Introduction and Staff Changes
Even as much of the world retreated to a more “virtual” existence last spring, researchers from the Virginia Transportation Research Council’s (VTRC) Pavements team strapped on their face coverings and climbed into (separate) cars and trucks and joined contractor quality control and VDOT quality assurance staff to make sure that a pandemic did not interrupt valuable asphalt research. Support from the highest levels of VDOT also enabled the research program to make a critical hire when most offices were frozen or shrinking under the global economic contraction.
In September of 2020, VTRC welcomed Danny Martinez as our very first Technical Resource Manager. He joined the VTRC staff after completing his Master of Science Degree in Civil Engineering from the University of Auburn. While at Auburn, Danny worked for the National Center for Asphalt Technology (NCAT). While there, he continued to build on private-sector experience to support NCAT’s applied research on pavement preservation and balanced mix design (BMD).
As the pavement program’s Technical Resource Manager, Danny will have the considerable challenge of coordinating technical support for six very demanding research scientists (and their associate director). The resources under his charge include two top-notch salaried technicians, a varying number of wage staff, numerous laboratory spaces, and state-of-the-art equipment that provide for both basic and advanced testing of conventional and not-so-conventional asphalt binders and mixtures. Danny is actively recruiting to fill several vacancies among our wage staff. He will also be busy working with scientists and technicians to replace several pieces of aging laboratory equipment and ensure everyone is trained and ready to support another busy construction season.
Balanced Mix Design (BMD) Research
Support for BMD Pilot Projects
As spring arrives this year, things continue to look much like last—face coverings and social distancing—but with hopes for a late summer/early fall that looks and feels more like what we used to consider normal. Regardless, our research program will press on with support for many existing initiatives and several new ones. Most visible among the continued activities include support for the BMD initiative, which involves several important sub-projects.
Stacey Diefenderfer, with help from Ilker Boz, Jhony Habbouche, and laboratory and field staff, will continue to provide support for both the scheduled BMD pilot projects, as well as any less-formal work-order trials that may get “negotiated” between forward-looking producers and the districts.
Indirect Tension Test for Rutting Potential
Ilker Boz, with help from the University of Virginia, is leading research that explores the indirect tensile test (IDT) to screen asphalt mixtures for rutting potential. If successful, contractors may have an economical way to tinker with design variants when they don’t have ready access to the Asphalt Pavement Analyzer (required for VDOT’s standard performance test for rutting resistance). Early results are promising, with some findings suggesting applications beyond just screening designs for rutting susceptibility.
Round Robin for IDT Cracking Test
Jhony Habbouche is working with Mike Dudley from the Virginia Asphalt Association to coordinate an early-season round-robin experiment centered on the primary cracking test for BMD. Jhony and Mike, with help from Stacey and Ilker, are working with a third-party laboratory to prepare a mixture that will be distributed to more than forty laboratories were specimens will be prepared and tested. This experiment, the second of two phases, will explore the effect of equipment type and loading rate, the impact of specimen fabrication/preparation, along with other factors such as test error, single operator variability, and multi-laboratory variability on the testing that characterizes cracking susceptibility of asphalt mixtures. If you operate a laboratory that supports asphalt production in Virginia, expect to receive material from Jhony and Mike by late spring.
Review and Acceptance of Recycling Agents
Jhony is continuing to lead research with North Carolina State University to better understand the myriad of chemistry packages that are being marketed nowadays to alleviate the potential brittleness of asphalt mixtures, particularly those that might incorporate higher fractions of reclaimed asphalt pavement (RAP). Jhony, with help from VTRC field support, contractors, and Districts Materials staff, spent part of last season collecting raw material from typical Virginia sources (virgin aggregates, RAP, and conventional binder) for transport to Raleigh. Those materials are being blended with a spectrum of recycling agents (i.e., rejuvenators and softeners) to create VDOT-
representative mixtures that will be studied as a complete mixture, as well as indicative components (e.g., binder and mortar). Most of this year’s work will happen in laboratories in Raleigh and Charlottesville. With any luck, strong guidance on what types of recycling agents to accept/reject will emerge from this work as Virginia enters the initial implementation stages of BMD.
Accelerated Testing of BMD Mixes
Some of the most interesting BMD-related research is happening at the Virginia Smart Road in Blacksburg. VTRC is working with the Virginia Tech Transportation Institute (VTTI) to test a series of BMD mixtures under the Heavy Vehicle Simulator (HVS). Late last summer, Salem District worked with Boxley to place five experimental mixtures and a control section on the testbeds for the HVS. Loading started shortly thereafter, and we’ve already learned how easy it is to “over-juice” an asphalt mixture when trying to meet cracking performance, even one with 45 percent RAP. On the bright side, our performance tests (using the APA criteria) are showing good agreement with what actually happened under accelerated loading (Figure 4).
Impact of Production Variability on BMD
We’re also continuing to work with our old friend Ben Bowers (now at Auburn University) to understand how normal variation of mix properties during production ultimately impacts laboratory performance indicators. In the past two seasons, we’ve sampled, packaged, and shipped components from six Virginia mixtures to the laboratories at NCAT for Ben and his army of graduate students. Unfortunately, university campuses’ activity came to a screeching halt last spring, and testing to support that work is only now beginning to catch back up again. We expect considerable progress this summer, and we’ll keep you posted.
Reflective Crack Mitigation
High Polymer (HP) Mixtures
Beyond the work that directly supports the department’s move to BMD, we continue to develop more expertise with asphalt modifiers. A persistent application for high-performing (and modified) binders is mitigation of reflective cracking, especially over jointed concrete pavements. Among the tools that we’re studying are High Polymer (HP) and rubber modified binders. Numerous field trials for HP mixtures have been conducted in Virginia in recent years. Documentation of those trials, along with early-life in-service performance reviews and a wealth of related information from beyond Virginia, will be included in a report that we will release early this spring. Jhony Habbouche led this work with assistance from Ilker and Brian Diefenderfer.
Hari Nair and Shabbir Hossain are following two field trials with rubber-modified mixtures, both of which were placed in Richmond District in 2019. The first is a gap-graded rubber-modified mixture placed on southbound I-85 in Dinwiddie County over jointed concrete. This two-layer “wet” process mixture was placed contiguously with a two-layer SMA overlay, which will serve as the control for in-service performance comparison. The second field trial used a “dry” process ground-tire rubber system in a dense-graded mixture. There are three sections in this trial totaling approximately four miles each in the east and west-bound directions. The two-inch rubber-modified mixture was placed over a THMACO interlayer, placed over jointed concrete patched with an asphalt base mixture.
Production and placement went well in both instances. Laboratory testing of the two modified mixtures suggests very good prospects for performance in one of the more challenging settings for an asphalt surface mixture. Hari and Shabbir will be completing some Texas Overlay Testing in the lab this summer and following up on the in-service condition for each trial (and control when available). Expect final reports for these projects by late fall 2021.
As spring arrives in Virginia, we look forward to another busy construction (and field research) season. They may still be behind a mask, but VTRC researchers will continue to be out among the folks who build, maintain, operate, and rebuild Virginia’s most visible public asset. Please ask them questions, and be generous with your ideas and opinions. And please, please be patient with them. They may be world-class nerds, but they mean well!