Overhauling hot mix asphalt
Joyce Taylor, P.E., MaineDOT Chief Engineer
The old MaineDOT adage that if you ask three pavement experts the same question you’ll get three different answers has certainly been in full force within the past five or so years. It all started with what appeared to be a moisture related, or stripping, problem in The County. MaineDOT pavements were meeting specifications, but the surfaces of new pavements were losing material. The initial fix was to incorporate an anti-stripping additive into the hot mix asphalt (HMA) mix. However, we soon discovered that this wasn’t solving the problem and the issue was more widespread than simply up north. There were many theories as to the cause: excessive dust or fines in the mix and aggregate degradation; perhaps poor quality RAP (recycled asphalt pavement) being used; the liquid asphalt isn’t as sticky as it used to be; or maybe the increased use of studded tires is at fault.
To address these issues, MaineDOT decided to take a more programmatic and pragmatic approach to improving our pavements. After all, if you consider all of our investments, pavements are the single most costly item. MaineDOT’s approach has been to work with the Maine paving industry, academia, the Federal Highway Administration and our colleagues from other state departments of transportation to identify the cause of this premature distress and to correct it.
Aggregate durability study
A significant aggregate durability study was undertaken by our pavement quality staff. We currently use the Micro-Deval test to determine aggregate quality characteristics on the combined aggregate gradation, not on individual aggregate sources. Aggregate blends are required to have a maximum Micro-Deval loss value of 18 percent for use in mix designs. This study was conducted to evaluate the performance of individual aggregate stockpiles in durability testing to determine whether aggregate quality should be a concern.
More than 100 coarse and fine aggregates sources were tested and analyzed with the Micro-Deval and L.A. Abrasion methods. The results suggested that individual aggregate sources vary significantly across the state and sources should be tested for mix design acceptance individually. Further work is being completed to develop a correlation between Micro-Deval values and actual pavement performance on Maine projects. I am extremely proud to say that our staff presented and published a paper at the Transportation Research Board 2013 meeting which shows the significance and credibility of the work completed.
Meetings with paving contractors
In 2011, MaineDOT staff and paving industry staff met to talk about our concerns and gain insights into ways that these could be addressed. Information was shared with contractors outlining the projects in which the distress was observed for each contractor. Industry experts shared items that they felt were contributing to the distress. Also, separate meetings were held with individual contractors to discuss potential factors leading to the evidence of the distress in some of their paving projects. These meetings provided invaluable information and helped point us in the right directions.
Tri-state: Pavement peer exchange
In the summer of 2011, MaineDOT hosted a tri-state pavement peer exchange with New Hampshire DOT and the Vermont Agency of Transportation. Participants spent three days discussing specifications and testing requirements, visiting contractors’ pavement plants and examining pavements in the field.
The key conclusions were: 1) there is likely too much dust or “fines” in our mixes (both New Hampshire and Vermont specify lower fine content in their mixes); 2) liquid asphalt quality has decreased in recent years; and 3) aggregate quality testing should be done on the sources and not just the composite blend (both New Hampshire and Vermont test aggregate quality on the sources). Our colleagues did agree that we had an early pavement distress problem and acknowledged that they were not observing this type of distress in their states.
Research: New test methods investigated
Meanwhile, MaineDOT engaged the Worcester Polytechnic Institute to test mix and liquid asphalts samples in order to gain a better understanding of the cause of our premature pavement distresses.
The traditional AASHTO test method for moisture susceptibility didn’t seem to match the performance observed on Maine roads, so a new method using the Moisture Induced Stress Tester (MIST) for conditioning samples was examined.
The MIST results have shown great potential for identifying good performing mix designs and work continues to develop acceptable criteria. In addition, MaineDOT has recently acquired a Hamburg Wheel Tracking Device and has begun to test mix designs using this well-established and approved AASHTO performance test.
Conclusions and changes we’ve made
Based on the aggregate quality study and observations from our peer states, we have revised our specifications to lower the acceptable amount of fines in our mixes. We now conduct aggregate quality testing on the individual aggregates, not just the composite gradation. We have also noticed that using modified binders seems to provide a more durable pavement, and we will be considering their usage on more projects, especially those with higher traffic volumes.
Our work with the MIST test is continuing with more mix samples being conditioned and evaluated. In fact, this work was recently recognized by AASHTO as one of the top research projects in 2013. Finally, a major effort to test mixes with the Hamburg Wheel Tracking Device is underway this fall. The goal of this work is to adopt the HWTD as a tool to screen our pavements during the mix design process.
It is too soon to determine the overall impact of changes that have been made, although there is agreement that the premature distresses have been reduced.
With the continued outstanding effort of our staff, cooperation from the paving industry, and improved materials and test methods, I am certain we will solve the problem.