Introduction: Cast Concrete Bench

About: I just occasionally make stuff, and if it's different enough, I'll make an Instructable to inspire others.

I really love watching my goldfish - it's relaxing. And I had an odd shaped space that needed a bench. The wife said, "No wood!" Therefore, my only option was concrete - something I've never worked with before. So, I set off to find some example projects... but there was none. No good Instructables, nothing on the internet, and still nothing at the library.

An Instructable is born.

This project is based on Fu-Tung Cheng's excellent book Concrete Countertops, which I recommend anyone attempting to replicate my work will read first. The way I see it, this bench is like a countertop that you sit on, and that's the way I built it.

I don't expect anyone would duplicate this design exactly, but it's real purpose is to explain how and why so you can make a bench that works for you.

Let's be clear: This was my first concrete casting project. Before this, all I've done is pour concrete for posts. So, this is not just a, "Look at what I can do!" Instructable, but one that you can make with the right tools and some space to work in.

Why concrete?

There's a lot of good reasons why concrete is ideal for this. First, it flows. When you pour it it literally flows to fill the any shape you mold. And when it's done it metaphorically flows in the space creating an appealing place to sit and enjoy nature.

Second, it's durable. It's extremely strong and gets stronger with age. It doesn't need maintenance; it won't rot or rust, and the color is embedded into it.

Third, it just has a wonderful, intangible quality to it. From it's solid feel to the slight imperfections in the surface - there's just something sensuous about it.

Step 1: Tools & Materials

You need a lot of tools, but you probably figured that out.

Here's the essentials:

  • Hand drill and preferably, a drill press
  • Circular saw, jig saw, mitre saw, band saw, reciprocating saw, table saw - or some combination
  • Rebar bender - bought or made with a 2"x4" and bolts
  • Wrenches, screw drivers, etc.
  • Levels - beam and torpedo
  • A wood or magneseum concrete float
  • A concrete mixer (one thing I regret not getting)
  • several buckets
  • heavy duty rubber gloves

As for the material, the sizes may change but here's the essentials:

  • A large sheet of melamine - I found a 6'x 2' sheet of shelving material.
  • A second sheet (or two) for the legs - I picked up two smaller sheets that I could use to create the boxes for the legs and the piece for the front of the mold.
  • 3/8" rebar - probably three 10' rods
  • black (preferred) silicone caulk
  • Lots of wood screws
  • 3" carriage bolts, washers, and nuts
  • a sheet of hardboard, 1/4"
  • 3" melamine edge banding, or something similar
  • scrap 2"x2"s and other wood
  • glue (I like Gorilla Glue)
  • Concrete (duh!)

Step 2: The Template

The first thing is to figure out what shape you want. For my spot, this kindey-like shape will blend in perfectly with the landscape. This be about five feet long - much bigger those paltry ones you can buy pre-made!

When making the template, think about what you have for the form - you need to allow the proper amount of overage in the form for the curved mold portions. Also, the more complex the form, the hard it will be to form, know your skill level. As you'll see, the easiest mold is a rectangle.

When I made the template, I just used corrugated paper from a water heater box and just marked it with a Sharpie cut it with scissors till I had a shape that was agreeable. The curves are pretty rough on the template, but since the actual mold curves are controlled by the material, having a rough template is OK. See how the hardboard makes a good arc compared to my rough template? The material will compensate. Of course, if you want the odd, unnatural shape, that's going to be more work to fight against the tendency of the material.

Back in my garage, you can see the template atop the bottom mold piece. I've left not-quite-enough of room on the edges with curves - the reason will be apparent why the space is needed and why I was too stingy.

Since the front of my bench is going to be flat, I'm taking advantage of that and using a solid section of melamine on that edge. As you can see, it's attached with screws every 8". The whole piece was ripped to match my desired thickness.

About thickness...

Too thin and your bench can look weak and will be prone to cracking, too thick and it will be hard to pour and weigh too much. What's the right thickness? I can't say for sure, but I found 2 1/2" to be just right. That gives me plenty of room for the rebar and it's wasn't too heavy. (Psst... you did consider how you're going to move this once it's done? I didn't!)

Step 3: The Curve Braces

I couldn't figure out the best way to secure the curved portion of the mold. You could stack several layers of pine boards or plywood, then cut it with a jigsaw to match your template... But I figured out this method for me because of what I had to work with.

I cut a 2" square (not a 2x2!) into segments as long as my mold is tall. After transferring the dimensions, then drilled a 3/8" hole in the middle of each one. This where the mitre saw and drill press come in handy.

You'll want enough for every six inches; more where the mold segments overlap.

Rip the hardboard into strips wide as the mold is tall. You'll want enough for two layers everywhere there's a curve.

Step 4: Starting the Curved Form

The hardest part is where the curve meets the straight. If you're doing all curved sides, or all straight sides, then skip this step.

With the template lined up against the straight edge, see how your hardboard mold strips bend. See how I push mine up into that gap? That's where it has to be secured very tight. To do this I made a wedge that I cut based on the angle I measured.

The wedges are first secured to the bottom mold, then the first hardboard strip is secured with screws. Remember how I said that the final product won't exact match the template? This is one point. It will be a smooth transition, but let the material guide you.

Step 5: The Curved Wall

Using your template as a guide, start bending the hardboard around the outside of the template. Every six inches, set one of your blocks by drilling a hole in the base, pushing a carriage bolt up, and securing it with a nut (and a washer if you want). Where piece of hardboard butt up against each other, set an extra block. Go all the way around, and when you get back to the end, just cut the excess hardboard. Secure with screws as necessary.

Important: push the bolt up from the bottom, so when the mold is filled with concrete, you can remove the nut, and the block, and the mold.

Remember when I said I made my template too big and didn't leave enough excess on the base? See where I had to use a 1x4 instead of blocks? It worked, but not ideal.

Step 6: The Curve Liner

After the first course of hardboard, install a second. This one should extend farther on the straight wall, and you want to use as few screws as possible, and where you do, make sure the heads are below the surface - any bumps here will be visible in the final product.

You need a smooth, non-porous material for the liner. I found some old vinyl blinds in a dumpster that I ripped to size. But if you can get melamine edge banding in the right thickness, that's the "proper" way. (My way is the cheap way.)

Glue that liner in securely, and wipe up any glue on the base or liner. At this point, you want the inside to be clean and smooth. Work slow, and clamp the sections as you go. This took me a long time to complete.

After the glue dries, spread a thin bead of silicone caulk where the liner meets the base. This keeps the wet concrete from weeping out. Cheng recommends black caulk since it's highly visible on the final product and you'll want to see where to clean it up when you're done.

I really can't stress it enough - get that caulk right - too little and it will be a mess when you pour in the concrete. But any lumps will be noticeable in the final product. One trick I use is to wet my finger, spread it, wipe off the excess, wet my finger, spread it, ....

Step 7: Leg Boxes

Let's take a break from the big mold and work on the legs. The legs get poured after the base has a chance to cure, but is poured in place. I thought separate legs would be much harder to get right. My way worked out OK. Since the main bench is poured upside down, the boxes get placed on top of the bottom and then concrete is poured into them, making a solid connection.

So, the leg boxes are just melamine cut to size and screwed together. You'll want them to be thick and look solid - mine are 4" wide, 10.5" long and 15.5" high. Remember, you want that as your inside dimension. Note the height was set to give an 18" overall height which seemed common.

Drill pilot holes, and screw the box together (box clamps help).

Learn from my mistake: apply a bead of caulk and squish that between the panels to create a watertight seal.

After the box is assembled, caulk the inside edges just like you did on the main mold.

Step 8: Leg Rebar

The best I could come up with is putting a loop of rebar in the slab, then having that looped up for the legs. This, I thought, would be a good method to attach the two components.

I used some 1/2" rebar I found - which was hard to bend this tight. I made a U, and used a clamp some wire to hold it at the width I wanted, since the legs are 10.5" wide by 15.5" tall, the U is about 8" wide and 13" tall. Note that the wire stays on permanently.

Step 9: Reinforcements

The bench is reinforced with wire mesh and 3/8" rebar. The mesh sheet gets cut to allow for a one inch gap from the edge - that's important because if it's to close, you get ghosting. Cut and bend the mesh to fit your shape.

Bend some of the rebar to create and ring around the edge and tie it to the mesh with wire. If your bench is wide like mine, use extra rebar ind middle lengthwise.

Cheng advises against overlapping rebar to prevent creating any tight spots that will lead to voids in the concrete.

Use 1" styrofoam block under the mesh and rebar.

The leg supports get tied into the mesh and rebar and set vertically (use a level). It's important that they secured tight, and can't flop around. You'll get a chance to adjust them a little after the concrete is poured.

Once you get it all set, (and it's going to take a lot of fidgeting), use some wire to secure the rebar to the frame by wrapping around a screw or one of the bolts. These wires will hold the rebar in place during the pour.

Finally, remove the styrofoam (we're done with it), clean the mold thoroughly, and go to sleep and dream about your bench, because you're almost done.

Step 10: The Concrete Mix

Let's go over some rules on concrete mixing:

  1. Mix enough concrete. You can always use excess for something else or find a spot in the yard to dump it.
  2. You want a thick consistency - like thick oatmeal. It may look to thick to work with, but once you agitate it, it will liquefy.
  3. If you add a colorant, record the ratio - you'll probably want the same color for your legs.
  4. Consider renting a concrete mixer. Doing more than two bags by hand is a lot of work. Get a mixer twice the size you need.
  5. No matter what, it will be messy. Buy some thick rubber gloves - concrete is mildly caustic.

I'm no expert, and this Instructable doesn't cover the basics, so consider reading Cheng's book about concrete mixing.

I recommend using a bags of "high strength" concrete you'll find at any home center. Look for something that's rated for 5000 PSI strength. Don't get something that's fast setting - you'll need the time to work with it. There's also concrete specifically designed for countertops that might be worth considering. How much concrete? Well, I can't say for your application (you'll have to do the math), but this bench took a total of four bags.

Step 11: Pour, Leveling and Vibrating

Once you have the mix ready, just start shoveling it into the mold. When it get full, get your gloved hands in there - just working all around the rebar and mesh, plunging in, swishing around... Now grab a scrap board and level out the concrete. You can see I'm trying to work around all the bolts and the rebar for the legs. Any excess should be cleaned up, try and keep things relatively clean - especially if in a garage.

Now we need to agitate, and if necessary ad more concrete.

Why agitate?

Because that trick concrete has lots and lots of pockets of air; and air isn't strong. We need to get as much of those air bubbles out of the concrete or else they'll be trapped in there forever and make the bench weak.

So, grab a mallet or hammer and beat the mold as hard as you dare. The first thing you'll notice is that the trick concrete now flows and runny - that helps it flow around the rebar and let the bubbles out. As you beat it, you'll notice lots of tiny bubbles coming up all over. Keep beating it. And more. And more. Seriously, you can't over agitate it and the more you work it, the better the end result will be, so whack that mold till your arm turns to rubber!

Step 12: What Can Go Wrong?

For me, the edge at one spot came loose, an other area leaked. It's not a big deal, but it did make the final bench have a few imperfections.

Other things that could go wrong would be the mix is too wet and when it cures it cracks.

Or, the form might not be strong enough and you'll have big leaks.

You're going to have something wrong. There's going to be some imperfections, that's concrete - the imperfections make it special.

Step 13: Smoothing Out

OK, so now that we're done agitating, we can cut the wires holding up the rebar. Cut them below the surface so nothing is exposed in the final product.

Check that the leg rebar is level.

Finally, using a wood or magnesium float, smooth out the concrete. Remember, this is the bottom and won't be visible, so we're not needing it to be perfect.

Now go hose down all your tools and then hit the shower yourself.

Step 14: Prepping the Legs

After a full 24 hours of curing, you'll be ready to pour the legs.

  1. Put a bead of caulk around the bottom of the boxes where they will attach to the base - don't be stingy, you need a good seal.
  2. Slide that box over the rebar. Make sure the rebar isn't closer than one inch and doesn't extend past the top. If it does... well you need a bigger leg box.
  3. Push done hard. You can use some weights to hold it in place.
  4. Realize you can't reach the caulk inside because the rebar is in the way. Yup, I didn't think about this. I figure it's not a problem since it's not visible at all.
  5. Let it dry.

Step 15: Pouring the Legs

When silicone is good and dry (don't rush!), you're ready to make another batch of concrete like before, pour, level, agitate, agitiate, agitate...

But after that big pour for the bench top, this will seem easy and you'll know what to expect.

You can see that I've employed an orbital sander (without paper!) to agitate because you don't want to go whacking hard on the sides and risk knocking the box loose - it's only held by silicone.

Tip: To help get out air, pour some concrete, then agitate, repeat until full.

Step 16: Removing From the Form

Has it been a week already?

If so, I bet you can't wait to pop it out and see how you did. If it hasn't been a week, stop. As Alton Brown always say, "your patience will be rewarded." You need a full three days of curing before it's strong enough.

OK, OK, let's do this.

Really, it's pretty much the opposite of building. Just unscrew and carefully pry as necessary. If you're thrifty like me, you be putting those bolts and screws back into their containers for another project. As for the mold - that's a one time use item, right there.

Mine probably weighed 400 pounds - it was not easy to remove and set upright. Use some blocks and wood as make-shift levers, but the best way is with many hands. Four of us used 2"x4" spanned underneath it to lift and carry it into place.

Oh, do you see the little holes? That's from air bubbles and even with all the agitation I did, there's still a lot of surface pits (that's OK).

Step 17: Years of Enjoyment

You could use an angle grinder to remove some imperfection or you could add a sealer to it... I was too lazy for either. Plus, it's exterior furniture, not a kitchen countertop.

No matter what you're going to get a lifetime or more of maintenance free use from this. There's no painting, rusting, bug infestation... just a big slab of cool concrete that's one of a kind.

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