Get Consistent Results in the Plant Lab—Part 2 for Gyratory Specimens

For the toolbox tips this month, we’re concentrating on quality control/quality assurance. When in the lab, look at the way you make gyratory specimens.

First.

Be careful when “handling” the mix so you prevent segregation from the get-go. You might want to re-think the use of a funnel for filling the mold. The free-fall through the funnel can encourage material segregation. Instead, you can place the hot mix in a half-round of PVC pipe, lay the mold on its side, and then slide the loaded PVC pipe into the mold. When you stand the mold upright, the mix falls as a mass into the mold.

Or you can place the hot mix on a flexible mat, roll the mat into a tube, lay the mold on its side, and then slide the rolled mat/tube into the tube. Then you discharge the mix into the mold by standing the mold upright.

Second.

Double-check the gyratory compactor’s internal angle. AASHTO T312 wants it at 1.16 degrees (+0.02 degrees). If the angle is too low, you’ll get less compaction of the specimen.

Third.

Double-check the gyratory compactor’s operating pressure. AASHTO T312 Section 4.1 wants it at 600 +18 kPa. If compaction pressure drops, you’ll lower the bulk specific gravity.

You want to get the compaction of the specimen consistently correct before testing can even begin.

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