Ceramic Floor Tile on Slab on Grade - Debonded or Tented

2011-10-29

Several methods are used to install floor tiles over concrete slabs, including the use of mortar beds and membranes of various types. This discussion focuses on ceramic floor tile bonded directly to a slab on grade with thinset mortar. Thinset mortar is often used to bond ceramic tile to a concrete slab because this method is generally fast and relatively inexpensive. Bond failures using this method of installation are often the result of a defect with the original tile installation, and may be related to the slab preparation, the thinset mortar, the quality of the workmanship, or a combination of these. Debonded floor tiles are susceptible to tenting, which is two or more floor tiles physically lifting up from the floor. This in turn makes the tiles more susceptible to cracking or breakage. Ceramic tile bond failures are often related to improper preparation of the concrete slab. The Handbook for Ceramic, Glass, and Stone Tile Installation published by the Tile Council of North America (TCNA) recommends that the concrete slab to receive the ceramic tile be "...well cured, dimensionally stable, and free of cracks, waxy or oily films, and curing compounds." It also recommends that the slab surface meet flatness tolerances. Both the TCNA and the Ceramic Tile Institute of America (CTIOA) field reports recommend that concrete slabs to receive thinset mortar have a steel trowel and fine broom finish. The TCNA recommends: "When (curing compounds are) used, mechanical scarifying is necessary." This means that for a proper bond between the mortar and the slab to occur, the slab surface should not be smooth. Slab preparation should include:

a thorough cleaning to remove all foreign materials such as oil or paint
roughening the slab surface, and
repairing existing cracks

The underside of the ceramic tile must also be rough and clean for a proper bond with the mortar. Ceramic tile bond failures also occur due to problems with the preparation and application of the thinset mortar. Problems with preparation may include using an improper ratio of water in the mortar mix and re-tempering the mix by adding water after it has already begun to set. In addition, the mortar used must be compatible with the tile. Problems with application include allowing too much time between the mortar mixing and the tile placement, not properly pressing or beating the tiles into the mortar, improper mortar coverage or thickness, and not allowing enough time before foot traffic or other loads are applied after installation. One or several of these construction defects are sufficient to cause bond failure over time. The everyday influences of concrete shrinkage, temperature, humidity, and minor settlement cause stresses in the mortar which may lead to bond failure of improperly installed tiles. The bond failure usually starts in small areas that grow larger with time. Bond failure may also lead to tenting, which occurs due to compressive forces that occur within the plane of debonded floor tiles. Compressive forces exist due to normal variations in humidity, concrete shrinkage, temperature, differences in material thickness and coefficients of thermal expansion between ceramic tile and concrete. In properly bonded floor tiles, these forces are reduced and distributed by contact and shear at the mortar interfaces. Once a larger area of floor tiles debond, these tiles are allowed to expand at a different rate than the slab, which leads to compressive forces building up in the debonded floor tiles. These forces are often relieved when the tiles tent or crack. Tented floor tiles are susceptible to cracking due to foot traffic and other normal loads. The following photograph shows tented and cracked ceramic floor tiles:

The next photograph shows debonded, tented and cracked floor tiles. Note the slab and thinset mortar visible beneath the tile with the failure planes between the mortar/tile and mortar/slab:

ADDITIONAL RESOURCES: TCNA - Handbook for Ceramic, Glass, and Stone Tile Installation CTIOA - Field Reports