Instructables

Concrete Garden Bench

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As an Architectural engineering student I learn a lot about building with concrete. The funny thing is that I never used concrete in my life. So I was going to change that and make something out of concrete. The first thing I came up with was a concrete garden bench. So here we go!

If you like this project please vote for me in the concrete contest.

 
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Step 1: Materials and tools

Picture of Materials and tools

Materials:

To make your mold you need a waterproof plywood with a smooth surface. Shuttering plywood is a material that's designed to make molds. It's got a very smooth surface and is water resistant. It's also very expensive. When you are planning on reusing the mold I will recommend shuttering plywood. Otherwise melamine board is a cheap alternative.

Melamine board got a smooth surface but it isn't water resistant at the sides. I found an old closet in the dumpster and used these boards to construct my mold.

  • Concrete

I used 10 premixed concrete bags (25kg each). The premixed bags are easy to use. When you want to make your own concrete you have to put the cement, sand and gravel together. 1 part cement, 2 parts sand and 3 parts gravel. When you change the size of the bench you have to calculate the amount of concrete you need. A 25kg bag of concrete is good for 12 liters.

  • Screws
  • Silicon sealant
  • Release agent: silicone spray, petroleum jelly, WD40, etc.
  • Masking tape
  • Rebar
  • Some pieces of scrap wood
  • Rope
  • Concrete coloring (optional)

Tools:

  • Power drill
  • Chaulking gun
  • Old orbital sander
  • Mixing bucket
  • Shovel
  • Hammer

Step 2: Making the mold

Picture of Making the mold
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The cutting of the pieces was done by my local hardware store. When everything fits correctly it's really easy to screw together.

The cutout was made with a circular saw. Place the two sidepieces on top of each other and clamp them down before cutting. Doing this you will get two identical pieces.

You can now screw everything together. Be sure to use a lot of screws because it has to withstand the pressure of the concrete.

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In thousands of years when the archaeologists are digging up our artifacts, your bench will still be going strong. You'll be famous!

I love the shape too. Very clean.

bookmatch1 month ago
There are lightweight concrete that can be use for benches, counteracts, etc. I'll try to find a reference or two and post them in another comment.
rubenrijst (author)  bookmatch1 month ago

The light weight concrrete at my hardware store was only 25% lighter but almost 100% more expensive.
I thought about filling the inside of the legs with foam and adding some pvc pipes to the seat just like hollowcore flooring.
Maybe the next time.

You can easily make lighter variations by the addition of other substances to normal concrete without particularly compromising the strength of it (like anything, there are always trade offs).

My brother experimented extensively with alternative forms of concrete for another project he was doing. I can ask him about it and post back here if you'd like.

At the risk of increasing the mold complexity, the obvious solution to the problem of weight is to cast voids into the legs and underside of the bench. Yes, there would be a reduction of strength, but not significantly so. Were it me, I'd probably have tried to incorporate polystyrene into the center of the pour so that none of it was visible but there would be less weight.

The whole weight/construction issue seems to me to be essentially about the weight bearing capacity of the bench. Ignoring the parts of the bench over the legs (because they'd be the strongest) you'd really need to identify the max load capabilities of the unsupported center section to really get an idea of what you can and can't do with materials.

Daveed cfuse1 month ago

In order to counter the reduction in strength, two pieces of rebar cut and bent to run the verticals (legs) and horizontal (seat) as one solid run of 1/2" to 3/4", embedded into both sides of the bench would be enough.

Then, the pieces of polystyrene (lightest alternative) or even fiberglass batting could then be placed as filler and embedded with the rebar. However, and this is the trickiest part, the rebar will need to internally connected with enough concrete that is structurally solid with the non-filler based concrete to give strength throughout the total structure.

Now, this is just my thought on how to approach reducing the weight while maintain strength. I will appreciate any feedback or suggestions. Who knows, I might even try to build one that way. Thanks.

rubenrijst (author)  Daveed1 month ago

I thought about using polystyrene and also pvc tubing. I made a sketch about how i would do this. What do you think?

lightwheightbench.jpg

Have you considered papercrete, much lighter. More info http://papercrete.wordpress.com/2011/09/10/abstrac...

rubenrijst (author)  kuder4rx20 days ago

That's a really good option and a cheap one too!

The only thing I'm worried about is getting a smooth surface. That seems to be the biggest problem.

Yes, that could be a problem, especially in a warm, humid climate like here in Florida. Do you suppose a final top surface could be embedded into the mold that would cleat to the papercrete mixture? Perhaps an initial spread of the original mixture--say, 1"-2" --laid into the bottom of the mold first (top surface of bench) and allowed to firm up slightly. A concrete sealer is a possible solution to help avoid water soaking into the paper fibers. Either that or some well-placed coatings of polyurethane or lacquer paint.

BTW, what do you think about the papercrete and modifications that we discussed earlier using the pvc tubing /rebar? Much lighter, yes. But where do you think it would stand on strength at that point?

cfuse Daveed20 days ago
BTW, what do you think about the papercrete and modifications that we discussed earlier using the pvc tubing /rebar? Much lighter, yes. But where do you think it would stand on strength at that point?

Cinder blocks have voids in them and people build houses out of them, so I don't automatically think that less concrete is going to be a structural problem here.

Daveed cfuse20 days ago

Yes, you are absolutely correct, and I believe that if the pvc tubing, rebar, or whatever reinforcement were incorporated into the seat, then the span wouldn't be an issue. The bench seat measures approximately 35 inches and would most likely need to be reinforced horizontally if you varied from the original design by much.

After all there are no unsupported spans using cinderblock that goes more than a few inches. The support is built from the foundation up by stacking a solid vertical span upon another solid span.

Now that would definitely reduce the weight, in either design. I assume that the two pieces of re-bar were the two reddish dots showing up in your cross-through sketches. Did you intend to tie the re-bar to the reinforcing wire? It might be more than needed but it is always better to be safe than sorry.

I'm sure many people here would like to know what your brother could tell us on techniques for concrete. Including lurkers like myself. :)

fermilab1 month ago

If you pour white vinegar on your hands after handling concrete, it near-instantly ends the dryness.

rubenrijst (author)  fermilab1 month ago

Wish i knew that a little but earlier. Thanks for the great tip though!

I do have a question, what grade of concrete did you use for this,also,what was the dry weight of the bags? This will help me to figure what type that is available locally for me here in the US.

rubenrijst (author)  Deathwraith51501 month ago

I don't know, they're just standard 25kg bags. They have strengthclass C20/25.

racoontnn1 month ago

Великолепная идея для дачи! Мои кирпичные стены скамейки постоянно облезает штукатурка или разрушаются кирпичи. Но никак не приходило в голову сделать бетонную, вечную скамейку. Спасибо за идею!

I love how almost every time I'm thinking about a new project the Instructables Newsletter comes out and has exactly what I'm looking for (or at least close). Great instructable! I was thinking of doing this but with 3 pieces, making the bench top as one rectangular piece and then using 2 cylindrical forms for the legs. Good tips on how to get the air bubbles out btw!

ktsolakidis1 month ago

Good job ! Well done on the finished bench. Looks great.

A year ago I tried to make a similar bench (only a little bigger) just to find out that it was IMPOSSIBLE to lift it and place it where I wanted...

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danzo3211 month ago

Good job! Other ideas.. Use a shovel to raise and lower the WHOLE mold rapidly, to shake out airbubbles. Or, Make the TOP surface bubblefree, then use dryer mix concrete for that textured look.

As an engineering student, maybe you could calculate how thin you could go and still have good strength.

Normally you wait maybe 2 days to demold, 5 seems excessive.

rubenrijst (author)  danzo3211 month ago

Thanks for the advice!

I did calculate the amount of rebar that's necessary. It's not a hard calculation, I calculated it with a 300kg force in the middle of the bench. At architectural engineering we always use big safety margins;). The outcome is you need 0,05% of steel in the section cut of the bench. This leads to a steel surface of 30mm2. A piece of 6mm rebar has a surface of 28mm2. So there isn't a lot of rebar necessary. I can tell you a lot more about this kind of calculations, but for this particular bench it isn't really that important. When you make the seat thinner though the needed amount of rebar increases rapidly!

I waited longer to demold the bench because i have to flip it upside. When the concrete cures for 2 days it's strong enough to demold. But when you flip the bench upside the corners of the legs will definitely be damaged and chip off.That's why i waited a little bit longer.

How long it takes to reach certain levels of ultimate strength depends on things like ambient temperature. 2 full days is plenty in memphis, where are you/ what is daytime high?
rubenrijst (author)  danzo3211 month ago

I'm from Holland. I made this in November when temperatures are around 10 degrees. I just wanted to be sure that's all but I could have demold earlier.

Daveed1 month ago

I like the project, the bench, and its usefulness. No one will ever steal it from the owners by simply picking it up and running off with it! ;-)

What I would be interested in finding out about is whether there is a suitable filler material that could be used to reduce the weight while maintaining adequate strength. I've heard of bamboo fibers being used for some projects, while fiberglass can also be used.

If so, does anyone have an idea regarding the ratios that could be incorporated to displace some of the concrete while still producing a strong product?

Love it! Thanks for posting it! I've never worked with concrete but see a garden bench in my future. Was there any time concern/issue with getting the mold properly filled before the concrete started to set up or does it stay loose enough to work at a regular pace?

rubenrijst (author)  JimInstructs1 month ago

There wasn't any time concern. I did two batches of five bags each. I think it took me 2 hours.

Shanderson1 month ago

Is it possible to put handles into it? Like on opposite ends, right? How would you do that?

Can I do that with handles formed from the rebar?

Normally you make handles by gluing short pieces of styrofoam (pink or blue) using silicone glue to the inside of the mold. These are 'reach-in' handles. If you let rebar protrude to be the handles, it allows rust to get deep into the cast. Rebar is used to form lifting hooks, but then cut off with a grinder after placement of big castings. You then cover the cut end with more concrete or epoxy paint.

rubenrijst (author)  danzo3211 month ago

I was going to say the same. After casting you can even dissolve the styrofoam with aceton when that's needed.

doc.kennedy1 month ago
250kg!!!! Otherwise your bench looks great! Clear and understandable instructions.

The weight is why we love our patio set. Wind, pets, kids and uncoordinated adukts have no chance of knocking it over...

Or of it ever being stolen! Unless you get hijacked by bodybuilders...

you can use a bit of sanding paper to remove any sharp corners from your bench :D

rubenrijst (author)  timothy demaegdt1 month ago

I did this a little bit, but when you sand too deep you will get pièces of gravel exposed. I disn't like that.

To sand on concrete we use a very hard bar called a Truing stone, sold in hardware stores. I agree, don't like to uncover the rock, but in a couple of years, rain will remove the top layer of cement and you'll see pebbles anyway. So maybe first concrete into the mold (becomes the TOP surface) could be Sand Mix, no pebbles.

makendo1 month ago

Great job, looks really nice... any reason why you didn't just cast it on its side?

rubenrijst (author)  makendo1 month ago

I thought about that but I wanted both sides to have a smooth surface. Also the edge Isn't round when you cast it on its side. When you are planning on placing the bench against a wall it's not a problém.

You can get wood corner molding which has a right angle with a concave hypotenuse and glue/screw it into any wood mold to give nicer corners.

pepe1231 month ago

Great job, i am surprised about the outcome.

Did you pour in the concrete at once or could you work in several days. 250 kg of concrete is a lot of carrying, mixing and last but not least still a time consuming work.

Do you think there could be a "light" version as well??

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