How to make exposed aggregate concrete

Written by lou paun
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Exposed aggregate concrete is beautiful and durable. The slightly uneven surface reduces slipping when used in a path. Because stones don’t stain easily, exposed aggregate concrete is less likely to stain than ordinary concrete. It’s not hard to make, either. A small project, like a stepping stone or a tile, is the best way to learn the technique.

Concrete is composed of a mix of elements: cement, coarse aggregate (usually gravel or crushed stone), fine aggregate (sand) and water. You can change the look of the finished concrete by making changes in the elements -- using white instead of grey cement, for example, or using crushed coloured glass in place of gravel. These choices can change the look of exposed aggregate concrete dramatically.

Skill level:
Easy

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Things you need

  • Concrete mix
  • Pigment in dry or liquid form (optional)
  • Water
  • Mixing bucket
  • Mold
  • Non-stick cooking oil spray
  • 2x4 board
  • Putty knife
  • Stiff scrub brush (fine wire bristle)
  • Hose with mist nozzle
  • Polyurethane sheet
  • Rubber gloves
  • Disposable respirator

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Instructions

  1. 1

    Decide what colour or colours you want for your finished tile and choose the appropriate concrete mix. Ordinary premixed concrete can make an attractive grey exposed aggregate tile. Some manufacturers have coloured mixes that also can create handsome tiles. If you mix your own concrete, you can choose the colours of the mix and the aggregate to create beautiful effects. For example, Quickrete in buff colour and a basic concrete mix containing 3/8-inch Roan River pea gravel make a beautiful tile.

  2. 2

    Mix the concrete according to package directions. Spray the mould with the non-stick cooking oil spray. Pour the wet concrete into the mould. Release air pockets in the wet concrete by stepping on it several times, shaking the mould and tapping the sides and bottom of the mould. Level the wet concrete with the 2x4 board. Shape the edges of the concrete with a putty knife.

  3. 3

    Wait about six hours until the concrete has set. If the weather is humid, you may have to wait slightly longer.

    Scrub the surface with the stiff scrub brush. Rinse frequently with a fine mist to remove the loosened grains of concrete. Rinse out the brush frequently as well.

    Scrub until some of the coarse aggregate shows clearly on the surface. Don’t brush too deeply, because any stones embedded less than halfway are likely to fall out later.

  4. 4

    Let the surface dry. Cover it with a polyurethane sheet. Leave the tile to cure in a warm, dry place for at least three days.

  5. 5

    Remove the tile from the mould. The concrete will cure for 28 days before reaching its maximum hardness, so handle it gently until the curing period has passed.

Tips and warnings

  • Wear rubber gloves when you work with concrete.
  • Wear a disposable respirator when you mix dry pigment or handle a dry concrete mix.
  • If you get wet concrete on your skin, wash it off promptly.

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