MAE : Mechanical performance and durability assessment of earthen elements and structures

Technical Committee MAE

General Information

Chair: Prof. Antonin FABBRI
Deputy Chair: Dr. Christopher BECKETT
Activity starting in: 2022
Cluster E

Subject matter

Definition of the TC subject and of the studied material

The goal of this TC is to define dedicated testing procedures for stabilized and unstabilized earth as a building construction material (crude earth). This construction material is currently increasing in interest for quite evident ecological reasons since the use of local earth as a building material can divide the embodied energy of a house by 2 and the transportation energy by 4.5 (cf. Morel et al., 2001), without increasing the energy consumption during service life of the building. Interest is also increasing because of the large number of earth-based buildings constructed before 1948: a good understanding of the behaviour of crude earth is crucial to develop proper methodologies for the rehabilitation and maintenance of these buildings.

The relevance of building with earth in the 21st century has already been shown in showcase examples built in Switzerland (by Martin Rauch), France (by Nicolas Meunier) and China (by Lu Wenyu and Wang Shu - 2012 Pritzker Prize). However, the prospects of earthen and bio-based materials of entering mainstream construction, and a fortiori as the main structural materials, are limited notably due to the lack of knowledge on the assessment of their performance and durability.

The first task of the previous round of this TC was to realize the “state of the art” report that presents a critical analysis of existing testing protocols for earthen materials and construction. This activity led to the publication of two papers in the Journal “RILEM Technical Letters” (Fabbri et al., 2018, 2021) and the aforementioned STAR book (Fabbri et al., 2022). On the basis of the results of this bibliographic work, a second stage of this TC comprised “round robin” testing whose aim was to determine and propose robust and reliable measurement protocols to estimate the performance of earthen materials. This second stage lead to recommendations concerning water vapour measurements and 3-point bending tests on compressed earth bricks (publications under finalisation).

The proposed technical committee would be a direct continuation of the previous committee on earthen construction, namely the TC-274-TCE (06/2016-06/2022). The primary goal will be to build upon the good work initiated on the study of mechanical behaviour and durability in order to publish RILEM recommendation papers and to inform global standardisation practices.

Level of investigation

Two scales of tests will be investigated: lab tests and on-site tests. A critical objective, for both scales, will be to define the type and minimal number of tests required in order to provide an accurate assessment of mechanical performance and durability of earthen materials and elements. We will start by using existing testing protocols, developed for similar materials, as these are familiar to researchers and practitioners. 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.

Importance will be given to the repeatability and the robustness of the tests. For laboratory tests, round robin campaigns will be performed, and, in function of the obtained results, the experimental protocols and even the nature of the test may be modified. For on-site tests, we will use instrumented earthen houses which are already studied by members of this TC. In particular, for that task, the TC will benefit from the results obtained within the framework of the French National Project on earthen construction.

Limit of the scope

This TC will be mainly focused on unstabilized earth. Time permitting, some exploration of methods to define properties of cement and/or lime stabilized earths may be included but no work will be done on new stabilization procedures (bio-polymers, enzymes…). Along the same line, this TC will only focus on traditional constructions techniques (i.e. Rammed Earth, Cob, Adobe, plasters, Compressed Earth Bricks). Newer construction techniques like printed earth and poured earth will be assessed in the two others technical committees which are submitted alongside this proposal.

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. This TC will focus on mechanical behaviour and durability, which means that thermal and hydrothermal behaviours will not be studied. Indeed, in our view, a first consensus had been reached during the TC-274 TCE among the community of researchers on earthen construction. At that stage, we think that hydrothermal behaviour of earthen materials should rather be treated alongside TCs on bio-based materials. Finally, only experimental studies will be performed within the framework of this TC; no numerical studies (e.g. finite element analyses) will be undertaken, except to provide support for publication if appropriate. All the theoretical developments underpinning the testing procedures examined under the proposed TC will extracted from a bibliographic survey and/or realized elsewhere.

Terms of reference

This TC is mainly based on experimental research and round-robin testing, which is quite time consuming. Thus, an estimation of 5 years should be quite realistic.

The TC will mainly comprise academics (researchers, research engineers) and engineers. The ideal number of members should be around 20 academics and engineers from 10 different laboratories spread throughout Europe/the UK.

Strong interactions with architects and masons will be fostered by holding dedicated workshops. This will disseminate the TC’s work to industrial bodies.

Detailed working programme

The TC will be divided into 3 main working groups (WG). These groups may be divided in a second stage into sub-working groups.

WP0/ Common dissemination & communication activities between the 3 TCs to increase impact of activities developed at RILEM.

This TC proposal for 5 years was designed together and in parallel with the following proposals: “TC on Resilience and durability of bio-stabilised earth-based construction materials: testing methods based on performances approach - chaired by Ana A. Bras and C. Perlot-Bascoules” and “TC on processing of earth-based materials – chaired by E. Keita & A. Perrot”. The objective is to ensure that the community stays together with a common focus on earth based materials characterisation at different domains for new construction and building rehabilitation.

Therefore, we propose a transversal WP (WP0) to ensure throughout the duration of the 3 TCs the dissemination of scientific results (writing of articles and recommendations), scientific animation (doctoral school, participation in conferences, organisation of a conference), but also for transfer of knowledge to industrialists (writing of a practical guide, technical days).

The content of the WPs is detailed below. Each WP is led by a person with recognised expertise in the field. The leader of WP0 would be a person that participates in the 3 different TCs related to earth-based materials mentioned above and submitted together with this proposal.

WG 1/ Laboratory tests to assess the mechanical performances

This WG will aim to finalise the work that was started during the TC-274 TCE. In particular, it was not possible to provide clear recommendation for the mechanical testing of earth-based material, except for the 3-point bending test. For CEB and rammed earth, testing protocols have been identified but round robin tests still need to be performed to order to reach a definitive result. Under this context, the main objective of this WG will be to:

  • Finalise the work (and write RILEM recommendation) on earth masonry elements
  • Provide some data on scale effect for rammed earth and cob and provide some guidelines to realise lab and on-site tests
  • Determine bond strength factors for reinforcement in earth, to be compatible with existing Eurocode approaches

For that purpose, the following milestones/deliverables have been identified (where T0+x refers to the start time plus x months):

  • 1 Identification of the laboratories involved in the RRT for masonry elements (T0+3)
  • 2 Identification of the test procedures to analyse de scale effect on RE and Cob and of the laboratories involved in the RRT (T0+24)
  • 3 Definition of a methodology to allow a compatibility between Eurocode approach and design of earthen constructions (T0+48)
  • 4 Bibliographic report on bond strength factors for reinforcement in earth (T0+54)
  • 1 Finalisation of the RRT for masonry and publication of the results (scientific paper or recommendation paper in function of the obtained results) (T0+18)
  • 2 Finalisation of the RRT for scale effect and publication of the results (T0+48)
  • 3 Publication of a methodology, compatible with Eurocode approaches, to consider bond strength factors for reinforcement in earth (T0+60)

WG 2/ Laboratory tests to assess durability

Durability issues were identified during the TC-247 TCE but the round robin testing was limited to the abrasion test. It follows that within the framework of this new TC we propose to:

  • Realise the experimental campaigns on water durability and erosion test and freezing-thawing behaviour in order to produce guidelines or recommendations.
  • Propose guidelines and indirect testing procedures to estimate fire/temperature (F-T) resistance of earthen materials.

For that purpose, the following milestones/deliverables have been identified:

  • 1: Identification of the test procedures to analyse (1) abrasion, (2) erosion (3) resistance to water up-take and (4) F-T resistance and of the laboratories involved that would be involved in the several RRTs (T0+18)
  • 1 : Finalisation of the RRTs on abrasion, erosion, resistance to water uptake and F-T resistance and publication of the results (T0+48)
  • 2: Publication of the guidelines for fire resistance (T0+60).

WG 3/ On-site testing

Earth is a complex material whose implementation requires skills not always known and/or mastered by masons, and which does not have normative dimensioning methods or ‘rules of thumb’ describing the conditions and processes of implementation of the works. The challenge of this WG will be to contribute to filling this gap by developing methods to monitor the quality of the raw earth elements of a structure during their implementation, following rehabilitation operations or whilst in service. To do so this WG will be divided in the following two sub—groups:

  • Quality control of new elements
  • On-site diagnostic methods

Let us note that the requirement of an increasing number of complex characterization tests is not appropriate for economic, ecological and practical reasons. Therefore, a strong stake of this WG will be to propose simple methods (that can be indirect) that can evaluate or control the quality of the implementation and/or the state of damage of earthen walls.

For that purpose, the following milestones/deliverables have been identified:

  • 1: Bibliographic study on diagnostic on-site measurements technics that may be used for earthen construction (T0+24)
  • 2: Identification of construction sites and/or demonstrator that can be used by TC members to tests these technics (T0+30)
  • 1: Summary of the results obtained by the several labs on the on-site testing campaigns through a common publication in M&S (T0+60)
  • 2: On the basis of the results obtained on WP1, WP2 and on M3.1, identification of simple tests that can be realised to perform on-site quality control of new earthen elements (T0+60)

A summary of the TC progress over the 5 years is presented in the GANTT chart below:

Technical environment

This TC is clearly in the continuity of the TC 274-TCE (cf. above).

This TC would be linked to the two other TC submissions dedicated to earthen construction.

In France, it will be linked to the “mechanical” and “on-site measurements” working groups of the “Projet National Terre”

In Portugal the TC will be linked to the 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 world 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 performance and limitations, earth-based construction is sustainable, environmental friendly, and safe.

Expected achievements

The goal is to provide harmonised tests methods for earthen materials through the publication of RILEM recommendation papers.  The direct benefits of these results are explained more in detail below.

In close collaboration with the two other TCs on earthen construction international conference on earthen constructions will be organised each two years during the lifetime of this TC. Following the same frequency, and possibly in connection with these conferences, doctoral schools on some scientific topics related to earthen construction will also be proposed.

Specific deliverables and outcomes are listed under each of the work packages.

Group of users

The short-term targets of this TC are academics and testing laboratories. The goal is to produce robust tests and increase the knowledge on the material. 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 construction. 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 material but is reusable (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 metres 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 techniques. The final result of this TC will thus counteract this problem by highlighting the particularities of earthen materials and by giving the tools to properly assess its performance. The scientific impact of this activity is therefore light, as new science will not be created. However, we believe that the engineering and economic impacts will be significant provided that these developments and regulations are accepted and adopted by earthen construction communities.

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 primary 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, the action of this TC will be focused on the development of the dedicated tests to assess the performance the earthen materials (both in laboratory and on-site) and to diagnose possible pathologies on existing constructions.