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How to Use Form Liners

Form liners are attached to the formwork in various ways, depending on the type of form. Sheet elastomeric form liners are epoxied to the form panel—either wood or steel. Elastomerics that come already mounted on plywood backing are screwed to the forms, typically from the back to keep from marring the form liner surface. Plastic form liners are screwed or stapled to the forms. One tip is to be careful what you use to patch over screw heads on plastic liners. "I always tell people not to use silicone or caulking," said Fitzgerald Formliner's Jill Richards, "use plumbers glue or ABS glue because then it's the same material as the plastic. Silicone will create discolored spots or streaks in the concrete."

Joints between form liners require special care, since leaks at these points will create discolored concrete or honeycombing. Always seal joints. Some liners come with interlocking edges. A simpler way is to design a rustication line or a non-textured area at liner joints. Liner materials will expand or contract with temperature, so watch for this as well.

A critical step for any form liner application is properly applying the form release agent. In general, any good reactive release works well. If the concrete is to be stained, do not use any sort of diesel oil or kerosene, since it can interfere with uptake of the stain. Reactive releases wash off cleanly after striping. Fitzgerald cautions that petroleum-based release agents or solvents can attack and destroy either elastomeric or plastic form liners. Spray and brush the release into the pattern to make sure every facet is coated.

High-slump self-consolidating concrete mixes will result in great detail and very few bugholes in form lined surfaces, also these mixes also require nearly watertight and well braced forms. But in general, more standard mixes will also work fine, although extra effort should be made to thoroughly vibrate the concrete. Scott says to use internal vibrators that are inserted into the previous lift and to avoid vibrator contact with the form liner. Fitzgerald advises use of an elephant trunk to avoid mix segregation and entrapping of air. Paul Nasvik avoids asking contractors to use special mixes on highway jobs, where "specifying a special mix can create a nightmare for the contractor. I maintain that regular concrete is fine—it really comes down to proper placing technique."

Another consideration with form liners is the clear concrete cover over the reinforcing steel, with an indented pattern. Position the patterns and steel to maintain 2 inches of cover in order to reduce the possibility of rust stains on the wall surface.

Form liners should be stripped as soon as possible—at least within 24 hours. The longer the liner is in contact with the concrete, the tougher it will be to strip. Karlson says that their expanded polypropylene forms will stick to the concrete the first time they are used but will come off with the forms on subsequent uses.

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