Dr Robert Rich
MUDSMITH
An article from EARTH GARDEN Magazine

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Poured earth, rammed earth and concrete

"Does poured earth need more concrete than rammed earth, and anyway, what is the difference between these two techniques?"

   Tom from Western Australia wrote me a long letter, including this question. Many others share his confusion, so he deserves a public answer.

How cement works

   How do you make chocolate crackles? Each rice bubble is coated in melted copha. This solidifies on cooling, gluing the rice bubbles together. Concrete is exactly the same. Cement is added to 'aggregate': stones ranging in size from 20 mm across to fine dust. The mix is agitated until every bit of stone is covered in a coating of cement. A little water will make the cement undergo a chemical reaction called 'hydration' (or 'going off' in the building trades). This turns the cement into stone, which glues the particles together.

   You can make rough concrete by mixing six units (by volume) of 20 mm crushed rock to one unit of cement. For a smooth finish, it's four units of 20 mm screenings, two units of sand and one of cement (4:2:1 mix). For brickie's mortar you use six of sand to one of cement and one of lime (6:1:1). The lime fills tiny holes between the grains of sand, making the dried mortar waterproof, less brittle, and more flexible.

   The amount of water needed for hydration is surprisingly little. The aggregate or sand is just damp, not wet. However, a lot more water is needed to make the concrete or mortar workable, and to ensure that neighbouring particles are in close contact with each other. Good mortar is like bread-dough in consistency, and so is good, strong concrete, allowing for the fact that the largish stones rattle around rather than move smoothly.

   The water in excess of that needed for hydration actually weakens the final product. It takes up space, which becomes empty holes once the water has evaporated. The six-to-one mix is actually far stronger than you would need if these holes weren't there.

   You now understand why it is important not to over-wet concrete or mortar. Use as little water as possible, given the need to have a workable mix.

Soil and its building uses

   Soil is almost infinitely variable, but for building purposes it is classified as 'sand' or 'clay'.

   Pure 'sand' consists of little, rough lumps of stone. If you wet sand, then let it dry, it becomes a powder again. If you make it just damp, then apply pressure, it has a tendency to form a three-dimensional jigsaw puzzle as the rough particles interlock. However, this doesn't provide enough cohesion to hold a lump permanently together.

   'Clay' has very much smaller, smooth particles, which tend to be like tiny flat plates. If you wet clay and stir it around, the flattish plates line up with one another, each coated by a film of water. As the water evaporates, the particles get in such close contact that electro-chemical forces attract them to each other. After drying, the clay becomes a very hard, cohesive lump. If the wet mud was made rectangular, you have a mudbrick. If it was placed within forms (like old floorboards clamped together) on top of a wall, then cut into brick lengths, you have 'formwork cob'.

   Most soils are mixtures of sand and clay. A clayey loam might have 75% clay, 25% sand. A soil like that will be excellent for mudbricks and similar techniques. The final result is not as strong as with pure clay, but plenty strong enough. The advantage is that the work of mixing is nowhere as difficult.

   A sandy loam might have 75% sand, 25% clay. If you compress it when it is just damp, the interlocking sand particles will be glued together by the clay, producing a very solid, durable lump.

Rammed earth

   Rammed earth is just that: sandy loam soil that is just damp, put within solid formwork, then thumped hard until it is so compressed that it rings. Water can't be compressed, so the technique doesn't work if the soil is too wet.

   Many soils like this will produce a solid, durable wall without any cement at all.. However, others are just not cohesive enough, and a tiny proportion of cement makes all the difference. My 'sandy soil' (actually, decayed granite) needs 5% cement: one unit of cement for 20 units of soil. Another soil I have used needs only 1% -- but without this the final surface tends to be a little crumbly.

   How can you determine the right proportion? Try a little systematic experimentation. Either hire/borrow a brick press (Cinva ram, Dalrac 'Mud Brick Press') for a day, or make up a small formwork strong enough to stand up to ramming a small amount of soil with a hammer handle, and dismountable. Produce a few samples with 0%, 1%, 2%, 2.5%, 5% and 8% cement. Scratch identifying marks in each, and let them thoroughly cure, at least two weeks. Then scratch the surface of each, pressing hard, using an object like a key or a 100 mm nail. A good sample is one in which you can't make a groove deeper than 6 mm. Choose the lowest cement content that achieves this.

   Rammed earth has one advantage over any other earth building technique: it needs no curing time between courses. You can build on top of, or next to a section immediately after taking off the formwork. It is very efficient in terms of hours of labour per cubic metre of wall.

Poured earth

   I have mentioned 'formwork cob'. This is an ancient technique. Mortar is placed within a formwork, and allowed to cure. This mortar can be clayey soil, suitable for making mudbricks; or a sandy-soil - cement mix. In terms of hours of work per cubic metre of wall, it is as efficient as rammed earth. With clay, it is THREE TO FIVE TIMES more efficient than making and laying mudbricks, and drying time is perhaps one-third. With sandy soil, it takes longer in elapsed time than rammed earth but the work is less heavy.

   In recent years, people have reinvented this technique, and given it a new name: 'poured earth'.

   How much cement does it need? No cement at all with clay. Adding cement to clay is a waste of money, for two reasons. First, you are adding glue to glue. Clay sticks together perfectly well without any additives. Second, cement works by coating every particle with a powder that will turn to stone. The small, flat particles of clay have a very large total surface area. To do any good, you would need a tremendous amount of cement. The gaps between the particles are tiny. To get the cement in, you would need a lot of water -- and excess water weakens the final product!

   A rough test will tell you if cement will do any good or not. Mix up a slurry of soil in a bucket. Make it the consistency of pea soup. Cement will work with this soil if there is clear water on the top after half an hour.

   With sandy soil, do the same kind of testing as with rammed earth: make samples with varying cement content, let them cure, then pick the lowest cement content giving a good, solid wall. If rammed earth with this soil needed cement, poured earth will probably need up to twice as much (because the mortar has more water, weakening it). Avoid using too much cement. You don't want a concrete wall -- it conducts heat too well.

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