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Rammed earth (pise) versus
mudbrick (adobe)
These two
techniques use different types of soil (See Any soil will
do, page 95).
It is
silly to decide on a building technique in the abstract and then
to have to buy and transport the materials for it. It is better
to adapt the technique to the soil on the site. Nevertheless, the
most frequent question asked about rammed earth is: 'How does it
compare with mudbrick?' Here is a comparison.
Advantages of rammed earth
*
Other things being
equal, rammed earth is about half as labour intensive as
mudbrick. There are two reasons for this.
First, rammed
earth is a means of constructing the wall in place. Any such
monolithic technique involves less work than the making and
combining of materials. Mudbricks have to be made (mixed and
shovelled into forms), handled several times during drying
(stacked and turned), even perhaps stored and later removed from
storage, transported to the job and laid with mortar mixed for
the purpose. Rammed earth involves preparing and transporting the
soil only once. The soil may have to be sifted and cement will
probably need to be mixed in, but then the soil is shovelled into
forms (or poured from a bucket), rammed, and there you are, with
a completed piece of wall.
Second, ramming
soil is used just damp, not wet and sticky. Mixing and handling
wet soil is much harder work than anything you need to do when
ramming.
* Rammed earth doesn't
shrink. There are no worries with cracking, no gaps at the top or
next to doors and windows. It is true that insufficiently
compacted rammed earth tends to powder away. This can usually be
repaired (see Preparation for rendering on page 153) but
it is a problem that's best avoided.
* There is a limit to
how many courses of bricks, stone or mudbricks a person can lay
in one day. Too much, and the still-soft mortar gets squeezed
out. There is no such limit with rammed earth. I have seen a team
of eight people complete a 4 metre long, 3 metre high wall on a
single day (this wall was three sections long and five sections
high).
* Mudbrick making and
mortar mixing require a lot of water. Ramming needs only a
little. This can be a big advantage if water is in short supply
or has to be brought in by drums or buckets.
Mudbricks take
time to 'cure' before they can be moved. This means that you need
either a lot of flat space under cover, or you will have to spend
extra time covering and uncovering the bricks as they dry. There
are no such space problems with rammed earth.
Advantages of mudbricks
* Mudbricks (and
compressed earth blocks) can be made in advance, for example
while your house plans are still being drawn up. In this way you
can put 'labour in the bank'. You could also employ a number of
helpers by having some people make mudbricks while others are
laying them. Rammed earth requires a small, practised team.
* Rammed earth usually requires the addition of cement (average of 5%), while ramming forms must be strong and solid. You need money for both of these. In contrast, mudbricks can be made using a mould made up of scraps, and a bale of straw (if needed) will last for hundreds of bricks. So choose mudbricks if you are strong and have lots of time, but little money.
* Mudbricks can be made in rainy weather, provided there is a flat area under cover. You may be brought to a standstill while ramming if rain over-wets your stockpile of soil, though soil dug out from a steep bank as needed will usually be dry enough.
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