Terrazzo is a veneer system applied to a concrete substrate, and as the concrete goes, so goes the terrazzo. A crack-suppression membrane over the slab is standard in terrazzo installation to inhibit any cracking in the substrate from telegraphing into the terrazzo.
Dan Stanton is president of Colorado Design Tile & Terrazzo in Denver and a 30-year veteran in the industry. The worst cracking he’s seen in terrazzo was in a new building that settled. “If the building moves,” he noted, “epoxy doesn’t hold it together.”
Cracks in terrazzo can occur for a variety of reasons, most of which can be headed off by proper substrate treatment and honoring the joints in the concrete.
- Do your homework: If you see there’s a crack in the substrate, do your homework and figure out why Mr. Stanton advises.
- Treat the substrate: If you treat the joints and the cracks in the substrate correctly, you’re not normally going to get cracks. If you don’t honor the joints, you will get cracks, Mr. Stanton warns.
- Treat the joints: In cement floors, especially if slab-on-grade, treat the cracks in the joints in concrete by shot-blasting them open and applying a crack isolation membrane. Use a rigid epoxy and apply a fiberglass scrim, then a crack isolation membrane.
- Use divider strips: Most cracks can be avoided by honoring the joint in the concrete with a divider strip in the terrazzo. Strips help prevent or repair cracks. Sometimes designers don’t want to put a strip, resulting in a crack. A metal divider strip is better than a crack. In an epoxy terrazzo installation, divider strips are more a matter of aesthetics than crack prevention. Cement-based terrazzo, however, must be poured in 4×4 or 5×5 panels to allow for shrinkage. In either case, divider strips can often provide a detour around the threat of cracks in your terrazzo floor.
- Use control joints: If slab-on-deck, use control joints. If you anticipate movement in the floor, two strips joined by a flexible membrane will help fix or avoid cracks.
As the authoritative resource on terrazzo, NTMA offers personalized technical guidance on the installation of terrazzo and related issues to all parties on a project. Call Gary French at 800-323-9736 ext 1 or email him at firstname.lastname@example.org.