Asphalt is the sticky black residue that is left over from the processing of crude oil. It has been used in paving for more than a hundred years. When asphalt first came into use, oil refiners would give it away. Today, however, it is a highly traded commodity that demands premium prices. These prices have increased dramatically. In 2002, asphalt sold for approximately $160 per ton. By the end of 2006, the cost had doubled to approximately $320 per ton, and then it almost doubled again in 2012 to approximately $610 per ton.

The rising price of asphalt had a major impact on the cost of constructing pavements, which increased interest in finding ways to reduce costs. Methods to reduce costs include minimizing the amount of asphalt in the mix, increasing the use of reclaimed asphalt pavement (RAP), and replacing part of the asphalt with lower cost additives. RAP already contains asphalt, albeit aged material that does not have the same properties of fresh asphalt.

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The DSR tests binder placed between two parallel plates about the size of a quarter. One of the plates moves and the machine measures the viscoelastic properties of the asphalt. The DSR is used to determine the maximum high temperature performance grade (PG) in degrees Celsius. These temperatures increase in steps of 6 degrees and are typically PG 52, 58, 64, 70, 76. They provide a maximum service temperature for the pavement. For example, a PG 70-28 binder would have a maximum service temperature of 70 degrees Celsius and a minimum service temperature of minus 28 degrees Celsius.

The addition of soft materials to asphalt will reduce the high temperature grade (for example, from a PG 76 to a PG 70). Several additives have been evaluated by industry and academia, including used frying oil, residues from corn stover, and even treated swine manure, for this purpose.

Analysis of liquid asphalt for the trace metals calcium, copper, zinc, and molybdenum provides a measure of the amount of REOB present. Sulfur and iron could also be analyzed, but because they occur naturally in asphalt, their use would confuse the analysis.

Because REOB is a waste product, its composition varies widely not only between producers but also between samples from the same producer on different days. The compositional analysis is also affected by the asphalt into which it is blended. However, by making many blends using different REOB samples and different asphalt binders, the variations largely can be averaged out.

To execute the plan, TFHRC provided the initial test method and 45blends of various REOB modified asphalt binders with REOB concentrations of 2, 5, 8, 10, and 20 percent. In total, the researchers prepared and shipped 720 blends.

The TFHRC researchers carried out a few mix tests (mixing the binders with aggregate) in 2015. The tests were not extensive, but they showed that at levels of 6 percent or more, the tensile strength of the asphalt dropped significantly. At a level of 3.5 percent REOB, the variation in the physical test methods was greater than the effect of REOB. In fact, it was difficult for researchers to assess whether REOB was present.

At TFHRC, researchers are planning a different way of looking at asphalt binders. Previously, all asphalt testing measured engineering properties such as stiffness. These tests do not show what materials had been added to the asphalt.

One sample received during the TFHRC study had a very strange analysis. The sample had the following test results: Superpave PG 64-28 with a high temperature grade of 67.3 Tcritical on the bending beam rheometer was 6.7 degrees Celsius. Chemical analysis indicated it contained approximately 1.7 percent phosphoric acid, 10 percent ground tire rubber, and 19 percent REOB. The addition of 1.7 percent phosphoric acid likely would make the asphalt very stiff. Ten percent ground tire rubber would make it even stiffer. Then 19percent REOB would soften it and bring it back within specification.

To address this issue and the expansion of new asphalt additives and extenders, TFHRC is starting a research program to use handheld spectroscopic devices, x-ray fluorescence spectroscopy, and Fourier transform infrared spectroscopy to enable analyses to be done in the field rather than having to take samples back to the lab. Fourier transform infrared spectroscopy can even find lime in the mix, as well as styrene-butadiene-styrene and styrene-butadiene rubber polymers. X-ray fluorescence spectroscopy can find REOB and phosphoric acid, and the handheld spectroscopy works for spot checks. These instruments can be preprogrammed and require no additional training or skills for operators. All of this testing can be done directly from the paving machine, or at the asphalt plant by an unskilled operator, saving time and associated costs. These methods are much more difficult to manipulate because they can almost always tell what materials have been added to the mix. They also enable the possibility of field spot checks and eliminate the possibility of sampling errors where the asphalt being used was not the same as received by the testing lab.

The TFHRC team will soon submit to AASHTO the draft test methods that transportation agencies can use to test for the presence of REOB in asphalt mixes.These test methods will help transportation agencies know what materials and additives are present in the asphalt mixes they are purchasing.

The Asphalt Art Initiative grant program supports projects that demonstrate the impact of asphalt art projects and encourage cities to develop their own processes for implementing these low-cost activations effectively. Previous grant rounds supported 65 projects in the U.S. and Europe, installing from 2020-2023. Newly awarded projects in 25 cities in Canada, Mexico, and the U.S. were announced in November 2023.

Released in March 2022, the Asphalt Art Safety Study, conducted by Sam Schwartz Consulting in partnership with Bloomberg Philanthropies, found that city streets with asphalt art became considerably safer for pedestrians after incorporating art into roadway redesigns.

Aggregates used for asphalt mixtures could be crushed rock, sand, gravel or slags. Nowadays, certain waste and by-products, such as construction and demolition debris, are being used as aggregates, which increases the sustainability of asphalt.

An average asphalt pavement consists of the road structure above the formation level which includes unbound and bituminous-bound materials. This gives the pavement the ability to distribute the loads of the traffic before it arrives at the formation level. Normally, pavements are made of different layers:

Asphalt is produced in an asphalt plant. This can be a fixed plant or even in a mobile mixing plant. It is possible to produce in an asphalt plant up to 800 tons per hour. The average production temperature of hot mix asphalt is between 150 and 180C, but nowadays new techniques are available to produce asphalt at lower temperatures. (See below).

To be able to provide the best performance to different applications, a large variety of asphalt mixes can be used. Due to the different requirements (amount of traffic, amount of heavy vehicles, temperature, weather conditions, noise reduction requirements, etc.) the respective mix used needs to have an sufficient stiffness and resistance to deformation in order to cope with the applied pressure from vehicle wheels on the one hand, yet on the other hand, they need to have an adequate flexural strength to resist cracking caused by the varying pressures exerted on them. Moreover, good workability during application is essential in order to ensure that they can be fully compacted to achieve optimum durability.

In general the asphalt layers are paved on a bound or unbound road base layer. Starting at the road surface, the first layer is called the surface course. The second layer is mostly called the binder course. The lower layers are the base courses.

NAPA advances the asphalt pavement industry through leadership, stewardship, and member engagement to support sustainable transportation infrastructure that paves the way for thriving communities and commerce.

NAPA is the only national trade association that serves the asphalt pavement industry. Membership is available to any asphalt pavement mix producer, paving contractor, equipment manufacturer, material supplier, equipment distributor, engineering firm, or consultant.

In this 2-part webinar series for industry and agencies alike, we delve further into our recently released roadmap for decarbonization of asphalt pavements. Explore sources of GHG emissions throughout the life of an asphalt pavement and uncover opportunities to reduce emissions using tools like specifications, mix design, and the Supply Curve.

NAPA is taking proactive steps to help the industry and agencies leverage unprecedented levels of federal funding reduce greenhouse gas emissions. This comprehensive roadmap documents specific steps pavement engineers, asphalt mix producers, paving contractors, policy makers, and other stakeholders can take to reduce embodied carbon emissions associated with asphalt pavements. 0852c4b9a8

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