Flexibility:
A needed trait in winter.
Asphalt is the best.
I have always enjoyed playing with haiku. I cannot say why, as I am not a poet or an artistic person at all. But there is something about the wordplay and finding a way to complete a coherent thought within the structure of 17 syllables in three lines (5-7-5 format) that is challenging to me. I think it may be because I typically overexplain things in writing which leads me to being really wordy. It is a challenge to be brief; I did not even bother with Twitter (X) until they got rid of the 140-character limit.
The constrained structure of a haiku is not unlike the constraint we see in asphalt mixtures at cold temperatures. Maybe a bit of a stretch to make that tie-in, but like asphalt, I can be flexible.
What happens to an asphalt mixture in winter that leads to cracking? Unlike other distresses that can be affected by many different factors, low-temperature cracking is largely related to the properties of the asphalt binder. Volumetric proportioning matters, as too little asphalt binder content in a mix can negatively affect its cohesion. The air voids in the mix matter as they also can impact the tensile strength of the mix. A mix with a high percentage of air voids will have lower tensile strength. Additionally, higher air voids cause higher permeability, meaning more pathways for oxygen to get into the structure and accelerate age hardening. Aggregate properties do not seem to matter much directly in determining low-temperature cracking performance. Much of the impact of the aggregate in a mixture lies in how it affects volumetric proportioning and effective asphalt binder content. A densely packed gradation will not leave as much room in the asphalt mixture for air voids and asphalt binder. An absorptive aggregate could result in lower-than-expected asphalt binder content if the absorption is not properly considered during the mix design.
The fact that aggregate plays so little of a role in the low-temperature cracking performance of an asphalt mixture always seemed odd to me before I came to understand that it was related to the thermal expansion and contraction of the mix constituents. The linear coefficient of thermal expansion for aggregate used in asphalt mixtures appears to be in the range of 2×10-6 to 1×10-5 m/m per degree C.
The linear coefficient of thermal expansion for asphalt binder appears to be in the range of 3×10-4 m/m per degree C. That may not seem like much of a difference because the numbers are so small, but what it means is that asphalt binder will expand or contract at least 30 times more than aggregate as the temperature changes by 1°C. Considering the differential rate of contraction between the thin film of asphalt binder and the aggregate as temperature changes, it is easier to understand why the properties of the asphalt binder play an outsized role in low-temperature cracking.
How do we manage our mix designs to reduce the likelihood of low -cracking in service? “NCHRP Report 673, A Manual for Design of Hot Mix Asphalt with Commentary”, discusses, among other things, performance tests during the mix design phase that can be used to address various distresses. In that report, the authors note that in most cases, mix performance testing is not needed as part of the mix design process to ensure good low-temperature cracking performance. Within the context of the PG specification, the proper selection of an appropriate asphalt binder, based on the climate in which it will be used, is really the most important factor needed. If a mix is “unconventional,” then the authors recommend the use of low-temperature indirect tensile creep and strength testing (see AASHTO T322). I will not try to explain what “unconventional” means – see my earlier note about overexplaining – just know that you can always do the performance testing if you are not sure.
With all the concerns over the factors that can affect rutting, fatigue cracking, and durability cracking, it is nice to have an elegantly simple focus for low-temperature cracking. Or in haiku form…
Cold temperature?
Simply focus on binder…
Performance managed.
For questions or more information on the research, testing, and training services provided by the Asphalt Institute Laboratory, please contact Wes Cooper or me. Every service comes with a free 5-7-5…while supplies last.
Anderson is the Asphalt Institute Vice President of Research and Laboratory Services.
What is a haiku?
An unrhymed Japanese poetic form consisting of three lines with a 5-7-5 syllable structure, traditionally having a seasonal reference.
