Earlier this year, I had some minor surgery that didn’t quite go as planned. I won’t go into all the details but suffice it to say that you don’t want to hear the phrase “unexpected hemorrhaging” from your doctor. The lower blood volume issue got fixed relatively quickly with IV fluids. What did not get fixed as quickly was the lower amount of hemoglobin in my blood. Adding fluids helped, but it was like adding water to a pot of soup; the volume increased but the composition did not change. The consequence was that I felt really fatigued for a while until my blood chemistry improved.
While I recovered, I considered the similarity between how and why I experienced fatigue and the way asphalt materials experience fatigue. My spouse considered a lost opportunity to test my cognitive processes.
Fatigue cracking is one of the more complicated distresses in asphalt pavements to address, with several materials, design and construction factors that can affect the ability of an asphalt pavement to resist fatigue damage. It is not all about material properties; if a pavement section is designed with inadequate thickness for the expected loading, even our most well-designed and well-constructed mixtures may not succeed.
So, what affects fatigue cracking? As an example, consider the fatigue life equation developed by the Asphalt Institute for the MS-1 manual on pavement design, shown in the following equation:
(Note: other equations are available that describe fatigue life, but they generally use the same variables, so the impacts are similar.)
Nf = A * 10(4.84*(VFA-0.69)) * εt -3.291 * |E*mix|-0.854
Where:
Nf = Number of load applications (ESALs) that can occur until reaching the asphalt mixture fatigue life
A = Constant that includes a shift factor related to the allowable cracked area (usually A = 6.167E-05 * 18.4 to represent 10% cracked area)
VFA = Volume of voids filled with asphalt expressed as a decimal, not a percentage
εt = Tensile strain in the asphalt layer, mm/mm
|E*mix| = Dynamic modulus (stiffness) of the mix, MPa
What are the implications of changes in those variables?
Fatigue life decreases as tensile strain increases.
The more you allow the mix to flex, the quicker it will fail. Think of straightening a paper clip and bending it back and forth. If you bend it a lot repeatedly, it will break quickly. If you bend it a little, you can probably go on doing so for a while before it breaks. The thickness of the asphalt pavement system affects the tensile strain. Thicker is better.
Fatigue life decreases as the VFA decreases below 69% (0.69).
VFA is a measure of how much effective binder (not including the asphalt that gets absorbed into the stone) fills the total space available for asphalt and air. Many mixes designed to have 4% air voids at the minimum VMA or higher (based on the NMAS of the mix) will have lab-mixed, lab-compacted VFA values greater than 69%. Higher asphalt binder content and lower air voids are better.
Fatigue life decreases as mix stiffness increases.
Mix stiffness is affected by a number of things, but the asphalt binder properties certainly have a role. Stiffer asphalt binders lead to stiffer asphalt mixtures. Increased aging of the asphalt binder leads to stiffer asphalt mixtures. Using the right asphalt binder for the environment in which the mixture is expected to perform is important.
Of course, traditional fatigue equations, like the one shown, were developed before the widespread use of modified asphalt binders. Many modified asphalt binders will exhibit higher stiffness but also greater strength and ability to resist the development of strain, leading to better fatigue performance in service. There are other characteristics of asphalt binders that are not accounted for in this equation that affect relaxation, strain tolerance, and strength.
In addition to the stiffness of the components, mix stiffness is also affected by the volume of asphalt binder in the mix. Asphalt binder, even modified asphalt binder, is much less stiff than aggregate, so increasing asphalt binder content decreases mix stiffness.
Both later points about mix stiffness highlight the importance of properly considering the properties of any RAP added to the mix. If we do not account properly for the effective volume of asphalt binder in RAP (referred to as RBA), we run the risk of using too little total asphalt binder content which reduces VFA, resulting in decreased fatigue life. If we do not account properly for the stiffness of the effective RAP binder by using a softer virgin asphalt binder and/or recycling/ rejuvenating agents, then we will see increased mix stiffness…resulting in decreased fatigue life.
Toward the end of my recovery, I shared this thought with my family: “We are just like asphalt…we age, become stiffer and lose flexibility. When our hemoglobin is low relative to our blood volume, we become fatigued, just like when an asphalt mixture has a lower VFA.”
The implication of that revelation is that my cognitive testing appointment has been scheduled.
For questions or information on the research, testing, and training services provided by the Asphalt Institute Laboratory, please contact Wes Cooper, Amma Agbedor or me. We bleed asphalt.
Anderson is the Asphalt Institute Vice President of Research and Laboratory Services.
