Introduction: Modular Concrete Bench

Picture of Modular Concrete Bench

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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


rhino (author)2012-06-24

Now you need cushions that lock into the waffle tops. Should be pretty easy to get done.

jgbataille (author)rhino2016-01-12

That was my first thought when I saw this. Brilliant design!

CharlotteE2 (author)2015-06-19

Hm, I didn't know that you could make your own concrete mold. That is good to know. I have found pre-made concrete molds in home improvement stores, but it would be fun it make your own. It would also make your home personal and unique. Thanks for your tutorials.

janetsellers08 (author)2014-10-19

We put silicone along all the seams of our concrete artwork, as it makes the edges just a bit rounded and thereby more safe. But anybody falling onto concrete at any age will likely have a boo-boo since the concrete is much harder than the person.

Rigidman (author)2012-10-05

I've been wanting to make some concrete chairs similar to a regular living room chair (not wingback) or an adirondac chair. the molds would have to be 2 piece. 1 for the sitting portion and 1 for the bottom with legs. I like your idea. Thanks

mrblint (author)2012-06-26

I am not very familiar with concrete molding techniques. Is there a way to chamfer the edges and round-off the corners? The edges and corners of the blocks are very sharp with potential safety issues. If such seats were placed in a public park rather than in front of an office-building, imagine a toddler stumbling on a paving stone and striking the edge or corner of the block with his/her head.

madmanmoe64 (author)mrblint2012-06-27

The pictures there aren't of the final installation, just a placement to take some glamour shots.

What we ended up doing was renting a concrete grinder for about £20 and using that to take the edges off of all the blocks.

It is possible to round off the corners in the mould, but the way we have ours set up it would take way too long to reset each time.

kludge000 (author)2012-06-25

have you considered cob? that would make the blocks lighter (a little) and made from renewable resources, also a lot cheaper to make.

thank you for your time and consideration
Peace Jeff

stealthop (author)2012-06-25

reminds me of minecraft , very nice

vern1530 (author)2012-06-25

Diesel in a pump sprayer works well to. We used it on big forms.

RayMit (author)2012-06-25

Nice project well described......try using foam cement, ie. replacing the stone with air bubbles. Makes the end result much lighter...also known as lightweight concrete.

ManifoldSky (author)2012-06-24

Nice work.
I made a few similar projects, and thought I'd share a few suggestions.
First, if you would like the units to be lighter and easier to transport and rearrange, you can insert either an empty box inside the mould (which you won't recover) or alternately, build the bottom first, insert a rectangular tube form (box with no top or bottom) into the center, fill the sides of the mould, forming your "rectangular tube", fill the inner form with packing peanuts or some other light substance for volume, pour additional cement into the inner form, and remove. Then fill to the top of the outer form. Now the center is mostly Styrofoam, and the final block is significantly lighter.
Alternately (or additionally) you might want to consider using hypertufa or papercrete in place of regular concrete. hypertufa is made using portland cement, and peat moss instead of the sand and aggregate. It is very strong and very light.
Papercrete is similar, but uses paper pulp, like pulverized newsprint, instead of peat. It too is very strong and very light.

The outer surfaces on hypertufa are very organic looking and pretty. One idea that works well is to attach moss samples onto the inner sides of the mould prior to pouring the hypertufa. When the unit dries, the moss is embedded into the sides, and continues to grow, creating a nice, soft, natural surface.

ElectroFrank (author)ManifoldSky2012-06-24

For units a public place, it has to be heavy, or kids will just roll them about for fun. If the individual units were made lighter, they would have to be cemented down and together to prevent vandalism.

madmanmoe64 (author)ElectroFrank2012-06-24

Absolutely right, whenever you have a project in a public space there's a whole bunch of stuff like that you have to satisfy or they just won't let you do it at all.

glorybe (author)ManifoldSky2012-06-24

Papercrete is very hard to get dry if there is much thickness to a block. Most people use too much cement and too little sand with the paper fiber and when it is mixed the mix needs to be on the dry side of the coin unless you have a year or more to get it to dry out. And it will really need shelter form the environment while drying. I do wonder if heat could be used like it is in making cbs blocks to speed the curing process.

ManifoldSky (author)glorybe2012-06-24

Part of the trick, of course, is just not to make it very thick! :)
To this end, you can add additional reinforcing elements, if it is intended to support much weight. Making it hollow, as detailed above, helps greatly, and also makes drying time not much of an issue, since it can dry from the inside out as well as the outside in. It helps to leave a small hole in the top to let the water vapour out as it cures.
But the real trick of course is just not to use too much water in the first place, as you said. Since the curing of cement is NOT due to it drying out, but rather due to the water being used up as part of the chemical bonding reaction, starting with a dryer mix assures that there will be little to no water vapour that needs to escape.

klixtopher (author)2012-06-24

Very cool. I'd recommend adding relief cuts radiating away from the top squares to allow water to drain away, so people don't have to sit in puddles.

madmanmoe64 (author)klixtopher2012-06-24

As I mentioned in an earlier comment we had a couple of solutions for that (check for picture)

MrRedwood (author)klixtopher2012-06-24

Agreed. Even more than sitting in puddles, those concavities will be a magnet for debris, and moss in damp environments, that would dissuade people from using them. Keep the top well drained.

suayres (author)2012-06-24

Very nifty--but how comfortable are they to sit on? I think you'd almost have to cushion them in some way. I can see why you noted Lego 1X1s as a comparable design. Good job!

madmanmoe64 (author)suayres2012-06-24

Surprising comfortable, I think 20 minutes while having lunch is the longest I've ever sat on them and I wasn't noticing any ill effects.

That's pretty much what they were designed for too.

juz4kix (author)2012-06-24

Great project! Saved the PDF in my future projects file.
You could put plastic water bottles in the mold to make it lighter. beats putting them in a landfill.
Also did anyone try to figure out if you could design the nubs and voids so you could stagger the blocks and interlock them horizontally as well as vertically?

madmanmoe64 (author)juz4kix2012-06-24

I wish I had the original concept art to show, but it was never really meant to be big lego bricks, the inspiration actually came from Roman columns, where each piece of stone work would have little lugs to lock into the piece above and below it.

The original idea was for series of pillars that were different in height and created setting, but obviously grounds people had concerns over how high they'd let us pile these things, so it because more spread.

So yes considered, but rejected, in line with the original design.

Verga (author)2012-06-24

Great Ible I loved the photos, very clear and well organized.

Also here in the US they do have powdered dyes that you can add to the mix to get different colors

I did wonder how many bags of sand and cement you needed to fill a single mold?

And how many castings you were able to get out of a mold?

Once again great job

madmanmoe64 (author)Verga2012-06-24

each mould was about 50 litres, so we generally filled one mould per mixer full of concrete (small mixer) we seemed to be getting through a bag of sand every three moulds, slightly less on cement.

We cast about 5 / 6 blocks from each mould. they were looking a bit ratty by the end, but I reckon we could have gotten 10 out of each before they started actually becoming unusable.

anode505 (author)2012-06-24

(I only breezed through) But are these solid??

I think I'd add in some glass or such and make them hollow to reduce weight, cost, and curing issues.

madmanmoe64 (author)anode5052012-06-24

Couple of things wrong with throwing glass into a concrete mix.
1. the alkali silica reaction between the glass and cement will actually break down the structural integrity of the block overtime and it will eventually fall apart.
- not such an issue in furniture but still not good practice (don't feel bad if you've never heard of this, I only did because my Uni was doing research into it)

2. we don't want to reduce the weight. because this was going to be installed in front of the building, security is an issue. We have to make sure that no one will steal it, and that it won't be knocked over and hurt anyone.

Lighter things like metal and wood benches have to be bolted to the ground, pain.
When we stick a couple of these together they are deemed both heavy enough that no one will run off with one and stable enough they won't fall on someone.

In short, for a home project glass can be a great way to reduce cost and weight, in a live, public project we had to consider different priorities.

Also mixing in polystyrene beads will greatly reduce weight if that's what you need :)

wobblycogs (author)2012-06-24

Nice instructable and an interesting design. I think as a piece of art it's great but as a bench I'm not sure how well it works. I can't help feeling the dips on top will just collect the rain and just end up filled with gunk which no one will want to sit on. Perhaps you could use an additional mould to make capping blocks.

madmanmoe64 (author)wobblycogs2012-06-24

You raise a couple of really interesting points. One plan was to fill in the top holes with clear casting resin, we could then embed more leaves 'n stuff in them.

We did also experiment with making a few 'capping blocks' which didn't have the top lugs. We only had one mould for this and so there are only a couple.

SuperPIXEL (author)2012-06-24


madmanmoe64 (author)SuperPIXEL2012-06-24

I only just started playing a few weeks ago, and that never occurred to me. Next, cobblestone texture.

archer9000 (author)2012-06-24

Macflame says:
Thankfully, I do not have small children, which could be a problem for others.

I disagree. Having the neighbor's kids come over to help. Now depending of there age they could be give different tasks. For the younger one's they could be given the task of collecting things from around the neighbor hood that would be used part of the molds decoration. This would give them pride and a sense of belonging and contributing to the neighborhood. So Invite your neighbors and there kids to come and help, also invite your family and there kids.

danzo321 (author)2012-06-24

As a concrete/casting pro I will grant you have a nice looking casting but not wild about the design. I think I would have gone toward a table with 2 legs design and kept casts under 1.5" thick. You can imagine how a slight dished surface with a weep hole in the middle could be a lot more comfy and clean. Legs could key into lower blocks with much less aggressive keys.
Oz is correct, the warmer it is, the quicker they cure. We try to turn over molds every day (start a new cycle.) BTW concrete does not dry, in fact it had better not. It needs the water in a correct ratio mix to form the molecules of cement.
A mold meant to hollow out a larger casting is called a core mold and they are carefully thought out and constructed, with the emphasis on draft, angles that make them easy to pull out.
Starting with white portland is more attractive, and colors mixed with white are much cleaner.
Nice woodworking anyway.

Questor (author)2012-06-24

I wonder how hard it would be to slightly dome the top with drainage channels running from the top indentations and not make the indentations quit so deep.
Also, with some slight modifications you could use them as a combo retaining wall/bench.
I wonder if hinges on the outside corners of the molds would work as well as using screws. Just pull the pins to open the molds.
Hollow with open bottoms. Inside corner around the top that fits into the open bottom

oz93666 (author)2012-06-24

Great article..... my understanding is that the higher the temperature , the quicker the set/cure, some pipe manufacturing companies cook set pipes with steam once out of mold, just need to keep items wet after concrete has set. Rather than wait 3/4 days a little calcium chloride or commercial accelerator could be added to the mix.

chandshar (author)2012-06-24

for release agent use diesel applyed with spray bottle
to reduce the wieght put a block of polystyreene in the middle
for another variation put coloured powder paint in the concreete mix to give different coloured blocks

CENTRALPARKTARZAN (author)2012-06-24

...Isn't my butt gona look like a crisp Waffle...hahaha Bad Design. Hot like Hell in summers...and freezing cold in the Winter...with hardness to match!...and Waffle fringe benefits...hahaha!

CENTRALPARKTARZAN (author)2012-06-24

...Isn't my butt gona look like a crisp Waffle...hahaha Bad Design. Hot like Hell in summers...and freezing cold in the Winter...with hardness to match!...and Waffle fringe benefits...hahaha!

oking (author)2012-06-24

I really like it.

Might use them for both seating and thermal batteries.

Keep up the good work.

drissel (author)2012-06-24

Arch department .... but of course ... where would anyone sit if he didn't have a corrugated butt? Looks good tho ...


bethmwl (author)2012-06-24

Love the project and thanks for the inbedded experiments. Can pick any or all that you have already shown how well they work. It seems like a very large undertaking, but I think it's fabulouse. Congrats.

Macflame (author)2012-06-24

I really love this instructable.
A very simple yet elegant design. The blocks stand alone as a piece of art, especially with the natural imprints, or you could construct raised beds or ponds with the cubes. They could also be used to divide a space and coated with yogurt to encourage moss. The possibilites are endless.
Scaling up would be easy to do if needed.
I was thinking of using hypertufa to make the structures lighter and more 'organic'. My garden (really a piece of scrub at the back of the house) needs uplifting and this could be just the thing to do it.
Thankfully, I do not have small children, which could be a problem for others.
Excellent work.

Jason Bedard (author)2012-06-21

What was used to colour those black bricks?

madmanmoe64 (author)Jason Bedard2012-06-21

Cement dye. They are powdered pigments specifically made for colouring cement.

Generally you only get black or red from building suppliers, but you can use almost any dye to colour concrete. As long as you aren't adding enough to throw off the mixture.

e.g. if you are adding liquid dye, do it as part of your water amount.

Also if you want lighter colours you need to use snowcrete as a base rather than standard cement.

Jason Bedard (author)madmanmoe642012-06-21

Thank you. That's very useful to know. Well, for me anyway.

Jason Bedard (author)2012-06-21

Love it. Thanks for taking the time to share ths.

themoose64 (author)2012-06-19

now this is cool

pmn9393 (author)2012-06-19

If i were there i wouldn't think it was for sitting on though. I'd think it was a sculpture.

SHIFT! (author)2012-06-19

This is great because it mixes my two favorite things with my least favorite exercise: Building with LEGO, Sitting, and lifting weights. Now I finally have an excuse for the third!

xcello (author)2011-02-09

PAM cooking spray makes for a great release agent too! BTW -Great project

About This Instructable




Bio: Studied Architecture at Brighton But now spend a lot of time building replica props or random gadgets.
More by madmanmoe64:Zig Zag Structure - Cardboard ChairSimple Japanese BridgeModular Concrete Bench
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