Mixing GFRC Backer Mix

GFRC backer mix is typically the most challenging of all the GFRC mixes to blend. The high fiber content and low water amounts dramatically increase its chances for clumping. We’ve discovered that a double-paddle mixer like those from Collomix or Benner Nawman do an excellent job of thoroughly blending GFRC backer mix quickly and with the fewest amount of clumps. Remember GFRC backer mix should have the pliable consistency of clay which allows us to lay it up on vertically formed surface.

Step 1 – Clay-Like Consistency

Structural Lay Up - Step 1 | Concrete Exchange
  • Do not wet the mix out like you would to spray it through a hopper or to cast it as usual.
  • Gradually add just enough water until the mix reaches a tacky consistency, with enough body to pack it on a vertical surface.
  • If the mix is a little bit wet, let it set for a while and it will firm up.
  • If the mix gets too dry while working, re-mix it and it will loosen up again.
  • The working time depends on the environment, but you’ll have at least 30 minutes to 1 hour to pack a form.

Step 2 – Thin Pads

  • Grab a small handful of the mix and flatten it out until it’s about 1/2″ thick.
  • Press it against the form wall, being careful not to let it get sucked away from the surface as you remove your hand.
  • If the pads are sliding down the wall, the mix is too wet, or the pads are too thick.
  • Try packing on thinner pads, or letting the mix firm up.
  • After the first coat has been applied, a second coat can be added until the piece reaches the desired thickness.

Step 3 – Spraying GFRC Backer Mix

  • New Sprayers are now available that allow the small contractor/ fabricator to spray GFRC backer mixes without the need for expensive peristaltic equipment.
  • Toolcrete’s 3-Jet Sprayer equipped with the Single Hole Extension Plate is a great way to place a lot of backer mix in a short period of time. In our tests we were able to place over 10 lbs of  CHENG D-FRC Backer Mix per minute.