Over the last decade, the Missouri Department of Transportation (MoDOT) has partnered with contractors to find innovative ideas to improve the roads and bridges in the state.
The result has been that MoDOT has won numerous awards for safety, innovation and project management. These roadway and bridge improvements have led to a greater than 80% approval rating of MoDOT from the citizens of Missouri. The partnering with industry also allows for rapid solutions to issues when they arise, such as some isolated jobs that used recycled asphalt shingles (RAS).
RAS has been used on MoDOT projects since 2005. In 2011 more than 80,000 tons of shingles were used in MoDOT roads. The vast majority of the shingles used in these projects were post-consumer, or “tear-off,” shingles removed from houses. This material used to be taken to landfills and lost forever. Today these shingles are reused to reduce the demand for petroleum and improve the rut resistance of roads. Roofing shingles generally consist of high-quality aggregate granules, fibers and stiff asphaltic cement; all of which have individually been used to improve the service life of our interstate highways.
Smooth makes the move
The dramatic increase in the use of RAS in Missouri occurred during the Smooth Roads Initiative implemented in 2005 by MoDOT as part of the Smoother, Safer, Sooner program. This initiative involved improving the quality of the major interstate routes by overlaying them with Superpave asphaltic concrete. Superpave mixes have a rigorous design methodology incorporating air voids, voids in the mineral aggregate (VMA), minimum asphalt content based on a dust-to-effective-binder ratio and voids filled with asphalt. These properties also were verified in the field through the quality-control and quality-assurance programs that MoDOT uses for project acceptance. Generally 2% to 5% by mix weight of shingles were used on these roadways. The successful use of shingles in these projects led to shingle usage in non-Superpave mixes used on lower-volume roads.
Due to record crude oil prices in 2008 and a shortage of styrene-butadiene-styrene (SBS) modifiers, shingle usage in Missouri tripled to 24,000 tons. In 2009 that figure rose to 53,000 tons. The use of RAS during these years allowed Missouri to save millions of dollars and led to more roads being paved statewide. At the same time the Smooth Roads Initiative was completed and ended the increased funding brought about by Missouri Constitutional Amendment 3. In 2011, MoDOT had a construction budget of approximately $1.3 billion; by 2013 funding had dropped to around $900 million. The decrease in funding has led to more thin-lift overlays to maintain the roads and increased competition for contracts.
Filling in with dust
The VMA of an asphalt mixture is based on the aggregate skeleton. VMA is the amount of air voids between the aggregate particles. These air voids must be filled with asphalt to produce a quality mixture. In general the higher the VMA, the higher the asphalt content. However, due to the way VMA is calculated, the asphalt content of the mixture and the bulk specific gravity of the aggregates will affect the calculation. One of the challenges of using RAS is how to determine the bulk specific gravity of a product that contains aggregate, binder and fibers. The traditional methods of determining bulk specific gravity for aggregates, AASHTO T-84 or AASHTO T-85, are very difficult to run on shingles. As a result many states have attempted to estimate the bulk specific gravity by using the effective specific gravity by means of AASHTO T-209. This method tends to overestimate the bulk specific gravity of the shingles, however the belief is that the impact is minimal, since a small percentage of shingles are used in the pavement. It has consistently been observed that asphalt mixtures that incorporate shingles have a very high VMA, and this has called into question the minimal impact of inflated specific gravities. Recently Gerry Huber at Heritage Research Group has shown that the increased VMA in RAS mixtures is due to the high-quality angular aggregates found in the shingles not due to mathematical manipulation.
Most states have a minimum VMA requirement during the mix design and production stages of an asphalt pavement. The combination of VMA and the voids filled with asphalt (VFA) is used to require a minimum asphalt in the mix. The impact of the higher VMA due to the presence of shingles in the mix is that the minimum mix VMA can be met with shingles and dust and not with virgin asphalt binder. For example, if a state requires a minimum of 14% VMA for mix design approval and the mix designer has 16% VMA, the mix designer can remove 0.2-0.4% virgin asphalt binder and replace it with dust. This substitution lowers the VMA but still meets the minimum specification. The mix still meets all of the volumetric requirements, but the result is a dryer mix that is more prone to problems in the field. The Superpave-mix-design method requires a dust-to-effective-binder range that prevents mixes from being loaded up with dust. However, some states do not have the same requirement on their lower-volume roads.
Lower gets higher priority
In Missouri, many lower-volume roads are treated with a mix referred to as a “plant mix bituminous surface leveling” mixture. This mix is usually placed 1 in. thick and thereby not subject to density requirements except that a minimum of three complete passes with a roller weighing at least 10 tons. In the past these mixes have performed very well, since they were fine mixes with a lot of binder in them. Due to more contractors having full testing labs, there was an option to design these mixes as either a gyratory compacted 50 gyration mix or a 35-blow Marshall mix design. They were designed between 3.5% and 4.5% air voids with a minimum 13% VMA. There was not a requirement for a dust-to-effective-binder range. These mix-design requirements were based on many years of field performance. Typically these mixes had a nominal maximum aggregate size of 3?8 in. but had a maximum aggregate size of ¾ in.
After many years of focusing on higher-volume roads in Missouri, there was a shift to inexpensive maintenance treatments of lower-volume roads. The surface-leveling mix was used extensively in 2012 on these routes. MoDOT noticed that in isolated instances, there was a loss of performance on some of these routes. Upon investigation it was found that these routes were typically laid at the end of the construction season, when it was cold and more likely to have rain showers, and the mixes were difficult to produce in cold weather. These mixes contained shingles. The shingles were showing up in the roadway as clumps. These issues were isolated to specific projects and were not statewide. More issues were encountered when there was a long haul to the project. On two of these projects there were significant failures the following spring.
In order to keep this from happening again, MoDOT issued a job special provision that limited the effective binder replacement by shingles on surface-leveling mixtures to 20%. This limited the amount of shingles in a mix to 4% or less by weight of mix. This requirement was only on surface-leveling mixes, not Superpave or other bituminous base and bituminous pavements.
Lay out ideas, asphalt
Since there was a strong partnering relation between MoDOT and the Missouri Asphalt Pavement Association (MAPA), a group of contractors that serve on the MoDOT/MAPA Technical Working Group were invited to Jefferson City to discuss this issue. MoDOT Construction and Materials Engineer Dave Ahlvers briefed everyone on their concerns and then in an innovative approach encouraged the contractors to meet independently for several hours to develop solutions to the issues. At the same time, a group of MoDOT engineers and technicians met to offer their ideas. In the afternoon both groups met and presented their solutions. The wily contractors proposed that even more standards be put into place to make sure that the roads performed well. The following recommendations were reviewed and included in the job special provisions of all contracts that use surface-leveling mixes:
- The gradation was changed to require 100% passing the 3?8-in. sieve in order to provide a mix that is more compactable when placed 1 in. thick;
- The design air voids are now 3.5%, and the minimum VMA was raised from 13% to 14.5%. This is more in line with the recommendations of the Asphalt Institute MS-2 Mix Design Methods for Asphalt Concrete;
- Dust-to-effective-binder ratio is limited to 0.8-1.6;
- Marshall mix designs can still be used at 35 blows; mixes designed on the gyratory compactor now use 35 gyrations;
- Statewide the bulk specific gravity for RAS was set at 2.600. There had been a wide range of values used in the past that did impact the VMA calculation of mixes;
- The 20% effective binder replacement from shingles was left in place and will be reviewed in the future; and
- Allowed for binder modification through the use of rejuvenators such as Hydrogreen or EvoFlex provided the modified binder still meets the requirements of AASHTO M-320.
In fall 2013 the impact of these changes has been that the total asphalt content of these mixes has increased 0.6% to 1.5%. The virgin asphalt content also has increased in these mixes. One oversight that was realized is that the aggregate producers were not included in this discussion. The changes to the gradation impacted the types of material that some Missouri quarries produce, and the aggregate producers were not informed of the changes until the contracts came out to bid.
The general consensus is that Missouri did not have as much of a shingle problem as it did have a mix-design-specification issue. The use of recycled materials will continue to increase, and undoubtedly we will need to continue to scrutinize our specifications, methods and quality-management practices. Through the strong partnering practices that MoDOT has developed with industry, these changes should occur rapidly and with many benefits to the taxpayers of Missouri. R&B