QUESTION

I am an inspector in the Sacramento area, and I need to know if any lab testing is relevant for determining the actual cause of indent fracturing.

I have looked at a shower with 12" x 24" white marble installed over site mixed fat mud. I put a 4 ft level horizontally on the marble wall. The two walls raised in the center.

I assume the mud, mortar or both have shrunk and caused the fractures. Of course, I can't report based on theory.

ANSWER

ANSWER - Indent fracturing in natural stone is due to some sort of shrinkage.  It could be excessively thick thin-set and/or too wet and rich mortar.  Also it could be due to the kind of underlying membrane or wire reinforcement that is part of the assembly.

The indent fracture is typically a slight depression and not a separation; unless it has been subjected to excessive stress.  If you put a metal ruler over the indent and put a flashlight behind it, then it should show light coming through over the indent demonstrating it is a low spot.

Not sure about your comment that the two walls are raised in the center.  That might be a separate issue or a related issue.

The only way to fully understand the situation is to intrusively remove tiles under different conditions and look for evidence to substantiate the cause of failure, which requires trained eyes to discern.

The only laboratory testing that you can do to verify indent fracturing is to substantiate the conditions that cause indent fracturing.  We have performed extensive laboratory testing to recreate the condition successfully, but that is an expensive process that may or may not be practical for the particular situation.