Rammed Earth Construction

Introduction to Rammed Earth

Rammed earth is a method of building walls whereby a mixture of earth is compacted in layers between forms.

In the family of building materials, rammed earth is a first cousin to adobe. It was used to create parts of the Great Wall of China.

Rammed earth buildings are extremely durable and can last for centuries. Experts claim that rammed-earth walls continue to harden - or cure, in their parlance - during the first year after construction. Although finished walls are somewhat water resistant, they can be stuccoed, plastered painted or left natural and sealed to better waterproofing them.

Like adobe, rammed earth buildings have the advantage of having very thick walls that retain heat. Their thickness, or thermal mass, helps to even out temperature fluctuations between day and night, making them easy to heat and cool.

Without the use of machinery, rammed earth construction tends to be very labour intensive. It can take many hours to properly tamp soil into the forms, then take the forms down and set them up again for the next section of a wall.

This laborious process can make rammed earth construction more expensive than traditional frame and stucco building, so most of the homes currently built in this way are custom homes. The use of a pneumatic tamper and a mechanical shovel can reduce the time and labour significantly.

BENEFITS OF BUILDING IN RAMMED EARTH

The many advantages of building with rammed earth include superior insulation, strength and durability, low maintenance, fire proofing, load bearing and pest deterrence. To say nothing of its beauty and the pleasure of building with a natural environmentally sound material.

Thermal Mass

The external walls of our rammed earth buildings are a minimum of 300mm (1 ft) thick, providing excellent protection from extremes in climate. The thickness and density of the material means that heat (or cold) penetration of the wall is very slow and the internal temperature of the building remains relatively stable, with the end result of it feeling warmer in winter and cooler in summer than the outside temperature. Rammed earth is a popular choice for buildings where temperature fluctuations need to be kept to a minimum.

Noise Reduction

The thickness and density of the walls mean that unwanted sounds such as traffic noise are kept out. Internal walls in rammed earth can also be extremely useful in providing sound insulation between areas with different needs, for instance between living and sleeping areas, between a public or family room and a study area, or in party walls between townhouses. CSIRO tests indicate a sound transmission rating of over 50 decibels for a rammed earth wall of 250mm.

Strong and Durable

As CSIRO research (and thousands of years of use worldwide) has proved, rammed earth can withstand pretty much anything nature can throw at it.

Low Maintenance

Rammed earth walls are maintenance free. They are features that stand alone and don't need finishing with gyprock, render, paint, wallpaper, tiles or anything else. But should you prefer any other finishes they can be applied to rammed earth walls in just the same way as to other masonry walls. Rammed earth walls are permanent and require no ongoing maintenance.

Fireproof

Earth doesn't burn. This is an ideal material for bush settings and leafy suburbs. CSIRO tests showed that a 250mm rammed earth block wall achieved a 4 hour fire resistance rating. A 150mm wall achieved a rating of 3 hrs 41 minutes. The walls are even stronger, being 300mm thick and of monolithic rather than block construction (so have no vulnerable mortar joints).

Load Bearing

Engineers recognize rammed earth as load bearing, so you are unlikely to need other structural framing for your home, cutting costs, fire and pest susceptibility. Rammed earth also provides substantial bracing to buildings usually well in excess of the minimum requirement achieved by most timber framed homes. It provides a feeling of stability and security in even the worst weather conditions.

Rapid Construction

Using pneumatic tamping, rammed earth walls can be fast to go up. The walls of an average home can be up and ready for the roof framing in as little as a week.

Cost Effective

Unlike most types of masonry, rammed earth walls don't need core filling or reinforcing. Nor do they need gyprocking, plastering, painting or wallpapering. This saves money and energy when you build, and goes on saving them for years.

Versatile

Architects are using rammed earth in commercial and domestic buildings of many styles and for many functions. Variations in texture, colour and finish are possible, and elements such as relief details, circular windows, alcoves and feature stones can be incorporated.

It is a natural product which blends beautifully with the environment and other natural building materials. Yet it also looks stunning when used in conjunction with man-made materials such as iron, glass and plasterboard, a combination used in several recent award-winning homes

Pest Proof

Termites and other pests simply aren't interested in rammed earth. And since the walls are load bearing structural timber isn't generally needed with rammed earth construction so the risk of termite or woodworm attack is even further reduced. There are no cavities in rammed earth walls for pests to live in or use as a route to the roof or other timbers.

Environmentally Friendly

Rammed earth is non-toxic, non-polluting and 'breathes'. This creates safer, more people-friendly buildings. It is very low in embodied energy, and extremely comfortable to live in.

Building With Rammed Earth

To create a masonry wall using rammed earth techniques, a soil mixture of stone (gravel), sand and clay is pounded layer upon layer by hand tamping or mechanical tamping into removable forms. The forms are then removed, leaving solid earth walls, 18 or 24 inches thick.

The soil mix needs to be carefully balanced between clay, sand and aggregate (gravel). The clay and moisture content of rammed earth is relatively low compared to that used for mud brick or other earth building methods.

The best mixture for rammed earth contains approximately 30 percent clay and 70 percent sand, with a small amount of cement added. These components are nontoxic, inexpensive and readily available.

A wider range of soils are suitable when a small amount of cement is added to the mix. Most rammed earth builders add some cement. The result, known as stabilised rammed earth', is a strong masonry product which provides excellent thermal mass.

Earth (i.e. screened fill from a sand and gravel yard) is mixed with 3% cement. It is sprinkled with water until it is damp (about 8% moisture).

Site soil is rarely used unless

• 1) it exactly meets the engineered soil requirements (i.e. the proper clay to sand ratio) and
• 2) either the material would have to be removed from the site and taken to a landfill or a large hole (for a basement or a swimming pool) is desired.

Unless these conditions hold true, the cost of screening the site soil will probably exceed the cost of purchasing the screened fill from a sand and gravel yard.

Each layer of earth is approximately 15 to 20cm (6 to 8 inches) deep. This soil is tamped down until its volume has been compacted by at least 25 percent. (A pneumatic tamper will compact 8" layers of soil-cement into 5" layers.) As each form is filled and the earth is stable, another form is placed above it, and the process is repeated.

This is continued until the desired wall height is achieved. Forms can be stripped off as soon as the form above is begun, as the compressed earth wall is self-supporting immediately. Most rammed earth builders use pneumatic rammers to compact the earth within the forms. The walls are typically 2 feet thick and are essentially man made stone.

The tamper should have a small head to enable the crew to tamp in corners and around plumbing pipes and electric conduit that are run in the walls. The final density should be around 125 pounds per cubic foot.

Wall widths are typically 18" or 24". Any wider than 24" is thermal mass overkill and any narrower would be difficult for the crew to work in.

Once compacted, a chemical reaction occurs between the cement and the water in the rammed earth mix. This reaction cannot be reversed by exposure to water and so erosion is not a concern.

A concrete bond beam is poured in the last 6" of the wall to insure stability against any possible lateral pressures. The roof is attached to the bond beam. When the walls are left exposed (un-stuccoed), the bond beam concrete can be dyed to match the colour of the earth walls.

PISÉ

Builder David Easton has developed a new, less labour-intensive way of creating a rammed-earth-type home. The process is called Pneumatically Impacted Stabilized Earth - named PISÉ by Easton in honor of the French tradition of rammed earth construction.

PISÉ is another earth-based building material that looks and performs much like rammed earth or adobe. PISÉ has all the environmental benefits of rammed earth, but with one important advantage - its quick-lock forming systems, which speeds up the building process. A trained crew can complete up to 1000 square feet of 18" thick wall per day.

The process is however, more complex than traditional rammed earth. A mixture of earth, cement and water is sprayed through the oversized hoses of a gunite machine - equipment normally used in the construction of swimming pools - and builds up against the one-sided forms. The carefully controlled mixture is just moist enough to stack up against the form without slumping.

The nozzle operator moves along the wall, spraying the PISÉ mix 18-to-24 inches thick and in layer two-to-three feet high. As the wall begins to firm up, any irregularities are shaved off to create a plumb, smooth wall. In 30 minutes to an hour, another two-to-three feet high layer can be added to the wall. Once the PISÉ dries, the forms are removed.

Because the forms against which the PISÉ mixture is sprayed are one-sided, they can be set up and removed quickly. Their open design allows plumbers and electricians to easily put pipes and conduits in place before the wall is cast. And one-sided forms make it easy to incorporate reinforcing steel - rebar - in the wall being sprayed, an important addition for earthquake safety. The addition of steel makes it possible to build taller walls that bear more weight.

PISÉ technology is relatively new, so it is not widely available. It requires sophisticated equipment and meticulous control over mix designs. PISÉ continues to be researched and developed to improve its quick-lock forming systems, versatile soil mixes, soil amendments, and improved delivery equipment.

BOOKS

The Natural House Book, David Pearson. Harper Collins Publishers.

The Rammed Earth House, David Easton. Chelsea Green Publishing Co. ISBN 0930031792 http://www.chelseagreen.com/bookstore/item/rammedearthhouse2

Bulletin 5: Earth Wall Construction, CSIRO. ISBN 064305449

Warm House, Cool House. Nick Hollo. Choice Books. ISBN 1947277226

Building with Earth Bricks and Rammed Earth in Australia, Earth Building Association of Australia, 2004. ISBN: 0975603604

Sticks, Stones, Mud Homes, Nigel Noyes, 2004. Hardie Grant Books. ISBN: 1740661478

UK Websites

Ram Cast - rammed earth company
http://www.rammed-earth.info/

Earth Structures UK - earth building company
http://www.earthstructures.co.uk/index.htm