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Because aggregate comprises approximately 95% of asphalt mixtures, it has a major effect on the performance of mixtures. The quality of filler and amount of filler greatly affect asphalt concrete performance. To insure satisfactory performance, procedures must be available that can be used to detect poor-quality mixtures and there must be techniques available for improving mixture properties.
The objective of this paper is to present data from various studies that show the effect of aggregate grading on performance of asphalt mixtures. Test methods to evaluate these mixture properties are discussed.
A well-graded, crushed aggregate should be used to provide the highest-quality asphalt concrete. Uncrushed aggregates such as natural sands and uncrushed gravels produce mixtures with lower stability and decreased durability, whereas a well-graded aggregate fits together more tightly during compaction, resulting in a lower required asphalt content and improved stability and durability.
The maximum aggregate size is important to control stability, skid resistance, and compactibility of the mixture. A larger maximum size aggregate produces a higher stability and usually requires less asphalt content. The larger size aggregate also provides for improved skid resistance. Compaction of thin layers is difficult when the maximum aggregate size is too small. The compacted lift thickness should be at least twice the maximum aggregate size.
The amount of mineral filler in a mixture greatly affects the overall mix quality. Since the filler tends to fill the voids in the mineral aggregates, an increase in filler results in a corresponding decrease in optimum asphalt content. Laboratory test results also indicate that a mixture design performed for an aggregate with higher filler content will result in a significantly higher stability than that for lower filler content. The type of filler used also affects the mixture properties.
The quality of the aggregate must be controlled to insure a quality asphalt concrete. Some properties that must be considered are maximum aggregate size, amount of crushed particles, and amount and quality of filler.
asphalt, bituminous concrete, compaction, density, filler, gradation, Gyratory Testing Machine (GTM), index, plasticity, rutting, shear, size, skid resistance, stability, stripping tendencies, voids in mineral aggregate (VMA)
Research civil engineer, Waterways Experiment Station, Vicksburg, MS
Consultant, president, EDCO Inc., Vicksburg, MS
Assistant research and development engineer, Jackson, MS