About the research
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 t op-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.