How to Construct Artificial Rocks

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Nature makes rocks that are sometimes too heavy to move without a lot of help. You can construct artificial rocks by using an old piece of cement or sculpting a form out of foam. Artificial rocks are lighter and easier to move. It takes practice to construct an artificial rock that looks like a real one, and it takes time. By constructing your own artificial rocks, you can landscape your yard with rocks made your way.

Things You'll Need

  • Foam
  • Chicken wire
  • Steel mesh
  • Tie wire
  • Pliers
  • Respirator
  • Wheelbarrow
  • Cement
  • Fine, clean washed sand
  • Un-densified silica fume
  • Fly ash
  • Glass fibers
  • Poly-plex or liquid latex
  • Liquid water reducer
  • Water
  • Spray bottles
  • Water-based latex paint
  • Trowel
  • Sponge
  • Fine-grit scraping stone
  • Stiff bristled brush --- not a wire brush
  • Water-based sealant
  • Carve the bulk of your rock form out of a piece of foam. Unless you want a brick-shaped rock or a round rock, don't make even straight sides or completely round sides because rocks have variations. Cover the foam piece with chicken wire or steel mesh to make it durable and also provide the concrete something to cling against. Cut a piece of tie wire at least 3 inches long and bend one end of the tie wire with a pair of pliers to make a 90 degree hook. Push the straight end into the foam with the hook on the outside to hold the wire in place. Push the hook into the foam to secure. You can also use an old chunk of waste concrete instead of foam. If you use the concrete you can eliminate the chicken wire or steel reinforcement.

  • Don a respirator before you begin to mix the concrete. The dust from silica, concrete and fiberglass are dangerous to your health.

  • Find a wheelbarrow to mix the cement in. Add one bucket of cement and three buckets of fine sand. Use sand that is clean and washed for better results; otherwise the cement may not set up properly. Add one and a fourth buckets of un-densified silica fume, a fourth bucket of fly ash and one handful of glass fibers.

  • Mix the dry ingredients thoroughly. You don't want any pockets of sand, cement or other material. Use a cement mixer use if you have one. Otherwise, a square head shovel works well.

  • Pour in a small amount of water and then mix the cement ingredients thoroughly. Add a little more water and mix the cement more. Do not add a lot of water at one time because you don't want to get too much. Also, the amount of water varies with every batch of cement made. If you want to make the cement strong, mix 26 ounces of poly-plex or liquid latex and 1 1/4 cup water reducer. The amount of water varies every time you make cement. It depends how wet the sand is and the amount of humidity in the air. This requires a lot of mixing.

  • Test the cement to see if you it's wet enough. Pick up some of the cement in your hand and form it into a ball. Squish the concrete ball through your fingers. If it breaks off into clumps then you need more water. Drop the concrete ball from the height of your knees. When it hits the ground it should flatten out into a circle but not totally flat.

  • Spray the foam piece with a water bottle filled with water. Apply the concrete to the top and sides of your foam shape, building to a thickness of 3 inches or more.

  • Sculpt the surface of the rock to give it a more natural look. Use the point of your trowel to mimic the veins and fissures. Dabbing a sea sponge over the surface gives texture.

  • Cure the artificial rock for three or more days. Soak the artificial rock with water as it cures. This keeps the surface from developing hairline cracks and aids in the curing process.

  • Take a fine-grit scraping stone and grind it over the surface of your rock. This gives your rock's surface a smooth finish.

  • Wash the surface of your artificial rock with a stiff bristled brush. Allow the surface to dry completely before adding any color.

  • Paint your artificial rock. Fill several spray bottles with different colors of a water-based paint. You may have to water the paint down so it goes through the mister. If you examine real rocks, you will notice that they have many different colors like blue, green or brown. Set the spray bottle to mist and apply multiple layers of color over the surface of the rock for a realistic look.

  • Paint the artificial rock surface with a water-based sealant. Apply three layers of the sealant, allowing each layer to dry according to manufacturer's directions. The water-based sealant needs to be reapplied every year.

Tips & Warnings

  • It takes one full month for concrete to thoroughly cure. Do not attempt to move your rock before that time or you may ruin it.
  • You can adapt the measurements for the amount of cement you need to make. The standard recipe is three parts sand to one part cement. The other ingredients are based on one part cement.

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References

  • Photo Credit Jupiterimages/Creatas/Getty Images
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