In a previous article, we covered how to put together three kinds of mixes of glass-fiber reinforced concrete. This time, we’re going to show you how to apply the GFRC after you’ve prepared it.
GFRC part one, in the previous issue, offered a lot of mix design recipes and lists. This lesson can be told mainly with pictures. Step-by-step instructions can be found in the captions of each photo.
If you want the whole story, information from last issue’s Technique section is available online at Concretedecor.net. A back issue of the magazine can be obtained by contacting Concrete Decor.
Step 1: Begin by applying any required mold-release to your mold.
Step 5 – above right and below: Spray. Begin by spraying into the corner of what will be the front edge of your piece. Your spraying technique will be the primary factor in the quality of the finish. When spraying GFRC, you always want to spray into a corner and work your way back, spraying into the face coat you’ve already sprayed. By doing this, you compact the face coat and force out any air bubbles. This is the secret to a superior finish for GFRC.
Step 8: Wait for the face coat to become stiff, but not dry, and then apply a thin application of the vertical back coat, an application also known as a scratch coat. Add a small amount of water, if needed, to thin the consistency of the back coat and make it more workable. Applying this thin scratch coat helps to prevent any air pockets from forming between the face coat and the back coat.
Step 9: Once the scratch coat has been applied, begin the application of the vertical back coat. Starting from the bottom, build the concrete up the vertical wall, holding the wall with one hand and using the other hand to smooth the top. You want to pull the mix over the top of the wall. This helps to keep the vertical back coat from pulling away or slumping.
If the vertical back coat does initially slump a bit, do not despair. Simply continue working your way around the mold. After a few minutes the section that was slumping should be firm enough for you to apply a second coat.