Inorganic hemp and flax shiv composites

Section 1. General information

The term "bio-based material" means material of plant or animal origin, which includes insulation hemp, flax, cotton, wood fibre or cork on the one hand, sheep's wool or duck feathers other. However, these insulation materials include in their composition from 7 to 20% of synthetic binder (e.g. polyester) to ensure their mechanical strength. The blocks of hemp and flax consist for their 100% shives and hydraulic lime, without other additions, which make it a material composed entirely of natural elements, the indisputable environmental qualities [1]. Mixed with a binder lime, hemp and flax shives can make mortars and concretes with specific characteristics and performance. These mortars and concretes benefit, in particular, an open porosity, which confer thermal and energy performance meets expectations sometimes contradictory the construction and the renovation [2].

Section 2. Expertise on production technologies

Concrete hemp can be made ​​into a workshop for the prefabrication of wall elements (blocks or prefabricated), or sites for implementation by hand or machine for the production or renovation of floors, walls, insulation roofing and coatings. [2]

HEMPCRETE BLOCKS

The manufacture of hemp block is carried out according a very specific method of cold-molding, followed by drying in the open air. Compared to other insulation materials, at equal thermal resistance, a block of hemp requires 4 times less energy in its production than terracotta brick and 3 times less than the aerated concrete. This consumption is estimated at 2 MJ / m².year for a lifetime of 100 years. This information is derived from the analysis of the life cycle of Hemp from INIES database. [1]

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Figure. Hempcrete blocks. 

 IMPLEMENTATION ON SITES

The on-site implementation of mortars and concretes of hemp can be done manually for the various applications. The manual application is however tedious and expensive as soon as the volume become important. In this case one can use machines adapted for the projection. Indeed, mortars and concretes of hemp are little fluid and not easily transportable with the tools normally used with concrete. Pneumatic transport of the dry mix of hemp aggregate and binder is therefore used, the water being provided to the spray outlet of the transport pipe. Concrete is projected on a temporary or lost framework (construction) or an existing wall (renovation). The implementation must be conducted in accordance with the Rules of Professional Hemp Construction and although the implementation does not pose particular difficulties, the Professional Rules require applicators raise their skills, which can be acquired by an appropriate training. [2] 

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Figure. Application of hempcrete. 

Section 3.  Examples of use

1)      outdoor and indoor insulation

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Figure. Installation of timber framed 'Chanvribloc' wall. 

 

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Figure. Interior side of the wall constructed with 'Chanvribloc'.  

2)      partition wall / self-supporting wall insulation 

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Section 4. Product characteristics – General Properties

Hempcrete blocks are good thermal and acoustic insulation materials. Table 1 presents characteristics of four types of 'Chanvribloc' [1].

Table. Properties of different types of hempcrete blocks.

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Hempcrete blocks generally have, depending on the ratio binder / hemp shives, a fire classification B, s1, d0 according Euroclasses system: they don’t spread the flames, they keep their mechanical properties and they give off little smoke during their combustion (CREPIM sources). To improve the fire reaction of this kind of materials, only the ratio binder / hemp & flax shives must optimized. Beyond their thermal conductivity or their inertia which can meet the performance requirements of construction, mortars and concretes have a triple hemp porosity: that of shives, the binders based on lime and the voids remaining between aggregates [2]. This triple connected porosity, allowing the material to stock and transport water vapor and water causes, thanks to the transfer and hygrothermal changes in the water phase, allows the regulation of temperature variations and moisture walls and therefore buildings. This hygrothermal regulation provides users of hemp concrete a sense of comfort and involve a significant reduction in energy requirements.

 

Section 5. Product characteristics – Durability

The hygrothermal behaviour of a given material is strongly related to its and to its porosity. Consequently, all materials don’t have the same characteristics in regards to hygrometric, hydric and thermal variations. The physics linked to the transfers of water vapour and liquid water within a given material are indeed complex. However, it is important to adequately characterize the behaviour of a material when confronted to these events, in order to optimize the material's use. The goal of this technical document from C&B[3] is to present an inventory of the publications concerned with the hygrothermal behaviour of bio-based construction products, as hemp concrete. 

Many studies are available concerning vegetable concrete. There is a lot of documentation and research around hemp concrete and hygrothermal behaviour. A few research teams have studied these materials: the ENTPE, Rennes 1 University, Bath University in England, Louvain University in Belgium and Reims University.

In 2005, Arnaud L. et al [4] [5] demonstrated the phenomena related to the evaporation-condensation of water in hemp concrete when subject to temperature gradients. When the temperature rose from 10°C to 40°C in the enclosure that matched outside climatic conditions, Arnaud L. et al. observed an increase in vapour pressure within the hemp concrete wall, illustrating the evaporation phenomenon. This evaporation phenomenon is accompanied by heat absorption, which leads to a reduction of temperature variations within a wall.

In addition, COLLET F. et al. [6] showed, in the context of their work, that vapour diffusion occurred in a homogeneous way throughout the material's thickness, and that the wall's framework did not disrupt the wall's hygrothermal behaviour.
EVRARD A. and DE HERDE A. [7] did a simulation with the WUFI software to study the hygrothermal responses of hempcrete wall subject to temperature variations. The results demonstrate a phase shift of approximately 15 hours and high thermal damping. The results show that the hemp concrete walls present a better “moisture buffer” capacities compared to cellular concrete, blocks, clay bricks and extruded polystyrene, mineral wools. We can link this characteristic to thermal aspects, since the phase change of water is accompanied by a release or absorption of energy. EVRARD A. and DE HERDE A. calculated that hemp concrete walls could release energy  (43kWh/m²) during night time.
 
TRAN LE A.D. et al [8] simulated the hygrothermal behaviour of a building made with hemp concrete. The results show that hemp concrete’s humidity absorption capacity moderates the humidity variations inside de room. TRAN LE A.D et al wanted to compare the hygrothermal behaviour of a hemp concrete unit to that of a unit made in cellular concrete. It is clear that the hemp concrete unit has a better moisture buffer value than the cellular concrete unit. Similarly, the researchers calculated that the hemp concrete unit consumed less energy than the cellular concrete unit.

In order to make better use of bio-based materials and to develop the entire industry, it is now necessary to quantify phase change in the material and to make links between phase changes within materials, hygrothermal comfort in a building and energy consumption.[3]

 

Section 6. Research activities

A lot of products with hemp concrete are available on the market at this moment. Their growth is regularly. But inorganic flax shives composites are unsung. Several university laboratories & industrial company work on the manufacturing flaxcrete for a building application. The goal is to compare these two materials (Compressive strength, Thermal conductivity…).

Mr. LANGLET Thierry  from “Laboratoire des Technologie Innovantes of Amiens University” has already worked on this theme.[9]The objective of this work is to present the thermophysical and mechanical properties of two types of concrete: Hemp and Flax concretes. This article highlights the benefits of using flax shives as aggregate for the formulation of biobased concrete. The compressive strength of flax and hemp concrete does not allow their use as structural elements. Both concretes fall into the category of filler materials.

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Figure. Hempcrete block during a compression test.  

The mechanical behavior of hemp and flax concrete is similar even if the flax concrete is almost twice as strong. Mechanical compression testing showed that the maximum stress for a flax concrete was about 1.09 MPa and for a hempcrete about 0.62 MPa. This observation can be explained by the high binder content used in its manufacture.

Furthermore, the high binder content reduces the insulating capacity of the material that is checked by the thermal conductivity of the flax concrete which is 40% greater than hemp concrete. For a hemp concrete with a density of 480 kg/m3, the thermal conductivity is assessed to 0.125 W/m.K at 20.5 ° C. For flax concrete with a density of 600 kg/m3, the thermal conductivity was estimated at 0.169 W/m.K at 20.5 ° C.

A decrease in the amount of binder will probably cause a decrease of the mechanical strength and the heat resistance of flax concrete will be rapidly optimized. 

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Figure. Compression test with cylindrical samples. 


Section 7.Case studies

A measurement campaign was performed in a hemp concrete house from February to October 2012 to assess the hygrothermal behavior of this kind of material in real conditions. This study was conducted by the CETE South West10] on a new house on two levels in Périgueux in France. 

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The previous researches stop either on the assessment of physical properties or on the performances evaluation of a hygrothermal wall. These results are not sufficient to accurately predict the comfort of home and the hygrothermal performance.

Thermography shows a homogenous temperature across the various walls of the house and the insulation quality of the material with a thickness of 30cm (hemp shotcrete). The measurement of air permeability shows leaks that may occur at peripheral walls between the different links due to the phenomenon of drying posts. Such defects need to be treated to maintain a good level of airtightness of the casing.

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The level of thermal comfort in the house is satisfied with homogeneous air temperatures between rooms. They vary in the range 20 to 24°C in winter and between 21 and 28°C in summer.

Monitoring both North and West walls helped to better understand the hygrothermal behavior of hemp concrete. This shows remarkable thermal performances; damping temperature of the order of 95% with a phase shift by more than 10 hours between the outer surface and the heart of the wall which tally with 15 cm of hemp concrete. The walls show a good water management with a stable relative humidity but which fluctuates around 80% in the heart of the material. 

Section 8. LCA

A Life Cycle Analysis (LCA) of bio-based concrete was conducted in 2012 by Codem Picardie under the direction of B. DUPRE.[11]Preliminary analyzes of hemp concrete showed a significant impact of the binder on all life cycle of the product. The impact of plant matter may seem little influence in the analysis of the complete system. However, the use of plant resources may result in changes in the quantities of all the components to be introduced, in particular the binder. This study compares bio-based concrete, made ​​from aerial lime and two sources of aggregates plant: hemp and oilseed straw. The life cycle analyzes were performed on products with the same performances (mechanical and thermal).

First, the results of this study highlight that the role of cultivation process and aggregate extraction plant isn’t determining on the results of the life cycle analysis of bio-based products like filling concrete. The major part of the environmental impact is due to the use of the inorganic binder. Although it is far less impacting other hydraulic products, it seems necessary to make choices in the formulation to reduce its proportions of this kind of materials, while ensuring the properties of use.

This has been clearly identified by the eco-design implementation in the work of R&D and therefore formulations. Thus, this study explicitly shows that depending on the natural fibers selected, environmental impacts can be greatly reduced.

We can notice that the comparison of the environmental footprint of 1 ton of hemp and 1 ton of oilseed straw is largely favorable to hemp due to a less greedy culture in fertilizers and pesticides and a lower mechanical intervention on the culture.

The environmental improvement of formulations of bio-based concrete should be more favorable consequences to their development, and in particular, a cost reduction because the hydraulic binder is the most present component of the point of view of weight.

Section 9.      References

[1] CHANVRIBLOC Book : “Le bloc de chanvre pour l’isolation et la construction”

[2] Construire en Chanvre website, Mortars and Concretes

[3] Current State of Knowledge on the Hygrothermal Behaviour of Bio-based Materials, Technical Document C&B, February 2013. (http://www.constructions-bioressources.org/wp-content/uploads/2013/05/Download-English-Version.pdf)

[4] L. Arnaud, D. Samri (2005). Hygro-thermal behaviour of building porous materials, in Proc. Third Int. Biot Conference, Oklahoma (USA)

[5] D. Samri, L. Arnaud (2006). Assessment of heat and mass transfers in building porous materials., Proceedings of the 4th European Conference on Energy Performance & Indoor Climate in Buildings, November 2006, Lyon.

[6] F. Collet, S. Prétot, C. Lanos.. Etude expérimentale du comportement hygrothermique d’une paroi en blocs de beton de chanvre avec ossature en bois. 29ème rencontres universitaires de génie civil. (2011). (pp 258-267)

[7] A. Evrard, A. De Herde. Hygrothermal Performance of Lime–Hemp Wall Assemblies. Journal of Building Physics OnlineFirst. (November 26, 2009)

[8] A.D. Tran Le, C. Maalouf, T.H. Mai, E. Wurtz, F. Collet. Transient hygrothermal behaviour of a hemp concrete building envelope. Energy and Building 42. (2010). (pp 1797-1806).

[9] T. LANGLET, Caractérisations mécanique et thermique de bétons agro-sourcés, Laboratoire des Technologies Innovantes – IUT d’Amiens, Ecobat Sciences & Techniques 2013.

[10] D. SAMRI, Etude du comportement thermique d’une maison en béton de chanvre, CETE du Sud Ouest, Ecobat Sciences & Techniques 2013.

[11] B. DUPRE, Agros-Betons et Analyse de Cycle de Vie (ACV) : le poids de la formulation en fonction des types de granulats utilisés, Codem Picardie, Ecobat Sciences & Techniques 2013.