In past articles we have profiled different methods for building artificial rockwork, including:
- building frames with rebar and lath, applying concrete over them, and hand carving shapes and textures
- making glass-fiber-reinforced concrete (GFRC) precast panels and erecting them on the jobsite
- using polymer cement mixtures rated for vertical application that can be built up to 2-inch thicknesses for vertical applications and stamped and textured to provide a unit masonry look.
Another method that also uses polymer cements is to mix polystyrene fine aggregates with port-land cement and polymer modifiers to create a mix that is very light and can be applied to vertical areas without sagging. Troy Epping, regional sales manager for Boulder Image, Phoenix, says that using this process they start by constructing a very basic shape for the rockscape. “Rubble that ends up in landfills, like broken concrete, concrete blocks, and foam can be used to do this,” he says. It doesn't have to be compacted because it only provides support during the construction process.” The elevation, length, and width of a rock formation are established in this part of the process. Blocks of polystyrene foam can also be used as filler material, and they are often added to form individual rock shapes. The polystyrene foam is often cut into shapes with chain saws and saber saws. There is no effort to cut them into realistic rock shapes.
After rough dimensional shapes are worked out, a filler mix weighing only 13 pounds per cubic foot (normal concrete mixes are approximately 145 pounds per cubic foot) is applied. Epping says his company uses recycled polystyrene foam to manufacture the 1/8-inch-minus aggregate for their mix. Batches are mixed in small jobsite electric paddle mixers or pan mixers, and workers, wearing long rubber gloves, hand apply it and work out basic shapes. In a single application they can build up a 3-inch thickness. Additional applications can be added to help refine the rock shapes.
When workers achieve the basic shapes, an application of polymer cement with sand aggregates is the next step. They use texture skins or rubber gloves to add refinements to both shape and texture. This cement mix can reach strengths between 5000 and 8000 psi, depending on the mix used. This helps create the different shading effects when you color the new rocks and maintains the imperfections you see in natural boulders, says Epping
Epping says there are a number of techniques and products available for coloring finished rockwork to provide realistic appearances. They include chemical- and water-based stains, water-based acrylics, and dry pigments that can be applied by hand or with basic spray bottles and airless sprayers. Virtually any natural rock can be replicated.
Typically, a concrete footing is used where a running water feature will be located. Epping says this is important since Mother Nature creates temperature changes and freeze/thaw movement. The polystyrene cement mix has an R-8 insulation value, and the filler material even helps to prevent damage to water supplies. He adds that trouble-free applications have been in place for eight years all over the United States.
This method of doing rockwork is frequently used around swimming pools, hot-tubs, waterfalls and water features, wine cellars, barbecues, and even mailboxes, and the look is entirely dependent on the creative skills of the contractor. Typical costs to consumers range from $35 to $70 per cubic foot.