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.

Step 1: 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

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.

Step 3: Adding Rebar

I used a piece of an old fence to function as rebar. I cut it down to the correct size with an angle grinder. You can also buy rebar at your local hardware store.

While making and placing the rebar you have to consider two things:

  1. You have to place rebar with an offset of at least 4 cm from the concrete surface. When you don't do this the rebar can rust and you can get spalled concrete. When you want to place your rebar closer to the surface you can use coated rebar or stainless steel.
  2. You have to place the rebar as far down in the seat as possible. When you sit on the bench it creates tensile forces in the bottom of the seat. This is the place you want your rebar to be.

I doubt if rebar is really necessary for this particular bench. The seat is 15 cm thick so it should withstand a lot by its own. But it's better to be safe then sorry.

Step 4: Aplying Silicone Sealant

When using melamine board you have to apply silicone sealant at the joints because the melamine board isn't waterproof at the joints. The silicone seals also creates a nice round edge. I used masking tape to create a tight seal. You can watch this video as an example.

NOTE: Before you seal the inside of the legs you have to place your rebar!

Step 5: Placing Rebar

I placed the rebar with some scrap pieces of wood and some string. The rebar is placed as shown in the drawing.

Step 6: Mixing the Concrete

Follow the instruction on the bag to mix your concrete. I used a big plastic tub and mixed the concrete and water with a shovel. A good tip is to first calculate the water you need and than add 2/3 of this in the tub. Add your concrete little by little while mixing it. At the end add the other 1/3 of the water.

I used black pigment powder to give the concrete a darker color. I did a shitty job mixing the pigment with the concrete. This is why the concrete has a patchy appearance. At the end I really liked this.

NOTE: When mixing concrete be sure too wear gloves. I did not know this and was touching the concrete, scooping the concrete in the mold and even mixing the concrete with my bare hands! The cement in the concrete is hydrophillic and will get your hands as dry as dessert sand. My hands where a disaster for three days. Always use gloves folks!

Step 7: Filling the Mold

Before you fill the mold you have to be sure the mold is placed level.

To make the mold easy to come off you have to use a release agent. There are special release agent on the market but it's not worth the money. I used silicone spray as a release agent. You can also use olive oil, WD40, petroleum jelly, almost everything that's greasy.

To get a smooth surface you have to get the air bubbles out the concrete. To do this you have to vibrate the mold, I used two different techniques for this:

- Tap the mold rapidly with a hammer. This is used to get the bottom of the mold smooth (seat part)

- Use an old orbital sander to press against the mold. This is a good method to get the air bubbles out of the legs and sides.

You can also use an drill hammer to vibrate the mold.

Knowing this you can now fill the mold with concrete. Be sure to spread the concrete and press it into the base of the mold. Fill small batches of concrete and tap the mold frequently between the batches. This is really important! Also use the orbital sander on the sides of the bench.

When you have filled the seat of the bench you have to put the piece of board in place before you can fill up the legs. When your finished filling up the legs you have to cover the legs with a plastic foil. This is done to prevent the water in the concrete from evaporating.

NOTE: I made extra bracing pieces to strengthen the sides of the mold. This wasn't necessary at all.

Step 8: Demolding

When you've waited for five days you can start demolding. Unscrew the boards and use a joint knife as a wedge between the wood and the concrete.

Step 9: Final Product

You've made yourself a nice looking concrete bench that will last forever and ever and ever... Good luck with moving the bench. This 250 kg beast is a real back breaker.

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    66 Discussions

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    masoon

    4 weeks ago

    Cool finished design! This is an amazing first project for concrete, I take it you haven't worked with it before since you didn't wear gloves?

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    Yardster

    Question 5 months ago on Step 9

    I assume this will experience freezing weather. Did you use an additive in the mix to minimize damage?

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    bjones93

    4 years ago

    You should of used a polystyrene filler to take the weight out off the bench

    2 replies
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    TomB263bjones93

    Reply 1 year ago

    How would this work? Would you just add it to the mix (beads) or can you use sheets? Also, would that change the way the reinforcements are used?

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    micksmom2

    1 year ago

    Very nice! I like the clean lines of this design.

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    RonW45

    2 years ago

    Nice project, thanks for sharing.

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    buck2217

    3 years ago on Introduction

    Nice but I suppose you need to construct near where it is going to be sited!

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    WilliamF4

    3 years ago on Step 9

    The proportions work very well, so important with furniture. Right after casting you can polish with diamond pads (expensive). A polished surface dries faster and holds less dirt, as well as looking very cool. The simplicity and permanence of the piece give it presence, great project!

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    azapplewhite

    3 years ago on Step 9

    Nice job on the bench, it looks fantastic and you did a great job on your instructions! Thanks.

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    selmaaah

    4 years ago

    congrats, trots ??

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    poofrabbit

    4 years ago on Introduction

    Congratulations on being a finalist in the Concrete and
    casting contest! Best of luck to you!

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    bjones93

    4 years ago

    Like you had the rebar on a hanger, do the same with the polystyrene

    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.

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    bookmatch

    4 years 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.

    2 replies
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    rubenrijstbookmatch

    Reply 4 years 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.

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    cfuserubenrijst

    Reply 4 years ago on Introduction

    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.