Try scratching the plaster to see if it is hard or soft.
Weak plaster scratches easily and good plaster is hard to scratch.
In weak pebble plaster, the pebbles come loose easily when brushing or scratching.
Are you getting a lot of loose pebbles?
This is difficult to tell the difference if you do not have a reference for good and bad plaster.
If the pH, TA and CH are all increasing continuously without adding fill water or chemicals, then the plaster is dissolving.
If the TA and CH increase the exact same amount, the plaster is probably dissolving.
Did you register the plaster with NPT?
Contact NPT and see what they have to say.
They can get a sample of the plaster by doing a core drill down to gunite and then examining the plaster for quality assurance.
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