Hi this is my first insturctable, so hope you forgive me if I don’t give enough details or I babble on to much. I decide to make a concrete chair, for as cheap as possible with simple tools and using mostly materials I had laying around. the chairs ended up evolved through a few stages as I had to overcome problems and tried to make it as light as I possibly could. The Mk2 chairs ended up weighing about 170lb which is not really light but movable. So here’s what I did..
For The Mould
- 18 mm (3/4inch) MDF - about a sheet will be plenty I used off cuts I had laying around. (Plywood will also work)
- L Brackets - I used 32, but I don’t think you can use to many.
- 3 x round 25 L Oil drums - I had these 3 lying about but any 1-1.5 mm plastic should work ( I tried plastic from a blue water drum. but I found that it was to think and it kept fighting against the shape of the form.
- Screws - lots and lots need to be less then 18mm as you don’t want them going through the MDF
- Tacking nails - lots and lots and lots. The one I used were about 15mm long.
- Expanding foam- at least one large can.
- Insulation board - I used the stuff they use in building houses. Think it goes between walls.
- 1 inch PVC pipe - 16 inch length
- 3 inch PVC pipe - 16 inch lengths
- Duct tape. - About a roll.
- Stanley/craft knife
- Hand/pad saw
For The Concrete
I’m assuming you could use a ready to mix bag of some description but I mixed my own.
- Sand - just good old general-purpose builders sand (9 shovelfuls - Per chair approx)
- Aggregate - I used a special lightweight aggregate ( its about 20% light then normal aggregates) but any normal gravel/stone will work. ( 6 shovelfuls -per chair approx)
- Cement - 1 Bag ( 3 shovelfuls - per chair approx)
- Concrete fibres -They are basically little pieces of fibreglass
- Water - H2O
- Oil - Anything to oil the mould, I used duck oil. But any oil should work.
- Cement mixer - you could just mix it on a board or in a bucket with a paddle mixer
- Offcuts of wood - for agitating and tamping
- Trowel - I modified mine to fit better in the mould (see pic)
- Paintbrush/spray bottle - for greasing the mould
For Grinding and Sealing
- Concrete grinding/polishing pads
- Variable speed is sander of some description
- Concrete sealer or concrete paint
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Step 1: Mini Versions
I did make a set of mini chairs and table before I attempted a full size Version I won't go in to much detail on them tho as I didn’t make a mini MK2 Version but thought I'd add this step just to show a few of the picture of the mini versions I made, and they look pretty cool and would make cool garden ornaments.
Step 2: Making the Mould MK1.
Ok. I started by drawing out the profile of the chair on to 3/4’ MDF and cutting it out with a jigsaw,be as precise as possible when cutting out arch as you will need both pieces
Then you need to prepare your plastic if like me you are using a 25l round oil drum start by cutting of the top and bottom and slicing down the middle to give yourself a large rectangle.I found a Stanley/craft knife worked pretty well, Hopefully you end up with a 16" by 35" sheet. Now using the tacking nails attach one of the long edge of the plastic sheet to the inside curve arch you cut out of the the profile. then attaching the other end to inside of the arch on the profile forming the arched section of the mould.
Next cut the MDF to size to form the side of the mould.
- Back 16 3/4" x 32"
- Top 16 3/4" x 3 1/2"
- Back Rest 16 3/4" x 16 1/2"
- Seat 16 3/4" x 14"
- Font 16 3/4" x 16"
- Bottom 16 3/4" x 21 1/2"
Its worth added 2 extra 16" bit to the bottom peice to keep the bottom edge of the curve straight.
Also cut one side on the top, and 1 side of the seat and both sides of the back rest pieces at a 15 degrees angle to make the joins meet up flush. The back rest angles need to be same Direction forming a parallelogram.
Now it's definitely worth giving these coat of lacquer/Varnish to protect the mould from the water in the concrete before fixing these to the sides of the profile piece using the L brackets and screws, use at least 2 brackets per side. Then attaching all adjoining sides using the same L brackets and screws, You'll had to bend the Back Rest > Top and the Back Rest > Seat brackets to the right angle. but this is easy enough to do with just your hands. Once you screwed everything together (and you can't over do it with the amount of brackets you use , the more the better.) That's the mould complete...(Well the MK1)
Step 3: Concrete Mix and MK1 Pouring
You'll probably want to make the MK2 modifications before you actually cast your first chair.
First you need to oil the mould, to make sure the concrete dosent stick to do it. a spray bottle or a brush work great.
Next you need to make up the concrete, either using a bag ready mix concrete or making you own.
I made mine in a 3- 2-1 ratio, being 3 shovelfuls of sand 2 Shovelfuls of aggregates and 1 shovelful of cement. I find it took around a 9-6-3 mix to fill the mould but this will depend the size of your shovel. I added about a handful of concrete fibers to this mix (These are definitely worth using if you can get hold of them, as they bond the concrete into one homogenous structure) Now adding the water slowly a bit at time as you don't want the concrete to wet, you'll have to use your judgement here as I can't really think a good way to describe the right consistency.
Using a small garden trowel to shovel the concrete from the mixer to mould, filling it about half way then taking an off cut of wood and poking and agitating the concrete into the corners and taking a wooden hammer and banging on the sides to bring the air bubbles to the surface, before filling the mould to the top and repeating the agitating and banging The more you Bang on the sides of the better, as this will give you a smoother finish to the concrete with less air bubbles. Before troweling the top smooth
Now we just wait....
Step 4: MK1 Fails
Ok the MK1 did work as a chair but there are a few problems The curve collapsed under the weight of the concrete I thought it would hold it shape but apparently this was wishful thinking. Also it Is a lot heavier than I anticipated I'm thinking it's got to be somewhere around 200 lbs +. So I definitely need to reinforce the curve and figure away to reduce the weight.
On to The MK2
Step 5: MK2 Mould
Ok the first problem with the Mk1 was the weight of the concrete collapsing the curve part of the mould. The easiest way I could think to solve this problem was to fill the cavity in the mould with some insulation foam I had lying around. you'll need to cut 4 layers the same size as the cavity and an off cut of MDF to the same shape to hold them all tight in place.
The next issue to address was the staggering weight of the MK1 so I decided to take a curved portion out on each straight edge side. I did this by first drawing on the MK1 to get a rough idea of what curves i can take out of each side once happy with there look ,Cut out MDF curves to the same size. mine were
- Back. - 22" by 2 1/2"
- Back Rest - 12" by 1"
- Seat - 12" by 1"
- Front - 8" by 1 1/4"
Once you're happy with your MDF blanks, make identical duplicates of each one. Then cutting up another plastic barrel for the plastic I cut it to a 16" and the using my MDF blanks as a guide and cut out the right length for each pair of curved banks. then tacking the plastic to the pair of curves with tacking nails use an off cut off MDF to brace between the 2 MDF blanks to give it a bit of support. Then fill the back of the curve with expanding foam to stop the curve caving in with the weight of the concrete pushing against it. once the expanding foam had dried cut any excess of the back, this curve is now ready to be attached to the MK1 do this by screwing throw the MK1 to the support piece on the curve. do this for all of the curved sections.
For transition between the Front and seat sides use a piece of PVC pipe to form the curve. cut a quarter section of a 1 1/2" inch PVC pipe by cutting it in half down the middle and a half again and tacking it to the front side of the mould( keep the other quarter of the PVC pipe for the table mould)
Use a half section of a 2 1/2" PVC pipe to form the curve for the Top side and tack in place with tacking nails.
Use duct tape to convert any nail holes and smooth out the transition between curved and straight sections.
MK2 mould should now be complete
Step 6: MK2 Pour
Exactly the same as the MK1 pour. (See Step 3)
Step 7: De-moulding
You want to leave the concrete to cure for as long as possible, at least 24hr ( you’ll see why it worth waiting on the next step) simply unscrew all the L brackets and carefully peel the sides off, the only tricky part is removing the main arch, I found it best of remove the installation from the arch to give the plastic room to peel away from the concrete. slow and steady is the key to removing the main arch.
Step 8: Ooops.
I was a bit eager demoulding my second chair. ( barely 12hours) And managed to split it in half, clean down the middle well removing the main arch! Fortunately I had some R kem+ lying Around from putting in some anchor bolts! Wasn’t sure if it would work but gotta say iv jumped up and down on it and it’s as solid as a rock!
Step 9: Grinding
You’ll want to smooth out the curves and some of the imperfections from the mould especially on the seat and back rest if it has bulged out, doing you’ll probably also want to remove the top layer of The concrete and expose some of the stone. I bought some stone and concrete polishing disks of eBay I got 30 grit up to 6000 grit, but I only used the 30/50/100 grit. I used these disk on a car polishing mop as it the only thing I had with variable speed, but in the orbital sander type thing should work fine.
Not much more I can say on this step just keep grinding away till you happy with how it looks and feels.
Step 10: Table Mould and Pour
I designed the table mould to be a modification of the MK2 mould, only needing 2 modification, You will need to make a mirrored duplicate of the MKII moulds front side including the pic pipe. (exactly the same process as in making the MK2 chair mould (Step 5)) and also a Top section by cutting a 21 1/2" by 16 3/4" pice of MDF with a 6' by 3/4' notch, 3/4 of an inch from the edge cut out to fit over the back rest part of the mould. and attaching everything together using the L brackets.
Pouring and demoulding is exactly the same as their chairs except I found a large 6-4-2 mix was enough to fill the table mould.
Step 11: Sealing
The last step is to seal the concrete against the elements as it will absorb water which would freeze and crack the concrete in harsh winters.
At first I used a colour enhancing sealer bit I decided this made the concrete look to dark and kind of grubby looking (There first 2 photos) so I bought a transparent stone sealer. Which made water bead and run off with no visible difference to the concrete finish. but what sealer you use is totally up to you. Apply the sealer with a brush or roller.
You could alway paint the chairs/table if you'd prefer to.
Job Done - now time to sit down and enjoy your beautiful table and chairs.
Second Prize in the
Stone Concrete and Cement Contest