A NATIONAL STUDY OF RUTTING IN HOT MIX ASPHALT (HMA) PAVEMENTS

The objectives of this study were 1) to identify the material properties, mix design parameters and construction procedures that affect rutting, 2) to provide information necessary to produce hot mix asphalt (HMA) mixtures that will perform satisfactorily, and 3) to provide information to identify those mixes with a tendency to rut under today's heavy traffic loadings. Forty-two pavements were sampled from fourteen states across the United States. Rut depth measurements were made across each pavement to quantify the amount of rutting occurring at each site. The mix design information, construction records and traffic counts were also obtained. A detailed laboratory testing program was performed on samples of the asphalt mixture from these rutted and good performing pavements. The data were analyzed to determine material and mixture properties and to identify procedures that are necessary for construction of rut resistant HMA pavements. Among the conclusions from this study are the following: (1) 69% of the pavements evaluated that were designed with the Marshall method utilized a 50-blow compactive effort. This resulted in high asphalt content and led to low in-place voids after traffic and subsequently rutting. (2) 33% of the sites had no construction history available, only 38% of the sites utilized laboratory compacted samples of the asphalt mixtures from the mixing plant during construction to verify that the air voids were within an acceptable range, and 53% of the sites that measured voids in laboratory compacted samples had voids less than 3%. (3) Most of the rutting occurred in the top 3-4 inches of the HMA. (4) In-place air void contents above 3.0% are needed to decrease the probability of premature rutting throughout the life of the pavement. (5) The shear strength of the recompacted mix as indicated by the GTM roller pressure had the best correlation with rutting of any single factor. (6) If the in-place air voids are above 2.5%, the angularity of the aggregate as measured by percent of coarse aggregate (plus No. 4) with 2 or more crushed faces and NAA Uncompacted Voids for the fine aggregate (passing No. 4) are highly correlated to rate of rutting. (7) The properties of the asphalt cements extracted from the mixtures are not closely related to rutting. (8) A rate of rutting of 0.00023 in. per square root ESALs delineated between good and rutted pavements for the pavements evaluated. (9) Rutting on high volume roadways can be prevented if angular coarse and fine aggregates are used and if the air voids in the mixture do not fall below approximately 3.0%.