The current version of the ACI “Specification for Shotcrete” is ACI 506.2-13 and purposely eliminated the core grading due to many problems that arose in the field in trying to equitably apply the procedure. ACI 506 has a new document ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation Technote,” that is intended to assist specifiers with evaluating shotcrete core quality for an intended use. So yes, the 506.2-95 version of the specification and the grading system it included is no longer supported as an industry standard.
Regarding grading, a shotcrete score of 5 under the deprecated grading system just because the core broke during coring is unreasonable. The coring operation exerts a massive force on the core, as it basically rips it out of the concrete. We have seen many instances where the core snapped where a large reinforcing bar horizontally crossed the core and effectively created a slip plane that reduced the shear resistance of the core section to the torque created by the coring. In those cases, examining the core hole for any evidence of shotcrete problems would be appropriate and then discounting the core itself.
In summary, my position and the ACI 506 Committee’s position is that core grading should not be used for evaluating shotcrete quality. The ACI 506.6T-17 Technote document should be the current reference for making a reasonable evaluation of shotcrete quality for the intended use.