Build a concrete bench

  • Photograph by Reed Young
  • Step 1 Photograph by Reed Young
  • Steps 2-6 Photographs by Reed Young
  • Steps 7-10 Photographs by Reed Young
  • Measurements diagram
  • Photograph by Reed Young
Date:27 February 2014 Tags:, , ,

Concrete furniture lasts forever, especially when you build it like a pro. By David Agrell and Eric Well

Tools and supplies
* Concrete mix
* Sheet of 16 mm white melamine chipboard
* Table and mitre saws
* Hot-glue gun
* 2 mm packing foam
* Shipping tape
* Black silicone caulk
* Paste wax
* 10 mm rebar
* Calibrated mixing container
* Three 25–litre buckets
* 100 litre refuse bin
* Electric drill
* Universal mixing paddle
* 60 cm screed or float
* Trowel
* Hammer
* 120-, 220- and 320-grit sanding blocks
* Masking tape

Eric Weil (photo, opposite) once studied wood carving in Ghana, but nowadays it’s the urban textures of New York City that inspire his work. Weil runs Oso Industries in the Big Apple, crafting furniture primarily out of concrete. It’s a material he values for its appearance, adaptability and strength.

We commissioned Weil to produce a bench project for PM readers that is less complicated than it looks: three slabs combine with elegant through-tenon joinery, so you won’t need any connecting hardware. The key is to fabricate accurate moulds; then just mix up a batch of concrete and pour.

Build the moulds
This bench relies on well-formed mortise and tenons for strength, so assemble the moulds with care. Create them as you would a piece of furniture. Make the sides and support blocks by ripping the melamine chipboard into strips with a table saw and cutting them to length with a mitre saw according to the materials list above.

Lay the remaining chipboard on a flat, level surface and use a pencil to trace the outline of the bench parts. Arrange the parts with enough surrounding space to allow for the support blocks.

Attach the side pieces along the layout lines with hot glue. Mitre the inside corners of the leg moulds where the shoulders meet the tenons, then glue support blocks 100 to 150 apart around the perimeter of each mould [1]. To create the curved sections of the leg moulds, install a block on the inside of each curve and bend acrylic (perspex) around it; make sure the straight sections of the perspex extend past the bottom of each leg.

The bench top’s mortises are created from two rectangular knockouts. Assemble with hot glue and wrap them in foam packaging material and a layer of shipping tape. This makes them easier to remove after the concrete has set; it also provides just enough wiggle room to make final assembly a snap. Trim the foam and the tape around the knockout [2].

Next, apply caulk neatly along the seams. Black silicone caulk is easy to see against the white melamine and can be cleaned up with soapy water.

Apply clear paste wax to all inside and top-edge surfaces [3]. Buff the wax with a soft cloth, but take care not to damage the moulds.

Prepare the concrete
Concrete has excellent compression qualities, which means it’s difficult to crush. However, it lacks tensile strength when used across a span, so you’ll need to reinforce your bench with rebar. Cut 10 mm rebar with bolt cutters, and bend the pieces into shape by hand or with a rebar bender.

Mix enough concrete to form both legs (see “Recipe: Eric Weil’s Concrete Mix”). Many of the ingredients are toxic and irritate the skin, so wear a cartridge respirator, eye protection and gloves. Use a calibrated mixing container to measure the cold water, acrylic additive and pigment, and combine them in a 100-litre refuse bin. Next, measure the cement and aggregates (the sand and gravel) into separate 25-litre buckets. Add half the cement to the liquid mixture, and combine using an electric drill and a universal mixing paddle. Then add half the aggregates; mix well. Add the remaining cement; mix again. Add the remaining aggregates, and mix once more [4]. The concrete should have the consistency of yoghurt.

Immediately pour enough to fill each leg mould to within 25 mm of the top, working it into the corners. Once the concrete starts stiffening, set the rebar into each mould [5]. Top up with the remaining mixture, screed the surface and flatten with a trowel [6].

Repeat for the bench top using the identical amount of concrete mix. Cover the parts with plastic sheeting and leave to cure for a couple of days.

Assemble the bench
Free the parts by knocking away the moulds with a hammer [7]. Smooth everything with a sanding block, starting with 120-grit paper and moving towards 320-grit [8]. Set the legs upright and squeeze silicone caulk onto the shoulders [9]. Get help positioning the bench top onto the legs. Tape around the mortise and tenons, and fill the voids with a sanded grout mixture, adding pigment to match the bench colour if desired [10]. After the grout has cured, remove the tape and clean with a damp sponge.

Recipe: Eric Weil’s concrete mix
This time-tested mix sets smooth and hard and can be honed to a granite-like finish. Make two separate batches of this recipe: one for both legs, another for the bench top. Not ready for concrete alchemy? Pick up bags of premix (check with the counter hand for the correct mix) at your building or home supply store.

*10 kg Type I white Portland cement
*15 kg multipurpose sand
*2,3 litres cold water
*200 g black alkali-resistant pigment
*1,6 litres acrylic additive

Parts lists

Quantity Description Material Size (mm)
2 Sides for bench-top mould 16mm melamine 32×1168
2 Sides for bench-top mould 16mm melamine 34×483
4 Knockouts for bench-top mould 16mm melamine 32×140
4 Knockouts for bench-top mould 16mm melamine 32×140
4 Sides for leg moulds 16mm melamine 32×70
6 Sides for leg moulds 16mm melamine 64×400
4 Sides for leg moulds 16mm melamine 64×172
4 Sides for leg moulds 16mm melamine 64×241
4 Sides for leg moulds 16mm melamine 64×64
24 Sides for leg moulds 16mm melamine 64×165
46 Support blocks for bench-top mould 16mm melamine 25×25
1 Backing sheet for all moulds 16mm melamine 914×2440
2 Sides for curved sections of leg moulds 2mm Perspex 64×762
2 Rebar for bench top 10mm rebar 1346
22 Rebar for legs 10mm rebar 837

 

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