Hemp Concrete Research Articles

A review of the properties of hemp concrete for green building applications

Abstract The growing concerns surrounding the rising carbon emissions have impelled the leaders around the world to make efforts to prevent catastrophic manifestations of climate change and global warming. This has led to the resurrection of vegetal concrete building materials using biomass, which have the added benefits of carbon sequestration apart from low embodied energy and renewability. Vegetal concretes are made up of an organic or inorganic binder, and biomass originating from agro-forestry industries such as rice husk, straw bale, hemp, kenaf, cork, and so on. Hemp concrete, a variety of vegetal concrete has been widely researched and is arguably one of the most researched building materials in current times. This paper presents a review of the state-of-the-art of hemp concrete research, with a view to identifying research gaps that shall guide future research for its implementation in the fast-growing green buildings industry. The reviewed aspects of hemp concrete include properties of hemp relevant to construction, binder characteristics, mechanical properties, durability, hygric and thermal properties, environmental credentials, manufacturing processes, and current applications. Several research gaps with regards to the hydraulicity of the binder, strength and durability, and fire resistance of hemp concrete were identified. It was also established that hemp concrete has very low embodied carbon and embodied energy, making it ideal for green building applications. The paper ends with a discussion outlining the need and direction for future research on improving the manufacturing processes and mechanical performance of hemp concrete for wider adoption by the construction industry.

Influence of hydric solicitations on the morphological behavior of hemp concrete

The use of bio-based materials such as hemp concrete in the field of construction allows limiting environmental impacts and improving the energy performances of buildings. The aim of this paper is to understand the influence of adsorption and desorption of moisture in hemp concrete on its internal morphology and its dimensional variations. That’s why, the high porosity and the adsorption capacity of hemp concrete were discussed. Then, an experimental cell was developed to follow the geometric evolution over time of hemp concrete microstructure under hydric solicitations: humidification and drying. The digital image correlation was used to determine the strains fields on the surface of the material. This technique showed the behavior of this hygroscopic material subjected to different hygrometries. Indeed, the hemp shiv undergoes larger strains than the binder, thus affecting the morphology of hemp concrete. The results obtained highlighted the influence of the hydric state of hemp concrete on its very heterogeneous microstructure. It has also been revealed that the durability of the material can be affected by the dimensional variations caused by the relative humidity variations.

Thermomechanical Characterization Of Particleboards From Powder Typha Leaves

New composite boards with low-thermal conductivity produced from a mixture of powder Typha leaves and a binder as gum arabic have been developed. The goal of this paper was to investigate the effect of binder content on compressive strength and thermal properties of typha panels. The results showed that the panels with a binder content of 33.33–50 % had the thermal conductivity values ranging from 0.055 to 0.083 W.m-1.K-1, which was close to that of many natural insulating materials. The compressive strength values obtained were comparable to those of the lime – hemp concrete. This study showed that typha possesses interesting capacities of insulating and its combination with gum arabic contributes to the improvement of thermal comfort, to the reduction of the energy consumption as well as to the emissions of CO2.

Water Transport in Bio-based Porous Materials: A Model of Local Kinetics of Sorption—Application to Three Hemp Concretes

The classic models describing the hygric mass transfers inside porous materials seem unsuitable in the case of bio-based materials. They are based on the assumption of instantaneous local equilibrium between relative humidity and water content (Kunzel in Simultaneous heat and moisture transport in building components—one- and two-dimensional calculation using simple parameters, Fraunhofer IRB Verlag, Stuttgart, 1995). These two parameters evolve according to the diffusive fluxes following the sorption isotherms. This study shows that it leads to predicting much shorter times of stabilization than those experimentally obtained. A new approach is presented here; it is free from the local instantaneous equilibrium introducing a local kinetics to describe the transformation of water from vapor state to liquid state and vice versa. The local kinetics of sorption is coupled with the well-known hysteresis phenomenon. It is adjusted from bibliographic data (Collet et al. in Energy Build 62:294–303, 2013) giving mass evolution of three hemp concretes under adsorption/desorption conditions. 1D cylindrical simulations allow an excellent fitting on the experiments. Finally, a semiempirical model is proposed, allowing to determine the kinetics parameters more easily.

Assessing the impact of calculation methods on the variability of Young’s modulus for hemp concrete material

Parameters influencing the variability of Young’s modulus for hemp concrete include, among others, the method by which it is calculated. At present, a range of methods are available in the literature, but as yet, little is known about how they might affect the obtained results, given that hemp concrete is a highly sensitive material. For the in-depth study of this issue presented herein, the Young’s modulus properties of two formulations of hemp concrete are analysed. Three main types of modulus – apparent, elastic and hardening modulus – are determined, using different calculation methods: floating, tangential, cyclic and hardening with 5 strategies. Therefore, for a stress–strain curve with 3 loading/reloading cycles, a total of 38 modulus values are calculated. Using the aforementioned methods, a comparative study of the results at 60, 90 and 180 days on 5 specimens for each formulation and each age reveals that the cyclic modulus method gives low values and the tangential modulus method gives medium values, while the floating modulus method gives higher values. In this latter case, low dispersion is observed, making this method more suitable to calculate hemp concrete’s modulus.

Lateral load-carrying capacity of hemp concrete as a natural infill material in timber frame walls

This study focuses on the racking behaviour of hemp concrete as infill material in timber frame walls, investigating and highlighting the parameters that significantly affect the lateral resistance of hemp walls. For this purpose, two different designs of timber frame walls are used as wall units, the first consisting of vertical studs and the second using diagonal bracing elements under compression. These two designs of timber frames are those most widely used as external walls in construction. In this research, an experimental investigation of large-scale timber frame walls with dimensions (2.5 m height and 1.25 m length), both with and without hemp concrete, was carried out to assess the contribution of hemp concrete as infill material against lateral loads. The hemp walls were then compared to empty timber frames to determine the real contribution of hemp concrete to lateral strength. Digital Image Correlation (DIC) was used in order to determine and track local displacement and shear-strain fields of hemp concrete as infill material in the timber wall. Based on the results obtained from the experimental tests, it is obvious that hemp concrete makes a slight contribution against lateral loads in a vertical stud timber wall of length 1.25 m. However, hemp concrete did not improve the lateral strength in walls with diagonal bracing elements. This study confirms, firstly, that the length of timber wall plays a significant role in the lateral strength of the hemp concrete. In other words, decreasing the length of the wall will significantly decrease the contribution of hemp concrete to lateral resistance. Secondly, the use of diagonal elements increases the overall rigidity of the timber wall, which negatively affects the contribution of hemp concrete and prevents hemp concrete from increasing the lateral strength of the wall.

A novel anisotropic analytical model for effective thermal conductivity tensor of dry lime-hemp concrete with preferred spatial distributions

This article develops a novel mathematical model using multi-scale homogenization approach to model the effective thermal conductivity tensors of a hemp shiv particle and lime hemp concrete (LHC). This bio-based material is an environmentally-friendly material that is used more and more in building construction. The thermal conductivity of lime hemp concrete which is generally either cast or sprayed, cannot be expressed as a scalar due to its anisotropic microstructure. The paper develops a novel model to predicting effective thermal conductivity tensor of lime-hemp concrete that takes into account the anisotropy and the size of hemp shiv, the preferred spatial distributions due to fabrication process and the imperfect particle-binding interfaces. Two probability density functions are used for modeling the particle size distributions of hemp shiv and the preferred alignment distribution of hemp particles. A good agreement is obtained between the model and experimental data provided in the literature. The presented analytical solutions offer a suitable tool for a fast optimization of the thermal conductivity of hemp concrete. Such promising future is very useful for the building design, in particular in three-dimensional (3D) simulation models.

Modeling and experimental characterization of effective transverse thermal properties of hemp insulation concrete

Hemp concretes gain more and more interest in house building field because of their thermal properties. They are expected to be suitable and sustainable insulation materials. They are heterogeneous composite materials mainly produced with natural cement and hemp shivs. As insulation materials, their thermal properties characterization remains a challenge due to their high porosity which induces internal heat radiation and their low thermal conductivities which causes lateral heat loose in the experimental setups. This study presents an inexpensive and reliable experimental setup development, based on hot film method. It allows to determine the transverse conductivity of materials. Laplace integral transform and Green function solution methods have been used to determine analytic solutions of the heat conduction in the tested specimens. The thermal properties have been evaluated based on the asymptotic approximations of these solutions. Two binders noted “B” and “V” have been characterized by XRD and used to produce hemp concrete mixes. In addition, three shiv-to-binder ratios (S/B = 0.40, 0.50 and 0.67 by mass) have been considered in view of the thermal properties assessment. This variation of S/B ratio induces a variation of the densities and the air void contents of the concretes. The thermal properties have been evaluated after 90 days of curing at 20 °C and 50% RH. The results indicate that the thermal conductivity decreases by about 8% and 30% respectively when S/B increases from 0.40 to 0.5 and 0.67, whatever the binder. They also show that the mixes produced with lime based binder “B” (70% of natural lime, 30% of hydraulic lime and pozzoulane) have higher thermal conductivity than those produced with binder “V” (natural prompt cement and citric acid) by about 15%–20%. The specimens’ thermal properties have been characterized before and after drying process. The results show a decrease by about 9%–16% of the thermal conductivity after drying. A self-consistent model widely used in the literature has been used to predict the thermal conductivity of the equivalent homogeneous hemp concrete. The comparison between the experiment data and the self-consistent model shows a relatively good agreement.

Microscopic hydric characterization of hemp concrete by X-ray microtomography and digital volume correlation

Hemp concrete is a bio-based material used in buildings construction. It presents particularly interesting hygrothermal properties. This very heterogeneous material is constituted by hemp particles embedded in a mineral binder. This paper focuses on the microscopic and hygromorphic characterization of hemp concrete based on non-destructive imaging techniques. X-ray tomography scans with different voxel sizes were performed on this material. The reconstructed volumes had a mean voxel size between 2.3 µm and 31.8 µm. 3D reconstructions of the real material showed that hemp concrete has several scales of porosity which depends on the orientation and position of the plant particles. Then, a tomography scan and SEM observations were made on the vegetable particle. The latter presents a complex and very porous microstructure. The hygromorphic behavior of hemp concrete under hydric solicitations was then studied. The technique consists in using X-ray tomography coupled to the digital volume correlation (DVC) to determine the three-dimensional strain fields in the studied three-dimensional volume. The results show a change in the internal morphology of hemp concrete subjected to different hygrometries. The humidification of the material leads to the appearance of anisotropic strains in the 3D volume. The strains of the hemp particles are greater compared to those of binder. In addition, the swelling of the hemp shiv causes the reduction of the porous space between the binder and the plant aggregates called interparticle porosity.

Characterization of a precast hemp concrete. Part I: Physical and thermal properties

Hemp concrete is seen as a possible solution for addressing the energy, health and comfort issues of sustainable buildings, thanks to its hygrothermal properties and environmental qualities. This paper aims to determine the physical and thermal properties of an industrial prefabricated hemp concrete block based on hemp shiv and a lime-metakaolin binder. When available, distinct measurement methods were applied and confronted on this precast material in order to evaluate the impact of the experimental procedure on the results. Concerning air permeability, the present study shows that a measurement method initially intended for regular concrete can be adapted to the specific behaviour of highly permeable material such as hemp concrete. The thermal conductivity was measured with a guarded hot plate and a hot wire. Hot wire measurements on different moist states of the material allowed the self-consistent scheme to be applied for the evaluation of the thermal conductivity as a function of the humidity. Two direct measurement methods, differential scanning calorimetry (DSC) on the components and use of a Calvet calorimeter on the hemp concrete, and one indirect method, through effusivity measurement, were adopted for evaluating the specific heat capacity of the material. The original method proposed using DSC was shown to be as accurate as direct calorimetric measurement. The direct measurement methods gave similar results while the indirect one led to a much lower value. A comprehensive analysis of the measurement values available in the literature on hemp concretes confirmed this trend.This first paper highlights the significant influence of the testing method on the determination of the thermal properties of a precast hemp concrete, especially on the heat capacity measurement. This result emphasizes the need for a large inter-laboratory experimental campaign, applied to bio-aggregate based concrete, in order to form the basis of recommendations for adequate physical and thermal characterization methods for these bio-based building materials. A second part of this study will investigate the hydric properties of the same material.

Optimization of lime and clay-based hemp-concrete wall formulations for a successful lime rendering

Different pathologies may arise on a wall finish if this is applied on a hemp-based rammed wall that is not completely dry. By simulating uneven drying and early-rendering of load-bearing and non-load-bearing hemp concrete blocks (made with natural hydraulic lime only or a mix of lime with clay) and studying them by means of chromatic, microbiological, textural and morphological investigations, we found that: it is advisable to wait at least ten weeks before rendering; adding clay to hemp concrete delays drying, leading to more intense deterioration of the render; natural hydraulic lime is the preferred binder for rendering hemp concrete.

Influence of binder on the multiscale properties of hemp concretes

Biobased materials are increasingly being used for building insulation. Hemp concrete is an efficient thermal insulation material containing renewable resources, providing living comfort thanks to its acoustical and hygrothermal properties. In this study, two hemp concrete ‘wall’ formulations are manufactured with the same type of hemp shiv and two commercial binders: natural cement and a lime-based formulated binder. Ninety days of curing (65% RH – 20 °C) ensure the hardening of the materials. Mortars are made up simultaneously to control the behaviour of the binders. The influence of binders on functional properties of hemp concretes and the relation with their microstructure are investigated in this article. The two materials exhibit similar thermal conductivity, acoustic performances and hydric properties, linked to their similarities considering density, open porosity, air resistivity, specific surface area and pore size. However, several other parameters describing the microstructure of the concretes are different depending on the binder, such as interparticle porosity, intra-porosity and tortuosity. Both formulations have low compressive strength compared with hemp concretes analysed in other studies. In our case, hydration of binders does not appear to occur. This could be due to an impact of the shiv extractible compounds on the hydration of the mineral binders.

Comparative life cycle assessment of magnesium binders as an alternative for hemp concrete

To counter the negative environmental impact, particularly greenhouse gas emission generated by the construction industry, many low-impact materials are being produced and researched having neutral CO2 emissions and low thermal conductivity. One of these materials is lime-hemp concrete, a self-bearing bio-based insulation material with low thermal conductivity and good CO2 uptake but with weak mechanical properties. In this study alternative magnesium binders are proposed to substitute the traditionally used lime binder in hemp concrete, comparing the environmental impact of these binder composites. To make the comparison, experimental mixtures with both traditionally used and alternative binder composites were produced and their mechanical and thermal properties tested. The magnesium binders showed promising results as these composites were approximately two times stronger, having similar density and thermal conductivity. Afterwards the Life Cycle Assessment (LCA) was carried out to evaluate and compare the environmental impact of all tested composites. Lime based binder composites achieved negative CO2 emissions, varying from −46.5 to −68.6 kg CO2/m3. Alternative binder, magnesium phosphate cement, demonstrated significantly greater environmental impact than all other binders due to its hardener, potassium phosphate, which is highly energy and resource intensive. Magnesium oxychloride cement showed promising results with bio-based filler, as their combined environmental impact was lower in most categories compared to lime-hemp concrete, and negative CO2 emissions of −37.38 kg CO2/m3 were achieved. These negative CO2 emissions were achieved with biogenic CO2 uptake from hemp growth and low binder content, thus achieving low thermal conductivity of 0.062 W/m2*K at 214 kg/m3 density.

Multicriteria analysis demonstrating the impact of shiv on the properties of hemp concrete

The shiv derived from the shredded stem of the hemp plant [1], when mixed with a mineral binder and water, produces hemp concrete (or “hempcrete”), which is considered as an eco-material because of its remarkable environmental properties (favourable CO2 balance, easy management and disposal at the end of the product life, etc.) [2]. However, much like any agricultural product, its quality may be affected by the soil where the hemp has been grown, the weather conditions, and a variety of other factors. As these parameters are rather variable, the produced plant matter tends to be somewhat irregular, which can hamper the development of this type of building solution. The study at hand tests the characteristics of the hemp (particle size distribution, water absorption capacity, etc.), to assess the impact of the variability of these factors on the performance of the hempcrete made with the shiv. To begin with, the thirteen types of shiv used in the study were compared by multicriteria analysis, and then classified into three different groups.Next, nine types of shiv were selected, so that each of the three groups was represented. In identical conditions (i.e. the only variable being the type of shiv), specimen of hempcrete were made up in the lab for experimentation. The aim here was to validate the results of the study characterising the different types of shiv. The results show that there are three main types of mechanical response when the hempcrete is subjected to a compression test, where the material behaviour is characterised by small, medium or large strains. The large specific surface area of shiv is known to be responsible for weak binding at the interface between the binder and the plant particles, leading to total failure of the test piece. In addition, its high water absorption capacity, which is the likely cause of the “false set effect” associated with the elongated form of the particles, is identified as being responsible for the high rates of deformability found. It should be noted that in the latter case, the specimens behave like a succession of layers, which accounts for the significant levels of strain.By multicriteria analysis of the results for the hempcrete specimens, we found that they can be also classified into three groups, matching the three detected by the multicriteria analysis of the types of shiv from which they were made. Unlike the previously published literature, this study leads to the conclusion that shiv types with small particle sizes and high water absorption capacities do not produce the best performance.

Acoustical properties of hemp concretes for buildings thermal insulation: Application to clay and lime binders

This experimental and modelling study provides a general overview of the acoustical performance of hemp-lime and hemp-clay for building thermal insulation at the material scale. It is based on statistically robust experimental results from more than 100 hemp-clay samples, together with the analysis of a large hemp-lime database.In hemp-clay mixes, our experimental results show the concentration of hemp in a mix has a first order effect on the acoustical performance, while binder fluidity and clay type have no effect. Another conclusion of this study is that hemp-clay and hemp-lime behave acoustically in a similar way. For both materials, experimental sound absorption and transmission curves can be modelled with a physical-based four-parameters approach. The close agreement between experimental measurements and modelling highlights the good level of understanding of the physical phenomena responsible for the acoustical behavior of hemp concrete.A classification is finally proposed in terms of density to be used as a general guideline to evaluate or optimize the acoustical performances of hemp-based concrete.

Utilization of hemp concrete using hemp shiv as coarse aggregate with aluminium sulfate [Al2(SO4)3] and hydrated lime [Ca(OH)2] treatment

This research presents the potential of hemp shiv for use as coarse aggregate in concrete. Hemp shiv was mineralized with aluminium sulfate Al2(SO4)3 and calcium hydroxide Ca(OH)2 before being used as a coarse aggregate to produce hemp concrete. The cement to sand ratio was 1:2, the Al2(SO4)3 to Ca(OH)2 ratio was 1:2, and the amount of superplasticizer was 1% by mass of cement. The compressive strength, density and water absorption of hemp concretes were tested. The results showed that at 28days the compressive strength of hemp concrete was in the range of 15.0–17.0MPa, density of 1420–1470kg/m3 and water absorption of 14.5–16.5%. The results indicated that the application of Al2(SO4)3 and Ca(OH)2 for mineralization increased the compressive strength of hemp concrete by four times compared to that with non-mineralized hemp shiv. The use of Al2(SO4)3 improved the compressive strength of composites and the increase in the amount of Al2(SO4)3 also accelerated the setting and hardening of the matrix.

Impact of the determination of the sorption-desorption curves on the prediction of the hemp concrete hygrothermal behaviour

Hemp concrete is a sustainable material with low embodied energy. However, its development as a building material requires a better evaluation of its moisture-thermal buffering abilities behavior, which are known to strongly depend on the amount of water contained in wall pores and its evolution. In this context, the aim of this paper is to study the experimental determination of sorption-desorption curves of hemp concretes following two methods and at two temperatures (23°C and 40°C). The former considers the total sorption-desorption loop, from the dried state to a relative humidity of 85%, while the second one is based on partial relative humidity cycles between 23% and 85%. At first, the difference between the curves obtained through these two methods were analysed. They were then implemented in a simple hygrothermal model to simulate dynamic hygroscopic and hygrothermal loadings. The comparison between the theoretical calculations and the experimental results eventually allows to scan which of these relations is more representative of the real dynamic behaviour of the tested hemp concretes.

Scale analysis of heat and moisture transfer through bio-based materials — Application to hemp concrete

In the context of energy transition, new construction materials are being developed, with the concern of being more eco-friendly all along their life cycle. This includes bio-based materials like hemp concrete. Based on experimental data, this paper documents a model predicting heat and moisture transfer through hemp concrete. Scale analysis allows to go further into the description by identifying the dominant driving forces both for moisture and heat transfer, for different classes of relative humidity. The case study deals with a material submitted to a variation in temperature and relative humidity on its surface. A significant change is noticed around a relative humidity ψ=95%. Below this threshold the temperature gradient is the main driving force for moisture transfer. The dominant mechanism becomes the capillary pressure gradient when ψ>95%. In the latter relative humidity range, phase change enthalpy becomes in the same order of magnitude as conduction heat transfer.

Temperature dependence of sorption isotherm of hygroscopic building materials. Part 2: Influence on hygrothermal behavior of hemp concrete

In the part I of the paper, a temperature effect was experimentally observed for the relative humidity measurement. This effect was correlated to the temperature dependence of the sorption isotherm. In this part II of the paper, the influence of this temperature dependence is investigated on the simulated hygrothermal behavior of hemp concrete. Sorption isotherm was modeled through three approaches: a unique curve (like in WUFI), a temperature-dependent GAB model or the Clapeyron equation associated to the isosteric heat of sorption evaluated in the part I of the paper. Comparison between simulation and experiment is performed for the sample presented the part I of the paper and for a wall subjected either to a slow temperature decrease toward 0°C or to a fast temperature increase due to solar radiation. Whatever the investigated case, results underline that accounting for the temperature dependence improve the relative humidity prediction. Nevertheless, a great sensitivity to the modeling approach and to the input parameter was observed.

Reefcrete: Reducing the environmental footprint of concretes for eco-engineering marine structures

The ecological value of engineered marine structures can be enhanced by building-in additional habitat complexity. Pre-fabricated habitat units can be cheaply and easily cast from concrete into heterogeneous three-dimensional shapes and surface topographies, with proven ability to enhance biodiversity on artificial structures. The net ecological benefits of enhancement using concrete, however, may be compromised on account of its large environmental footprint and poor performance as substrate for many marine organisms. We carried out a pilot study to trial alternative cast-able “Reefcrete” concrete mixes, with reduced environmental footprints, for use in the marine environment. We used partial replacement of Portland cement with recycled ground granulated blast-furnace slag (GGBS), and partial replacement of coarse aggregate with hemp fibres and recycled shell material. We calculated the estimated carbon footprint of each concrete blend and deployed replicate tiles in the intertidal environment for 12 months to assess their performance as substrate for marine biodiversity. The hemp and shell concrete blends had reduced carbon footprints compared to both ordinary Portland cement based concrete and the GGBS based control concrete used in this study. At the end of the experiment, the hemp and shell blends supported significantly more live cover than the standard GGBS control blend. Taxon richness, particularly of mobile fauna, was also higher on the hemp concrete than either the shell or GGBS control. Furthermore, the overall species pool recorded on the hemp concrete was much larger. Community compositions differed significantly on the hemp tiles, compared to GGBS controls. This was largely explained by higher abundances of several taxa, including canopy-forming algae, which may have facilitated other taxa. Our findings indicate that the alternative materials trialled in this study provided substrate of equal or better habitat suitability compared to ordinary GGBS based concrete. Given the growing interest in ecological engineering of marine infrastructure, we propose there would be great benefit in further development of these alternative “Reefcrete” materials for wider application.