In order to fix an old chair I found without it's legs and seat I made a mould for concrete. This is a step by step guide about how I did it.

You need the following:

Wood

Concrete mix

Power drill/impact drill

Steel beams

Screws

Step 1: Making the Seat Mould

In order to achieve a natural print of wood on the seat I made a layer of wooden beams. (Optional)

I started by making a copy of the seat. (See pic. 1)

Then I measured the exact angles the seat on the chair had and used this angle to create the outline of the seat inside of the mould. (See pic. 2) In order to make space for two wooden triangles which keeps the legs together inside of the chair I made two identical pieces which I then added to the mould. I then added more wood to create an even height along side the mould to insure an even surface. I also added a back plate which will be the outside of the front legs. Please note how I've added a bit of air on the sides next to the black plate. This will be the thickness of the legs. (See pic. 3) Afterwards I added a plate and sides to what's going to be the front legs of the chair.

Also note how the added wooden pieces and triangles "float" approx. 1 cm above the wooden beams in order to make the finished concrete seat fit neatly into the chair which has a slightly lowered seat. See picture 1 and look how the wooden seat fits into the chair.

Step 2: Bending Metal

I bent 4 small metal rods in order to make the concrete more durable and stronger once cured. You can either bend the metal using a machine or by hand. I made 2 short bars and two longer. I did this because the long bars wouldn't fit the sides of the mould. (Because of the wooden triangles, see step 1)

Step 3: Mixing the Concrete

To mix the concrete I used two bags (40kg in total) which I poured into two buckets and added water along the mixing process to insure I didn't use too much water. To much water will make it wet and hard to handle but too little water will make it hard to get into the corners.

The concrete used was pre-mixed with sand and small stones (0-4mm). I just added water.

I used about 1.5 - 2 L water to make the concrete a bit more wet to get it into all of the corners inside the mould.

Step 4: Pouring the Concrete

After mixing the concrete it's now ready to be poured into the mould. I started making a small layer filling up approx. half the mould of the seat. I then lowered the metal rods into the still wet concrete and filled up tjhe mould completely covering the rods.

I closed the mould of the seat with a piece of wood.

Then I poured the concrete down from the top to form what would end up being the front legs of the chair.

To get all air bubbles out I used an impact drill to create lots of vibrations on the outside of the mould. Vibrating the mould makes the concrete stronger as it remove the air bubbles which inevitably have formed during the process. (See pic. 2)

After pouring the concrete into the mould I put plastic bags on top to make sure the concrete doesn't cure too fast which will make it crack. (See pic. 3)

Step 5: Removing the Mould

Step 6: Final Assembling

After removing the mould I carefully placed the concrete into the chair.

If you want to it's possible to treat the concrete with some wax to give it a cleaner look but for this project I personally wanted the raw concrete to clash with the old chair.

Now you've finished your project! Hope you had fun and learnt a thing or two :)