SGC Tests for Influence of Material Composition on Compaction Characteristic of Asphalt Mixtures
Compaction characteristic of the surface layer asphalt mixture (13-type gradation mixture) was studied using Superpave gyratory compactor (SGC) simulative compaction tests. Based on analysis of densification curve of gyratory compaction, influence rules of the contents of mineral aggregates of all sizes and asphalt on compaction characteristic of asphalt mixtures were obtained. SGC Tests show that, for the mixture with a bigger content of asphalt, its density increases faster, that there is an optimal amount of fine aggregates for optimal compaction and that an appropriate amount of mineral powder will improve workability of mixtures, but overmuch mineral powder will make mixtures dry and hard. Conclusions based on SGC tests can provide basis for how to adjust material composition for improving compaction performance of asphalt mixtures, and for the designed asphalt mixture, its compaction performance can be predicted through these conclusions, which also contributes to the choice of compaction schemes.
OPTIMIZATION OF CONSTRUCTIBILITY AND RESISTANCE TO TRAFFIC: A NEW DESIGN APPROACH FOR HMA USING THE SUPERPAVE COMPACTOR
Traditional mixture design methodologies for HMA are mainly focused on using a standardized compaction effort to evaluate volumetrics and simple mechanical properties at a target volumetric proportioning. The more recently introduced Superpave volumetric design procedure deviates slightly from this tradition by evaluating the overall densification characteristics and setting target limits of air voids at more than a single compaction effort. The Superpave Gyratory Compactor (SGC) presents a convenient way to evaluate the densification characteristics of asphalt mixes. The current method of interpretation of the results from the SGC and the design criteria are however biased towards the performance under traffic and do not give proper consideration for constructibility of mixtures. It is believed that the SGC data is underutilized in mixture design. This study is an attempt to take a new look at mixture design requirements using the SGC results to optimize the densification characteristics under construction and densification characteristics under traffic.
DEVELOPMENT OF A MIXTURE DESIGN PROCEDURE FOR STONE MATRIX ASPHALT (SMA)
Stone Matrix Asphalt (SMA) has been used successfully in Europe for over 20 years to provide better rutting resistance and to resist studded tire wear. Since 1991, the use of SMA has increased steadily in the United States. At present, some states routinely use SMA even though a standard mixture design procedure is not available. A mixture design procedure that provides guidance on material properties, aggregate gradation, determination of optimum asphalt content, and mixture properties is needed. This paper presents a mixture design procedure for SMA mixtures developed by the National Center for Asphalt Technology. Data for the development of the procedure were collected from a laboratory study conducted with various samples of aggregates, fillers, asphalt binders, and stabilizing additives. Compacted mixtures were tested to evaluate the effects of aggregate structure, asphalt binder, and binder-fine aggregate mortar. Specific conclusions from this study were: (1) The Los Angeles abrasion loss showed good correlation with aggregate breakdown. (2) It appeared that the 3:1 or 2:1 flat and elongated particles provided much better classification for the various aggregates than a 5:1 ratio. (3) The flat and elongated particle ratio showed excellent correlation with aggregate breakdown. (4) In a SMA mix, the percent passing the 4.75 mm sieve must be below 30% to ensure proper stone-on-stone contact. (5) The percent passing the 0.02 mm sieve did not show a correlation with mortar stiffness. However, the dry compacted volume, as obtained from the Penn State test method, did show a good correlation with mortar stiffness and can be utilized to characterize the shape of fillers. Generally, a more angular filler tends to produce a higher air voids result in this test. (6) In-place results from about 86 projects showed that very little rutting has occurred in SMA pavements constructed in the U.S. since 1991. However, for the pavements with air voids falling below the 3% range, some rutting was observed. (7) A voids in mineral aggregate (VMA) significantly lower than specified VMA can be obtained due to aggregate breakdown. Hence, the mix designer must consider aggregate type, compactor type and compactive effort along with the gradation in meeting the required VMA criteria. Specifying a minimum asphalt content can result in different requirements for aggregates with different specific gravity. (8) Fifty blows of Marshall hammer were found to be approximately equal to 100 revolutions of the Superpave gyratory compactor in terms of resultant density. The Superpave gyratory compactor was found to produce less aggregate breakdown than the Marshall hammer. (9) Fiber stabilizers were found to be more effective in reducing draindown than polymer stabilizers. However, mixes modified with polymer showed better resistance to rutting in laboratory wheel tracking tests.