NRRA Newsletter: February 2018
RAP and RCA Used for Base
Recycled Asphalt Pavement (RAP) and Recycled Concrete Aggregate (RCA) have been getting so much attention because we have so much of it. Hundreds of millions of tons of RAP and RCP are produced nationally each year and are reused into our transportation system.
The reuse of these materials has multiple benefits for the environment and the budgets of state DOTs nationwide. A lot of the conversation revolves around reusing this material for the surface of roads, but the NRRA will be researching its use in in road bases.
The NRRA Geotech team is doing long-term research into using both materials in the aggregate base. In some of the test cells, the team has mixed RAP and RCA using different gradations and proportions, and it is assessing performance compared to base composed of virgin aggregates. There are studies on virgin aggregates, and studies using RAP and RCA successfully, however studies on different gradations and proportions of recycled aggregates are limited.
It is hoped that the results of this study will assist agencies in specifying recycled aggregates, so that the performance and cost effectiveness of recycled aggregates will meet or exceed virgin aggregate base.
These recycled bases sections overlay two different subbases, sand and large stone 6” minus material. These research cells are part of a larger research project and will have data collected for the next two years. Then there will be forensic study of these cells with cores and testing.
Reflective cracking in asphalt pavements
By John Garrity and Dave Van Deusen
This is the second in a series of two articles.
Last month we covered two questions that pavement engineers are often asked: Why do new asphalt pavements crack? And when will new asphalt pavements crack? A distinction between two pavement types, new pavement and resurfaced pavement, was made to help address these questions. We focused on new pavement and discussed the fact that, a new asphalt surface will crack eventually, and it will crack when the tensile strength of the asphalt mixture is exceeded. The forces that cause a crack to propagate to the surface, and the strength of the mix to be exceeded, depend on many factors.
Reflective cracking occurs predominantly in resurfacing situations such as asphalt overlays on existing concrete or asphalt pavements. Existing distresses, or even design features (such as transverse joints in concrete pavement), may cause stress concentrations as the underlying pavement expands/contracts producing differential movements. These movements transfer to the new, overlying surface. If the movement causes the material to exceed its strength, a reflective crack results. Reflective cracking can also be affected by traffic. Existing joints or cracks in the underlying pavement cause pavement adjacent to either side to deflect vertically as axle loads move across. This motion places the overlying mix under stress causing fatigue, and thus cracking. It is much more challenging to engineer a crack resistant pavement system for construction on top of an existing, distressed pavement. Unlike new asphalt pavements, selection of a PG binder will not necessarily delay reflective cracking in these situations.
Resurfaced pavements abound in all manner of age, composition, and condition. Maintenance and rehabilitation of existing roadway pavements constitute a significant portion of available pavement rehabilitation construction resources. Therefore, any gains that can be made through improved overlay designs and materials selection have the potential for a significant payback.
In 2017, MnDOT, in partnership with National Road Research Alliance (NRRA) members, designed and constructed a series of twelve asphalt overlays on the existing concrete pavement of westbound I-94 adjacent to the MnROAD mainline. The overlay sections consist of both single and two-lift construction. https://youtu.be/6va0jzOJCF4
The roadway selected for these rehabilitation alternatives was the original alignment of westbound Interstate 94. This section of roadway has been in place since the early 1970s and consists of 9.5-inch thick, reinforced, concrete pavement with 27-foot doweled/skewed joints.
NRRA members recognize the need for a) establishing an engineering practice for design of overlays, b) assessment of pre-overlay concrete pavement condition, and c) recommendations for improvements to existing pavement prior to overlay construction. This study will address current state-of-practice for asphalt overlay design by evaluating the cost, constructability, and benefit of rehabilitating concrete pavements with asphalt overlays that vary in thickness and composition. Final evaluations will consider cost, impact of construction, and service life of each technique.
Specific objectives of the proposed study are to develop a simple decision tree-based tool for selecting suitable asphalt mixtures and overlay designs to prolong overlay lives by lowering reflective cracking and improving in-situ density. Recommended guidance from this study will incorporate consideration of constructability (time and effort), performance over time, and life cycle and cost-benefit analysis. It is anticipated that implementation of the tools and materials recommended from the results of this study will translate to savings in construction costs and time, improved serviceability of the roadways for users, and reduced life cycle costs.
Besides varying the thickness and composition, other factors are under investigation, including:
- alternate mix designs
- highly modified, crack resistant mixes
- the use of permeable asphalt courses as a first lift
- pre-overlay stabilization of subsurface concrete pavement support
The study will also examine whether enhanced density improves performance of asphalt overlays. States across the country have been adjusting asphalt mixes in a variety of ways to try to attain better overall mixture performance by achieving higher in place field density.
MnROAD personnel will monitor the performance of these sections for a number of years to come. Stay up-to-date by monitoring the NRRA Flexible Team website.
Also plan on attending the 2018 NRRA Pavement Workshop, May 23-24, 2018, to learn more.
Associate Profile: Caterpillar Paving Products
Even before Caterpillar Paving Products joined the NRRA, its employees and equipment were familiar fixtures at MnROAD’s 2017 construction last summer. Cat® compactors, equipped with Intelligent Compaction technology, were part of many experiments. Moreover, Caterpillar brought a team of equipment designers to MnROAD near the end of the season to see how the compactors they developed operated in the field.
Todd Mansell, Product Application Specialist with Caterpillar Paving Products, says collecting and sharing data among machines is becoming more of a focus for contractors, designers and manufacturers. Collecting the data and providing access to it in a meaningful and useful way is an undertaking with many significant technical challenges; but it is one that Caterpillar, amongst others, is engaging in.
One technology Caterpillar offers right now includes compactors with Intelligent Compaction, temperature mapping and accelerometers. That last piece of technology is used on asphalt compactors as an indicator of the stiffness of a new pavement structure, or in the case of soil compactors, it’s an indicator of the stiffness of the aggregate base structure (Mansell was very careful to use the term stiffness, so there wasn’t any confusion with measuring density, which accelerometers do not measure). Generically, this indication of stiffness is derived by measuring the rebound of the drum as it vibrates during the compaction process and assigning a number, generically referred to as an “Intelligent Compaction Measurement Value” or ICMV for short. Many variables affect the “stiffness” readings, but of particular note is that stiffness values are influenced by the base and subbase materials to various depths ranging from approximately 2-ft to as much as 6-ft below the surface.
This technology on compactors is not meant to replace conventional performance testing, says Mansell, but rather is part of an effort to establish a correlation between traditional field tests such as the DCT, the DCP and plate load tests. If they can establish a reasonable correlation with ICMVs, it helps build confidence for operators that the job is being done right and compaction targets are being achieved. At this point, IC technology is not suitable for quality assurance, or acceptance testing. Conventional testing methods still apply. This is just one more tool in the operator’s tool kit to assist in achieving uniform compaction. It’s also important to note that IC technology covers 100 percent of the mat, rather than just a representative sample of the job determined by traditional testing methods.
Sharing data between machines is achieved using local Wi-Fi networks, says Mansell. This way, one operator on a roller, for example, can see the total number of passes that have been made over an area by all the rollers on the jobsite. Caterpillar’s compactor data can be imported to Veta software, which allows it to be analyzed alongside data from other manufacturer’s systems. Veta software was developed by MnDOT and the FHWA with the intent to import and analyze data from mixed equipment fleets. The compaction analysis tools in Veta software are geared toward asphalt compaction, such as creating a “compaction curve” showing the number of passes versus density. True density values are input from core or density gauge measurement in the field.
Mansell says technology is about more than simply increasing machine performance– it is also an effort to help contractors with one of their greatest concerns: finding, hiring and retaining qualified and engaged workers. “There’s a lot of new technology going into paving equipment, and finding people with a good mix of low-tech and high-tech skills is a challenge. Machines that are easy to use reduce training costs for new hires and enhance the skills of experienced crews. The result is a better built road that lasts longer, and a contractor with a happy, engaged crew. That's good for taxpayers, good for travelers and transportation, good for the contractors and their crews, and ultimately good for Caterpillar and other manufacturers as well,” says Mansell.
2018 Pavement Workshop: Save the Dates
The National Road Research Alliance 2018 Pavement Workshop will occur May 23 and 24, so please save the dates. The Conference will feature:
- A tour of the 2017 construction/research projects at MnROAD
- Four research tracks for concurrent sessions: Flexible, Rigid, Geotech and Preventive Maintenance
- Keynotes, awards and networking with your peers
- Workshops and a BBQ dinner at MnROAD
Please help us get the word out. As the conference develops and the agenda is finalized, all details for the workshop will be updated.
It’s not too late to help us tailor the conference to your needs. If you have topics we need to cover, please forward them email@example.com
March Research Pays Off Webinar
*We are switching to Skype for Business this month. Details for connecting to the video and audio will be on the above website.