By Dr. Buzz Powell, Ph.D., P.E., Technical Directors, Asphalt Pavement Alliance and Dave Johnson, P.E., Senior Regional Engineer, Asphalt Institute
Most of you are probably familiar with the Discovery Channel program, “Myth Busters.” For 15 years they would address various myths or urban legends to try to determine if they were true or not. We enjoyed the application of science and engineering to determine whether a commonly held strong belief was actually false.
Typically, a strong true or false was established for a particular subject; however, that was not always the case. Sometimes, they found that circumstances and conditions would influence the outcome. In such cases, the answer would begin with, “it depends on…”
Similarly, within the asphalt world, there are many long and often strongly held beliefs that may need to be examined objectively to establish if they are true or not. Together, we have over 70 years of experience designing, constructing, maintaining and experimenting with asphalt pavements and will tackle some myths.
We have a willingness to question and challenge various “truths” we “know” about the design, construction and maintenance of asphalt pavements.
MYTH
“Tack coat can be skipped when paving multiple lifts in short order as the fresh materials will still bond together.”
Most asphalt roads are constructed by building in successive layers to reach an overall design thickness. Inherent in the determination of that overall design thickness is an assumption by the designer that all the layers will be bonded together so that they act as a single unit and not independently. This is why we call it “structurally adequate” bonding. Much like a glulam that might be used in the building of your house, asphalt construction includes the use of a gluing layer known as a tack coat. Most commonly this glue will be a light application of an asphalt emulsion, which admittedly, can be a pain for the contractor to deal with.
Because it can be a pain, it’s not uncommon for field personnel to believe there is no need for interlayer tack when paving multiple lifts. The root of this belief goes back to pre-Superpave mixes almost exclusively produced with neat asphalts and all virgin aggregates.
Typically, these mixes were also produced with higher asphalt content. Modern mixes are often produced with lower total binder content, virgin binder can contain polymer(s) and significant recycled binder can come from RAP and/or RAS. The notion that tack can be skipped was completely disproven on the 2012 NCAT Pavement Test Track when debonding between the new base and binder layers resulted in the premature failure of a structural test section
The objective of the section was to use higher percentages of RAP to improve sustainability, improve performance, and reduce costs. Total pavement thickness was only six inches thick to produce significant deflections that would result in failure within the 10 million equivalent single axle loading (ESAL) three-year research cycle. The base layer was produced with a highly modified binder and 35 percent RAP. The binder (middle) layer was produced with neat asphalt and 50 percent RAP. Emulsion was shot at a rate of 0.06 gallons per square yard between the adjacent layers. Materials testing and modeling indicated this section would be the best performer in the experiment (that also included sections built with recycled asphalt shingles and recycled tire rubber); however, transverse fatigue cracking was observed in both wheel paths throughout the section after only approximately 2.5 million ESALs.
A forensic study at NCAT determined the pavement had debonded between the base and binder layers. The unbonded top portion of the pavement then responded as if it were a thinner pavement, which quickly failed in fatigue from “middle-up” cracking. A laboratory study further revealed the need for a much higher tack rate, and the section was rebuilt using a rate of 0.10 gallons per square yard of the same emulsion. As a result of this corrective action, the test section supported over 15 percent more traffic than the next best performer. Additionally, the rate of progression after the onset of cracking was less than any other section. The worst-performing section became the best-performing section, demonstrating the importance of interlayer tack in modern mixes with higher RAP contents and stiffer modified binder.
Still having doubts? A simple common demonstration of the lack of bonding between existing asphalt layers occurs routinely when a road is cored. Many times, the core that is produced exhibits a separation of its layers during the coring. The only reason this happened was that structurally adequate bonding was not present. Skipping the tack coat is certainly suspected in these cases. If it wasn’t skipped, then poor tack practices likely came into play, compromising the bond
For more information on the importance of tack coats, how to best apply them in the field, and relative cost information, read FHWA’s Tech Brief on the subject.
MYTH
“Mixes with higher RAP and/or RAS cool faster than virgin or lower RAP mixes.”
The same thermodynamic relationships apply to all mixes, regardless of RAP and/or RAS content. Namely, as most mixes consist of about 94-96 percent aggregates, and it is the aggregates that are the thermally dominant component of a mix, under the same external conditions all mixes will cool at about the same rate.
Mixes with higher RAP and/or RAS contents do not cool faster than any other mixes; however, the relationship between mix stiffness and temperature can be very different. Although they are not cooling faster, it is true that recycled mixes are stiffer at the same temperature. In other words, at equal production temperatures, it takes the same amount of time for a conventional mix and higher RAP and/or RAS content mixes to cool to a certain temperature, but the conventional mix will be more workable and more compactable at that temperature. If a field crew thinks the mix is cooling faster because they perceive higher RAP and/or RAS mix to be stiffer, that may be an indication the mix either needs more virgin binder, a recycling agent, or some other change to reduce the mix stiffening as a function of cooling mat temperature.
Two free proven tools (PaveCool and MultiCool) are available which model asphalt cooling characteristics. Please use your favorite search engine to locate them for use within your browser. Mobile apps for them also exist, but not across all platforms.
MYTH
“Laying an asphalt layer greater than four times NMAS will result in poor compaction.”
Most practitioners agree the minimum compacted thickness of asphalt pavement ranges from three times the nominal maximum aggregate size (NMAS) for fine mixes to four times NMAS for coarse mixes. These minimum thicknesses are necessary for mix compaction purposes. It may not be possible to comply with density specifications in thinner layers as aggregates will fight each other, resisting compaction. For example, a 37.5 mm NMAS fine mix should have a compacted thickness of at least 12.5 mm thick.
These same practitioners commonly believe it is impossible to meet density targets when the thickness exceeds four times NMAS. The logic behind this limitation assumes that the effective depth of compaction is approximately five times NMAS, beyond which density will be reduced. Research on the 2018 NCAT Pavement Test Track disproved this belief when they successfully compacted a 12.5 mm NMAS surface mix at up to 16 times NMAS. Additionally, the states of South Carolina and Utah have both achieved excellent compaction on projects with 12 times NMAS and more.
At the NCAT test track, sections were built with both neat and highly modified binder. In all cases, cut cores were sliced into thirds to develop a density profile. These data demonstrated that density is effectively a “sandwich” where the greatest density is in the middle third of the “ThickLay” core and the upper and lower thirds were more or less a mirror image of each other. In subsequent track rebuilds at the test track, ThickLay paving has become a standard practice to expedite construction, avoid the use of interlayer tack and completely eliminate the possibility of interlayer shear failures.
In Utah, a 12.5 mm NMAS mixture with low design air voids easily exceeded the target density of 96 percent without an alteration of the contractor’s typical compaction process. This mixture has handled over four million commercial vehicles at the port of entry along Interstate 80 at Wendover, Utah without any rutting, bleeding or other noted distress in a very challenging location.
For more information on either the NCAT 2018 cycle or the Utah Wendover installation, please visit NCAT Report 21-03 or Thick and rich in Utah.
Where to next?
Have a mix question? Email the Asphalt MixBusters at asphaltmixbuster@gmail.com with your suggestions.
