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INTRODUCTION TO HEMPCRETE :: AN INSULATIVE BIO-AGGREGATE
Though structures made of hemp-lime aggregates are found to date back to at least the 6th century A.D., the current material formulations and methods took a leap forward as a result of research into the use of hemp as an aggregate in concrete mixtures in France during the mid 1980’s.
While hempcrete is technically speaking a type of concrete, its physical properties and performance characteristics are very different from what most people consider concrete. Like all living material, hemp sequesters CO2. The production of lime requires energy, and thus there is CO2 produced in the process- much of the CO2 produced is reabsorbed in the subsequent carbonation of the lime in the mixture after it is placed in the wall. Claims have been made to the effect that a hempcrete wall is carbon negative, but we are unaware of any comprehensive analysis that shows this to be true. In a doctoral study by Anne Rouse at the University of Kentucky, information is presented showing that relative to all other building materials, hempcrete has a significantly lower carbon footprint- we are looking forward to the public release of this information by Ms. Rouse.
The use of hempcrete is particularly indicated in high humidity environments and in climates where extreme seasonal temperature variations can cause dew point condensation within the exterior wall assembly. The inherent mold mitigation of the hemp- lime material addresses the most pressing issues related to mold activity in the building envelope.
PROPERTIES OF HEMP / LIME MIXES
Lime is the oldest processed building material in use today, with a history stretching back thousands of years. Commonly mistaken as a substitute for Portland cement, lime is in fact a unique material that embodies very different properties than common cement.
Lime-based binder is a critical structural and performance component of a hempcrete system, and the use of the correct binder formulation is absolutely essential to a proper application. In general terms, it performs the following functions:
Lime-based binder is used as the structural bonding agent in the creation of the material matrix.
Hemp contains an unusually high silica content when compared to other plant-based materials, which makes it uniquely suited to mixture with lime. The lime forms a chemical bond with the hemp material, enhancing structural integrity and preservation of the bio-aggregate.
Hygroscopy is the ability of a substance to hold and attract water molecules from the environment. As part of the wall system, lime acts to mitigate changes in the internal environment and to provide a buffer between differences in internal and external humidity. Water is bound within the lime and may be held indefinitely without negative effect to the material and is released when the surrounding humidity decreases.
Lime has a pH of 12, placing it high on the alkaline scale, and acts a preservative to the bio-aggregate and any encased wood structural components.*
The alkaline nature of the combined materials are extremely hostile to mold, mildew and subsequent degradation of the wall. Additionally, this makes it resistant to both insect and rodent incursion*.
The part of the hemp plant used in bio-aggregate construction is composed primarily of the woody, inner core of the plant, referred to as hurd. Most processed hurd also contains some amount of bast fiber, which varies from one processor to another.
Although processed hurd has inherent anti-microbial properties, it should be stored in dry conditions.
The thickness and width of hurd varies, but the more important consideration is the fiber content of the material, which can have an effect on the mix consistency and set-time. Excessive fiber can result in uneven distribution of lime in the mix, affecting the stability of the material. In addition, and unlike the hurd, the fiber contains both pectin and lignin, which can negatively affect the ability of the lime to set properly. When used in textile applications, the pectin and lignin are removed from the fiber through an alkaline bath process.
1) Short hurd with minimal fiber content.
- Consistent mix with minimal “balling” of material.
- Short hurd length has smaller matrix and interconnecting properties between individual pieces of material.
- Medium cost
2) Medium hurd length with minor fiber content
- Stronger matrix
- Higher cost
3) Long hurd with high fiber content
- Difficult to mix
- Unless processed to remove pectins, set time will be affected
- Lower cost
In the choice of hurd, but more specifically the lime/hemp ratio, there are a number of variables that must be taken into consideration. In the majority of applications, all structural concerns are taken into account in the structural framing, whatever form it might take (see Structural Requirements below), and the function of the hempcrete is to provide maximum thermal value, while maintaining sufficient structural integrity of the insulative infill. The lime-hemp mix ratio should be followed carefully per manufacturers recommendations. Where additional structural integrity or density is desired, additional lime may be added with a corresponding increase in the amount of H2O in the mix, this will decrease the r-value of the wall, but increase its hygroscopic capacity. In no case should the amount of lime be decreased from the manufacturers specification without testing and evaluation by the supplier.
Types of Lime:
There are two basic types of lime which may be used as a binder, Natural Hydraulic and Hydrated. These vary both in chemical composition and finished properties.
Natural Hydraulic Lime (NHL) is considered the highest quality available, is sourced from Europe and Britain, and available in the U.S. from multiple distributors. The setting of the material is primarily achieved through the chemical reaction of the lime with water- thus the term hydraulic.
NHL is available in raw form and is the primary ingredient in commercial binder products. NHL contains native pozzolans, but typically will have additional pozzolanic material added to achieve particular performance properties.
Hydrated Lime (HYD): (CaOH)2 is available domestically from the sources noted below. It is essential that HYD has a minimum 95% Calcium Carbonate content. Hydrated lime contains no natural pozzolans, and proper pozzolanic content is essential, or it will fail to cure.
Defined by ASTM(1) C618 as: “a siliceous or siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided form in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties”
Pozzolanic materials consist mainly of reactive silica (SlO2) and alumina (AL2O3). As noted above, NHL contains naturally occurring pozzolans; HYD will always require the addition of pozzolans. If faster set times are desired, the addition of up to 8% Portland cement may be incorporated into the mix. The addition of Portland to the mix also enhances the structural capacity of the wall, but will reduce the hygroscopic performance and R value of the system.
The time involved in the curing process for the mix can vary considerably, and should be taken into account in the context of building performance and construction scheduling.The amount and composition of pozzolan in the lime will affect both its set time, structural and hygroscopic performance.
On its own, lime hardens as a result of reaction with water and absorption of CO2 from the atmosphere, a process called carbonation, in which the material slowly returns to its original state. The time required for this to take place can vary significantly, and as a result, suppliers typically add a small amount of either Portland cement or Natural Hydraulic cement to their product.
While an ideal wall would have only natural pozzolans, in practice the addition of small amounts of cement, either Portland or Hydraulic, are helpful in allowing an efficient thru-line in the construction process. *
While it is possible to formulate a lime binder from raw ingredients, it is recommended that binders be purchased from an established supplier (see Material Sourcing below) and all mixing ratios be followed per their specifications. an improper binder formula will result in a failed wall. Any alteration to the mix rations should be undertaken in controlled test batches and evaluated for quality and performance before being incorporated into a finished structure.
The specific formulation and use of binder is determined based on the following considerations:
Lower density mixes, i.e. lower lime to hemp ratios, will have a higher insulative value. As a general rule, the best value that may be achieved while maintaining adequate structural integrity is an R of 2.4 per inch of material.
As noted above, in most instances the function of hempcrete is to perform as an insulative bio-aggregate infill. In instances where structural performance takes precedence over thermal capabilities, increasing the lime content will significantly increase the compressive and tensile strength of the mix. A hemp lime mixture can not achieve the compressive strength of concrete. it should never be used below grade. Any application that anticipates the use of the material as a structural load bearing or shear element should be independently tested and/or confirmed by a licensed structural engineer.
Consideration must be given to the reaction of the lime to certain materials. The chemical composition that makes hempcrete desirable in so many respects must be taken into account in the following instances:
All ferrous metal must be galvanized, painted or have a physical barrier such as plastic or foam between themselves and the mix. As a general practice, it is advisable to allow ONLY galvanized nails on site. Coated deck screws are acceptable with no further isolation.
Penetrations for plumbing and wire are best run in PVC chases.
The chemicals in pressure treated wood (PT) can react with the lime and cause degradation of the wood. This is counter-intuitive to many, including building inspectors, when they are accustomed to using pressure treated sills as a matter-of-course. PT may used provided it is isolated from the hempcrete by a physical barrier. In the case of how the sill might be constructed, the substitute of a composite lumber product, or adequate isolation of an untreated sill from masonry will be sufficient.
Stray electrical currents will create galvanic reactions within the wall, which will result in deterioration of less noble metals. All electrical wiring and connections must be installed within alkali resistant material or casing. All electrical boxes are to be plastic, and any wire exposed to lime shall be UF wire.
MIXING PRACTICES & HEMPCRETE INSTALL
- It is possible to obtain different material properties by the following changes in ingredients and mix ratios:
- Addition of Portland or Hydraulic cement to basic binder
- Reduced set time
- decrease in insulation value and vapor mitigation
- slight increase in compressive strength
- Increase ratio of binder to hemp
- Increase in compressive strength proportional to increased binder content
- Decrease in insulative value proportional to increased binder content
In its dry powdered form, the alkalinity of lime is a severe irritant to skin and mucous membranes. Proper precautions must be taken when handling the material to prevent chemical burns. Review and implement all precautions as outlined in the MSDS for the product.
The most common method of mixing in the U.S. is done with a large, paddle type mortar mixer. The established lime binder is added to water and mixed with the prescribed measure of water and mixed for 5 minutes into a consistent slurry. This part of the mixing process activates the lime and is essential to the performance of the wall. Failure to follow this procedure may result in structural issues, as the lime will not set properly, nor bind sufficiently to the hemp material.
After the slurry has been mixed and activated, the hemp is added to the mix and allowed to mix for another five minutes.
As an alternative to a mortar mixer, a pan-mixer has the capability to mix larger quantities of material in less time. The Imer Models 360 & 750 are ideal. The same process as described for a mortar mixer is to be followed. Alternatively, a mortar mixer may be positioned on a platform for the creation of the slurry, which may then be poured into the pan mixer from above. This enables the creation of a slurry for a following mix, while the full water/ lime/ hemp mix is being prepared in the Imer mixer.
One person should be designated as responsible for the mixing of material, in order to ensure consistency and accountability, with an additional two persons for the placement of the hempcrete in the forms.
Immediately prior to placing the material in the forms, apply a very light mist to the wood studs. This prevents the wood from absorbing water from the mix and negatively affecting the cure of the material. Now, the hempcrete is placed in the form to approximately 8” depth. Using a tamping tool not more than 1 1/2” wide, the interior and exterior of the wall adjoining the forms are firmly tamped to compress the material. This provides strength to the wall and a superior surface for plastering.
A 2×4 is used as a tamping tool at the edge of the hempcrete form.
The next lift of material is then placed and the process is repeated. When the top of the form is reached, the section should be allowed to dry for a minimum of 24 hours before moving the forms up for the next pour. After moving the forms, inspect and evaluate the surface and hardness of the material for sufficient strength before adding the next course. Binders that do not have added cement may take up to a week to achieve sufficient “set”- depending on the size and scheduling of the project, this may be acceptable; if not, the addition of a small amount of Portland may be warranted.
In most cases, the hempcrete is considered a bio-aggregate insulative infill, and as such is not attributed with any structural attributes. Structural loads are typically carried by internal wood framing, timber frame, or concrete columns.
In the case of extreme wind loading or seismic considerations, additional structural and shear wall considerations may be necessary to provide necessary resistance to forces on the building. Shear may be accomplished by the use of diagonal galvanized strapping, steel brace frames, timber framing, moment frames or reinforced concrete columns. The choice of any of these methods should be analyzed in the context of local engineering requirements, budget, building form and aesthetics, and should be developed in conjunction with a licensed structural engineer.
It is desirable to have the structure under roof prior to the casting of hempcrete in the wall to facilitate the installation process, allow workflow that is not affected by rainfall and facilitate the drying of the material. All structural shear elements must be in place prior to installation of roof framing and decking.
Strapping used for shear installed before hempcrete.
The first course of roof decking at the eaves should be screwed down; this will allow the deck to be temporarily removed if necessary when the hempcrete pour reaches the top plate. While this is not necessary in all applications, it can significantly reduce installation time.
ELECTRICAL/ PLUMBING ROUGH-IN
As a general rule, with any exterior wall in colder climates it is best to avoid running plumbing in exterior walls, to avoid potential issues with freezing. When water lines must be installed at the exterior, follow these guidelines:
Do not allow contact between metal of any kind and lime. Use foam insulation on all lines when water is supplied to interior, as in showers, etc. Lines penetrating wall to furnish exterior water supply should be with frost-proof hose-bibs, insulated and contained in PVC chase- fill chase with expanding foam to prevent air infiltration.
Maximize insulation between water lines and outside of wall. The best approach is to fur wall to interior to provide a chase for water lines and fixtures.
There are three primary methods, listed in order of desirability, for the installation of AC electrical distribution outlets:
Prior to installation of hempcrete, electrical boxes are installed to the interior plane of formwork on plywood or OSB extensions mounted to wall studs. These boxes are brought flush to interior or exterior of wall. PVC or flexible conduit is installed horizontally to connect boxes as required. Conduit should be placed as close to center of wall as possible.
PVC conduit is stubbed up vertically from the floor and electrical boxes installed flush to plane of formwork. Secure boxes to stud as required for stability. This method is less desirable than the first method above, as it does not allow for as much strength in the hempcrete surrounding the conduit.
This third method should be confirmed as acceptable with your building inspector prior to the hempcrete installation. With this method, no electrical installation is performed before installing hempcrete. Following the removal of all formwork, cut or route channel in the walls for the installation of wiring and outlet boxes. Building Code typically requires a minimum of 1” plaster over all live wire connections. While if may be acceptable to use UF wire embedded in this fashion, we recommend that all wiring be contained within PVC or flex conduit, and secured in the channel so as to ensure proper depth of plaster over electrical lines/ conduit.
SUGGESTED RESOURCE MATERIAL
Bio-aggregate-based Building Materials: Applications to Hemp Concretes, John Wiley & Sons, Inc. ISBN 978-1-84821-404-0
This book contains extensive testing and evaluation data for hemp-lime material properties and performance.
Report on Tradical Hempcrete- Material Evaluation
The KubAia WashAtko Architects, Inc.
Selecting Building Materials to Reduce Environmental Impact
Graduate School of the Environment Centre for Alternative Technology Machynlleth, Powys
Pre-formulated Hempcrete binders:
Limeworks U.S (NHL) 3145 State Road, Telford, PA 18969 LimeWorks.us (NHL & HYD)
Transmineral (NHL) 2105 S McDowell Blvd, Petaluma, CA 94954 limes.us
Graymont (HYD) 965 East College Avenue, Pleasant Gap, PA 16823 graymont.com
Hemp-Technologies, Manitoba, Canada hemp-technologies.com
Lime Technologies (NHL & HYD) Chicago, Illinois americanlimetechnology.com
Geoplast S.p.A. Via Martiri della Libertà 6/8 35010 Grantorto (PD) Italy
Imer 360 and 750 available from the Imer Group www.imergroup.com
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