NRRA Newsletter: July 2017
FHWA Reviews the Rolling Density Meter
MnROAD Weekly Construction Updates
This is an important summer with all of the construction going on out at MnROAD, so we are helping you keep up with the progress on a weekly basis. The updates describe each week with photos, updated schedules, videos and a look ahead at the rest of the construction season. The best way to learn about the updates is to follow the NRRA on Twitter or Facebook. Updates will be posted on both sites.
Where Does the Water Go? Swales Research at MnROAD
People who construct roads spend a lot of time thinking about water—how much is present, where it goes, what’s it going to do if/when it freezes.
This quote was posted during our recent Pavement Conference and Workshop, and it attests to the role water plays in road building. That’s one reason why MnROAD has recently accepted a proposal to continue research into what happens to water once it exits the road surface.
The first (pilot) phase of this research into swales and how they handle storm water happened last fall. The next, full-scale research, begins this summer at four locations including MnROAD, says David Fairbairn, a research scientist with the Minnesota Pollution Control Agency.
He and his team are trying to measure the input and output of water in a swale under a variety of conditions. The measure they concentrate on is infiltration, i.e. the way the swale draws water into the ground. Good infiltration results in a variety of good outcomes including:
- Replenishing ground water
- Reducing flooding downstream during storms
- Saving money spent on handling storm water
That last one is particularly important if you build roads. If a swale does its job and you can prove it—through this research--it means less money needs to be set aside in your project for additional storm water solutions.
So what kinds of things does Fairbairn’s team want to find out?
How construction affects swales
When you are constructing a road, it’s important to be careful not to damage swales by driving heavy equipment through them. The thought is, if you compact the soil in a swale, its ability to filter storm water into the ground is decreased. By how much? No one knows, and that’s one of the things this research will help quantify.
How construction affects vegetation
An integral part of the swale’s ability to filter water into the ground is its vegetation. Damage that during construction and you also compromise the swale’s ability to handle storm water. In addition, the vegetation can help a compacted swale regain its ability over time to recover and handle water like it did before. How fast and how well does vegetation do this? Again, that’s what Fairbairn’s team is going to research.
How runoff from construction affects swales
The water that runs off a road construction site can pour sediment into a swale. That can affect how well it infiltrates water into the ground. How much does sediment affect the system? That too is what Fairbairn hopes to find out.
This research project will go through the monitoring season of 2018 with a report ready by mid 2019. The preliminary data from the pilot phase last year was encouraging, said Fairbairn. In addition, he thanks MnROAD for all the help it has given him in this research and for providing a controlled environment for his experiment.
July Research Pays Off
We are glad to welcome David Newcomb from the Materials and Pavement Division of Texas A&M Transportation Institute for this month’s webinar. He will be presenting on Balanced Mix Design.
“Dr. Newcomb joined the Texas A&M Transportation Institute (TTI) in September 2011. He has been a Principle Investigator (PI) on two NCHRP projects dealing with aging of asphalt mixtures and the behavior of foamed asphalt mixtures. He is currently co-PI on another NCHRP project to provide an experimental design for the validation of cracking tests. Newcomb has also worked on projects for the asphalt pavement industry and the Texas Department of Transportation.”
Please join us for Research Pays Off on July 18 at 10 a.m. (CDT).
As always, we are eager to hear from you about future topics for this webinar series. Please forward this invitation to others in your organization who might benefit from the material presented.
MnROAD Test Cell Profile: Benefits of Stabilized Full Depth Reclamation
Three test cells at MnROAD are getting a lot of attention because they have out-performed their design expectations and really show the benefits of successfully using stabilized full depth reclamation (SFDR) as a viable rehabilitation option.
MnROAD cells 2, 3 and 4 were originally built in 1994 and are located on MnROAD’s mainline Interstate-94 westbound lanes – they were in very poor condition before the 2008 rehabilitation. Each test cell contained multiple transverse and fatigue cracking (both wheelpaths) causing poor ride conditions. Typically, in the past, asphalt “mill and fills” were used as the most common rehabilitation technique by removing 2 inches of the upper asphalt layer and then placing a 3-inch new asphalt overlay as a driving surface. The problem is the existing cracks still remain in the lower, remaining asphalt layers and will eventually reflect back through over time. This is where SFDR can provide longer life by grinding up all the asphalt layers including some of the underlying granular base. That breaks up the cracking pattern so cracks cannot reflect through. Then by adding an engineered asphalt emulsion (stabilizer) to this material it adds additional strength to support the new asphalt driving surface, complimented by new innovative designs to prevent cracking and rutting. This is also very green technology because materials are recycled in-place, which reduces the amount of both trucking and new materials.
The MnROAD SFDR test cells built in 2008 consist of only 3 inches of new asphalt layers, which is very thin for an interstate roadway that receives 30,000 one way average daily traffic---more than 14% of that is trucks. In cells 2 and 3, each contains 1 inch of ultra thin bonded wear coarse (Novachip) and 2 inches of hot mix asphalt Superpave mix (PG64-34 asphalt). In Cell 4 there is just 3 inches of Superpave hot mix asphalt (PG 64-34 asphalt).
The original design life was expected to be five years or 3.5 million equivalent single axle loads (ESALS). Now each test section has lasted more than nine years (2008-2016) or over 5.9 million ESALS. This is four more years (80% increase) of hot summers and cold winters and more than a 68% increase in axle loadings than we anticipated. The performance has been exceptional. The ride (International Ride Index in m/km) is still very smooth as of 2016, and there is very little distress (cracking) as noted below:
2016 Cracking Observations
Cell 2 = 0.95
• Minimal transverse cracking (no reflective cracking)
Here’s a video of what these cells looked like a few days ago.
Note that cell-4 does have a higher (rougher) ride because it originally had no granular base under the asphalt layer. This thinner design makes it harder to reduce the effects of seasonal variations of frost heave and loss of strength in the spring time during the thaw compared to the other two test cells. They both had a granular base under the SFDR layer. Maintenance is also more important for this rehabilitation option because with these thinner surfaces, cracks do need to be sealed as soon as possible to protect the surface.
MnROAD is looking forward to learning how many more years/amount of traffic these test cells can continue to take as they continue to outperform our expectations. MnROAD will continue to monitor and report on their performance. If you want to learn more about these test sections please contact Shongtao Dai or attend an upcoming NRRA sponsored conference developed by Asphalt Recycling and Reclaiming Association on October 16-19, 2017, here in Minneapolis.
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