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!
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Step 1: Materials and Tools
- Melamine Board or shuttering plywood:
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.
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.
- Silicon sealant
- Release agent: silicone spray, petroleum jelly, WD40, etc.
- Masking tape
- Some pieces of scrap wood
- Concrete coloring (optional)
- Power drill
- Chaulking gun
- Old orbital sander
- Mixing bucket
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:
- 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.
- 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.
First Prize in the
Concrete & Casting Contest