How We Gained Extra Air Voids
BY Jayanth Kumar Rayapeddi Kumar, P.E.
Using activity tracking devices is a popular way to track the number of calories burned. I’ve always worn mine on my left hand. Recently, because of a cut on my left wrist, I wore it on my right—dominant—hand. When I checked the activity tracker at the end of the first day, I was thrilled to see I had burned about 500 calories more than usual. The same trend continued for the next seven days, and I was so pleased with the calorie upturn, I happily indulged myself with one double fudge brownie after dinner every night.
When my left wrist healed, I swapped the activity tracker back and noticed the number of calories I burned was closer to pre-injury numbers. Although this surprised me, I conveniently blamed it on my inactivity. To my bewilderment, the next few days followed the same trend. Sadly, this put a temporary stop to my double fudge brownie gratification.
A Google search and several days of research later took me to a user review that explained how users need to set up their activity trackers based on which wrist they prefer to wear it on. The calorie-burn calculation is different based on the user’s preference to wear the tracker on the dominant or the non-dominant hand.
My mistake was changing the tracker from my non-dominant to dominant hand without changing the settings. The tracker kept indicating higher calories burned.
We relate this to asphalt by looking at the settings.
The Voids Changed
Earlier in 2023, the lab team at Jas. W. Glover Ltd formulated an asphalt mix with design air voids of 3.5%. The mix was approved for production for a large project. Before the production day, we made a test batch in the lab to verify the mix volumetrics. To our shock, the average of two compacted samples resulted in air voids between 8 and 9%. We tried molding two more samples, but the result was no different.
This set off alarm bells.
We started to investigate the root cause. We reviewed the batching sheet, talked about the sample preparation, confirmed mixing and compaction temperatures, ensured correct conditioning time was followed, verified the number of gyrations, checked the gyration angle, and so on. We hoped something we looked at would be amiss. But everything was okay. To eliminate the possible culprit, the theoretical maximum specific gravity (Gmm) test was repeated, and two new samples were molded. Both Gmm and air voids weren’t much different from the first time. The symptoms were clear, but we couldn’t diagnose the problem.
Standing in front of the compactor totally confused, I remembered the compactor was calibrated between the times the mix was formulated and verified. So, I started checking the compactor settings. It was a eureka moment when I saw the sample diameter on the compactor was set to 100 millimeters (mm).
We were molding 150-mm diameter samples.
The calibration technician had left the compactor settings at 100 mm after calibration. What should have taken less than a minute to adjust—the setting—took us about 14 hours. We ended up batching and testing additional samples, and more importantly, putting ourselves through unnecessary grief, before realizing the problem was with the setting.
Follow the Science
We know from physics: Stress = Force / Area. The gyratory compactor applies a constant consolidation pressure of 600 kPa (87.02 psi) to compact the mix. A 100-mm (3.94-inch) sample diameter has an area of 12.2 square inches and a 150-mm (5.91-inch) sample has an area of 27.4 square inches. The force applied to compact the sample changes based on the area of the sample to maintain the constant pressure of 600 kPa. With the compactor’s settings set to 100 mm, the compactor was applying a force of about 1,060 pounds instead of about 2,386 pounds (applicable to 150-mm diameter samples) to compact the samples. The lower pressure resulted in higher air voids.
After changing the setting to 150-mm diameter, to our great relief, the sample air voids read closer to the design air voids of 3.5%. The lesson learned was that there is always a reason for everything.
Thanks to our quality control measures, we didn’t allow subpar material to be produced or shipped. All the team “lost” was time. What we gained was experience through this lesson. Whether it’s the tracker device or gyratory compactor, the input parameters must be checked.
Jayanth Kumar Rayapeddi Kumar, PE, ENV SP, currently serves as the quality control engineer for Jas. W. Glover Ltd. in Hawaii, and on the ASTM D04 committee on Road and Paving Materials and the ASTM E36 committee on Accreditation and Certification. Reach him at email@example.com.