Indent Fracturing of Stone Tile: What is it… and how can it be prevented?

Photos courtesy WJE

by Daren S. Kneezel, RA, and Michael J. Scheffler, PE
Soon after a stone-tile floor is installed for a hotel lobby, an architect hears from the client: “In the early morning, the tile exhibits surface depressions and has a wavy appearance—not necessarily at joints between tiles, but randomly throughout the tile. When we review the tile close up, the waviness disappears. It’s also less apparent in the afternoon. How do we fix this? Is it because of your design or is it an installation issue?”

The condition is likely indent fracturing—permanent distress manifested as slight depressions in the tile, often accompanied by very fine, barely perceptible hairline fractures. An example of a tile installation exhibiting indent fractures is shown in Figure 1. Typically developed within weeks or months of installation (i.e. through the curing and drying phase of the mortar materials), this distress is most apparent under oblique reflected light conditions, such as in the early morning—it may not be immediately noticed unless these specific conditions are present.

A stone-tile installation exhibiting indent fracturing, where evident, is often found to be undesirable by owners and can often be rejected or be subjected to a warranty claim. Where the indents are extensive throughout an installation, the most economical method of repair typically involves full removal of the tile installation, including the thin-set setting mortar, as large-scale replacement of individual tiles is rarely cost-effective. However, where distress is limited, individual tile replacement is possible and may be economical, provided matching tiles are available.

Removal and replacement of stone tile can be labor-intensive, disruptive to building occupancy, and the cause of significant material waste. Knowledge of a few key factors can help avoid this potentially costly, irreversible issue, which is not attributed to lippage or exceeding setting tolerances.

Figure 1: Typical indent of stone-tile assembly (circled on left; close-up view on right). The indent fracture location is visible at an offset in the reflection on the tile surface (arrow).

Factors leading to indent fracture
Several factors have been shown to directly contribute to the development of indent fractures in stone-tile installations. They include:

  • shrinkage of the mortar setting bed and crack development;
  • stone tile water absorption;
  • tile dimensional changes upon wetting;
  • tile strength;
  • construction timing (e.g. allowing mortar to be set in lifts and cure before setting the tile, and providing sufficient time to construct and evaluate mockups); and
  • construction detailing—particularly lack of shrinkage restraint when the mortar is set on a flexible substrate.

These critical factors have been identified by this article’s authors, through examination of individual elements of stone-tile assemblies. The tested system and components included several types of nominally 9.5-mm (3/8-in.) thick stone tile set in varying mortar thicknesses over a sound-attenuation mat on a concrete substrate. They represent various conditions noted in a floor system comprising several thousand rectangular stone floor tiles. The frequency of indent fractures varied by stone type and mortar thickness.

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