Tips for using fibers when making GFRC

Don’t think making GFRC is as simple as just adding some fibers to your concrete mix design. There are many important considerations to remember. Here are a few:

Amount of Fiber Present– GFRC relies on a high load of glass fibers. Without sufficient fiber the concrete will be unable to resist cracking and breakage when faced with a high tensile load. Fiber content varies, but is at least 3% of the total mix weight. Some mixes go as high as 10% fiber content. The more fiber present the stronger the GFRC, fiberglass chopped strand mat but increased fiber does lead to decreased workability and even to compromised compaction.

However, decreased fiber leads to the worse problem of less strength. Some concrete countertop teachers recommend only 2% fibers. I’m not sure what the motivation behind this is, but 2% is not sufficient. 3% is the minimum.

Orientation of Fibers– Orientation of the fibers in the mix is also important. Truly random fiber orientation means more fiber is needed since many of the fibers will be pointing in the wrong direction. See below for

Some concrete countertop teachers recommend creating a fluid backer mix and pouring it into the forms, effectively an “SCC” backer mix. This should not be done, as it results in random fiber orientation. See below and read this article for more information about why this is problematic.

Method of Reinforcement Used– There are three different levels of reinforcement used in general concrete and GFRC. Each type carries different benefits.