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I have another thread on the forum dealing with my issues testing a bentonite mix. The questions are sort of linked. The Federal lab that is also doing plasticity indexes on the material are getting huge numbers. My numbers average 8. Another lab is getting around 10. The Fed lab averages 14. We compared techniques and it came out that the Feds are grinding by hand using a rubber glove. Everyone else is using a rubber tipped mortise and pestle. AASHTO R58 does not mention gloves or by hand. Has anyone else heard of this hand method?


Best to prepare proficiency samples.

The fed lab prepares a sample using their hand method. Your lab prepares a sample using the rubber tipped mortise and pestle. Each lab prepares enough representative samples to be supplied to all laboratories that have been previously involved. All labs test as per their regular testing regimen.

If all labs submit similar/identical results for each preparatory method then you can rule that the fed lab's preparation method is the cause of their outlier results, as I'd assume they would be using the method of determining the plastic limit as per everyone else, which in its self is a very variable test as it relies heavily on operator interpretation of when plastic limit is achieved.

Does the fed lab include AASHTO R58 on their test reports/certificates? If so, and AASHTO R58 does not mention within it the use of gloves and hands to produce the prepared sample, then the fed lab should not be including this method on their test reports.

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