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
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CharlotteE22 months ago

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. http://www.pyramidconcrete.net/services.html

janetsellers0810 months ago

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.

Rigidman2 years ago
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
mrblint3 years ago
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)  mrblint3 years ago
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.
kludge0003 years ago
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
stealthop3 years ago
reminds me of minecraft , very nice
vern15303 years ago
Diesel in a pump sprayer works well to. We used it on big forms.
RayMit3 years ago
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.
ManifoldSky3 years ago
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.
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)  ElectroFrank3 years ago
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.
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.
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.
klixtopher3 years ago
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)  klixtopher3 years ago
As I mentioned in an earlier comment we had a couple of solutions for that (check for picture)
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.
suayres3 years ago
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)  suayres3 years ago
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.
juz4kix3 years ago
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)  juz4kix3 years ago
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.
Verga3 years ago
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)  Verga3 years ago
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.
anode5053 years ago
(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)  anode5053 years ago
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 :)
wobblycogs3 years ago
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)  wobblycogs3 years ago
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.
SuperPIXEL3 years ago
madmanmoe64 (author)  SuperPIXEL3 years ago
I only just started playing a few weeks ago, and that never occurred to me. Next, cobblestone texture.
archer90003 years ago
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.
danzo3213 years ago
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.
Questor3 years ago
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
oz936663 years ago
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.
chandshar3 years ago
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
rhino3 years ago
Now you need cushions that lock into the waffle tops. Should be pretty easy to get done.
...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!
...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!
oking3 years ago
I really like it.

Might use them for both seating and thermal batteries.

Keep up the good work.
drissel3 years ago
Arch department .... but of course ... where would anyone sit if he didn't have a corrugated butt? Looks good tho ...

bethmwl3 years ago
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.
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