Introduction: Modular Concrete Bench

About: Studied Architecture at Brighton But now spend a lot of time building replica props or random gadgets.

UPDATE: There have been lots of questions about safety and practicality aspects. The pictures here aren't of the final placement of the seat, because they wanted to put it in a bunch of architectural galleries first, so I have added another step addressing things you need to consider for long term placement of something like this.

This was a summer project run within the architecture department at the University of Brighton.
It was a group project, has gone over a year with no further development, so I can't imagine anyone will mind me showing the process here.
If you try this yourself or take inspiration from here please cite 'University of Brighton Architecture Department' as a source.

See here for all the people involved in this project

The Brief:
Design a chair / seat predominately made of concrete.

The Competition:
Consisted of several rounds finally producing a winning design

The Winner:
Was a design for modular blocks which could be stacked to create different seating arrangements.

The winner couldn't be there for the build so we developed the deign adding lugs onto the bricks to help them fit together more securely.
We then began to experiment with different textures to add to the blocks.

The Build:
Initially took two weeks, then about a month coming in every 3 - 4 days to break the moulds, clean them and cast new blocks.
Read on for details

Step 1: Testing

We took inspiration for our design from Roman columns. To secure each piece of stone to the one below it they would have lumps projecting out of one stone and holes in the other that would fit snugly together.

We used laminated chipboard and constructed a simple box. In one side of the box we cut out a circle and sunk a small plastic dish in there (sealing it up with silicone).

On the opposite side we placed a plaster filled dish straight on the side.

We made two of these boxes and cast them.

The finished bricks came out really well. To me they look like giant versions of lego 1x1 bricks.

For the final version we used 4 square lugs on top (including 2 lugs on the sides).

Step 2: Playing With Finishes

Possibly the best bit, of any concrete project, is playing around with the finishes.

This is your chance to metaphorically slap anyone who has ever said: 

"Concrete? Isn't that a bit dull?"

Most of this was relief work (putting stuff inside the moulds to create patterns in the surface of the concrete).

You can also add dyes to the mix or stain the concrete afterward.

Build yourself some small wooden boxes and try as many different materials as you can get your hands on.

Among my favourites are the glue gun patterns and the leaves, the mixture of organic and man made, plus it kind of looks fossilized.

We also experimented with using glass as an aggregate, but this requires lots of grinding / polishing to see the bits of glass.

Step 3: Designing the Moulds

Make sure you have your design locked down by this stage. You're going to make a few moulds that all need to be the same.

When designing a concrete mould take several things into account:

Your mould will be taken apart and put back together several times, so use screws, not nails or glue. 

The concrete will pick up great detail. A wood grain effect can look really nice, but if you want a clean smooth edge then use laminate board. Also make sure any holes are filled and that you grease or oil your mould to stop the concrete soaking into the wood.

Concrete is deceptively heavy, It will bust out of your mould if you're not careful. Then you will have a broken mould and concrete everywhere.
Also you are going to be knocking the moulds around a lot when agitating the concrete. 

Try not to screw into the side of a sheet of wood. Instead screw into a batten fixed to the sheet (see building the mould for more info)

To each step I've attached a diagram of how our mould was assembled. I've also attached the full vector PDF here.

This seems as good a place as any for a materials list.

  • 18mm Plywood sheet
  • 18mm² Timber batten
  • Wood screws


  • Cement
  • Sharp sand
  • Gravel
  • Plasticiser
  • Water
Tools (bare minimum):
  • Drill & bits
  • Mitre saw
  • Large clamps
  • Wood glue
  • Wood filler
  • Screwdriver
  • Trowel
  • Pencil, ruler, etc.

Step 4: Building the Moulds Part 1

Depending on how many bricks you want you may only need a couple of moulds. Each brick will take about 3 days to cure so 1 mould will produce 2 bricks a week.

We made 5 moulds which gave us about 30 bricks over the period of 3 weeks.

This could conceivably be done without masses of shop tools, but you will need to be able to control the angle of your saw cuts, many of the pieces have chamfered edges.

The diagrams show you how it all went together, you'll probably be drawing up your own measurements, but if anyone needs it I can provide exact sizes and a cutting list.

We cut all of our pieces out of 18mm plywood, using a table saw. We sanded off the rough edges and put the pieces together as follows.

For Bottom Piece:
Using a square and ruler mark the location of the four lugs. 
Spread a thin layer of wood glue on the back of each lug and place it on the board, pressing firmly. Make sure each lug is lined up straight then flip the board and put two screws into the back of each lug (through the board).

Check the lugs are still correctly aligned then screw on the 18mm wooden battens, using glue and a couple of screws, per batten.

For Positive Side:
Using a square and ruler mark the position of the lug, carefully glue the lug in place flip over and screw the lug in place. Screw a piece of timber battening to the short edge.

Once cut to size this piece is done, nice and simple.

Step 5: Building the Moulds Part 2

Top Piece:
Arrange the pieces and make sure everything looks right / fits. Begin to assemble to pieces using wood glue. When it is all glued then clamp the whole piece and weight it down (so no pieces pop out)

Once this is dry un-clamp it and screw on the back board and battening.

Negative Side Piece:
Same as above but with less bits :)

Now that you have all the pieces assembled you need to make sure the faces are in good condition. This means filling in any knots or visible screws.

Once you have done that, sand the whole face removing any odd lumps or edges.

Step 6: Preparing the Moulds

Once you have all the bits ready you can assemble you mould. Notice that because of the design of the mould we aren't screwing into the edges of the boards, we are screwing into the timber battens behind them. (see second picture)

This is stronger, will last longer and there is no danger of splitting the panels

Now that you have a completed mould you can do whatever you want to make it more interesting, or just cast it plain.

We put leaves, glue gun patterns, bubble wrap and other things in the moulds, as well as burning the surface of one mould to get a really cool wood texture.

The final stage is to use some kind of mould release. If your mould is shiny (laminate) you won't need this, otherwise you will need to grease / oil you mould a lot.

We started out using Vaseline, but for the majority we used WD40 because it was faster and easier to get into small gaps.

You should do this as close to casting as possible, and the moulds will need re-oiling after each use.

Step 7: Casting

This will be a lot easier if you have a cement mixer, but for smaller batches you can just mix things up on a board using a trowel. 

The ratios quoted here are in volume. They can easily be scaled up, we used a small bucket to measure materials but you can measure by trowel-fulls.

A standard mix will do fine for this build, 1 part cement, 2 parts sharp sand and 3 parts gravel.

In addition to this we are using a plasticiser which should stop the concrete cracking in cold weather.

Measure out the dry parts into your mixer and start it up. Once the dry parts are mixed then add your plasticiser, it comes in large bottles, and you only need a small amount per mix.

Now you can begin to slowly add water. If you haven't done this before, go really slowly. It may seem like nothing is happening but the concrete will suddenly hit the right consistency and you can easily go too far and end up having to add more sand and cement to compensate.

To get an idea of the right consistency look at the last two pictures. You should be able to shovel the concrete and have it retain some form, but when you agitate it it should settle down and flatten out.

Once the mould is filled you need to agitate the mould. This not only flattens off the top, but gets rid of any air pockets and allows the concrete to get into all the small gaps of the mould, picking up all the detail.

There are several ways to do this:
  • On a sturdy surface rock the whole mould back and fourth quite violently (watch your fingers) 
  • Take a big rubber mallet and whack the sides / base of the mould.
  • Find a power tool you can make vibrate (drill with an offset weight on it) and strap this to the mould.
Once you get tired of shaking the mould around then level off the top with a bit of wood and leave it to cure (3 - 4 days)

Where you leave the mould will make a difference in curing. Extreme high or low temps will mean the concrete is weak and crumbly.

Step 8: Batch Production

Now that your first batch is cured it's time to break out the moulds. 
Carefully unscrew the moulds, and remove all the panels, if you used enough mould release this should be easy, if you have any trouble try sliding a palette knife between the mould and brick to sepatate them.

Be careful! You are now handling a very heavy weight that will happily break your toes/ fingers if dropped.

Once all the bricks are out clean the moulds and go back to step 6 sanding the moulds, assembling them, oiling them and casting.

This method is known as batch production and becomes more efficient the larger the batch is.

Step 9: Installation

The plasticizer alone won't stop your bricks from cracking if left exposed to the elements.

You will need to seal them somehow. The simplest way of doing this is to coat them in PVA. Go pick up a big tub from your local hardware store, paint it onto the concrete and it will soak into the surface and , when dry, provide a rudimentary barrier against water.

For this set up to be safe for a public environment it needs to be stable and theft proof. 

When you are happy with the arrangement of your seat, take it apart and start reassembling it, but this time place a good lump of grip-fill between each lug connection. Build the whole seat up this way and once the grip-fill is set, your seat will be completely immobile and a permanent fixture (so make sure it's in the right place).

Of course other weatherproofing and fixing solutions are available and I encourage you to research them.

Now enjoy the large gallery of final pics.

Step 10: Practicality and Longevity

If you want your project to survive for a long time outside and be safe there are lots of things you have to consider.
I mentioned most of them already, but people keep asking so I'm collecting them in this extra step.

  In a public setting you are liable for anyone who hurts themselves on or with your project.

- Sharp edges, you can rent a concrete grinder for a day and just take off all the sharp edges, makes sure not to leave anything likely to catch on clothing or cut skin.

- Weight, a lot of people assume that you want it be as light as possible, which you can do, but concrete is still heavy and will injure people if it falls on them, this is tied in with...

- Stability, make sure your project won't fall over on someone and that they can't pick it up. Normally if it's too heavy for anyone to lift by hand it is considered safe, however if it is too light then you will have to bolt it to the ground, which is a pain. So having a heavy project can be helpful.

- Theft, the problem with the public is that it includes everyone, and some of them are dicks, if we left this project as it was then someone would think it was funny to either steal them or move them around, which is dangerous and a pain in the ass. Our bricks were pretty heavy already, when they are finally installed on site they will be glued together (using grip fill or similar) and they will become one unmovable mass, safe and stable.

  Concrete is generally hardwearing, but it still won't last long outside if not properly protected.

- Frost Resistance, if water gets into holes etc in the concrete and freezes it will expand and can cause cracks. We added a plasticiser to the mix to help alleviate some of the effects, but sealing the surface of the concrete with PVA and filling any large gaps where water will collect will also help.

- Waterproofing, essentially the same mesures as frost resistance but for different reasons, you don't want water pooling, if it is supposed to be a seat. Our seating had lots of holes in the top so we came up with a couple of solutions, firstly we made a few 'capping blocks' without lugs in the top (pictured) secondly we filled in some of the top holes with clear resin to stop water pooling in them.

- Don't use glass as an aggregate, someone mentioned throwing glass into a concrete mix to lighten and cheapen the load. Over long periods of time glass reacts with cement and will actually weaken the concrete. Most likely not enough to matter for a seating project, but still not a good practise to get into.