Well perhaps it is still a "brittle" material, but the tensile strength of granite and similar rock is in the range of 4 - 8 MPa (as compared to concrete that would be 1 - 3 MPa). For the application shown it still appears to be gratuitous overkill given the thickness of the slab used, the support arrangement (e.g. pinned on 3 edges, fixed on one), and the fact that there appears to be no additional or superimposed load acting on the surface.
Even without the PT, the FOS would be ~3 as a conservative estimate (from my basic calcs) against a service load crack.
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Member of the SSPIA Senior Committee. Have a question? Go pester Tony.
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