Membrane
If possible, one should avoid using elastic membranes below the mortar. If an elastic substrate is desired (for reasons such as soundproofing or for isolation of substrate cracks), the stone tile properties and mortar thickness must be carefully considered.
Mockups
As the potential for indent fractures can be compounded by a number of contributing factors, precise failure criteria for each factor cannot be established. For example, a highly moisture-sensitive thin stone tile may perform well when set in a thin mortar application. Consequently, when schedule permits for large tile installation projects or those including the factors noted in this article, the authors recommend construction of mockups and monitoring for at least four weeks prior to installation to evaluate the potential for indent fracturing. However, monitoring should take place for as long as the construction schedule allows, as the cracks may take longer to develop in some applications. Indent fractures are most easily identified on mockup assemblies in
a dimly lit room with oblique lighting conditions, and also using a straightedge and flashlight.
Conclusion
A few essential factors are necessary for indent fractures to occur, but this distress may be aggravated when the other contributing factors act in combination. Modification of factors related to material selection may be impractical where certain aesthetic or performance characteristics are desired, assembly depth is predetermined, or budget/schedule do not allow for extensive comparative testing between different materials. Addressing factors related to construction timing, such as proper schedule allowance for application of mortar in lifts and creation of mockups, is likely a more practical approach to avoiding the development of indent fractures.
Each of the factors that result in indent fractures can be avoided through proper preparation and specification, and at minimal additional cost to the owner relative to the significant expense of replacement. Armed with an understanding of how to mitigate the various factors that are described in this article, an informed specifier should be able to confidently design a stone tile floor without fear of indent fractures.
Daren S. Kneezel, RA, is a senior associate with Wiss, Janney, Elstner Associates (WJE). He has been involved in projects ranging from investigation of high-rise building façades to laboratory testing of stone and other construction materials. Kneezel has extensive experience performing laboratory testing of various materials with a concentration on stone and stone-faced honeycomb composite to evaluate structural performance, durability, and to determine compliance with project specifications and industry standards. He is a member of ASTM technical committee C18 (dimension stone). Kneezel can be reached at dkneezel@wje.com.
Michael J. Scheffler, PE, is a principal with WJE. Over more than 35 years, he has been involved in thousands of investigations of deterioration and distress in buildings and other structures. Scheffler also has extensive experience performing laboratory and in-situ testing of building assemblies related to stone material performance, durability, and structural performance, and has performed long-term monitoring and instrumentation of stone distress and construction. He is a member of ASTM technical committees C18, C24 (building seals and sealants), and E06 (performance of buildings). Scheffler can be e-mailed via mscheffler@wje.com.
