The importance of constructing pavements to maximize their life is in no way a new idea.
In fact, Henry Kassan, Western Manager of The Barber Asphalt Company, stated to an 1898 convention of their superintendents, “The durability of asphalt pavements depends wholly on the suitability of the asphalt for the purpose. It must be of such a nature as to permanently and thoroughly cement together the sand and limestone powder forming the body of the pavement. It must be elastic, independent of the residuum oil required on making the paving cement and in no degree brittle.”
More specifically, the need for flexible pavements to be built to an acceptable degree of density has also long been understood to be a key to long life and a high level of performance. How density has been measured and to what it is referenced has changed throughout the years.
Historically, core densities were compared to a laboratory value established via Marshall or Hveem methodologies. Today, nearly all specifications are written so that the measurement is obtained from a density gauge or core and compared to the theoretical maximum density (TMD).
What has been considered acceptable density has also changed throughout the years. There was a time when 90 percent of theoretical maximum density was commonly considered acceptable. As recent as 2007, despite significant advances in equipment, mix designs and the use of additives, over one third of all state highway agencies still reported a density requirement of less than 92 percent of TMD.
Over time, the agencies that recognized the value of specifying higher densities gradually began to increase their density specifications. In most states with increased requirements, specifications were introduced that added incentive/disincentive pay factors to reward exceptional quality or punish for poor quality workmanship. This was especially true when measuring densities along longitudinal joints. Contractors in these states soon recognized the economic advantages of achieving higher densities to not only avoid penalties but to earn the bonuses where they were offered. As a result, contractors began to educate their personnel on best practices, and they began to invest in newer compaction technology to successfully build asphalt pavements to higher densities.
Federal efforts to improve density
A concerted effort has been ongoing at the Federal Highway Administration (FHWA) Asphalt Pavement Technology Program to encourage state highway agencies to increase their density requirements. Led by Tim Aschenbrener, FHWA has taken a multi-pronged approach to this effort including a review of past research on this topic, an evaluation of the current state specifications, the development of a workshop on the subject, and the solicitation of volunteer state highway agencies to demonstrate their ability to achieve higher densities (or decreasing air voids) on their construction projects.
FHWA enlisted the assistance of the National Center for Asphalt Technology (NCAT) to report on past research that investigated the effect of increased in-place density when constructing asphalt pavements. Dr. Nam Tran took the lead for NCAT and they published their findings in “NCAT Report Number 16-02, Enhanced Compaction to Improve Durability and Extend Pavement Service Life: A Literature Review.”
A review of 39 previous works provided significant reinforcement of the quantifiable benefit of increasing density and improved performance. Some of the key takeaways from the NCAT work shows that for both laboratory and field experiments, increasing densities resulted in better resistance to several typical distresses. Rutting and fatigue resistance, two major distresses, were significantly improved.
The NCAT report conservatively estimated a 10 percent improvement in durability for each percentage that the air voids were reduced. Tran and his associates further reported that when they performed a life-cycle analysis using this 10:1 ratio for improved fatigue life and air void reduction, that $88,000 savings in net present value for every 1 million dollars spent on asphalt pavements. Therefore, huge savings annually could be realized nationally if just a single additional percent of density is achieved on asphalt paving projects.
Concurrent to NCAT’s literature review, the Asphalt Institute (AI) worked with the FHWA to produce a six-hour workshop on the topic entitled, “Enhanced Durability through Increased In-Place Pavement Density.”
Senior Regional Engineer Dave Johnson took the lead for AI. He worked closely with FHWA and the rest of AI’s engineering staff to produce the workshop. Twenty-eight states from Florida to Alaska hosted the popular workshop.
The workshop took a holistic approach to the problem of achieving higher density, by not just focusing on compaction, but taking the participants from materials selection, through mix design to construction best practices.
Field density demonstrations
The third item that FHWA has employed, this time in partnership with state agencies, are demonstration projects. To date, three separate phases have occurred of these demonstration projects. Field documentation of the projects was performed by NCAT. Participating states reported the details of their project and what they observed.
Ten agencies volunteered to participate in the initial phase of demonstration projects. FHWA encouraged each of the agencies to approach their projects as they saw fit. This allowed the agencies to actively review what they were currently doing, and then to try to improve their densities by addressing the factors they deemed most likely to produce improved density. Thus, agencies did not just simply specify higher densities.
Each of these initial ten demonstration agencies were required by FHWA to host a workshop before field work commenced. A wide array of approaches were employed in an attempt to increase field densities. Strategies included changes to the nominal maximum aggregate size (NMAS) of the mixture; changes to the NMAS and the compacted thickness; changes to the gradation in terms of it being fine or coarse-graded; changes to the gyrations used for mix design; the target mix design air voids; alterations to the roller pattern and the use of one or more of the newer technologies presented in the workshop.
Each agency had a control section which used “typical” or “standard” methods, and from one to seven test sections. A total of 38 sections were constructed. Details on Phase I approaches and results can be found in NCAT Report Number 17-05.
Agencies and contractors were generally able to construct higher density/lower air void pavements. Eight of the ten states saw their test sections surpass the densities of their control sections by at least one percent. The success of these Phase I demonstration projects led FHWA to move to a Phase II and now Phase III.
Phase II saw nine new states participating with demonstration projects. Once again FHWA allowed the states significant freedom to how they chose to attempt to improve field density. As in Phase 1, a control section using the standard methodologies for that state was constructed and compared to a test section or sections.
As many as five unique test sections were constructed which incorporated a wide variety of approaches to the achievement of added density. A total of 28 sections were built by the eight reporting states. Many of the same approaches that were reported in Phase I were again tried by Phase II states. In the case of Phase II, six of the eight reporting states saw at least a 0.5 percent improvement in density for their test sections versus the control. Another NCAT report for Phase II is anticipated in the coming months.
Phase III demonstration projects were constructed in 2018 or will be in 2019 in ten states. A variety of strategies are being utilized by these states.
Ongoing tracking of Phase I states has seen modifications to many of their specifications. Some of the more notable changes include improvements to their density measurement methods by correlating nuclear gauge readings to cores; targeting TMD instead of laboratory density and raising their minimum density requirements. Increases to their upper specification limit as well as a shift to a Percent- Within-Limits (PWL) system from lot averages were also seen. Multiple state highway agencies have shifted to or increased incentives for densities achieved on their projects.
Half of the participating states have made changes to their mix design requirements to get higher effective asphalt contents as an aid in achieving greater in-place densities. While the additional asphalt will assist in improving durability, long term monitoring of the individual test sections will be needed to verify that rutting resistance has not been compromised.
In Phase II, fully two-thirds of the participating state highway agencies reported notable increases in field densities. Similar approaches to the Phase I projects were again employed yielding favorable results. Phase III is ongoing but is also expected to produce valuable information regarding density improvements resulting from various approaches to the goal of higher construction densities.
Tracking of sections from all three phases is encouraged. From both the short- and long-term perspective, it will only be with tracking that any differences in performance will be documented.
To get updates on all three phases of the ongoing demo projects, plus the latest information on the Federal Highway Administration’s efforts to improve durability through increased density, please visit: www.fhwa.dot.gov/pavement/asphalt/density/index.cfm.
Asphalt Institute Senior Regional Engineers Wayne Jones and Dave Johnson collaborated on this article.