Indent fracture mechanism
Based on these findings, the physical/mechanical mechanisms resulting in the occurrence of indent fractures can be best described as follows:
1. The stone tile elongates and can slightly warp upon wetting from moisture in the setting mortar. A greater amount of available water in the mortar results in more wetting of the stone tile.
2. The stone tile then becomes restrained in its wet and lengthened condition as the mortar develops a bond with the stone, altering from a plastic state to a partially cured and rigid one.
3. The drying shrinkage of the mortar setting bed results in shrinkage cracks in the mortar, particularly for stone types with high absorption or where restraint is not provided by the substrate below (such as where there is a flexible sound-attenuation mat or crack-isolation membrane).
4. Mortar shrinkage results in tensile stress being imparted to the underside of the tile to which the mortar is bonded at the mortar crack.
5. Tile cracks result where the applied tensile stress from mortar shrinkage exceeds the tensile strength of the stone tile. The greater width of cracks at the tile’s bottom surface—and resulting downward curvature—is likely due to greater applied stress (and induced shortened length of tile) at the bond surface between the stone and mortar than is present at the top surface.
The mechanism of indent fracture distress is inherent in the stone-tile assembly materials, and no external applied forces (e.g. service or construction loading) are required. Of course, external loading of a tile can result in cracks, but these are not likely to exhibit indents coincident with the crack, as mortar shrinkage shortening the bottom of the tile is needed to locally indent or curve the tiles at cracks. It may be difficult to differentiate the exact cause of cracks between loading-induced types and mortar shrinkage-induced indent fractures where both are suspected; however, loading-induced cracks are typically observed only locally to where loads were applied. Indent fractures are very likely to occur throughout the installed assembly as long as the aforementioned factors are present either singly (for essential factors) or found in combination.
Limiting potential for indent fractures
While most stone-tile installations do not exhibit indent fracturing, there are a few key contributing factors that can, when acting in combination, result in this distress. These factors can be modified to minimize the potential for indent fracturing of a stone tile installation. Each of the contributing factors alone is not likely to result in indent fracturing, provided the other factors are appropriately accommodated.
Mortar
Mortar should be installed in lifts where necessary, and its thickness minimized. When setting beds thicker than 6.4 mm (¼ in.) are required, consideration should be given to installing an initial thick bed of mortar, allowing it to fully cure, then installing a second thin-set bed for adhering the tile. This limits the mortar drying shrinkage stresses applied to the tile, and also the amount of tile moisture expansion prior to bond formation and the subsequent stresses from tile shrinkage.
Stone tile
The stone-tile thickness should be increased and a less-moisture-sensitive stone selected. If a particular stone type with high absorption or low tensile strength is desired for its aesthetic, a greater stone-tile thickness may be appropriate. Increased thickness provides greater stone tensile strength, as well as greater dimensional stability and less potential for curling where a hairline fracture occurs—this makes any distress that occurs far less noticeable.
Additionally, increased thickness may slow the rate of free moisture evaporation from the mortar, resulting in reduced mortar cracking from drying shrinkage. Application of an impervious tensile reinforcement mesh to the back surface of the stone may be an alternative means to increase the tensile strength of the tile while reducing the rate of mortar shrinkage.
