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”. What you are doing is routing out ±10 mm from a ±30 mm slab, which will leave ±20 mm remaining.  But due to the stress riser effect, the end result will be weaker than if the entire slab were a uniform 20 mm.  My third concern is the fact that this is exterior, and there is considerable difference in thermal expansion rates between the steel and the granite.  The granite will expand at roughly 4.4 x 10-6 in/in/°F, while the steel will be nearly double at 7.3 x 10-6 in/in/°F.  While all granites are different, most seem to fracture at somewhere about 600 microstrain.  While the steel/granite differential is considerably less than that, I would expect that repeated cycling would eventually crack the granite due to the greater elongation of the steel.

Q: We’ve got a general contractor who is crawling around on our newly installed Crema Marfil floor installation with a gloss meter.  He’s threatening to reject it because he says the meter shows that the gloss isn’t uniform.  Is there something that says this is to be expected?

A: There is no industry standard that specifies minimum or maximum gloss meter values for various finishes.  The main reason that no standard exists is that it would be a different standard for every stone, and in many cases would be different for specific minerals within the stone.  A stone like Crema Marfil, which has veining of relatively high contrast to the balance of the stone fabric, would undoubtedly have variability in gloss meter readings across the same piece.  This is addressed in the Great Big Book of Everything (a.k.a. The MIA’s Dimension Stone Design Manual), on page 13 8 where it says “Due to the heterogeneous composition of natural stones, variable mineral hardness exists within the stone, producing variable reflectivity of light energy. Most stones, and especially travertine marbles and honed-finish surfaces, will appear to reflect light unevenly.”

Q: What is the industry acceptable allowance for broken pieces in a shipment?

A: There is no industry established allowance.  The anticipation of pieces breaking would be the responsibility of the supplier, and the inclusion of extra pieces to replace the anticipated broken pieces would be the normal way to cover this situation.