To make this stool you will need:
- 1.5 metres of concrete impregnated fabric with reinforcement (concrete canvas or similar) of 5mm thickness. The standard width for that thickness is 1m.
- autocad, rhino or your prefered CAD software.
- large format laser cutter
-2 thick cardboard tubes (like the ones that come inside fabric or paper rolls) cut in 50 cm pieces
- 60cm diameter cardboard circle
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Step 1: Designing the Pattern
Design a cut and joint pattern using AutoCad, Rhino or another CAD software.
It is important first to make a circle of 60cm diameter at the centre where you will seat. Then offset this circle to start defining your cut and joints areas. The marked areas are the cuts, in between the cuts where you don't see anything are the joints. It is important to remember that the pattern of cuts will define the 3D volume so test different options. The following criteria is very important to consider:
- The cuts on a flat material need to be offset to achieve a concave geometry. The spacing between the cuts needs to consider how the concrete will expand and avoid extreme clustering that will result in long thin elements that can buckle given a very small distance from the edges. If the cuts are too far apart the pop-up will be too shallow. For the stool a minimum section of 30mm is needed otherwise it will collapse.
- The joints between cuts affect the stability of the overall structure. The joints are the areas where there is no cut. The best results are obtained by staggering the joints as the system becomes more rigid. For the stool joints need to be between 40-50mm to create rigid conditions.
Step 2: Cutting
Once you have decided on a pattern, make all your lines the same colour and export for laser cutting.
It is important to consider the dimensions of the laser cutter and its maximum cutting area, which generally is smaller. Files are sent to the laser cutter at 1:1 scale. Therefore the file should be prepared to the size of the stool. Laser cutters normally work with pdf files but is better to ask the administrator.
Before you start cutting place a piece of wood below the concrete canvas piece (1.00 by 1.5m) and make sure that the laser cutter only cuts the fabric and doesn't go all the way through the wood. This is to make sure that you don't loose a lot of concrete when picking up your piece from the laser cutter, as this will weaken the overall structure.
In this case we also tested a robot with a circular cutter attached which works fine for larger pieces where a laser cutter is not big enough.
The fabric impregnated concrete has two membranes one is fibre and one is pvc with reinforcement and cement in between. Make sure that you are cutting with the fibre side up, otherwise the reflection might break the lens of the laser cutter and it will not be able to cut. For the robot is the same the fibre side is better up.
Step 3: Hydrating
After your piece is cut the next step it to hydrate it as much as possible in its flat state.
The concrete canvas will absorb a lot of water. How much depends on your environment and temperature in this case 9 litres was enough. The material can not be over hydrated so an excess of water is always better.
If you manipulate the fabric before hydrating you will loose a lot of concrete, as its powdery- and weaken your overall piece. When hydrating is important to remember to keep your fibre side up, otherwise the pvc will not allow for the water to go inside the material. The pvc membrane main function is waterproofing.
Once the concrete is fully soaked you have 3 hours to manipulate it into shape.
Make a cardboard disk of 60 cm diameter to attach to the seating area and keep it flat.
Cut a cardboard tube, like the ones inside the fabric rolls or plotting paper rolls - in 60 cm sections.
Place both of the above elements under your concrete fabric piece to lift it. The pattern of cuts will give it different degrees of flexibility, if you did less cuts it will be shallower and less flexible. Without cuts it will barely lift.
Once you have lifted it into position, staple the bottom edge to a wooden board to make sure that it remains flat, all the pieces in between will cascade, you can play around with the bottom edge while stapling it to give different configurations.
After you are satisfied with the shape add more water, approximately 5 more litres and leave to settle for 48 hours. If the environment is very wet you might need more time
Step 4: Concrete Stool
After the 48 hours remove the formwork and you have a rigid concrete stool!
The concrete can be painted to achieve different effects.