A shared story
There was an article late last year in “Ars Technica” online that caught my attention, titled “In 1982 a physics joke gone wrong sparked the invention of the emoticon.” You can look it up if you want to read more, but the TLDR (too long, didn’t read) version is that a faculty member at Carnegie Mellon University posted something on a physics discussion board that they intended to be a joke. But without any way to indicate their intended sarcasm, their fellow faculty thought it was serious and it caused a bit of an uproar. Ultimately, that led to a proposal, followed by refinements to the proposal culminating in the smiley face emoticon – 🙂 – being accepted as an indication that what was just stated in writing should not be taken seriously.
A small thing, maybe, but the emoticon has since led to major changes in how we communicate. Thumbs up, emojis and memes have inserted themselves into our online/text interactions. We worked so hard as humans over millennia to develop the written language only to see smart phones encourage us to basically return to hieroglyphics.
The process of discovery described in the development of emoticons led me to reflect on another idea that was “introduced” fifteen years ago to the asphalt world – Delta Tc (ΔTc). I see a lot of parallels in the introduction of Delta Tc and the introduction of the smiley face emoticon. Neither were truly original ideas but were adaptations of what others had suggested but maybe not in the same form.
In the late 2000s, the Asphalt Institute lab was part of the research team exploring non-load-related distresses on airfield pavements. General aviation airfields are akin to low-volume roads in that traffic loading usually does not lead to distress, but oxidation and loss of durability do. As part of the research seeking to quantify the progression of the loss of durability and the onset of cracking, Dr. Gayle King suggested we consider how the continuous grade temperature (Tc) changes as the asphalt binder ages. This idea was presented by researchers (Dr. Ray Robertson et al.) exploring oxidation in the early 1990s. Specifically, we looked at BBR data and discovered that as the binder aged to a greater extent, the continuous low-grade temperature got warmer. Not a surprise there. But what we also observed was that the continuous grade temperature as defined by m-value (Tc,m) was more affected by aging than the continuous grade temperature as defined by stiffness (Tc,S). In other words, as the aging progressed, the difference between the two continuous grades became greater. To quantify this, the research team coined the difference in continuous low-grade temperatures as “Delta Tc”. In retrospect, we probably could have been more creative in our naming.
During the research, we found that the newly named parameter, ΔTc, was related to a DSR parameter explored by Dr. Charles Glover in a study on oxidative aging of asphalt pavements. He tied that DSR parameter to ductility, which was tied to work reported in the 1970s by Dr. Prithvi (Ken) Kandhal that showed cracking was correlated to low ductility after aging. In short, Glover found values of his DSR parameter that related to ductility values of 5 cm – considered the warning limit for future cracking – and 3 cm – considered to be the point where cracking was imminent, if it had not already occurred. These values translated to ΔTc values of -2.5 and -5.0, respectively.
As an aside, the research leading to the definition of ΔTc originally reversed the terms, making an asphalt binder appear to have higher (more positive) values after aging. A suggestion by another colleague, Gerald Reinke, offered that the terms should be reversed so that the convention would be that aging would cause lower (negative) values of ΔTc. Since aging was a negative effect, it seemed appropriate that the response should be negative as well.
Thus, we progressed from a research report to a technical paper published in the AAPT Journal in 2011. Incidentally, the discussion on that paper by Dr. Geoff Rowe led to a refinement of the Glover DSR parameter into what has been termed the Glover-Rowe Parameter, or GRP.
An article in this magazine in 2016 highlighted Delta Tc as a possible parameter of interest. In 2019, the Asphalt Institute published IS-240 offering a state-of-the-knowledge on the ΔTc parameter and its potential use in quantifying the effects of aging and loss of relaxation. Now, in 2026, fifteen years after its “introduction,” ΔTc is one of the parameters being used by some agencies and researchers to signal possible durability issues.
Although there appear to be other parameters that may be better suited for a specification, the original work on Delta Tc led to a renewed understanding of durability changes in asphalt materials as they age. Like the development of the smiley face emoticon, it was not a breakthrough as much as an iterative path with multiple contributors – the true scientific process.
Anderson is the Asphalt Institute Vice President of Research and Laboratory Services.
For questions or more information on the research, testing and training services provided by the Asphalt Institute Laboratory, please contact Wes Cooper or me. Despite the temptation, we will respond using actual words.
