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Rockwork

Artificial rockwork has been manufactured for over 30 years. It is quickly becoming more and more common to see decorative structures in the commercial swimming pool and water park markets for their aesthetic appeal.  The two most common methods of creating artificial rockwork are by “hand-carving” (shotcreting) or by using glass fiber reinforced concrete (GFRC).

  • Hand Carving
    • Hand carved rockwork is also called positive rockwork because there is no molding involved. This process starts with engineered three-dimensional concrete walls joined to footings with reinforcing steel.  When the reinforcement is placed, a special clip to the reinforcing bars is attached to secure a polyester weave "back-up" fabric to the frame (chicken wire).  This creates a back-form surface for the shotcrete and ensures that concrete can be sprayed around the reinforcement, positioning it towards the center of the placement.  2 to 4 inches of structural cover is placed over the reinforcement.  The structures are then carved to simulate the desired rock texture.
  • Glass Fiber Reinforced Concrete (GFRC)
    • GFRC panels are created by taking impressions from real rock formations. The textures and shapes are then brought together in a mold, typically about 8 feet x 8 feet.  Several molds are created with different formations and textures so that, once assembled, repeat features are unnoticeable.
    • Portland cement is super-blended with silica sand, admixtures, and water to produce a mix that is then sprayed onto the mold at high velocity (approximately 45 psi at 25 cfm). Fiberglass roving (bundles of fibers that look like rope) are chopped into 1 inch segments. The fibers mix with mortar in the nozzle, and the mixture is projected against the mold, which drives out most of the entrapped air voids.  The balance of the air is forced out by hand or with rollers.  By volume the mix is approximately half glass fiber and half cement mortar.  Approximately, a ¾ inch thickness of material against the mold is built up, allowed to cure, and removed from the mold the next day.
    • Some manufacturers have different construction approaches. After footings are placed in all areas where rock displays will be constructed, a rebar frame for the entire project will be constructed, roughly the shape of the rock structure, covering the frame with chicken wire and lath.  Then manufacturers shotcrete the structure, leaving pieces of rebar exposed to attach the GFRC panels.  The panels purchased have pieces of rebar molded into them to serve as attachment points.  They are then welded to the exposed pieces on the frame.  When everything has been solidly connected, concrete is placed between the casting and the shotcrete frame to complete the structural part of the job.
    • GFRC rock-textured panels are lightweight, weighing between 10 and 14 pounds per square foot. They are especially useful for projects where weight is important.
    • GFRC also has an advantage in strength. GFRC can have tensile and flexural strengths as much as seven times those of plain concrete, with compressive strengths as high as 14,000 psi.
    • To give the rockwork a natural look, panels can be hung upside down or sideways. A panel with 12 to 18 inches of relief can be cracked and folded to provide much more relief and a completely different appearance.  Panels have the ability to be broken into many pieces and mounted on a frame in three-dimensional ways that completely mask the panel's original appearance.
    • GFRC panels must be “embossed” when positioned onto the frame because gaps between the panels will exist that must be filled. These spaces can be filled one of two ways.  The first method is one in which the gaps can be filled with concrete when the voids are filled between the panels and the shotcrete frame.  Using the second method, the gaps are filled by cutting diamond-lath mesh to fit the gaps.  Then, ¼ inch holes are drilled through the castings along the gaps, and tie wire is used to fasten the lath to the castings.  A "scratch coat" of Portland cement and sand is then plastered onto the lath and allowed to dry.  After embossing, it should be very difficult to detect the seams between the panels.
    • Rock formations can be colored to match any stone type or color scheme. Coloring merges the embossed areas and panels and makes everything look like one material.  Coloring is done via a water-based acrylic stain.  Careful cleaning needs to be done to ensure a good bond between the concrete and the acrylic.  Water is sometimes added to thin out the colors and make them more translucent.  Black washes can also be applied to transition the colors and bring everything together.

GFRC rockwork is the more expensive option when compared to shotcreting. GFRC rockwork costs about 35% more than hand-carved rockwork.  Hand-carved rockwork will cost approximately $34 to $39 per square foot of surface area, while GFRC will cost between $50 and $55 per square foot of surface area.  Some rockwork experts recommend that 50% to 60% of a project use GFRC and the rest using positive rockwork, with GFRC castings being placed in the more prominent areas.

GFRC rockwork provides the most realistic appearance and is therefore the most desirable. Hand-carved or positive rockwork should be the second option.  Lath-and-plaster should not be used for swimming pool applications since the reinforcing bars are not encapsulated completely in concrete and corrosion would be an issue.

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