Types of Form Liners
The first thing to decide when considering form liners is how many times you hope to reuse it. Some materials will withstand up to 100 pours or more, while others are intended for a single use. The price varies accordingly. Most form liner manufacturers have a selection of standard liner textures and most will make custom liners.
There are basically four materials used to make form liners, each with its advantages and disadvantages:
- Elastomeric urethane rubber
- Polystyrene or polypropylene foam
ELASTOMERIC RUBBER (URETHANE)Buck Scott, Scott System, Denver, has been making elastomeric form liners longer than anyone--since about 1970. He notes that "Elastomeric means a material that is stretchable from a point within itself in all directions. Mostly for form liners that means urethane rubber. Plastic is flexible but not stretchable." This stretchable quality allows elastomeric liners to create nearly any surface texture imaginable and even allows some undercut textures.
Custom elastomeric form liners can be made in very large sizes to create special effects, like this locomotive motif that now graces the Demonbreun Street Viaduct spanning Nashville's historic railroad yards. Scott System.
Form liners made with the casting face of urethane rubber are the longest lasting, with 100 or more reuses possible and can be produced with patterns as deep as 5 inches. Within the category of urethane liners, there are some manufacturers who have different grades. Paul Nasvik of Milestones, Hudson, Wis., only makes urethane liners, but has three grades. "Even what we call our single-use liners have a urethane facing and are cast in the same mold as those that will withstand many uses. For some applications, the contractor intends to use the liner only once. So we cut down on the thickness of the urethane facing and use a lower grade of plywood backing. They still aren't as cheap as plastic liners but are about half the price of our high reuse liners."
A concrete block pattern will be created on this tilt-up panel using an elastomeric rolled liner. Fitzgerald Formliners
Elastomeric liners can be supplied as rolled rubber sheets, as flat sheets, or cast onto a plywood backing. The contractor glues the rolled or flat liners to a plywood backing or directly to the form panel. Milestones places polystyrene foam between the rubber facing and the plywood backing to bond the two together. Scott System has what they call Hydro Edge liners, where the plywood, including the edges, is embedded into the urethane, protecting the edges from damage from wet concrete.
The cost of elastomeric form liners varies from about $14 per square foot to as much as $75 for custom patterns and patterns with deep relief. "The liner price is determined by the amount of relief in the surface," said Scott. "If you have deep detail, all that has to be solid rubber. It's sort of sold by the pound. If a liner weighs 10 pounds per square foot, it will cost $50 or more per square foot. If it's a light sand blast texture that's only 2 pounds per square foot it would be only $14 or so. The price is primarily based on the cost of the rubber."
PLASTICPlastic form liners are typically made using a vacuforming process. Plastic liners can be single-use or can go up to 15 reuses, depending on how thick the plastic is and what type of plastic is used. Fitzgerald Formliners makes plastic form liners of styrene for single use applications, like tilt-up panels, or from ABS for cast-in-place uses. Fitzgerald also makes an extruded plastic formliner for applications where the designer wants long sections without seams, but this only works with ribbed patterns.
Plastic form liners are easy to handle because they are very lightweight—as little as 0.4 pounds per square foot, compared to an average of about 6 pounds for elastomeric liners. A wide variety of plastic patterns are available, although the more typical plastic form liners are those used for ribbed patterns. Plastic patterns can't achieve the deep relief of rubber liners (maximum is about 1 inch) and can't get the sharp detail of elastomerics. Allowable form pressures (controlled by pour rate) are lower with plastic liners—600 to 750 psf. For deeper plastic patterns, wood strips are often added behind the liner to provide added support.
Plastic form liners cost from $1.50 to about $7.00 per square foot, depending on the type of plastic and the relief.
POLYSTYRENE OR POLYPROPYLENE FOAMStyrofoam can be used in formed concrete as a block out to create various patterns or even logos. To make that work, however, it must be waterproofed. One clever technique is to form patterns with Styrofoam then to actually melt the foam out with acetone when stripping. Styrofoam single-use form liners are also available in rock patterns.
One fairly recent addition to form liner technology is expanded polypropylene liners available from Karlson Forming Specialties, Amery, Wis. Produced in roughly 3x3-foot panels, these liners are very lightweight and will last from 6 to 8 pours. They are easy to trim to size with a utility knife or a circular saw. To install, they are nailed to the forms with finish nails or attached with double-backed tape. Cost for these liners is $8.00 per square foot.
Karlson Forming Specialties
FIBERGLASSFiberglass form liners were once quite popular, but are seldom used today since elastomeric liners are simpler to make and perform as well for most applications. "Fiberglass is slowly getting outdated," says Richards. "We had a fiberglass liner not too long ago with 8-inch relief. That's a great application for fiberglass—anything over 3 inches, but I haven't seen one in 3 or 4 years. It's an old technology but we still offer it because every once in a while someone wants it."
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