StoneDimensions

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.  Stair treads require a higher than typical abrasion resistance due to their in-service exposure.  We’ve all seen the near century old marble staircase in a courthouse or other historic building that shows the evidence of wear over time. 

At the ends of the treads, they will have nice, sharp 90° corners, and toward the center of the treads, where people have been stepping on them since the opening of the building, they’re so worn that they nearly have a bullnosed edge profile.  That is why the DSDM recommends that stone used for stair applications have an abrasion resistance of at least 12.0, and frankly, more is better.  What the test laboratory is citing is ASTM C568, which is the Standard Specification for Dimension Limestone.  This standard lists a minimum abrasion resistance of 10.0, but that is merely saying that it is expected that a limestone sold as dimension stone would have an abrasion resistance of this value or above.  It is not saying that a limestone with an abrasion resistance of 10.0 is necessarily going to perform well in all conceivable types of applications where stone products could be used.  So yes, there are limestones on the market that comply with the ASTM C568 standard, but would not be considered appropriate candidates for certain applications, like for instance stair treads. 

Conversely, there could be applications where a limestone that doesn’t comply with ASTM 568 might still be considered appropriate, because the specific application doesn’t demand that level of performance.

Q: We have some cubic granite pieces that are pinned into a concrete foundation, and the spec says to epoxy the pins into the holes in the granite.  But the holes drilled by the fabricator are about 1½” diameter, and the pins are only 3/4”.  That’s going to take a lot of epoxy – is there a cheaper product that can be used?

A: I think I would opt to just use a larger diameter pin.  Over-sizing a hole diameter to 2x of the pin diameter is not a good idea.  Some engineers limit the hole diameter to no more than 1/8” greater than the pin diameter.  Epoxy, or any adhesive, for that matter, does not perform well when used in thick layers.  The smaller this annulus region is, the higher the capacity of the pin. I would also suggest using a threaded, or otherwise deformed pin to gain the advantage of a mechanical interlock between the epoxy and the pin.  If tolerances and alignment issues mandate that the hole must be significantly oversized, then either switch to an epoxy grout, or add sand to your epoxy.  The sand will not only save on the amount of epoxy product you need to purchase, but it will produce a better end result.  Epoxy is exothermic during cure, meaning it produces heat, and when large amounts are used, the heat can be great enough to negatively affect its bonding capability after cure.  The sand will reduce the amount of epoxy, thereby reducing the amount of heat generated, and also provides some thermal mass which can absorb some of the heat.  If the pin is required to have capacity in tension, I would also recommend testing its pullout capacity just to be sure of its adequacy.

Q: The MIA Dimension Stone Design Manual specifies lippage to be less than 1/32” on flooring.  This seems unreasonable with large pieces, especially when there’s some warpage in the stones.  Is there any chance that this will be relaxed in the future?

A: I wouldn’t expect that that will ever happen.  The lippage spec for cladding in the manual is increased to 1/16”, for exactly the reasons that you state.  In the case of cladding, it is only a visual issue.  In the case of flooring, it is an occupant safety issue.  Lippage of 1/32” can be approximated with a typical credit card, as the thickness of a credit card measures to nearly exactly this dimension.  Particularly when a stone floor has narrow joints, and/or has minimal (or no) chamfer at the stone edges, a lippage of 1/32” is significant.  If you’ve ever encountered an actual installation such as this, you will find that it already is an uncomfortable flooring surface to walk across, and anything more would have to be considered a potential safety issue.