Depending on the type of form that is being used, liners can work in a variety of different ways. For instance, there are sheet elastomeric form liners, which attach to the form panel, wood or steel, through epoxy. There are also concrete form liners, which can be mounted in different ways. It is very important, regardless of the type of form liner you use, that you are always very careful about how you patch over the screw heads, particularly if you use plastic. It is very important that you don’t use caulking or silicone to do this. Instead, ABS or plumber’s glue works a lot better. Using the wrong patching will lead to streaks and discolored spots, particularly on concrete.
Between form liners, there will always be joints and these have to be cared for in specific ways. This is because, if it is done wrong, you will see honeycombing or discolored concrete. Joints always have to be sealed, therefore. It is possible to get form liners that have interlocking edges, but you can also design a non-textured area or a rustication line at the joints. All liner materials respond to temperature changes by expanding or contracting, so that is another thing to look out for.
Whenever you apply a form liner, you have to make sure that the form release agent is used properly. A good reactive release is generally all that is needed. If you want to stain the concrete, then you shouldn’t use kerosene or diesel, because this will affect the way the stain is absorbed. If you use a reactive release, it can simply be washed off after stripping. Petroleum release solvents and agents can destroy plastic and elastomeric form liners, so the release should be brushed or sprayed into the pattern in order to cover every facet.
If you use self-consolidating, high slump, concrete mixes, you will get far fewer bug holes and a lot of beautiful detail. However, this does need well-braced, near watertight forms. Usually, however, a standard mix will do the job, although you do have to make sure that the concrete is properly vibrated. Internal vibrators, which are put into place in the previous lift, work really well, so long as they don’t make contact with the form liner. An elephant trunk is usually the best way to stop the entrapment of air and mix segregation. It is important to not ask a contractor to use a special type of mix if they work on the highway, as this can pose real difficulties for the contractor. They usually know what is best, so let them do their job.
Make sure, finally, that the form liner is stripped within 24 hours. If it is in contact with the concrete any longer than that, it will become very difficult to strip it. Polypropylene forms, however, are often very sticky at their first application, but become increasingly easy to remove afterwards, which is important to remember so that you don’t think something has gone wrong.