Local calibration of the MEPDG prediction models for pavement rehabilitation and evaluation of top-down cracking for Oregon Roadways
The Oregon Department of Transportation (ODOT) is in the process of implementing the recently introduced AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) for new pavement sections. However, the vast majority of pavement work conducted by ODOT involves rehabilitation of existing pavements. Hot mix asphalt (HMA) overlays are the preferred rehabilitation treatment for both flexible and rigid pavements in Oregon. However, like new work sections, HMA overlays are also susceptible to fatigue cracking (alligator cracking and longitudinal cracking), rutting, and thermal cracking. Additional work was therefore needed to calibrate the design process for rehabilitation of existing pavement structures. 38 pavement sections throughout Oregon were included in this calibration study. A detailed comparison of predictive and measured distresses was made using the MEPDG released software Darwin M-E (Version 1.1). It was found that Darwin M-E predictive distresses did not accurately reflect measured distresses, calling for a local calibration of performance prediction models was warranted. Four distress prediction models (rutting, alligator cracking, longitudinal cracking, and thermal cracking) of the HMA overlays were calibrated for Oregon conditions. A comparison was made between the results before and after the calibration to assess the improvement in accuracy of the distress prediction models provided by the local calibration. While the thermal cracking model could not be calibrated, the locally calibrated models of rutting, alligator cracking, and longitudinal cracking provided better predictions with lower bias and standard error than the nationally (default) calibrated models. However, there was a high degree of variability between the predicted and measured distresses, especially for longitudinal cracking, even after the calibration. It is believed that there is a significant lack-of-fit modeling error for the occurrence of thermal cracks. The Darwin M-E calibrated models of rutting and alligator cracking can be implemented, however, it is recommended that additional sites, which would contain more detailed inputs (mostly Level 1 ), be established and be included in the future calibration efforts and thus, further improve the accuracy of the prediction models.
Recently, the Oregon Department of Transportation (ODOT) has identified hot mix asphalt concrete (HMAC) pavements that have displayed top-down cracking within three years of construction. The objective of the study was to evaluate the top-down cracked pavement sections and compare the results with the non-cracked pavement sections. Research involved evaluating six surface cracked pavements and four non-cracked pavement sections. The research included extensive field and laboratory investigations of the 10 pavement sections by conducting distress surveys, falling weight deflectometer (FWD) testing, dynamic cone penetrometer (DCP) testing, and coring from the cracked and non-cracked pavement sections. Cores were then subjected to a full laboratory-testing program to evaluate the HMAC mixtures and binder rheology. The laboratory investigation included dynamic modulus, indirect tensile (IDT) strength, and specific gravity testing on the HMAC cores, binder rheological tests on asphalt binder and aggregate gradation analysis. The FWD and DCP tests indicated that top-down cracked pavement sections were structurally sound, even some of the sections with top-down cracking showed better structural capacity compared to non-cracked sections. The study also found that top-down cracking initiation and propagation were independent of pavement cross-section or the HMAC thickness. The dynamic modulus testing indicated that cores from all the top-down cracked pavement sections except one section (OR 140) possessed stiffer mixtures than that of non-cracked pavement sections. All four non-cracked pavement areas were found to be exhibiting fairly high IDT strength, and low variability in IDT strength and HMAC density when compared to top-down cracked sections as indicated by the IDT strength tests and air void analysis. Asphalt binder rheological test result indicated that asphalt binders from all the top-down cracked sections except OR140 showed higher complex shear modulus (stiffer binder) compared to non-cracked pavement sections. The study concluded that top-down cracking could be caused by a number of contributors such as stiffer HMAC mixtures, mixture segregation, binder aging, low HMAC tensile strength, and high variability in tensile strength or by combination of any.