Robert S James

Effect of Aggregate Degradation on Volumetric Properties of Georgia's Hot-Mix Asphalt

Various degrees of degradation of aggregate particles can occur in hot-mix asphalt (HMA) production during the mixing and compaction process. This breakdown may significantly increase the dust fraction of the aggregate gradation relative to that of laboratory specimens. This increased dust fraction could cause a loss of voids in the mineral aggregate (VMA) and air voids. After evaluation of 22 variations of HMA mixtures during production and construction, it was determined that aggregate degradation did result from plant mixing and field compaction activities. Aggregate degradation varied somewhat with respect to where the mixture was sampled within the production and construction processes. Aggregate degradation resulting from binder extraction that uses the ignition method, relative to the solvent extraction method, was also examined. Mixture volumetric properties (i.e., air voids and VMA) were significantly affected by aggregate degradation. On the basis of this study, before a volumetrics specification for mixture acceptance is investigated for future implementation in Georgia, a change in the Georgia Department of Transportations mix design specification is recommended. The minimum VMA values for Superpave® mix design should be increased by 1% for all dense-graded mixes to compensate for the amount of aggregate degradation and loss of VMA during HMA production and construction. It is believed that this research-based policy change for mix design will better maintain the balance between proper durability and rut resistance of asphalt pavements in Georgia.

Development of Mix Design Criteria for 4.75-mm Superpave® Mixes

The Superpave® mix design system includes design criteria for a range of mixes with nominal maximum aggregate sizes (NMASs) between 9.5 and 37.5 mm. Many agencies in the United States have expressed an interest in using a 4.75-mm NMAS mix. Such a mix could provide benefits; that is, it should provide a very smooth riding surface, could be used for thin lift applications, could correct surface defects (leveling), could decrease construction time, could provide a use for manufactured screening stockpiles, and could provide a very economical surface mix for facilities with low traffic volumes. A study was done to establish the standard mix design criteria needed for 4.75-mm mixes. On the basis of the findings of the study, the recommended Superpave mix design criteria include a specified gradation control of 30% to 54% passing on the 1.18-mm (No. 16) sieve and 6% to 12% passing the 0.075-mm (No. 200) sieve. During design, the following were recommended: a design air void content of 4%, minimum voids in mineral aggregate (VMA) of 16% for all traffic levels, maximum VMA of 18% for mix designs with more than 75 gyrations, voids filled with asphalt (VFA) of 75% to 78% for mix designs with 75 gyrations and above, and VFA of 75% to 80% for mix designs with 50 gyrations. The results of the study showed that 4.75-mm NMAS mixes can be successfully designed. These types of mixes should provide economical mixes for low-volume roadways.

State of Knowledge for Cantabro Testing of Dense Graded Asphalt

AbstractThis paper uses information collected over several years to determine where the Cantabro test usefully characterizes dense graded asphalt (DGA). The Cantabro mass loss (CML) procedure is si...

Bottom-Up Fatigue Cracking of Low-Volume Flexible Pavement Analysis from Instrumented Testing

This paper provides general design guidance for bottom–up fatigue cracking of thin flexible pavements. Throughout the paper the need for the data is shown alongside the methods for obtaining the data and potential uses of the findings. The data were gathered as part of a study to evaluate geosynthetics in thin flexible pavements. Data were collected from strain gauges, pressure cells, and thermocouples in instrumented sections on a low-volume road in Arkansas after approximately 2,100 passes of controlled traffic with an FHWA Category 5 vehicle. More than 30,000 asphalt strain measurements were obtained. The distribution of asphalt strain measurements was heavily skewed, while the distribution of pressure measurements in the base layer was normal. Finite element analysis was used to account for temperature effects, which made the asphalt strain distributions fairly normal. Plots of damage were developed using transfer functions, finite element modeling, and the measured strains. The plots are central to the design approach, which improves the state of knowledge in mechanistic–empirical low-volume road design.

Development of Design Gyration Levels for Airfield Asphalt Pavement

The objective of this paper is to provide guidance for adapting Superpave gyratory compactor (SGC) procedures to design airfield asphalt mixes (specifically to select the design asphalt content) with comparable properties to Item P-401 protocol. Three methods were used for selecting the SGC design compactive effort: (1) evaluate in-place density in the general manner used during development of the Marshall mix design airfield procedure; (2) compare specimen bulk specific gravities compacted with the Marshall hammer and SGC; and (3) test airfield mixes using confined repeated load permanent deformation testing to determine the binder content at which the mixtures become unstable. The result of this analysis was a gyration level related to tire pressure.