AC on AC overlay

10 weeks ago by
When performing a pavement design using AASHTOWare Pavement ME for AC on AC Overlay, the pavement distress prediction for bottom-up (alligator) cracking and AC thermal cracking seem to give false results. First, the Reliability for both these distresses seems fixed at 50%. It does not allow me to increase the value.
Secondly, if I go ahead and run the program for 10 years the two distresses almost always remain zero (or close to zero).

Any ideas on this?

1 Answer

10 weeks ago by
Please remember, you can change the reliability for total fatigue cracking and total transverse cracking which includes the cracking in the existing AC layer that reflects through the AC overlay and new cracks that develop in the overlay. The reliability is set at 50 percent for new fatigue cracking in the AC overlay and new transverse cracks in the AC overlay. The reason the reliability is set to 50 percent is because it is impossible to separate new crack development in the AC overlay from the reflective fatigue and transverse cracks in the existing AC layer that reflect through the overlay. As such, the reliability is applied to the total fatigue and total transverse cracks which combines new crack development with reflective cracks.

To provide an answer or opinion to the second part of your question, more information is required. AC overlay thickness and the location of the site in terms of climate is important relative to version 2.3.1. If the site is located in the south or in a mild climate, version 2.3.1 will almost never calculate transverse cracks – they will be close to “0” when using the global calibration coefficient for transverse cracks. If the AC overlay thickness is relatively thick, then it will take a long time for additional fatigue cracks to develop in the existing AC layer because the tensile strains are significantly reduced. The version to be released in July of this year has a coefficient for transverse cracks that is climate dependent.

Thank you for the answer.
So from what I understand, it would just make sense to look at total cracking (reflective+fatigue/thermal) to check the performance of the pavement over the design period.
This would depend a lot upon the parameters inputted for the existing pavement condition and quantification of the distresses therein. This information coupled with the modulus from FWD testing would give the best picture for an overlay design.

As for the second part, I have done most design analyses in regions of North Indiana (fairly cool) with overlay thicknesses as thin as 1 to 1.5 inches. Running these for almost 9-10 years has also yielded unrealistically low values (almost 0) for fatigue cracking and thermal cracking. But, as you mentioned, the fatigue and thermal cracking by themselves may not be the best parameters for evaluation.

written 9 weeks ago by Sagar Shah  
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