By Pamela Turner and Christine Hall
You may have heard of a new asphalt binder parameter for evaluating age-related cracking potential. Pronounced delta tea see, it’s spelled out ΔTc. It’s defined as the numerical difference between the low continuous grade temperature determined from the Bending Beam Rheometer (BBR) stiffness criteria (which is the temperature at which stiffness, S, equals 300 MPa) and the low continuous grade temperature determined from the BBR m-value (which is the temperature at which m equals 0.300).
If you aren’t sure how to determine low temperature continuous grades, you can find instructions in ASTM D7643, Standard Practice for Determining the Continuous Grading Temperature and Continuous Grades for PG Graded Asphalt Binders. Don’t forget to subtract 10°C from your BBR test temperatures when determining low temperature continuous grades.
For example, let’s say a set of BBR tests yielded the following results at two test temperatures:
-18°C : Stiffness = 243 MPa and m-value = 0.309
-24°C : Stiffness = 400 MPa and m-value = 0.256
Using these results, the low continuous grade temperature for the stiffness criteria (Tcont, S) equals -30.5°C, and the low continuous grade temperature for the m-value criteria (Tcont, m) equals -29.0°C.
Once you have the temperatures, subtract Tcont, m from Tcont, S to get the value of ΔTc.
For this example:
ΔTc = -30.5 (-29.0) = -1.5.
It’s that simple. The ΔTc parameter can be measured on any asphalt binder, whether it’s a virgin asphalt binder or binder that’s been extracted and recovered from a sample of asphalt mix.
Put it to Work
Now that you know what ΔTc is and how it’s calculated, let’s discuss how it relates to asphalt pavement performance, and what it shows us that we can’t get from BBR stiffness and m-value results alone. Asphalt Institute Engineer Mike Anderson first proposed ΔTc in 2011 to measure the ductility loss of aged asphalt binder as part of a study examining relationships between asphalt binder properties and non-load related cracking. In particular, the study focused on finding a parameter to explain block cracking in airport pavements.
Block cracking is a non-load related cracking phenomenon similar to thermal cracking that causes cracks to develop in both longitudinal and transverse directions. This results in a square or “block” pattern. Block cracking is most often seen in significantly aged pavements with low traffic volumes. The lack of traffic allows the asphalt binder to develop a type of internal structure (thixotropic hardening) that will exhibit brittle behavior when exposed to thermal stresses. Although it’s similar to thermal cracking, studies have shown that block cracking may be more dependent on the age of the asphalt pavement than on the environmental conditions. In other words, an older pavement that does not experience environmental conditions that would cause thermal cracking may still experience block cracking.
Ductility is defined as the ability of a material to be stretched without breaking. This is important in flexible pavements because the thin films of asphalt binder between the aggregate particles must have a certain amount of ductility to withstand stresses in the pavement due to traffic or temperature changes.
Prior to the Superpave Performance Grading (PG) system, ductility was used as a surrogate relaxation parameter for asphalt binders and was considered a way to distinguish cracking performance. In general, research has shown that ductility may be an important factor in cracking performance as asphalt pavements age. In particular, pavements with low ductility binders tend to exhibit poor cracking performance, even when the overall asphalt binder stiffness values is similar to that of pavements that perform well. These observations imply that asphalt binder stiffness and relaxation may not change at the same rate due to aging and that the loss in relaxation (ductility) may have a more significant effect on cracking performance than the increase in stiffness does.
Our current Superpave PG system does not include a direct measure of ductility. Instead, it relies on relaxation parameters such as the phase angle measured by the Dynamic Shear Rheometer (DSR) at intermediate temperatures and the BBR m-value at low temperatures to predict cracking performance.
While these parameters are suitable for determining the relaxation properties needed for other types of cracking, they do not provide a relationship between stiffness and ductility that may be needed to control block cracking.
Other studies have shown that as some asphalt binders age, their low temperature relaxation properties, as measured by the BBR m-value, deteriorate significantly faster than their low temperature stiffness increases. This leads us back to the possible use of ΔTc. There is currently not an official criteria for ΔTc, but an unofficial minimum value of -5 has been proposed for asphalt binders extracted and recovered from mixes containing recycled asphalt shingles (RAS). Possible criteria for other binders are still being researched and evaluated.
In summary, the ΔTc parameter has been proposed as a relatively simple method for measuring the loss of relaxation properties of asphalt binders. While still in the evaluation phase, it could potentially be used as an asphalt binder parameter for predicting cracking performance of asphalt mixtures. Potential applications may include evaluating binders from mixtures containing recycled materials and evaluating the effectiveness of rejuvenators, recycling agents, etc.
Reprinted with permission from the Spring 2017 NCAT Newsletter.