the TC (year 5). It will be focused on the testing procedures and it will be dedicated to both scientists and practitioners. Its goal will be to present the final results of the TC.
Group of users
The short term targets of this TC are academics and testing laboratories. The goal is to produce robust tests, increase the knowledge on the material etc… However, since this TC aims at serving as a basis for the development of dedicated standards for earth-based construction materials, longer-term targets are practitioners. Even if the direct participation of practitioners to the TC faces logistical issues (language, time, funding…), the link between them and the TC development will be made through their numerous local collaborations with the TC’s members.
Specific use of the results
The stabilization of the atmospheric greenhouse gas concentrations requires our emissions to drop well below current levels, and thus requires a drastic reduction in our energy consumption. Since the building section accounts for about 40% of global greenhouse gas generation (Dixit, et al., 2010), the development of earth-based buildings appears to be a sustainable alternative to conventional constructions. Indeed, earth is a local material that can be taken and used immediately on the construction site or nearby and does not require industrial processing (Morel, et al., 2001). It is not a renewable but is a reusable material (depending on the stabilization); it requires no treatment to be reused and therefore has a very low impact in terms of embodied energy (Habert, et al., 2012). Moreover, the sustainability of this material was recognized long ago, as highlighted by the significant heritage of earth construction all around the world. An iconic example of this is the city of Shibam in Yemen where buildings with more than eight stories reaching heights of 30 meters were built with earth blocks. Shibam is a UNESCO world heritage site and is known as the most ancient skyscraper city in the world (Houben & Guillaud, 1994).
However, the development of this ancestral building technique notably suffers from the lack of appropriate standards. In consequence, they are disadvantaged compared to conventional construction technics. The final result of this TC will thus allow counteracting this problem by highlighting the particularities of earthen material and by giving the tools to properly assess its performance. In particular, it lacks dedicated standards which take into account the particularities of earth when it is used as a building material.
In addition, we must keep in mind that buildings built before 1948 in most of European Countries (about 1/3 of the existing housing) are mainly composed by load-bearing structures in prime materials (stone, wood and crude earth). In 1987, according to Michel and Poudru (1987) there were in France about a million earthen buildings. And, due to an increasing demand in thermal comfort, for aesthetics reasons, or simply for maintenance, a major issue concerns the diagnostic, rehabilitation and renovation of these buildings. Indeed, the lack of knowledge of the material behavior can lead to apply common procedures and solutions, which are suitable for other building materials but which may be non-adapted or even harmful when they are applied to earth buildings.
To remedy this situation, a first action of the TC will be the publication of a report on the particularities in behavior of earth material. Thanks to this publication, the role of water in the thermal and mechanical behavior of the material will be clearly stated and the inconsistencies of some insulation systems and rehabilitation procedure will be clearly emphasized.
The second action will be the development of the dedicated tests for the earthen materials. Indeed, thanks to this information, new strategy for thermal enhancement and renovation of buildings will become possible.