Surecrete 45101003 XS Bag Backer Mix Xtreme Series GFRC Backer Mix is a Glass Fiber Reinforced Concrete blend available in white and gray (DFRC) for reinforcing cast concrete pieces. SureCrete’s high fiber cement backer mix can be colored with both our standard and XS color additives. XS Backer is the perfect medium for residential, commercial, and industrial applications.