biosystems engineering 103 (2009) 474–479

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/issn/15375110

Research Paper: SEdStructures and Environment

Mechanical properties of lime–hemp concrete containing shives and fibres Paulien Brigitte de Bruijna,*, Knut-Ha˚kan Jeppssona, Kenneth Sandinb, Christer Nilssona a

Department of Rural Buildings, Swedish University of Agricultural Sciences, Sundsvaegen 3, 23053 Alnarp, Sweden Division of Building Materials, Lund Institute of Technology, Lund University, Lund, Sweden

b

article info The effect of using different binding agents in combination with hemp shives and fibres in Article history:

Lime–Hemp Concrete (LHC) building material was examined. LHC is a light composite

Received 3 July 2008

building material with building lime as binding agents and hemp (Cannabis sativa) as

Received in revised form

a renewable raw material from agriculture. Contemporary LHC only uses the woody core

2 February 2009

part of the hemp, the shive. However, using both hemp shives and fibres may improve the

Accepted 11 February 2009

mechanical strength, eliminating the need for a fibre separation process.

Published online 3 July 2009

The aim was to elucidate the feasibility of using the entire fragmented hemp stalk in an LHC, and to determine some important material properties such as compressive strength, splitting tensile strength, water sorption and frost resistance. LHC with varying inclusions of the lime-based binders were tested, as were five mixes using the binding agents hydrated lime, hydraulic lime, and cement. Specimens were cured for 12 weeks at room temperature and 40 days in a carbonation room (4.5 vol% CO2), and tested for mechanical properties, water sorption and frost resistance. Using both shives and fibres in LHC may be advantageous for countries such as Sweden where facilities for separating hemp from shives are not commercially available. ª 2009 IAgrE. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

To build sustainable and affordable housing for the future it is advantageous to create links between agriculture and the construction industry. Sustainability can only be possible when construction uses renewable materials or materials recycled from construction wastes (Peris Mora, 2007). The use of the renewable material hemp (Cannabis sativa) in combination with the binding agents hydrated lime, hydraulic lime and cement as a building material was examined in the present paper. Cultivation of hemp provides a fast-growing annual crop that can reach a height of 1.5–4 m in northern Europe, with hemp

grown in Sweden measuring between 1 and 2 m in height (Osvald, 1959). The hemp fibres are situated in the bast of the hemp plant, and have a high tensile strength. Inside the stem of the hemp plant is its woody core, the shive. In the past, hemp was used in Sweden to produce ropes, sails and other textiles. However, growing industrial hemp was prohibited in Sweden in 1965 but it has since been legalised, in 2003 (Holstmark, 2006). In the five years that hemp has legally been grown in Sweden, a viable commercial market has yet to be established. The majority of the hemp cultivated in Sweden is used as biomass fuel, where the hemp is pressed into briquettes and combusted for energy purposes. The remainder of the hemp grown is used for trials and research projects (Rolandsson, personal

* Corresponding author. E-mail addresses: [email protected] (P.B. de Bruijn), [email protected] (K. Sandin). 1537-5110/$ – see front matter ª 2009 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2009.02.005

biosystems engineering 103 (2009) 474–479

Nomenclature A Aw a d E F Fmax

cross-section area of the material, m2 1 water sorption coefficient, kg m2 s ⁄2 2 specimen surface area, m diameter, m Young’s modulus, Pa force, N maximum force, N

communication, 2008). Using hemp in building materials could create a more viable market for hemp in Sweden. Since the legalisation of hemp cultivation in 2003, the total cultivated area has increased from 24 ha in 2003 to 792 ha in 2007. However, in 2008 it decreased to only 389 ha, probably due to the lack of products and commercial possibilities (Rolandsson, personal communication, 2008). Therefore, new products and possible uses must be found for hemp grown in Sweden. To date, little has been reported about the mechanical properties of building materials containing hemp grown in Sweden. Hemp fibres are used, e.g. in pulp and paper, insulation material, and bio-composites for automotive parts. The most important use of hemp shives is as horse bedding (Karus, 2005). Cultivated hemp contains more shive material than fibres. Karus (2005) noted that the benefit of selling hemp shives is important for the overall economics of hemp utilisation. The process for separating fibres from shives is complex and costly, and requires a viable market for hemp fibres. Since no such commercial fibre separation facility currently exists in Sweden (Rolandsson, personal communication, 2008), it is of interest to identify possible applications for hemp material consisting of both the fibres and the shives. It should be noted, that because of the shredding process, some dust is present in the hemp material. According to Li et al. (2006), hemp fibre is a good reinforcement material because it has a high tensile strength and it has a strong tolerance to alkaline environments. Due to the low density and high porosity of the hemp shives, the combination of hemp and a cementitious binder creates a building material with mechanical, thermal and acoustic properties that differ from those of conventional concrete. It has a lower density, a lower thermal conductivity, and better acoustic insulation properties, and thus it could be advantageous to use in construction (Evrard, 2003; Cerezo, 2005; O’Dowd and Quinn, 2005; Arnaud et al., 2006). Toleˆdo Filho et al. (2003) emphasised the importance of the use of vegetable fibres in concretes, especially in nonindustrialised countries. They point out that vegetable fibres are cheap and readily available, require only a low degree of industrialisation for their processing and a low amount of energy for their production, and thus costs are low. Hemp shives, hydraulic lime, hydrated lime, water and admixtures constitute a building material called Lime–Hemp Concrete (LHC) that can be used for walls, etc. in combination with a load-bearing wooden framework. Depending on the composition, it can also be used in floors and roofs (Arnaud and Cerezo, 2001; Evrard, 2003). Today, several companies in Europe offer a variety of hemp and lime products that can be used to produce an LHC. A well-functioning wall must be load

l m T t 3 r s smax

475

length, m mass, kg splitting tensile strength, Pa time, s strain material density, kg m3 stress, Pa maximum stress, Pa

bearing, but it should also fulfil the demands for thermal insulation, sound insulation, air tightness, fire protection and moisture protection. Conventional walls of brick or wood are built up of several layers, where each layer fulfils one or more of the above-mentioned functions. Apart from a load-bearing wooden framework and a lime rendering, LHC does not require any other additional building materials to create a well-functioning wall (Cerezo, 2005). In modern LHC, hydrated lime is used in combination with hydraulic lime and some cement. Hydrated lime is formed when water is added to quicklime: CaO þ H2 O/CaðOHÞ2 þCO2 Carbonation occurs when calcium hydroxide (lime) in the binder reacts with carbon dioxide from the air and forms insoluble calcium carbonate. CaðOHÞ2 þCO2 /CaCO3 þ H2 O This is a very slow process, and causes the pH value of the material to decrease. Evrard (2003) studied four different LHC mixtures and recorded compressive strengths of 0.2–0.5 MPa and Young’s modulus values of 3–26 MPa. Arnaud et al. (2006) found compressive strength values, depending on the mixture, ranging from 0.4 to 1.2 MPa and Young’s modulus values of 40–90 MPa. Data from other studies show similar values (Arnaud and Cerezo, 2001; Cerezo, 2005). However, the compressive strength of these LHCs is not sufficient for the material to be load bearing. In the above-mentioned studies on LHC, only hemp shives were used. The main aim of the present study was to determine the feasibility of using LHC as a load-bearing material with the inclusion of both hemp shives and fibres. A further aim was to determine some important material properties of different LHC mixtures, using varying compositions of the lime-based binder. Different ratios of hydrated lime, hydraulic lime and cement were used to achieve an increase in compressive strength and to determine the influence of cement on the properties of LHC. Using not only the shives of hemp but also the fibres could increase the mechanical strength of LHC. It would also eliminate the need for a fibre separation process and it could create a market in Sweden for the entire fragmented hemp stem.

2.

Materials and methods

The hemp cultivar Futura 75 was acquired from a local farm in the province of Scania, Sweden. It was sown on 21 April 2005

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biosystems engineering 103 (2009) 474–479

and allowed to dry before it was harvested on 15 April 2006. The entire hemp plant was harvested, baled and stored. The hemp bales were processed in an industrial shredder where the shives, fibres and dust were not separated but used as the hemp material for this research. The composition of the hemp material by weight was 35% fibres, 62% shives and 4% dust (