By Mike Anderson, P.E.
Little known fact about “Casablanca” – Humphrey Bogart’s line wasn’t intended to be “We’ll always have Paris. We didn’t have, we lost it until you came to Casablanca. We got it back last night.”
In football terms, he called an audible and changed it from the previously undiscovered original script. The quote should have been “We’ll always have Paris and his Law. We didn’t have, we lost it until Erdogan joined him. Now we can model crack propagation.” What a shame that he changed the line; the movie could’ve been a classic.
In the original models for Superpave performance analysis, the Paris Law (or Paris Erdogan Law) for linear elastic fracture mechanics was used as the basis for crack propagation or the rate of growth in crack length. It is also important in several of the more recent tests for asphalt mixture cracking – a subject in which there is currently a great deal of interest in the United States. I think the following sums up the way most asphalt technologists feel today:
“Asphalt mix designs are becoming more complex due to the increased use of recycled materials, recycling agents, binder additives and modifiers, and WMA technologies. These changes have altered the performance of mixtures both positively and negatively so that volumetric mix design alone is not sufficient for evaluating the potential behavior of asphalt mixtures, especially cracking behavior. Thus, there is an urgent need to establish and implement reliable performance tests that can be used to eliminate brittle mixes or provide input to models that predict asphalt pavement cracking.” – NCHRP Research Results Digest 399, August 2016
There are many tests that are currently in use for evaluating the laboratory cracking performance of asphalt mixtures. The NCHRP 09-57 research report (the basis for the Research Results Digest quote on page 42) does a great job at outlining the various tests and their utility. It would be impossible to adequately cover all the tests in a few short paragraphs, so instead I’ll focus on summarizing the three versions of the Semi- Circular Bend (SCB) test that are in use by researchers and some agencies today. For information on the Texas Overlay Test, I’d refer you to the article by John D’Angelo in the October 2017 “Asphalt” magazine.
Semicircular Bend (SCB) at Low Temperature
Method: AASHTO TP 105, “Determining the Fracture Energy of Asphalt Mixtures Using the Semicircular Bend Geometry (SCB)
Temperature(s): PG Low+10°C and PG Low-2°C
Geometry: Semicircular; 150-mm diameter, 25-mm thick; single notch length (15-mm)
Loading: Monotonic; Displacement rate of 0.0005 mm/s based on Crack Mouth Opening Displacement (CMOD) gauge
Output: Fracture energy (Gf), Stiffness (S), Fracture toughness (Kic)
Semicircular Bend at Intermediate Temperature – Louisiana (SCB-LTRC)
Method: Louisiana Transportation Research Center Draft Standard, “Evaluation of Asphalt Mixture Crack Propagation using the Semi-Circular Bend Test (SCB) at Intermediate Temperature”
Temperature(s): 25°C
Geometry: Semicircular; 150-mm diameter, 57-mm thick; three nominal notch lengths (25.4-mm, 31.8-mm, and 38.1-mm)
Loading: Monotonic; Displacement rate of 0.5 mm/min
Output: Critical Strain Energy Release Rate (Jc)
Semicircular Bend at Intermediate Temperature – Illinois (SCB-IL)
Method: AASHTO TP124, “Determining the Fracture Potential of Asphalt Mixtures using Semicircular Bend Geometry (SCB) at Intermediate Temperature”
Temperature(s): 25°C
Geometry: Semicircular; 150-mm diameter, 25-50 mm thick; single notch length (15-mm)
Loading: Monotonic; Displacement rate of 50 mm/min based on Load Line Displacement (LLD)
Output: Fracture energy (Gf), Secant Stiffness (S), Post-peak slope (m), Displacement at peak load (w0), critical displacement (w1), Flexibility Index (FI)
Whether a version of the SCB, or another of the cracking tests, the important thing is that users start evaluating their asphalt mixtures.
The industry needs a cracking test. Researchers provide a cracking test. I think this is the beginning of a beautiful friendship.
Anderson is the Director of Research and Laboratory Services for the Asphalt Institute.