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Contents

Foreword

Chapter 1. Environmental, Economic and Social Context of Agro-Concretes

1.1. Sustainable development, construction and materials

1.2. Standardization and regulation: toward a global approach

1.3. The materials: an increasingly crucial element

1.4. The specific case of concretes made from lignocellular particles

1.5. What does the term “Agro-concrete” mean?

1.6. Conclusions

1.7. Bibliography

Chapter 2. Characterization of Plant-Based Aggregates

2.1. Microstructure of the shiv particles

2.2. Particle Size Distribution (PSD)

2.3. Compactness and compressibility

2.4. Water absorption capacity

2.5. Bibliography

Chapter 3. Binders

3.1. Portland cements

3.2. Lime

3.3. Lime-pozzolan mixtures

3.4. Plaster

3.5. Summary

3.6. Bibliography

Chapter 4. Formulation and Implementation

4.1. Objectives

4.2. Rules of formulation

4.3. Examples of formulations

4.4. Installation techniques

4.5. Professional rules for buildings using hempcrete and hemp mortars

4.6. Bibliography

Chapter 5. Mechanical Behavior

5.1. Composite material

5.2. Modeling of the mechanical behavior

5.3. Toward the study of a stratified composite

5.4. Conclusion

5.5. Bibliography

Chapter 6. Hygrothermal Behavior of Hempcrete

6.1. Introduction

6.2. Heat conductivity

6.3. Hygrothermal transfers

6.4. Thermal characterization of various construction materials

6.5. Modeling of coupled heat- and mass transfers

6.6. Conclusions

6.7. Bibliography

Chapter 7. Acoustical Properties of Hemp Concretes

7.1. Introduction

7.2. Acoustical properties of the material on the basis of the main mechanisms

7.3. Modeling the acoustical properties

7.4. Application of the model to the acoustical characterization of shiv

7.5. Conclusion

7.6. Bibliography

Chapter 8. Plant-Based Concretes in Structures: Structural Aspect – Addition of a Wooden Support to Absorb the Strain.

8.1. Introduction

8.2. Preliminary test

8.3. Test on a composite panel of a wooden skeleton and hempcrete

8.4. Results and comparative analysis

8.5. Conclusions and reflections

8.6. Acknowledgements

8.7. Bibliography

Chapter 9. Examination of the Environmental Characteristics of a Banked Hempcrete Wall on a Wooden Skeleton, by Lifecycle Analysis: Feedback on the LCA Experiment from 2005

9.1. Introduction

9.2. Description of the products studied

9.3. Method for environmental evaluation of bio-sourced materials

9.4. Lifecycle Analysis on hempcrete – methodology, working hypotheses and results

9.5. Interpretations of the lifecycle, conclusions and reflections

9.6. Bibliography

List of Authors

Index

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Foreword

I am writing this foreword soon after finishing reading Sur la route du papier (On the Paper Trail) by Erik Orsenna: a wondrous journey, a lesson in history but also, and above all, a revelation about the workings of globalization. Paper is a harmless fibrous pulp – originally created from old rags, and later on and to date, from wood – which, filtered in the form of a thin layer, has enabled the most abstract creations of the human mind to be promulgated and become immortal.1 Though it has long been decried for the environmental consequences of its production, paper has now acquired a stamp of eco-friendliness thanks to the constant improvement of forestry and forest management, the manufacturing procedures and recycling. Is there any other “bio-sourced” material that has had a more profound impact on the development of civilization than paper? Assuredly not.

In another context, is there any other material that has made a greater contribution to the human race’s economic development for over a century, and to the mass development of infrastructures that that development has required, than concrete? Along with petroleum (for mobility) and silicon (for information and communication technology (ICT)), this artificial rock is one of the material foundations of our so-called developed societies. Concrete has enabled us to harness the energy from rivers, to build ports and airports, levees, sewage systems, roads, bridges, tunnels and more buildings than any other material.

Yet this material, a mixture of cement, sand and gravel which we simply call “concrete”, is in fact only one of the representatives of a broad category. After all, what is a concrete, if not a composite material made up of granular particles and a “glue” or binder holding everything together? According to that logic, bitumen concrete – that of the asphalt paving that now covers our roads in an almost monopolistic fashion – may be a serious challenger to cement concrete for the title of kingpin material in our infrastructures. Between them, these two materials alone constitute almost the totality of the “skeleton” of our lands. However, the intensive usage of these materials is not without consequences, either through greenhouse gas emissions or by the exhaustion of natural resources, be they fossil or mineral.

It might be tempting to leave the topic at that, unless for anecdotal purposes. However, there are at least two other concretes which merit our attention. The first is at least as widespread as cement and bitumen concretes on a worldwide scale; yet it is largely overlooked in our societies. Quite simply it is crude earth (not fired or baked like terracotta), which should, more correctly, be dubbed “clay concrete”, because it is its fine-grained constituent – clay – which, upon interaction with water as in the case of cement concrete, ensures the cohesion of the larger grains. In various forms – compacted, molded, pasted or plastered – it provides shelter to over a quarter of the world’s population. In France alone, the patrimony built of crude earth represents over a million houses. In the hands of master craftsmen and expert architects, the use of earth is fully capable of delivering on our desires for comfort and aesthetic beauty, whilst also satisfying our desire for eco-modernity.

This book is devoted to a fourth concrete, or rather a fourth family of concretes; original and promising from more than one point of view, which would seem to exhibit all the advantages of paper and earth, whilst still offering the convenience of use of our major industrial concretes. Contrary to popular opinion, sand and other granular (particulate) minerals are not an inexhaustible resource. Unless we wish to inflict irreparable damage on the environment, the time has come for recycling, or for using bio-sourced particulates, which is essentially the same thing. This is the path adopted by agro-concretes and, in particular, hemp concretes. France is the largest producer in Europe of Cannabis Sativa, whose fibers have been used to make rope for centuries. Yet this fast-growing plant, well adapted to temperate climates, harbors many other resources. Its stem, of a highly porous and therefore very lightweight wood, when ground up makes a surprising aggregate. Surprising, not on a mechanical level – the only level which truly counts for mineral aggregates, with cleanliness and shape in joint second place – but surprising, primarily, on a functional level: the level of hygrothermal equilibrium and acoustic properties.

Looking at the proliferation of synthetic materials available on the market, one might think that thermal, hydral and acoustic comfort is a domain in which the materials available – particularly when these materials are used in combination – have nearly reached the optimum desired. Polymer foams and organic and inorganic aerogels have extremely low thermal diffusivity and air permeability, which are difficult to better in the race toward very low values. Yet they lack inertia. When combined with other materials – or, even better, when a solid-to-liquid “phasechanging” material such as paraffin or a salt is added into the mixture – they (apparently) acquire the thermal inertia that they lack, by absorbing and reflecting the latent heat of fusion. In spite of their remarkable performances, these insulating materials still lack the “breathability” of certain natural materials, related to the capacity for absorption, transfer and phase-change of water in vapor and liquid form – all properties which depend on the characteristics of the porous space of the material and the thermal and hydric coupling which manifests itself in that space.

In the face of the complexity of combinations of synthetic materials employed to ensure an acceptable degree of comfort, agro-concretes and hemp concretes in particular offer a simple solution, which draws upon the exceptional porous texture – nearly always hierarchical – of certain plant structures. However, in order to take advantage of this property in terms of hygrothermal exchanges, the binder used must be able to work with the granular material rather than counteracting its properties.

The authors of this book present us with the elements, drawn directly from research, that help us comprehend the properties, function and formulation of agro-concretes. It is undoubtedly true that such concretes can never stand up to high- or ultra-high-performance mineral concretes, but it is not their intention to do so. First and foremost, they are intended to be insulating materials. Therefore, their primary intention is to sustainably ensure the comfort and durability of the dwelling, including the moderately dense dwellings towards which we are now tending.

This book has another virtue. It leads us to reflect on the physical bases for our criteria of “high environmental quality”, which are still largely founded upon the segmentation and selection of a few physical properties. The very least that can be said is that this manner of proceeding is not hugely well-adapted to materials in which there is extensive coupling between properties. This is precisely the case where bio-sourced materials are concerned. Let us hope that this book is distributed as widely as possible, so that a global, “performance-oriented” approach can finally emerge.

Henri VAN DAMME
IFSTTAR
December 2012

1 And, recently – but this is less noble – to package practically all the goods that we produce, in the form of cartons, boxes and bags.

Chapter 1

Environmental, Economic and Social Context of Agro-Concretes

Chapter written by Vincent NOZAHIC and Sofiane AMZIANE.

1.1. Sustainable development, construction and materials

After decades of virtuous and limitless consumption, the evidence is incontrovertible: human activities are not without impact on the environment and on humans themselves. It was not until 1987, with the Brundtland Commission [UNI 87], that this observation gave rise to a new concept: sustainable development. The report published by this commission, Our Common Future, defines the term as follows:

“Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” [UNI 87]

Thereafter, this concept has pervaded modern societies, ultimately becoming a political and economic issue, and an issue of the very survival of the human race… All human activities – industry, construction, agriculture, energy, transport, etc. – now have to deal with so-called “sustainable development” issues. The report unveiled by the United Nations Environment Program (UNEP) [UNE 09] constitutes an overview of the evolution of our societies since the publication of the Brundtland Report. The following quote, taken from that text, highlights the enormity of the challenge:

“There are no major issues raised in Our Common Future for which the foreseeable trends are favourable.” [UNE 07]

1.1.1. Environmental impacts of the construction sector

Above all, we must remember that the concept of sustainable development dealt with locally is often linked to problems on a worldwide scale, such as global warming or the gradual exhaustion of resources. These two criteria constitute the points of no return for our civilization.

As regards the climate, the scientific works of the IPCC1 serve as a referential framework. The second assessment report (SAR) published by this organization in 1995 [IPC 95] concludes that the “the balance of evidence suggests a discernible human influence on global climate”. A mere two years later, on the basis of this report and the UN Framework Convention on Climate Change [UNI 92], the international political debates culminated in the Kyoto Protocol [UNI 98]. This text commits the countries which have ratified it to reduce their GHG2 emissions by 5.2% in comparison to their level in 1990 over the period 2008–2012. The protocol came into force in 2005 and therefore will conclude in 2012. Owing to its use of nuclear and hydroelectric energy, which do not produce much GHG, France is committed to maintaining these levels of emissions.

For its part, the construction sector (residential and tertiary), much like the agricultural or industrial sectors, finds itself facing significant challenges in terms of reducing GHG emissions and energy consumption. The figures speak for themselves, but they must be analyzed seriously. Indeed, it is not always entirely clear what data have been taken into account when producing the figures, particularly in terms of drawing the distinction between a building’s function and its construction:

– Total GHG emissions from both energetic and non-energetic sources (e.g. agriculture, forestry, etc.): 7.9% on a global scale [IPC 07], 40% on the scale of the US [USD 11] and 18% on the scale of France in 2007 [CGD 10] for all residential/tertiary, institutional and commercial consumption (heating, specific electricity, hot water, cooking, etc.);
– Final electrical energy consumption3: 41% on the scale of the US in 2010 [USD 11] and 43.4% on the scale of France in 2008 [RIL 06; CGD 09] for all residential/tertiary, institutional and commercial use. These figures do not include the fossil energy required to produce the electricity.

However, while climate change represents an alarming phenomenon, it is not the only point that needs to be taken into account. The natural resources needed for the perpetuation of human activities and societies are, for the most part, finite and exhaustible. Similar to the threat of global warming, exhaustion of resources – be they minerals or arable land – is a major point of concern which will inevitably lead us to change our ways before the current century is out [OEC 08]. The activities relating to construction and to public projects, while they do not necessarily require materials to be used which come from exhaustible sources (with the exception of road infrastructures, which consume bitumen), constitute the single greatest cause of consumption of natural resources (31% in Europe [SER 09]). Furthermore, this consumption causes a large amount of waste production, even though 97% of the waste produced by construction and public works in France are inert [IFE 08] and are subject to a policy of value-creation. In France, the amount of waste generated by this domain equaled 343 million tons in 2004, 44% of the total mass [PEU 08].

In summary, the construction sector battles four main impacts on the environment:

– Its GHG emissions;
– Its energy consumption;
– Its consumption of natural resources;
– Its waste production.

1.2. Standardization and regulation: toward a global approach

1.2.1. Standardization and regulation in force

The legislation in charge of regulating these major impacts of the sector is a relatively recent phenomenon. The European framework was solidified in 2002 with the publication of the Energy Performance of Buildings Directive (EPBD). In the context of France, the successive “réglementations thermiques” (thermal regulations) RT 2000 and RT 2005 [FFB 09] follow this document. More recently, the loi Grenelle 1 (First Conference Law) of 3 August 2009 defined the Plan Bâtiment Grenelle4 (Conference Building Plan), launched in January 2009 (see Figure 1.1):

“The aim of the Plan Bâtiment Grenelle is to guide the implementation and deployment of the measures prescribed in the program to reduce buildings’ energy consumption and greenhouse gas emissions” [GRE 09].

The approach deals with buildings’ performances with a view to satisfying two main objectives:

38% goal: to achieve a 38% reduction, by 2020, of the energy consumption of the existing residential/tertiary sector in comparison to its level in 2008;
Factor of 4 goal: to achieve a fourfold reduction, by 2050, of GHG emissions by the existing residential/tertiary sector in comparison to its level in 1990. This commitment was made in 2003 as part of the National Climate Plan.
– The foundations for the new thermal regulation RT 2012, which is currently coming into force, were laid by Article 4 of the First Conference Law. This new regulation, which applies to new buildings in the residential and tertiary sectors, includes three main objectives:
– a numerical objective (CMAX coefficient) regarding a reduction in energy consumption by 50 kWh/m2/year, with a variety of criteria modulating that figure such as geographical location (see Figure 1.1). In terms of heating, this corresponds to an average of 15 kWh/m2/year. In terms of controls, the emphasis is placed on the measure of the building’s air-tightness, and on the monitoring of consumption;
– a significant technological and industrial change in the design and construction of buildings, for all areas of energy expenditure (heating regulators, lighting or water heating systems, etc.). However, air conditioning should be avoided, as the design of the building should take account of comfort in the summer months (TIC coefficient – Interior Conventional Temperature);
– a balanced energy provision, which emits little in terms of GHGs and contributes to the country’s energy independence (incentive to use renewable energies).

The primary goals of these new thermal regulations are to reduce GHG emissions by the construction sector, but also that sector’s energy bill from a socioeconomic standpoint and in terms of exhausting fossil resources. With the RT 2012, a global design of the function of the building, including consumption in terms of heating as well as air conditioning or lighting, is prescribed. This is the first step towards a “cradle to the grave”-type approach taken from the process put forward by the Analyses de Cycle de Vie (LifeCycle Analyses – LCA). The publication, between 2010 and 2012, of the European norms NF EN 15643 [AFN 10] also moves in this direction, proposing a system for evaluating buildings’ contribution to sustainable development, based on an LCA approach. Design and diagnostics tools have been developed in the same vein by the CSTB (such as Team, Cocon or Elodie).

Figure 1.1. Construction plan from the environmental conference, relating to energy consumption and reduction of greenhouse gas emissions by the construction sector

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Note also that in the context of the project’s dwellings plan, the R&D program PREBAT5 conducted a very exhaustive study which offers an overview of the international initiatives to reduce the consumption of construction [PRE 07].

1.2.2. Limitations of the normative and regulatory framework

As we saw above with regard to European and French legislation, the issue of a building’s ecological impacts is, for now, centered on the period of time for which it functions. The impacts of the material or the waste products (from the building site, or from demolition) do not enter into the debate. Approaches do exist which encapsulate more criteria than this, but on a voluntary basis.

Table 1.1. The 14 targets of the HQE approach (High Environmental Quality) [HQE 01] and the importance of the construction materials in this approach

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In France, many associations bringing together professionals in construction, public institutions or regional bodies have given rise to labels certifying a building’s environmental quality in accordance with a global standard. Such is the case, in particular, with the HQE approach6 (see Table 1.1), which involves three applications: tertiary building, individual home or collective/group accommodation. Other certifications exist in France and offer similar approaches, such as Habitat et Environnement (Habitat and Environment).7 All are based on a top-down approach – i.e. working from the project’s aims to the materials to be used [JUL 09]. Yet the HQE approach [HQE 01], while it is more complete than the regulatory framework detailed in the previous point, does not include a target as regards the choice of primary materials and their sustainability.

1.3. The materials: an increasingly crucial element

1.3.1. Role of the materials in energy consumption

The distribution of the energy consumption between the heating post and that devoted to the materials and construction of a conventional house (see Figure 1.2) reveals the greater relative weight that this second post as the building plan is gradually applied [PEU 08]. Indeed, we see a rise from 8% to 60% in the relative proportion of the materials and the construction in the energy consumption for a building with a lifespan of 100 years, while the consumption from heating plummets from 200 kWh/m²/yr to 15 kWh/m²/yr (RT 2012).

Figure 1.2. Distribution of the energy consumption of buildings, due to heating and to materials and construction respectively, depending on their energy performances and their lifespan [MAG 10]

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In addition, the durability of the construction material used is also of crucial importance. If the lifespan drops from 100 to 50 years for a building that consumes 15 kWh/m²/yr, the relative proportion of the materials and construction in the building’s energy consumption jumps from 60 to 75%. Thus, we can understand the important issues which will affect the construction materials market in a not-too-distant future, be it in terms of new construction or renovation of old buildings.

1.3.2. What is a low-environmental-impact material?

The points touched upon in the previous section outline a new set of specifications as regards the elaboration and choice of construction materials. Thus, we can define a new category: low-environmental-impact materials, or “ecomaterials”. At present, there are no clearly-defined criteria and even fewer norms that can be used to classify a material as an eco-material [PEU 08; ESC 06]. In fact, in addition to the technical characteristics typically required of a product for the home, we look for it to satisfy the specifications of sustainable construction in terms of respect for the environment, the comfort of the dwelling and the health of the users (see Table 1.2).

1.3.3. Constantly-changing regulations

The new criteria defined above are all points which need to be taken into account when creating new materials in the laboratory or research center, and when choosing a material for a particular construction project. Currently, lifecycle analysis (LCA) is the most widely-used method for approximating a product’s environmental impact. This tool was standardized in the ISO 14040 series [AFN 06] on environmental management. There are other methods to define the impact of a manufactured product – particularly the carbon balance put in place by ADEME – but the application of these methods is limited.

Table 1.2. Qualities sought when creating an eco-material for construction. Overview of different definitions [AMI 09; MAG 10]

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In France, the implementation of FDESs8 (type III environmental declaration) by the AIMCC9 requires material manufacturers to carry out a LCA on their products, based on the aforementioned standard and to publish information about the hygiene, safety and health aspects of their products. They are the subject of the French norm NF P 01 010, and confer ISO 14025 certification [AFN 10]. Currently, FDES are not a legal obligation. For the time being, the introduction in 2008 of the European regulation REACH (Registration, Evaluation and Authorization of Chemicals – [REA 06]) is the main regulation which directly or indirectly affects construction materials. A more targeted legislative move was the French government’s decree no 2011-321 of 23 March 2011, relating to the “labeling of products for construction or wall/floor coating and paints and varnishes as regards their emissions of volatile pollutants”. This legislation requires manufacturers, importers, distributors of construction and decorating products, building contractors and buyers of such products to “indicate on a label, placed on the product or on its packaging, the characteristics of its volatile pollutant emissions once it is used”.

A non-obligatory form of labeling (Type I environmental declaration) on construction materials is also available, but again its use is limited. The ACERMI10 label [ACE 09] also relates to industrial insulating products delivered in rolls or in slabs. Products commercialized on the European market can also be granted the Natureplus11 label, which is a reference point in terms of requirements because it relates only to products made up of at least 85% of renewable primary materials or materials of practically-inexhaustible mineral origin. To aid in the making of technical choices, the databases of software packages such as Cocon provide technical information above and beyond that provided on the FDESs for many construction materials (thermal conductivity, specific heat, resistance to water vapor diffusion, embodied energy and carbon impact, on the basis of a LCA).

1.4. The specific case of concretes made from lignocellular particles

The marriage of vegetable or animal materials and mineral binders is by no means a recent phenomenon. There are many vestiges in the past that bear witness to the durability of this type of mixture. The centuries, or even millennia, offer us an observation: it is possible to locally create construction materials that will stand the test of time.

Figure 1.3. Ksar d'Aït Ben Haddou, Morocco, 13th Century. A constructive mix combining stones, earth and lignocellular vegetable matter

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Currently, nearly 60% of dwellings in the world are built of earth or a mixture and earth and plant matter [HEG 10]. Such is the case of the traditional Berber habitat, built of rammed earth, in southern Morocco (see Figure 1.3).

In this particular domain, countries’ industrialization has caused locally-created materials to be gradually replaced with industrial materials. Yet, if we look at the goal set by the plan bâtiment of 400,000 renovations a year, two million tons of straw a year would serve this purpose, which is around 4% of France’s annual production [AMI 09].

1.4.1. Development of agro-concretes in the context of France

It is certain that with the now-omnipresent “sustainable development”, the use of so-called renewable materials (if they are managed correctly) and local materials presents a growing advantage in the world of construction materials, in France [ALC 07] and in the rest of the world [OEC 04]. Around the main markets generated by a cereal or petroleum culture, there are a great many secondary markets springing up, which will facilitate as complete a value creation as possible. Such is the case, in particular, for hemp [BOU 06], for which the areas of the market are as varied as the automobile industry, for the fibers, foodstuffs for the grain or indeed the wood of the stem (known as shiv) for construction. The quantities and the sources available are abundant [FRD 11]. Hence, vegetable biomass has a bright future.

1.4.1.1. Environmental and socio-economic issues

In the area of construction, France finds itself facing a fairly complex problem. For decades, in response to the poorly-constructed buildings of the post-war period [AMI 09], architects have congregated around a new design of a building: bioclimatic design. They were the first to reuse local materials to erect their buildings [PEU 08]. This new school of thought became progressively more popular in the circles of ecologists, and gave rise to numerous associations of selfconstructors and a new generation of artisans [LAU 07]. For them, the use of eco-materials coupled with an eco-construction approach to building must, first and foremost, be a factor in local development and social links [AMI 10]. Thus, when these topics are taken up by industrial and scientific actors, they are understandably uncertain.

“Eco-materials are usually produced from local resources, employing a local workforce, mobilizing local skills and savoir-faire, integrating themselves into the local art of construction and stimulating an economy that protects workers’ social rights and redistributes the wealth that it creates. This approach flies in the face of industrial production of standardized materials for standard homes, which form a uniform landscape which does not adapt to regional architectural or climatic peculiarities” [AMI 09].

1.4.1.2. The pitfall of novelty: technical opinions

While this approach is possible in certain European countries – particularly the Scandinavian countries, where the certification system is more favorable [PRE 07] – it remains complicated in France. The conventional construction materials, such as concrete or brick, have clearly-defined techniques for their use which are set down in Documents Techniques Unifiés (DTUs – Unified Technical Documents), which often fit in well with a normative framework. Thus, it is easy to commercialize a product whose technical framework for use is the topic of one or more DTUs.

In order to have a hope of being used, innovative construction materials, such as lignocellular concretes, must be subjected to a more complex certification approach. This is the condition sine qua non for master craftsmen and artisans, who are subject to a review every ten years, to be able to safeguard their work. Therefore, from the very start, manufacturers and suppliers of innovative materials are constrained to provide guarantees in terms of performance and implementation. In the case of France, it is only after obtaining a technical assessment about a construction product, delivered by the CSTB and renewable every five years, that insurers will usually agree to a ten-year cover. It is clear that without this insurability, project managers cannot run the risk of using an innovative material. In the case of a product or system of construction whose goal is to satisfy a local demand, the financial burden of having a technical assessment carried out often proves too great to bear.

In this particular context, the role of associations such as Construire en Chanvre (Build with Hemp) is very advantageous, because they lend credibility to the methods by bringing together all the actors, from the grower to the researcher. As has been the case for hemp construction methods, the work of these groups may lead to the implementation of professional rules of execution which constitute an intermediary step before the DTU.

1.4.1.3. Training professionals and offering incentives to the general public

Beyond the issues of the ten-year guarantee and the securing of technical appraisals for innovative products, there is a lack of training for professional building contractors, who continue to employ conventional solutions [LAU 07]. One of the major problems for an artisan lies in the seasonal nature of plant-based concretes, which cannot be put in place in wintry conditions. Hence, year-round outdoor use requires industrial molding which enables the concretes to be dried beforehand. This type of molding could also be a significant limiting factor as regards the variability of the finished products generated by the plant-based primary material in combination with manual installation. Also, the use of plant matter from agriculture and whose yields vary from year to year is a further source of variability in terms of the users’ supply. This is even more so in the absence of a specific process in charge of creating the distribution network.

Often, the development of such processes, at one time or another, requires the provision of help to private actors in order to get the market off the ground by financially guiding people’s choices. Such assistance may be provided by the state itself or by local collectives at all levels (region, district, commune, etc.) [LAU 07]. The use of eco-materials may carry with it fiscal incentives such as tax credits, much like the purchase of any other insulating material [DGF 09]. Depending on the intended application, the material must satisfy the set specifications in terms of thermal conductive resistance (i.e. insulation).12 We shall see later on (Chapter 2) that a lignocellular concrete, such as hemp concrete, possesses intrinsic properties which cannot be boiled down to its thermal performances alone. Thus, these dispositions do not favor the use of a material like hemp concrete.

It is also clear that the lack of accumulated scientific knowledge about this type of composite materials is a deterrent for decision-makers, project managers and master craftsmen. The most critical point relates to their durability and the sustainability of the procedures for their implementation. Although archaeological observations stand in evidence of the durability of earth/organic mixtures, such as wattle and daub, (see section 1.2.1), they merely attest to their potential in terms of durability.

1.5. What does the term “Agro-concrete” mean?

1.5.1. General definition

A concrete in the conventional sense of the word consists of a heterogeneous mix between a mineral binder and aggregates (also mineral in origin) of graduated dimensions. Similarly, that which we define as agro-concrete will therefore consist of:

“A mix between aggregates from lignocellular plant matter coming directly or indirectly from agriculture or forestry, which form the bulk of the volume, and a mineral binder”.

This definition will not cover mixtures including:

– a low proportion of lignocellular aggregates;
– lignocellular plant fibers to reinforce conventional concrete.

Indeed, many projects aim to create construction materials using one or more forms of lignocellular matter as a reinforcement to the structure rather than as a lightweight aggregate with an insulating purpose. The materials used are generally fibers which serve to improve the traction resistance, ductility and post-fracture behavior of composite concretes made in this way. The scientific study of fiber-reinforced concrete (FRC) created from mineral or synthetic fibers began at the start of the 20th Century [BRA 08]. More recently, projects have been carried out to enhance the value of organic fibers to substitute industrial fibers. They are drawn from various sources, such as wood [COU 05; TON 10], coconut [GHA 99], sisal [LI 00; TOL 03], palm [KRI 05], bamboo [SUD 06], bagasse [AGG 95; BIL 08] or indeed diss [MER 07]. It is interesting to note that countries such as Brazil which have an exceptional range of flora have a wide range of fibers to experiment with, and research in this domain is very active [SAV 00; AGO 05].

1.5.2. Lignocellular resources

Many lignocellular substances have been the subject of research, with the aim of integrating them with mineral binders as a lightweight aggregate. Table 1.3 offers an overview, by country and by material, of the research carried out hitherto on what has been defined as agro-concrete. The table only includes studies performed on lightweight aggregates, which yield concretes with a dry density of less than 1000 kg/m3. It is interesting to note that France, where agriculture and forestry are prevalent, finances research on numerous resources such as hemp, flax, wood, sunflower or beetroot. For concretes reinforced with natural fibers, the first scientific studies date from the turn of the century [ARN 00; BOU 98], although experiments had been carried out previously. The trajectory followed by the development of these materials is said to be bottom-up, i.e. from the building site to the laboratory.

Table 1.3. Overview of research into materials mixing mineral binders and lignocellular products for the making of lightweight concretes

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All these resources have a common point: they are either co-products or byproducts, or industrial waste. This is not an exhaustive list, because industrial projects are conducted with other plant matter such as miscanthus or wheat straw [FRD 11]. These materials, readily and cheaply available, therefore logically hold a growing interest for many uses – particularly for creating agro-concretes. Their increasing value also facilitates a reduction of the environmental impacts as opposed to traditional building insulation systems. Indeed, these materials are renewable, biodegradable, neutral in terms of GHG emissions and require little energy to be transformed [BAL 05]. However, not all of them can be used, and it is necessary to define a set of specifications to guide their selection. A recent study carried out jointly between ADEME and FRD13 makes the point about sources and worldwide availability of plant fibers [FRD 11].

1.5.3. General characteristics of lignocellular agro-resources

1.5.3.1. Chemical composition

The resources chosen as a matter of preference to create agro-concretes are said to be lignocellular. The etymology of the word reflects their composition, primarily, of cellulose and lignin, which are the two most common compounds in plant biomass (≈ 70%). Two other major molecular compounds are to be found in the stems of these plants: hemicelluloses and pectins. All these substances are made up of organic macromolecular chains which constitute polysaccharides. A few minor elements such as waxes and proteins are also present.

Cellulose is a polymer of glucose which is one of the main components of the plant cell wall. This biopolymer is responsible for most of the mechanical resistance in plants which have no secondary tissues. Its organization, which is mainly crystalline, renders cellulose insoluble in most solvents, and particularly water, although the compound is highly hydrophilic.

Lignins manifest themselves in the form of three-dimensional polymers. Their complex structure varies from one species to another, but so too do morphological elements (fibers, vessels, etc.). They lend rigidity and impermeability to plants containing them, as lignins are highly hydrophobic compounds. Finally, they are involved in the cohesion of the fibers in the lignocellular woody parts of the xylem and provide them with significant compression resistance.

Hemicelluloses are shorter-chain polysaccharides than cellulose, with an amorphous structure. They are hydrophilic, and notably they are able to swell when they come into contact with water. It is this swelling that means wood cannot be relied upon to retain the same size. In addition, hemicelluloses are water-soluble and can be removed from the wall, notably by soaking in an alkaline substance.

Pectins are acidic polysaccharides. They are present in large numbers in the middle lamella, where they hold adjoining cells together. Similarly to hemicelluloses, pectin is a water-soluble compound. It should also be noted that the composition and structure of the hemicelluloses and pectins varies from one plant species to another, and therefore is very complex to apprehend fully.

1.5.3.2. Structural organization

Lignocellular plants can be described at three levels: macroscopic, microscopic and molecular. In the case of stems or stalks, which represent the greatest potential for use in agro-concretes, the macroscopic structure can be generally characterized as shown in Figure 1.4. The stems of lignocellular plants may be of two types: monocotyledons or dicotyledons. In the case of perennial plants such as those used for straw, these two types of stems have the same composition but are organized differently. It is only at the start of the plant’s second year that secondary tissues begin to be formed, constituting the age rings [WIE 05].

Figure 1.4. Planes of a cross-section characteristic of lignocellular stems [RAV 70]

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Depending on the resource available and the co-valuations, the stem may be valued either partly or entirely for the making of agro-concretes. The use of the stem in its entirety, of course, increases the complexity the phenomena of interaction between the plant matter and any binder used. Indeed, the chemical composition of the different plant structures is subject to variation. In fact, the carminot-green de Mirande coloration on the cross-section shown in Figure 1.5 clearly distinguishes the zones with a high lignin content, with the other areas being richer in cellulose [GUY 10].

Figure 1.5. Transversal cross-section of a young dicot stem (flax) colored with carminot-vert de Mirande and detail on the associated structures [GUY 10]

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On the microscopic scale, it is interesting to note the multiple porosity that characterizes these plants, the reason for which is to cater for the plant’s physiological needs. For instance, the xylem and the phloem serve respectively for the circulation of raw sap and the elaborated sap [RAV 70].

A three-dimensional representation of the xylem of soft woods is given by River et al. [RIV 91] (see Figure 1.6). It stresses the organization into cells, mainly oriented in a longitudinal direction. Some cells do not have an end wall, and may constitute veritable tubes or tracheids that can transport sap (Figure 1.6). The lateral walls are dotted with punctuations: openings integrated into the walls, whose role is to transfer fluids between the different cells.

Figure 1.6. Three-dimensional representation of the structure of a soft wood [RIV 91]

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1.5.3.3. Cellular organization

On the cellular scale, lignocellular plants are made up of long cells with complex organization, with the exception of cellular marrow with a beehive structure (see Figure 1.7). The middle lamella is made mainly of pectins which play the part of binders between the cells. The primary walls are made of cellulose, surrounded by microfibrils of hemicelluloses, and are linked by a web of pectins and proteins [SED 07]. The secondary walls have the same constituents, but also contain lignin, which intervenes and blocks cellular expansion. This process is carried out in the case of trees when a new layer of soft wood is laid down to replace the previous one in the spring. However, in annual lignocellular plants, the secondary wall does not have time to form. Hence, the role of the cell is to channel nourishing fluids (sap, water, etc.) through a conduit: the lumen.

Figure 1.7. Structure of the plant cells that form annual lignocellular plants [WIE 05]

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The different structure scales of plant matter described (see Table 1.4) are all parameters which need to be taken into account when making a technical choice [GAR 08]. The size of the structures, their organization and their molecular make-up will indeed exert an influence on the behavior of the plant in the alkaline milieu formed by the hydrated mineral binder. These parameters will be detailed more completely and comparatively between the plants used during the discussion later on in this book.

Table 1.4. Characteristic dimensions of the structures of stems of lignocellular plants

Type of structure Characteristic dimension Source
Diameter of pores in the primary wall 1.7–2 nm [GAR 08]
Diameter of an open pore 200 nm [GAR 08]
Diameter of the lumen of the tracheids 4–25 μm [GAR 08]

1.6. Conclusions

The objectives are unambiguous, but there are genuine economic and scientific hurdles to overcome before we have at our disposal new products – often composites – that satisfy the same technical criteria but have enhanced qualities in terms of the environment, sanitation and comfort [ESC 06]. It is therefore necessary to create certifications and labeling to incite manufacturers to further develop their offering in terms of eco-materials, and thereby encourage construction professionals to use them. Undoubtedly one of the most promising avenues relates to lightweight concretes that combine mineral binders and plant particles.

1.7. Bibliography

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[CGD 10] COMMISSARIAT GÉNÉRAL AU DÉVELOPPEMENT DURABLE, Chiffres clés du climat, Ministère de l'Ecologie, de l'Energie, du Développement durable et de la Mer, 2010.

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