This wood and concrete outdoor bench features a really sturdy base, a modern, minimalist design and it's pretty cheap to make. The concept is pretty simple: you build a mold using plywood which you cast concrete in, with J-bolts attached. Then you drill through some 4x4 wood and attach the wood to the concrete bases. Once you've made the molds you can reuse them and create more bases to make additional benches!
There's a video that goes over all the steps in detail, so make sure to watch that for a better perspective of the process.
Step 1: Making the Mold
The first step is creating the molds. I used 1/2 inch plywood and these are the cuts that you need per mold (you'll need two molds per bench, or you can reuse the same mold twice, however it's easier with two molds.)
Cut list needed per mold :
2 @ A - 14 x 17 1/2 inches (35.5 x 44.5 cm)
4 @ B - 3 x 9 inches (7.5 x 23 cm)
2 @ C - 9 x 9 inches (22.9 x 22.9 cm)
1 @ D - 17 1/2 x 8 inches (44.5 x 20 cm)
Step 2: Cutting the Profile
Cut A needs to be trimmed down a bit to achieve the right profile. I used a bandsaw, however you could also use a jigsaw to accomplish this cut.
Step 3: Assembling the Mold
Once I had all my cuts, I assembled the mold using screws. To make sure the concrete didn't get stuck in the corners, I decided to use silicon caulk.
Step 4: Positioning the Molds
Since the concrete bases will be so heavy once made (about 120 lbs each), I wanted to pour the concrete into the molds, close to where I ultimately want the bench. I placed a 4x4 on top of the two molds with a level on top, just to make sure they were sitting correctly on the ground.
Step 5: J-Bolts
You will need three j-bolts for each mold (one j-bolt for each 4x4 piece of wood). After positioning the 4x4 on top of the mold, I marked out how far down each j-bolt should sink into the concrete. You want to have enough space so you can screw on the washer and bolt.
Step 6: Concrete
I used 1 1/2 80lbs bag of fast setting concrete for each mold. Once the mixed concrete was poured into the mold, I first tapped it all around with a rubber mallet to reduce the amount of bubbles. Next, I marked out where the center of each 4x4 will go and then I placed each j bolt at that position. I tried to be really careful to make sure the bolts went in just right, and that they were nice and square.
Step 7: Removing the Molds
I waited about 6 hours before removing the molds on the sides, and then I waited overnight to remove the bottom piece. Everything went very smoothly, and the concrete didn't get stuck or anything.
Step 8: Cutting the Wood
At this point I positioned where I ultimately want the bases, and measured how long the wood needs to be. Overall, I decided to make my bench about 5 1/2 feet, however this would be easy to adjust depending on what size space you have to work with. I then cut three 4x4 boards to size, accounting for about 1 1/2 inch overhang on each side of the base.
Step 9: Drilling
Next I marked out where the J-bolt will go through on each 4x4. I began with drilling a large, shallow hole with a 1 1/4 inch Forstner bit on each side of the board (this is so the washer will fit). Next I completed the cut with a 5/8 bit all the way through.
Step 10: Attaching the Wood
Now it was time to attach the boards to the j-bolts. I used a rubber mallet to get the wood in position. If you attached the j-bolts right, and drilled the holes square, it all should go smoothly. If not, you can always enlarge the holes slightly to create more wiggle room. Once the j-bolts were through the wood, I secured them in place with a washer and a bolt. I used a reciprocating saw to cut off the part of the bolt sticking up.
Step 11: Grinding the Metal
To make sure the bolts are absolutely smooth, I used a grinder to remove any bits sticking up at all. Later on I'm planning on filling all the voids with epoxy so that water won't be able to collect in this space.
Step 12: Sanding and Finishing
At this point I sanded all the wood, and finished the bench with some water based Spar Urethane to add some protection.
Step 13: Conclusion - Watch the Video
For a much better perspective, make sure to watch the video that goes over all the steps in more detail.