This table takes advantage of a unique property of glass fiber-reinforced concrete (GFRC). The material can be bent roughly 20 minutes after casting, when it has entered a false-set. I made this mould, and the one for the blue table, to demonstrate this technique for the 3-day advanced fiber-reinforced concrete training at work.
This had to be a quick process because notice was short on thursday with training on monday, so there was only time for a little bit of ideation, sketching, paper models, and 1:1 cardboard mock ups before the rubber needed to be cast.
The final piece is bent, but it starts out as a flat shape, so mock ups were cut out of cardboard to get a feel for how the shape could bend, and to determine what the final height would be.
After some small adjustments to the cardboard, it was ready to be used as a template to trace onto a sheet of 3/4″ melamine. The shape was laid out, cut out with a jig saw, and the edges were sanded with an orbital.
The positive gets stuck to the base board, some walls are built around it, and then polyurethane rubber is cast to create the mould. To keep air bubbles from escaping from underneath the positive, it is sealed with silicone to the base board and fixed with a few screws so it won’t slide around.
The edges of the melamine positive should be sealed before casting rubber, or at least they should be smooth, because any detail in the positive will be transferred to the rubber, which will then show up in the finished concrete piece. A quick and dirty way to seal wood is to use clear packing tape, which will easily release from the rubber without a messy texture. The inside of the form is blown out, cleaned with denatured alcohol, and sprayed with release before casting rubber.
The form walls are about 1″ from the positive, and the rubber is poured over the form until it’s about 1/4″ – 1/2″ thick. Too thin and it will distort, too thick and it’s just a waste of rubber. This is a two part polyurethane with a shore hardness of 60, meaning it’s about a medium hardness – rigid enough to hold it’s shape, but flexible enough to be bent.
To minimize air bubbles the rubber should be poured from one place. Thorough mixing is important because any un-mixed rubber won’t cure, and that is not good.
The rubber can be removed from the form after about 12 hours. If the casting surface was level and there were no problems with the mix, you’ll now have a usable rubber mould that will last for many many many castings if taken care of.
After it’s cured, the mould can be used immediately. It should be on a flat and level surface, and in this case, with table vibrators mounted nearby to drive out the air and help consolidate the concrete mix.
A GFRC mix does not contain rock aggregate like a regular concrete mix, instead relies on a range of sizes and types of fiber to make it strong (10,500 psi compressive strength, 2500 psi flexural strength!). Even when relatively thin, it’s still quite strong, without any additional reinforcing.
After about 20 minutes the piece is bent into shape and left to cure overnight.
When the bracing is removed, the piece still holds the shape, almost like magic.
The rubber releases easily from the concrete and the piece can be flipped over for finishing.
