Contents at a Glance...
PREPARING THE FRAME
FILLING THE PANELS
The use of a mixture of sawdust, sand and cement for making wall panels has been fairly common in parts of Northern NSW for many years.
The history of this technology goes back to at least the 1930's, and it has been researched and applied in parts of the USA, UK and Germany. In some instances, the materials (with various adaptations) have been used for flooring as well as walling.
The possibilities for this medium are probably endless. There may be no reason why we can't make bricks, ceiling panels, moulded furniture or whatever.
In construction, the material is used as a non-loadbearing infill in a way not totally dissimilar to the traditional wattle and daub. There appear to be a couple of basic structural approaches:
1. The roof is supported on a post-and-beam frame in the same way as it can be for a mud brick house. The space between posts is further subdivided by light hardwood stud framing which supports the infill panels. These can be up to a couple of metres wide. Stud spacing of 600-1200 mm are more common however, and probably more manageable.
2. The other approach is to support the roof with framed walls which have studs spaced at up to 1200 mm centres. In this case, roof point loads should be carried directly over studs or heavier top plates. Whilst infilling between studs is usually carried out in the vertical position as described below, there have been cases of infilling first then lifting the walls into place. The panels are light enough to be used as walls which are supported by stumps and bearers. They are also stiff enough (once dry) to provide bracing to the building, although other diagonal bracing such as steel rods or timber battens should be incorporated within the panels as required.
Some people may find that the lightness of the materials makes it a good material for high gable ends or even second storeys on a mud brick or rammed earth building.
The most common mixture used for walls consists of 3 parts sawdust, 2 parts sand and 1 part cement. For smaller panels, 4 parts sawdust is usually satisfactory.
The sawdust should be from hardwood - which is low in tannin, gums and oils - for the best results.
Most people mix the ingredients in a large, shallow metal tray using hoes or rakes. Only enough water to activate the cement should be used.
The mixing procedure should be as follows:
- First, blend the sand and sawdust - do this thoroughly;
- Next, add the cement and blend again - until the entire mix is of a consistent colour;
- Now add the water and mix again - garden watering-can is useful in distributing the water.
To ensure consistency, once the amount of water is established, all materials should be delivered to the mixing point via gauging buckets or boxes. Work in the shade to avoid premature setting of the mix and ensure that it is in place within about thirty minutes of mixing.
Squeezing a ball of the mix in the hand should produce no excess water running through the fingers. Excessive water will mean that the panel may slump and even collapse before it sets.
PREPARING THE FRAME
Make sure that the overall structure is strong and secure without reliance on the sawdust cement infills.
On drying, there will be a certain amount of shrinkage of the panel away from the framing. It's a good idea to seal the edge of framing to prevent the mix sticking to it and subsequently cracking off while drying. For the same reason, it is good to trowel a pencil-round at the edge of panels rather than leaving a brittle feathered edge.
To avoid having a daylight gap around the panels, a bead or metal strip as per the diagram can be used. This will have an additional effect of holding the panel in place.
Another holding-in method used is to stretch 12-gauge soft galvanized sire between the frames in the middle of the panel thickness. Fix wires with U-shaped staples or through holes in the woodwork. Wire can tensioned by inserting a rod or screwdriver and twisting.
It is normal to use a sheet material fixed to one side of the panel frame with G-clamps or screws. The ideal is concreter's Formply, as it is very strong and unlikely to deflect. Formply is expensive but can be used repeatedly over many years if looked after.
If a less substantial plywood or other sheeting is used, it can be reinfororced with cleats as required. Formwork shouled be well sealed to prevent the sawdust/cement mix sticking to it when dry.
FILLING THE PANELS
Some people use only the backing form described above and push handfuls of the sawdust-cement mix up against it, around the wires and beads, and well into the corners. The surface is patted to "an informal flatness" and allowed to dry. This system is suited mainly to smaller panels.
Another technique is to use a couple of 150 x 25 boards fixed to the second face of the studs and pack the mixture between these and the backing form. The boards can then be leap-frogged up the walls, as per the diagram.
Unevenness in the surface can be lightly trowelled out as the work proceeds.
Each panel should be laid up in one session. Cracking is almost certain to result between fresh and dry areas.
Allow up to twenty-four hours drying before removing the backing form, depending on panel size and drying conditions. Panels should be protected from drying out too quickly.
New panels can be damaged by the vibration caused by adjoining building activities. Give them a chance to harden before subjecting them to such stresses.
Panels made in this way are not inherently weatherproof. Internally, walls may require no applied finish although most people will want to decorate in some way.
The surface can be painted with commercial paints *, but this will be expensive as the surface will soak up lots of paint.
Traditional lime-based whitewash * can be used. The addition of one cup of linseed oil per ten litres of limewash can work well and oxide powders can be added to create the right colour. The further addition of one cup of a PVA (such as Bondcrete) per twenty litres, is another possibility.
For areas particularly exposed to weather, a possible finish is as follows:
- 1 part Silasec - a proprietary brand cement sealant
- 5 parts water
- 7 parts Portland Cement - oxides and/or hydrated lime can be used to produce the desired colour.
Having been aware of the sawdust, sand, cement medium for many years, I'm excited by its potential, now that I've seen it. No, I'm not suddenly an expert on the subject, but I hope the above comments will encourage readers try their hand with the material.
(Russel Andrews is the editor of Owner Builder Magazine - see under Non Timber Building: Earth, Adobe, Mudbrick in the Books section of the Alternative Directory)
* See also Non Toxic Paint Suppliers in the Alternative Directory.