Introduction: A Less “Visible” BMD Presence
Since the 2018 construction season, support for Virginia’s Balanced Mix Design (BMD) field trials has been the most visible element of the Virginia Transportation Research Council’s research program. In 2021 that support assumed a more secondary role, primarily limited to cataloging key characteristics from a series of BMD pilot projects awarded as part of the annual resurfacing schedules. This less intense BMD role allowed us to explore other material trials with partners from the Virginia Department of Transportation’s T’s districts and our more progressive contractors. The advanced material trials from 2021 were of interest for a couple of reasons. First, they offered alternatives for achieving particular performance qualities and were thus indirectly relevant to our aspirations for BMD. Second, each trial mixture incorporated something that would typically be considered a waste material and, therefore, potential contributors to a more sustainable paving program.
Hybrid Rubber Modified Asphalt
The first field trials of the summer were conducted in the NOVA District in partnership with Virginia Paving Company and Superior Paving Corp. They were designed to explore the prospects of hybrid rubber modified asphalt (HRMA). The HRMA mixes used a binder package that included 75% ground-tire-rubber (GTR), 20% styrene-butadiene-styrene (SBS) and 5% rejuvenating additive. Not only does HRMA represent another method for consuming a waste material, GTR, but this application is also a potential alternative for more conventional asphalt modifiers to improve resistance to cracking and deformation under heaving loads. Led by Jhony Habbouche and Hari Nair, our research team documented the constructability of the modified mixtures and will be following up with laboratory performance testing and an evaluation of initial field performance.
Recycled Plastic Modified Asphalt
Jhony and the Research Council’s field support team ventured south for the second series of trials. In this case, our researchers worked closely with Richmond District Materials and Colony Construction, Inc. to develop a field experiment that would help evaluate methods for modifying asphalt using recycled plastics. The entire experiment, which happened over several nights and a couple of weeks, was designed to benchmark recycled plastic modified asphalts (RPMA) against VDOT’s convention D and E mixes. Plastic 1 (Figure 2a) was a complex arrangement of polyethylene(PE)-based polymers, and Plastic 2 (Figure 2b) featured the use of polyethylene terephthalate (PET)-based amorphous polymers. The modified designs were intended to both extend and enhance the base binder. Original sources for these plastics include things like water and soda bottles, food packaging and containers.
Similar to the HRMA trial in NOVA, researchers were there to document produce-ability, constructability and initial field performance and collect samples for follow-up testing in the laboratory. The RPMA study will also include an attempt to detect and quantify the presence of microplastics, which might be generated from pavement wear and mobilized via storm-water runoff. Microplastics typically break down from larger plastic pieces and have been found to accumulate in places that can adversely impact aquatic and other terrestrial organisms (including humans). While RPMA represents a promising way to reuse (and dispose of) plastics, researchers will be working hard to ensure we don’t solve one problem while creating another. The Pavement’s Team will be assisted by the Research Council’s Environmental Research program as Lewis Lloyd assists Jhony to lead this multidisciplined study.
Construction Quality Assurance
VDOT and its industry partners have made important progress in recent years with the in-place density of new dense-graded asphalt pavements. Significant contributors to those improvements include both tweaks in mix design criteria and aggressive quality incentives. Despite these positive developments, what we know about compacted quality continues to be extrapolated from very limited, often destructive sampling. The summer of 2021 was an opportunity to begin testing technology that may give us a complete view of the quality of the compacted mat. In late spring, courtesy of another FHWA State Transportation Innovation Council (STIC) grant, VTRC took delivery of a rolling density meter (RDM). The RDM is a ground-penetrating radar (GPR) based system that can quickly and non-destructively conduct continuous sweeps of the surface mixes dielectric properties, which have demonstrated strong correlations to in-place voids.
Hari Nair and Brian Diefenderfer have conducted testing on several paving projects this year and are making plans to include more work next year. Objectives for this work are to assess a higher percentage of the surface, which we hope will enable incentives that better recognize and reward uniform compaction—with much less coring!
A Little Light Reading—Recent Asphalt and Pavement Related VTRC Publications
A number of asphalt-relevant projects have been completed and final reports released during the 2021 calendar year. A quick review of recently available titles is included here, along with a QR code to read the full text of the VTRC reports.