StoneDimensions

Q: We’ve been asked to rout out the bottoms of a granite overhang at an outdoor kitchen and epoxy in a 3” x 3/8” strip of steel to reinforce the stone. We’re nervous about routing out so much of the stone in that we think it could make it more fragile – has this been done before?

A: A variety of embedded reinforcement measures have been attempted in natural stone, some of which have been successful and others which haven’t. The particular description that you provide makes me share in your concern. First, it is unlikely that the 3” x 3/8” deep flat stock will actually prevent fracture of the stone. It may prevent a catastrophe, in that it would keep the overhanging stone portion from falling to the ground if it did fracture. But it wouldn’t be rigid enough to make a sufficient support contribution prior to fracture of the stone. My second concern, similar to yours, is that routing out a large, rectangular cross-section of the stone does compromise the strength. In a brittle material, any abrupt geometrical discontinuity creates what is called a “stress riser”.

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Q: We’re having trouble interpreting the tolerances for stone panels. In the thickness tolerance, it says “± 1/8” – is this both directions so that the total tolerance is actually ¼”? And then in the face tolerance, it says ”non-cumulative” – what exactly does that mean?

A: Dimensional tolerance is indeed one of the most common areas of questions that we field. Interpretation of them is far from uniform, and trying to reconcile them between different associations and standards writing bodies makes it even more complicated. Then add the conversion factor required because the United States rarely uses SI (International System of Units) units of measure, and we’ve got the perfect recipe for confusion.

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Q: We’re installing a mesh-backed stone tile interior wall project. The architect is insisting that we use epoxy thinset, since the MIA Design Manual says that is required. We think this is overkill – is there any way around using the epoxy?

A: We have addressed this on a case-by-case basis on a number of occasions. The mesh is applied using an adhesive that is most commonly epoxy-based, but can also be of polyester or other adhesive families. As a general rule, portland cement based adhesives do not bond well to mesh-backed stone units. Within any industry, there is a constant race between standards and technology, in that when technology advances, the standards must be adjusted to address the performance advantages of the new, technologically improved products. This is exactly such a case, in that a few of the modern thinset adhesives actually do produce a serviceable bond to the resin adhesive coated mesh-backed surfaces. Particularly in the case of a wall, when there are no traffic loads on the tile, there may in fact be a portland-based thinset available that achieves adequate bond without having to use epoxy based thinsets. Unfortunately, the testing and evaluation needs to be done on an individual project level, since we have no control over what mesh is used or what adhesive was used to apply it. So while a combination may exist that is workable using a given thinset and a stone from a given supplier, the same thinset may not produce the same results on stone from another supplier.

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Q: We recently submitted shop drawings on a small cladding job. We didn’t specifically locate the anchors. We just showed them on the detail and noted “anchors located at 1/4 points, typical”. When the shops came back, the note was changed to 1/5th points. We have no problem doing this, but we’ve always used 1/4 points as a rule of thumb – is there some reason that fifths would be better?

A: Quarter-point anchor positions seem to be a rule of thumb for many in the industry, and they likely stem from the traditional, classic building designs of years ago. If one looks at the stone cladding in these classic buildings, one will note that it was common, if not standard, to use a staggered jointery pattern (similar to a running bond in brick masonry). In this case, quarter-point anchor positions make sense, because when an anchor is shared between two courses which have 50% offset, quarter-point positioning in one course is also quarter-point positioning in the next course. And because the stone panels used at that time were generally thick, cubic stones, bending stresses were a nonissue.

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Q: We’re proposing the supply of slate pavers for exterior walkways. These will receive pedestrian traffic only. We’re planning on suggesting ¾” thickness, but in the MIA’s Dimension Stone Design Manual it calls for 1¼” minimum thickness for any exterior pavement. Is this an absolute minimum, or simply a rule of thumb?

A: It’s more of a rule of thumb. There are probably many more examples where an exterior paver needs to be thicker, but there are some cases where thinner is acceptable, and the installation that you describe is perhaps one of them. Paver thickness is influenced by the traffic loads, the paver unit size, the stone’s bending strength, and the type and rigidity of the bedding.

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Q: At what point is a stone no longer “natural stone”. We’re working with more and more slabs that have backers and fillers and resined faces. It seems as if we no longer have the right to call them “natural”. Where do we draw the line here?

A: There probably isn’t going to be consensus on this, although I know where I draw the line in defining “natural stone”. At StonExpo, and possibly a few other venues during a typical year, I teach a very basic class covering rudimentary information on stone types, formation, mineralogy, properties, and definitions.

The class can serve as either a very informative primer for those relatively new to the industry, or it can serve as a sure cure for insomnia, depending on the student. At the start of that class, I offer a definition of natural stone as “Stones which have been harvested from their in-situ position in the earth, then cut and machined into final products without alteration to the internal fabric of the material.”

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Q: I’m an engineer that designs floor frame systems for ultra-high end custom homes. When I’m designing the floor frame, the finished surfaces usually haven’t been picked yet, but just by looking at the floor plan, I usually have a good idea which areas of the home are likely to receive tile or stone floor finishes. Where can I get a general idea of the deadloads that these floor finishes typically add? The second question I have is that I have an older version of your Dimension Stone Design Manual which limits deflection to L/720, but not to exceed 7/32”. In the current version, I no longer see the 7/32” limit; has this been dropped?

A: The best source to find assembly unit weights for a variety of residential finish floor systems is the TCNA Handbook for Ceramic, Glass, and Stone Tile Installation. In the 2012 version, Appendix B contains this information and can be found on pages 281 through 287. You will note that stone tile assemblies are typically a couple of pounds per square foot heavier than a comparable method using ceramic tile. Another adjustment that might be necessary is that some stones will be cut to slightly greater thicknesses due to fragility or unit size requirements. Certain stones, particularly black igneous rock types, have much greater densities than other stone varieties, so this will also influence the deadload. Of course, there are deflection limits and subfloor/underlayment requirements for stone that are more stringent than those for tile. So the safe thing to do would be design the floor system for stone finish, and then it will be adequate for either stone or ceramic, albeit a bit over-designed for the latter.

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Q: We are quoting the supply of a limestone for exterior pavement in the Chicago area. The supplier of the material says it has been proven freeze/thaw resistant for 48 cycles. Is there anything that documents that this is sufficient for Chicago?

A: There isn’t any table or other selection guide that would make this decision for you. Forty-eight cycles is a fairly common default for testing via the European EN-12371 method, although there are numerous different methodologies used to determine the number of cycles run in that test. From the data that I’ve seen on accelerated weathering of limestone, if significant degradation occurs, it is more likely to occur later, from about 50 to 125 cycles. It should be clarified that within the United States based ASTM system, we do not yet have a standardized procedure for accelerated weathering testing of dimension stone. The lack of a standardized procedure makes it really difficult to establish any formal guidelines as to how many cycles is significant, since any data we have comes from a variety of non-standardized procedures.

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Q: I’m looking at a spec that references ASTM C482, Test Method for Ceramic Tile Shear, although in this case it’s being used for stone, not ceramic. Why do they test it in shear? Wouldn’t we be more worried about tension? The tiles aren’t going to slide off the floor, are they?

A: ASTM C482, Standard Test Method for Bond Strength of Ceramic Tile to Portland Cement Paste, is specified frequently for applications that are not ceramic tile. Perhaps the primary reason for this is the fact that it is really the only published shear bond test that currently exists for tile applications. The test is actually somewhat limited, in that it is only a laboratory test, and can be performed only on specimens specifically prepared for the laboratory.

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Q: I recently had some limestone tested for abrasion resistance per ASTM C241. The standard that I was citing was the Dimension Stone Design Manual, which states that because my application is stairs, the stone should have a minimum abrasion resistance of 12.0. The test laboratory says that the minimum recommendation per ASTM is 10.0. Are these two standards in conflict with each other?

A: No, they are not in conflict with each other. They are actually talking about two different things. The reference of 12.0 minimum in the Horizontal Surfaces Section (Chapter 14) of the Dimension Stone Design Manual is a recommendation of what is required for the application, regardless of what type of stone is used.

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