Make Fake Rocks with Concrete

Making artificial rock can benefit anyone, from the casual garden enthusiast to the landscaping professional who wants to spice up their garden life. Combining basic construction skills and artistic creativity, you can create artificial rocks with concrete that are virtually indistinguishable from naturally occurring stone. Sculpting landscape accents from concrete is an economical and lightweight alternative to large rock installations.

Steps

Building a Form

  1. Choose a material as a base for your rock's shape. You can use a variety of materials to create the shape of your rock. There are several common items you can choose from:
    • Styrofoam
    • Cardboard
    • Crumpled newspaper
  2. Create the rough shape of your rock. Cut the cardboard or styrofoam to the shape you want your rock to be. Combine different materials with glue to create oddly shaped rocks.
    • Use a plain cardboard box for a roughly square shaped rock.
    • A hot wire foam cutter works well to shape styrofoam.
  3. Cover your rock shape in chicken wire or hardware cloth for a better look. Use a metal mesh to wrap the rock shape. The metal gives strength to your artificial rock and provides a structure for the cement mortar mix to adhere to.[1]
    • Use wire twist ties to secure the wire frame to your rock base.
  4. Refine the curves of your rock. To make the most natural looking rock, bend and shape the wire form around your rock shape. Natural rocks have dips and creases; simulate these shapes by pushing your wire form in various places to create uneven surfaces.

Mixing the Mortar

  1. Combine dry ingredients for mortar mix. Mix 3 parts sand with 1 part portland cement. Add all the ingredients to a wheelbarrow or concrete mixer, depending on the size of rock you are creating and amount of mortar you are mixing.
    • You can reduce the sand, and add 1 part peat moss to create a more porous artificial rock.[2]
    • If you want to use the fake rocks in an area exposed to water, use a hydraulic cement mix instead.
  2. Add water to the dry mortar and sand mixture. Slowly add 1 part water to the dry mixture; you may need to add more or less water depending on humidity and temperature. The mixture will turn into a thick paste as you add the water.
    • Stir the water into the mortar mix as you add it.
    • Watch closely as you add the water so your mixture does not get too wet.
  3. Stir the mortar mix for several minutes. For small amounts, turn the mixture over in the wheelbarrow repeatedly, or stir with a paddle attached to an electric drill. For larger amounts, use a concrete mixer. You will need to mix the mortar to the consistency of cookie dough.[3]
    • Ensure the mixture is fully mixed and uniformly wet.
    • Add more water if necessary to get the consistency of a thick paste. You do not want the mixture to be runny.
    • Unmixed blobs of sand will cause weak spots in the finished rock; be sure to mix everything completely.
    • Keep track of what you added and adjust until you reach the desired consistency. Write down the formula you find that works best. Follow this formula and use the same measuring device for the water each time to keep a consistent mix from batch to batch.

Sculpting the Rock

  1. Apply the mortar mix to the wire form. Use a flat pointed trowel to apply a 2-3" layer of mortar to the wire frame.[4]
    • Build the rock from the bottom up.
    • Make a layer of mortar around the base of the rock and work upwards around the wire frame.
  2. Add texture to the mortar. Create a realistic looking rock by adding contours and patterns to the surface of the mortar.
    • Use your trowel to create dips and creases in the surface of the mortar.
    • Press a real rock into the mortar to make imprints of the rock's texture.
    • Press a sea sponge or scouring pad into the rock to create pock marked look.
    • Wrap a plastic bag around your hand and press it into the mortar to give a wrinkled look.
  3. Cure the rock for 30 days in a dry location. The curing process is a result of a chemical reaction, not the cement drying out. Although 75% of the curing is completed after one week, it may take up to a month for cement to cure completely.
    • Mist the surface of the rock every few days as it cures.
    • Keep the cement out of direct sunlight to prevent cracks.
    • Cover the rock with plastic sheeting as it cures.

Finishing the Rock

  1. Scrape the rock to smooth the edges. Use a scraping stone or a hard wire bristle brush to rub the surface of the rock. Scrape off any sharp or pointy edges that exist on surface of the rock.
    • Allow the rock to cure for a week prior to scraping to prevent crumbling.
  2. Wash the rock. Thoroughly rinse the surface of the rock. Brush the surface with a wire brush while washing to remove any loose bits of mortar. Make sure to flush out any creases or dips in the rock to get rid of rock dust.
  3. Stain the rock. Use a penetrating concrete stain to cover the surface of the rock in the color of your choice. You can apply multiple colors to provide the most natural look. For a more obvious design element, shiny aggregates or even glow in the dark powders are also available to add to the mix.
    • Brush the stain on rock with a paint brush.
    • Add depth to the coloring by using more than one color.
    • Apply more stain in some areas for darker contrasts.
  4. Seal the rock. Use a water or solvent based concrete sealer to protect your artificial rock from the elements. Some sealants provide a glossy finish while others have no sheen but still provided protection.
    • Brush on 3 coats of sealant. Wait about 15 minutes between coats.
    • Maintain the sealant by reapplying a coat of sealant every 1-2 years.
  5. Remove the inner base from the rock. Decide which side is the bottom of the rock, and cut it open so you can remove the inner structure. The rock's shape and strength come from the mortar and wire frame; the inner materials do not provide structure after the concrete has cured. Removing them will prevent them from decomposing.

Landscaping with Fake Rocks

  1. Decide where you want to place your fake rock. Fake rocks can be used as a part of a water feature, lining pathways, or as garden accents. Determine the best place for your rock based on its size and appearance.
    • Unless you used hydraulic cement mix, keep the fake rocks away from water. Standing in water or heavy water splashing can cause regular cement to break down.
  2. Dig a small indention where the rock will be placed. Place the rock in place and trace the edge of the rock with a stick or shovel. Dig a 1-2” pit in the shape of the rock. Placing the edges of the rock underground will provide a more natural look of a rock outcropping.
  3. Place the rock in the hole. Push dirt and other small rocks against the edge of the rock to integrate it with the landscape. Build multiple rocks to create elaborate rock landscapes.



Warnings

  • Do not attempt to use artificial landscaping rocks as weight bearing installations for swimming pools or hot tubs.
  • Use caution when working with cement. Lime can cause chemical burns if it gets on your skin or in your lungs. Wear gloves and a mask when mixing cement, as well as proper protective clothing.

Related Articles

Sources and Citations

  1. ↑ http://homeguides.sfgate.com/cast-faux-rocks-landscaping-32323.html
  2. ↑ http://extension.oregonstate.edu/lincoln/sites/default/files/hypertufa_container.pdf
  3. ↑ http://www.waterbugdesign.com/pond/rock_making.html
  4. ↑ http://mold-making.com/fake_rock.htm
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