Building with Earth: Design and Technology of a Sustainable Architecture

Formwork

With traditional formworks, the boards on

both sides are held apart and kept together

by spacers (5.1). These spacers pierce the

wall, causing openings that must be filled

in after removal of formwork. A system with

very thin tensile spacers (4 x 6 mm) pene-

trating the wall has been developed at the

Building Research Laboratory (BRL) (5.2).

In order to completely eliminate this dis-

advantage, spacer-free systems have been

developed (see p. 56 in this chapter).

As shown in 5.4, formworks without inter-

mediary spacers which are braced on both

sides require a lot of space and hinder site

movement considerably.

With a special formwork, rounded corners

and curved walls can also be formed (5.5).

A circular barn built in 1831 in Bollbrügge,

Germany, with 90-cm-thick rammed earth

walls is shown in 5.6.

Common formwork systems used in con-

crete technology can also be used for

rammed earth, but usually turn out to be

too heavy and expensive. In Europe, timber

panels of 19 mm thickness are commonly

used. They need to be stiffened by vertical

members at approximately 75 cm intervals.

If this is not done, they will bend outwards

during ramming. Therefore, it might be

more economical to choose thicker boards

of 30 to 45 mm thickness, which need stiff-

ening only at intervals of 100 to 150 cm.

If the soil is very clayey, the form should not

be wrenched off, but instead slipped off the

rammed earth smoothly along the surface,

thus preventing it from being spoiled by

clayey particles sticking to the form. Further-

more, it is neither desirable to have a surface

that is too rough (such as saw-cut timber),

nor one that is too smooth (such as var-

nished and planed timber).

If the formwork is not optimised for this

technique, then up to 30% of total labour

input could be invested simply in erecting,

adjusting, and dismantling the formwork.

Therefore, the following points should be

borne in mind:

• Boards must be stiff so that they do not
  bend outwards while ramming is underway.


• All parts must be light enough to be car-
  ried by two workers.


• The formwork should be easy to adjust
  in both vertical and horizontal directions.


• Variations in the thickness of the wall
  must be controllable within a specified toler-
  ance.


• It is preferable that the edges require no
  special formwork. Therefore, the formwork
  should allow varying lengths of wall to be
  cast.

53 Rammed earthworks

5.3Formwork with-

out intermediary

spacers

5.4Typical formwork

with bracing used in

China

5.5Formwork for

rounded and curved

walls

5.2

5.3

5.4

5.5