Performance-Based Mix Design – The Next Level
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Since the inception of SUPERPAVE™ mix design, a multi-level design process was envisioned. This process would be as simple as AC content, gradation, and volumetrics to a performance mix design. The issue then and the issue now is – how do you measure the performance of a mix and know that it will work (or not work)? Millions of dollars have been spent trying to answer this question. Much of the challenge has been with the equipment, but challenges such as specimen preparation and testing procedures were just as great. So what has changed? Technology!
Recently, VDOT announced the formation of a task force to address performance-based mix design. Unlike the current approach, asphalt mixes will be designed and tested in the lab to assess their properties against certain thresholds. Eventually, these thresholds will be compared to the condition of the mixes in the field to ascertain performance, i.e., do they crack, rut or have durability issues?
Virginia is not alone in this quest for performance-based mix design. Recently, NCAT installed sections on the test track that will be assessing performance. Additionally, NCAT is performing a research project to evaluate the various lab performance tests available and in use. For VDOT, three tests are being considered for day-to-day design, production and mix monitoring – IDEAL-CT, APA, and Cantabro. IDEAL-CT is a lab test using a gyratory pill; this test is one of several approaches to assess the cracking potential of a mix. APA is a lab test using gyratory pills as well; this test is used to determine the rutting susceptibility of a mix. Finally, the Cantabro test involves placing pills in an LA Abrasion machine, tumbling the pills for a set number of revolutions and measuring the mass loss. A quick and easy test to perform, it is used on mixes such as porous friction courses to set the minimum AC content.
As 2018 draws to a close, contractor labs will be running these tests on their current mixes to establish baseline data. In the future, new mixes using more recycling, different binders, mix additives and so forth will be compared to these initial baselines and the specifications. Over time, lab results and field performance will be linked together to determine the effectiveness of these tests. As experience and knowledge evolve over the next few years to a decade, improvements will be made. But for now, the Virginia paving industry is taking the next step in the mix design evolution.
Recently, VDOT announced the formation of a task force to address performance-based mix design. Unlike the current approach, asphalt mixes will be designed and tested in the lab to assess their properties against certain thresholds. Eventually, these thresholds will be compared to the condition of the mixes in the field to ascertain performance, i.e., do they crack, rut or have durability issues?
Virginia is not alone in this quest for performance-based mix design. Recently, NCAT installed sections on the test track that will be assessing performance. Additionally, NCAT is performing a research project to evaluate the various lab performance tests available and in use. For VDOT, three tests are being considered for day-to-day design, production and mix monitoring – IDEAL-CT, APA, and Cantabro. IDEAL-CT is a lab test using a gyratory pill; this test is one of several approaches to assess the cracking potential of a mix. APA is a lab test using gyratory pills as well; this test is used to determine the rutting susceptibility of a mix. Finally, the Cantabro test involves placing pills in an LA Abrasion machine, tumbling the pills for a set number of revolutions and measuring the mass loss. A quick and easy test to perform, it is used on mixes such as porous friction courses to set the minimum AC content.
As 2018 draws to a close, contractor labs will be running these tests on their current mixes to establish baseline data. In the future, new mixes using more recycling, different binders, mix additives and so forth will be compared to these initial baselines and the specifications. Over time, lab results and field performance will be linked together to determine the effectiveness of these tests. As experience and knowledge evolve over the next few years to a decade, improvements will be made. But for now, the Virginia paving industry is taking the next step in the mix design evolution.
