By Mike Anderson, P.E., CAE
I was listening to the radio recently and heard “Learning to Fly” by Pink Floyd. I had forgotten how much I enjoyed it since much of my experience with their music ended with the album “The Final Cut” in 1983. As I listened it occurred to me that two other songs I also loved – “Given to Fly” by Pearl Jam and “Learn to Fly” by Foo Fighters – had almost the same name and were all released within about a decade of each other. Odd that three great songs would have so similar names and be in my listening consciousness around the same time.
Three is a powerful number. Superstition holds that bad things happen in threes, like celebrity deaths. Three has relevance in religion (the Holy Trinity in Christianity), mythology (the Three Fates), and society (the troika in Russian culture). Even entertainment – Shakespeare’s “Macbeth” and Aaron Spelling’s “Charmed” – make use in their respective plots of three witches who are sisters. Yes, I just mentioned William Shakespeare and Aaron Spelling in the same sentence. Please do not judge me harshly.
So, the fact that there are three parameters being considered for use in the PG asphalt binder specification to address the same issue isn’t a surprise. The power of three is, well, powerful.
The PG asphalt binder specification has been around for three decades now, having been designed to address three principal distresses – high-temperature rutting, low-temperature cracking and intermediate-temperature fatigue cracking. The problem is that no asphalt technologist has ever been completely happy with the intermediate temperature parameter, G*sin δ, to address cracking. It does seem to have value in characterizing the stiffness of an aged binder, but it mostly misses the relaxation component so important to asphalt binders as they age and become more brittle. This has led researchers to propose an additional specification parameter to better capture the behavior of an asphalt as it ages and loses flexibility. The three leading parameters? In no particular order:
1. Delta Tc (ΔTc)
Proposed by the NCHRP 09-60 research to address the loss of flexibility in an asphalt binder during aging in service. It can also capture colloidal instability in the binder. What does that mean? Generally speaking, asphalt binder is a blend of asphaltenes, saturates, resins and oily fractions. If that blend becomes imbalanced then the properties of the binder may change. Some additives may affect this balance.
Delta Tc is calculated using Bending Beam Rheometer (BBR) data at two or more temperatures. Technicians know the test procedure and perform it routinely. The calculations are pretty straightforward. The drawback is in performing testing at multiple temperatures – resulting in a little more work. More negative values of Delta Tc mean poorer relaxation properties.
2. Phase angle at a fixed modulus
Phase angle (little delta this time, δ) at a fixed shear modulus, G* has been proposed by some experts. It is determined using the Dynamic Shear Rheometer (DSR) at intermediate temperatures. It captures the same basic information related to loss of relaxation and colloidal instability.
Testing is needed at multiple temperatures to bracket the selected G* value, but this is automatic for the DSR once the sample is loaded. A minimum phase angle (δ) is needed at the selected shear modulus, G*, to address the potential loss of relaxation or instability.
The small sample size is a positive, making it great for forensic analyses. It has the advantage of testing at temperatures in which cracking due to loss of relaxation is most expected to occur. A lower phase angle at a specified G* means poorer relaxation properties, in general.
3. R-value
Developed as a measure of the shape of the curve of modulus or stiffness as a function of loading time or temperature. “R” stands for “rheological” and is dependent on characterizing the asphalt binder properties compared to its glassy modulus (the point at which asphalt behaves as a glassy solid). This parameter has been proposed by the NCHRP 09-59 research.
R-value can be calculated with BBR data at a single temperature that is used to determine the binder low-temperature grade. The calculations are straightforward. It is sensitive to temperature, meaning the result can change if you don’t define the proper temperature (actually stiffness) range. It assumes a standard glassy modulus, which some technologists dislike. Higher R-values mean poorer relaxation properties and/or instability.
Having three options is powerful, but eventually we asphalt technologists will need to decide which parameter is best for use in a purchase specification. One may be the loneliest number, but for specification purposes, it is needed. Reference here is to the song “One” by Three Dog Night. Again, we can’t seem to escape the power of three.
For more information on the research, testing and training services (three) provided by the Asphalt Institute Laboratory, please contact Mike Anderson, Gary Irvine or Wes Cooper (three). By the pricking of our thumbs, asphalt questions this way come.
Mike Anderson is the Director of Research and Laboratory Services at the Asphalt Institute.
More information on Delta Tc can be found in IS-240 as a free downloadable PDF on the AI website. asphaltinstitute.org/engineering/delta-tc-technical-documents