Formwork

Formwork is the basis of rammed earth. Formwork allows a loose material to be placed directly into a wall with no intermediate steps apart from mixing. Although there has been some work done with pre-fabrication recently the advantage of building in-situ is homogenous walls from materials which are lower specification than for bricks or blocks.

There are essentially two types of formwork, a movable type and a static type. Moveable means each formwork assembly is filled, disassembled and then set up again. Typically a building will be built horizontally, with the first course being completed before the second is begun.

Static formwork by contrast normally rises vertically to between 2m-4m. Care has to be taken in the assembly and disassembly but generally this is a rapid way to build. It allows the use of system built formwork which may be hired or bought.

Moveable, horizontal formwork.

This type of formwork is found across the world, very similar typology from Morocco to Peru, France to Afghanistan and Bhutan to Australia. Traditionally it is carpenters who assemble a formwork and a ramming team, as they are the ones who understand the tooling.

Much of the skill in moving formwork is in the rapid assembly and disassembly of the basic components. Its also in the choice of materials which must be strong enough not to deform or break while at the same time being light enough to be manoeuvrable. The fixings between the two sides need be both strong enough to control the loads but also not too fiddly or liable to  jam. Many systems use wedges to tighten and untighten and for adjusting plumb. Threaded rods are also used to good effect, but these can be more time consuming than simple rope ties and tourniquet's.

The most common material for this type of formwork is timber. It has both the strength and flexibility as well as elasticity that however often it is twisted out of shape it keeps springing back. Modern timber materials like plywood have these same characteristics although they may lose surface quality quicker than timber. Composites like chipboard and strand board are to be avoided unless the effect of bulging and splintering surface finish is the desired one.

One material which works very well is timber scaffold planks. Regular in shape and size, 35mm thick and evenly graded these boards either new or second hand take a lot of beating. A set of moveable forms would have a lot of life in them.

Steel formwork also works but the relationship between weight and strength has to be carefully assessed. Although ramming may take place manually, building up layers centimetres at a time the overall effect is to produce a lot of pressure. While the formwork contains it there is no problem. As soon as the pressure begins to deform the sides there is no going back and with steel this mean the steel is bent once and for all.

System formwork as shown can also be used as moving formwork, panels which are 1.2m long by .6m high each weigh 35kg and so are just about man-handleable. With steel ribs which are 90mm deep and a 15mm epoxy resin bonded face they are very resilient. However they are heavy and the smooth finish can lead to issues with removing them from fresh work.

Formwork leaves its mark on rammed earth, timber grain is often clearly visible on walls that can be a hundred years old. For this reason it is worth considering the material the form sides are

Hybrid formwork

In China and in Serbia there is another kind of formwork which slip forms in a different way, using vertical fixed sections to contain movable form sides. Once the vertical sections are fixed the work proceeds very quickly, with very long sections being rammed as a single piece with no joins.

More recently we have been using a hybrid of this system fully boarded, not slip formed, but in shorter static setups, see image at bottom. So formwork has a lot of potential solutions, what is described above covers most of the options so far, but by no means all of the myriad differences in detail.

Finally we have also done some work with textile formwork, but that's another story...

Static formwork

Static formwork really comes from the concrete industry. Separate elements are built up to make a single large section. In concrete work the whole thing is assembled before the liquid is poured in, with rammed earth the sections are assembled as the work proceeds, allowing access into the formwork for people, materials and rammers. With thicker walls the sections may be built up higher, in a formwork which is 400mm wide or more there is room to manoeuvre but with walls that are 300mm thick a formwork higher than 900mm becomes awkward.

The advantages of static formwork are that the more vulnerable vertical and horizontal joins in moving formwork are now contained in an ever expanding box, so finish is easier to control. Also the major setting up is in the first lift, the formwork which follows more or less just clips on.