Hempcrete or hemplime is biocomposite material, a mixture of hemp hurds (shives) and lime, sand, or pozzolans, which is used as a material for construction and insulation. It is marketed under names like Hempcrete, Canobiote, Canosmose, Isochanvre and IsoHemp. Hempcrete is easier to work with than traditional lime mixes and acts as an insulator and moisture regulator. It lacks the brittleness of concrete and consequently does not need expansion joints. The result is a lightweight insulating material ideal for most climates as it combines insulation and thermal mass.
Hempcrete is developed from a mixture of hemp shives, and a lime based binder. The lime-based binder typically consists of either hydrated lime or natural hydraulic lime. Hydrated lime is made from pure limestone and set through the absorption of CO2 during the carbonation process. When dealing with time constraints, hydraulic binders are used in combination with regular hydrated lime because the set time for hempcrete will be less than that of regular limes (about two weeks to a month to gain adequate strength). Occasionally, a small fraction of cement and/or pozzolanic binder is added to speed up the setting time as well. The overall process creates a mixture that will develop into a solid, but light and durable product.
The typical production process of hempcrete is contained in a few steps. At the manufacturing location, hemp is stored in a storage room, and the lime is stored in silos. The hemp and lime are added to water and thus mixed. This process produces about 0.6 m3 of hempcrete mixture. The mixture then travels through a conveyor belt where a machine shapes it into blocks. The blocks are then taken to an area to cure. Once cured, the blocks are separated into 2 m3 batches and loaded on pallets. They are wrapped up with polyethylene packaging film and polypropylene straps to be transported to a construction site.
Applications and specifications
Hempcrete has been used in France since the early 1990s, and more recently in Canada, to construct non-weight bearing insulating infill walls, as hempcrete does not have the requisite strength for constructing foundation and is instead supported by the frame. Hempcrete was also used to renovate old buildings made of stone or lime. France continues to be an avid user of hempcrete, and it grows in popularity there annually. Canada has followed France's direction in the organic building technologies sector, and hempcrete has become a growing innovation in Ontario and Quebec. There are two primary construction techniques used right now for implementing hempcrete. The first technique consists of using forms to cast or spray hempcrete directly in place on the construction site. The second technique consists of stacking prefabricated blocks that are delivered to the project site similar to masonry construction. Once hempcrete technology is implemented between timber framing, drywall or plaster is added for aesthetics and increased durability.
The typical compressive strength is around 1 MPa, around 5% that of residential grade concrete. It is a low density material and resistant to cracking under movement, thus making it suitable for use in earthquake-prone areas. Hempcrete walls must be used together with a frame of another material that supports the vertical load in building construction, as hempcrete's density is 15% that of traditional concrete. Studies in the UK indicate that the performance gain between 230 mm (9 in) and 300 mm (12 in) walls is insignificant.[clarification needed] Hempcrete walls are fireproof, transmit humidity, resist mould, and have excellent acoustic performance. Limecrete, Ltd. (UK) reports a fire resistance rating of 1 hour per British/EU standards.
In the United States, a permit is needed for the use of hemp in building.
Hempcrete's R-value (its resistance to heat transfer) can range from 0.67/cm (1.7/in) to 1.2/cm (3.0/in) , making it an efficient insulating material (the higher the R-value, the better the insulation). The porosity of hempcrete falls within the range of 71.1% to 84.3% by volume. The average specific heat capacity of the hempcrete ranges from 1000 J/kg K to 1700 J/kg K. The dry thermal conductivity of hempcrete ranges from 0.05 W/mK to 0.138 W/mK. The low thermal diffusivity (1.48 x 10-7 m2/s) and effusivity (286 J/m2Ks-1/2) of hempcrete influence longer times for temperature change and create a warm sensation on the touch. Low thermal diffusivity directly relates to increased thermal comfort within the building.
Benefits and constraints
Hempcrete provides high vapor permeability because of the mixture’s ability to easily absorb or release water vapor from the air. In frame structures, hempcrete mixtures can be used as filling materials in infill walls. Increasing the density of the mixture allows the production of roof or floor insulation hempcrete materials. Decreasing the density allows the production of indoor and outdoor plasters. Hempcrete block walls can be laid without any covering or can be covered with finishing plasters. This latter uses the same hempcrete mixture but in different proportions.
The fact that the mixture contains a plant-based compound introduces the caution against water and rising damp levels. Hempcrete walls need to be built with a joint between the wall and the ground in order to avoid capillary rising as well as water runoff at the wall base. Moreover, hempcrete block can only be installed above the ground level. External walls need to avoid rotting of shives by implementing protection by the rain gale with sand and lime plaster. The exterior of a hempcrete based assembly needs these protections, but the interior side of an assembly can stay exposed.
Life cycle impacts
Just like any crop, hemp absorbs CO2 from the atmosphere while growing, so hempcrete is considered a carbon negative material. Accordingly, this CO2 will be stored in the hempcrete block after fabrication and for the duration of the block’s life allowing positive environmental benefits. The specific amount of carbonates in the blocks actually increases with the age of the block. The amount of CO2 capture within the net life cycle CO2 emissions of hempcrete is estimated to be between -1.6 to -79 kg CO2e/m2. There is a correlation that increasing the mass of the binder which increases the mixture density will increase the total estimated carbon uptake via carbonation.
The main cause of environmental impacts for hempcrete comes from the production of the binder. Reports have estimated that 18.5% - 38.4% of initial emissions from binder production can be recovered through the carbonation process. The binder is produced by the calcination of lime which takes in kilns at very high temperatures. The transport phase poses embodied energy impacts since it involves the consumption of diesel. The diesel consumption also occurs due to the functioning of machineries used for hemp shives production. Abiotic depletion is caused from the consumption of lead and cadmium in the electricity generation process, which is largest in the manufacturing of the hempcrete block inside the company.
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