Elongation of mockup assemblies without external loading
Elongation of the stone in mockup assemblies exhibiting indent fractures typically increased rapidly to an initial maximum positive elongation, followed by a more gradual descent to a negative elongation (shrinkage) and another ascent to positive elongation. The initial positive elongation of the stone is likely caused by moisture absorption of water into the stone from the mortar during curing.
The shrinkage suggests the stone is in compression resulting from mortar shrinkage after bond development. The subsequent positive elongation indicates tile cracking at mortar shrinkage crack locations.
The elongation for the assemblies that did not exhibit indent fractures typically had a similar initial increase, followed by gradual descent. However, the negative elongation tended to stabilize, indicating the mortar shrinkage stresses did not exceed the strength of the stone tile. Several of the tile types exhibited indent fractures in-situ, but not in mockups when observations were halted after four weeks. However, the stone tile in these mockups was continuing to shrink or had begun to elongate again—this indicates indent fractures may occur even beyond four weeks.
Essential factors leading to indent fracture
Based on testing and observations of indent fracture development in mockup assemblies, several factors result in indent fracturing of stone-tile applications. Appropriate modification of the following essential factors can significantly mitigate or eliminate the potential to develop indent fractures in thin-set stone-tile applications.
Mortar thickness
The stress applied to the stone is proportional to the thickness of the mortar. Additionally, a thicker mortar setting bed yields a greater amount of free moisture. This causes elongation of the tile prior to restraint from the mortar and greater subsequent stresses due to stone shrinkage as the moisture evaporates. As a result of both these factors, greater mortar thickness will contribute to a greater likelihood of indent fracturing. This was confirmed through the greater frequency of crack development in mockup assemblies with greater mortar thicknesses.
Stone tensile strength
The use of a stone type with a greater tensile strength or greater thickness (if an inherently weak stone is to be used) will provide greater resistance to cracking from the applied mortar shrinkage stresses.
Contributing factors that can lead to indent fracture
Appropriate modification of the following contributing factors can minimize the potential for indent fracture formation. (However, the aforementioned essential factors must also be sufficiently addressed.)
Elastic or low-permeability membrane below mortar
A primary contributing factor is the use of an elastic substrate such as a sound-attenuation mat, or bond-inhibiting surface, which will not restrain shrinkage of the mortar bed, as would be provided by a more rigid and bond-developing substrate such as concrete. As a result, tile applications with an elastic or bond-inhibiting substrate have a greater potential for mortar shrinkage cracks and associated indent fractures. Cracks originate in the setting mortar at the contact with the flexible sound-attenuation mat before extending upward through the mortar and into the more-rigid stone.
Additionally, low-permeability substrates will result in a greater amount of free moisture availability for absorption into stone tile. This means greater moisture expansion of the tile prior to restraint from the mortar, increasing potential for indent fractures.
Stone moisture sensitivity
All other factors being equal, stone tile with greater total absorption (and/or rate thereof) is more likely to exhibit indent fracturing than tile with less absorption or a reduced rate. This is probably due to more-rapid mortar curing adjacent to a more-absorptive tile, resulting in a greater potential for shrinkage cracks to develop in the mortar. However, low stone-tile absorption alone does not preclude development of indent fractures, as stone types with very low absorption can also exhibit indent fractures in-situ.
Stone types with greater moisture expansion are also likely more vulnerable to indent fractures due to the greater stresses after bond is developed and the tile recovers to its original pre-installation dimensions. Where two or more stone types are being considered for a project, the less-moisture-sensitive stone is less likely to exhibit indent fractures.
Mortar shrinkage magnitude
Mortar exhibiting greater total shrinkage likely contributes to a greater frequency of indent fractures, as increased shrinkage after bond development leads to greater applied stresses to the stone tile.
