Sep 24, 2019
Get Consistent Results in the Plant Lab
BY AsphaltPro Staff
For the toolbox tips this month, we’re concentrating on quality control/quality assurance, which means attention to detail. For the lab, let’s look at the way you make gyratory specimens. Quality results stem from a quality start, right? Here are a few ideas to improve the uniformity of the asphalt specimen.
Consistency in the specimens will build the confidence in the test results down the line. To improve all of this, start with the consumables in the process. It’s surprising how such a simple thing can affect the end result, but studies have shown that the non-glossy paper disks you use in preparing the specimens matter. Paper disks with different surface textures and thicknesses will generate different specimen densities. If your lab produces its own paper disks from whatever paper stock is available, you’re not saving money; you’re adding variability and inconsistencies.
You might not have thought of your mold assemblies as consumables, but the components may need replacing after time and wear. Monitor the wear of the mold and end plate so all mold assemblies remain consistent in size. Minimum clearance between the end plate and the mold surface should be 0.15 mm. Maximum clearance is 0.70 mm. If one assembly is at the minimum allowable size and another is at the maximum allowable size, you have variation creeping in. One source showed you can expect to see a bulk specific gravity difference of 0.020 per millimeter.
Also, 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.
Be sure to 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.
And double-check the gyratory compactor’s operating pressure. AASHTO T312 Section 4.1 wants it at 600 +18 kPa. If compaction pressure drops like a 1970s Robert Palmer song, 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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