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1 CHAPTER 1 INTRODUCTION AND PROBLEM STATEMENT Proper compaction of hot mix asphalt (HMA) mixtures is Numerous factors can potentially affect the permeability of vital to ensure that a stable and durable pavement is built. For HMA pavements. In a study by Ford and McWilliams (5 ), it dense-graded mixes, numerous studies have shown that initial was suggested that particle size distribution, particle shape, in-place air voids should not be below approximately 3 percent and density (air voids or percent compaction) affect perme- nor above approximately 8 percent (1). Lower percentages ability. Hudson and Davis (6 ) concluded that permeability is of in-place air voids can result in rutting and shoving, while dependent on the size of air voids within a pavement, not just higher percentages allow water and air to penetrate into the the percentage of voids. Research by Mallick et al. (7 ) has also pavement, leading to an increased potential for water dam- shown that the nominal maximum aggregate size (NMAS) age, oxidation, raveling, and cracking. Low in-place air voids and lift thickness for a given NMAS affect permeability. are generally the result of a mix problem while high in-place Work by FDOT indicated that lift thickness can have an voids are generally caused by inadequate compaction. influence on density and hence permeability (8). FDOT con- Many researchers have shown that increases in in-place air structed numerous pavement test sections on Interstate 75 void contents have meant increases in pavement permeability. that included mixes of different NMAS and lift thicknesses. Zube (2) showed in the 1960s that dense-graded pavements Results of this experiment suggested that increased lift thick- become excessively permeable when in-place air voids exceed nesses could lead to better pavement density and hence lower 8 percent. Brown et al. (3) later confirmed this value during the permeability. 1980s. However, due to problems associated with coarse- Thus permeability, lift thickness, and air voids are all inter- graded mixes (those with a gradation passing below the related. Permeability has been shown to be related to pave- maximum density line), the size and interconnectivity of ment density (in-place air voids). Increased lift thickness has air voids have been shown to greatly influence permeability. A been shown to allow desirable density levels to be more eas- study conducted by the Florida Department of Transportation ily achieved. Westerman (9), Choubane et al. (4), and Mus- (FDOT) (4) indicated that coarse-graded Superpave mixes selman et al. (8) have suggested that a thickness to NMAS can sometimes be excessively permeable to water even when ratio (t/NMAS) of 4.0 is preferred. Most guidance recom- in-place air voids are less than 8 percent. mends that a minimum t/NMAS of 3.0 be used (10). How- Permeability is also a major concern in stone matrix asphalt ever, due to the potential problems of achieving the desired (SMA) mixes that utilize a gap-graded coarse gradation. Data density, it is believed that this ratio should be further evalu- have shown that SMA mixes tend to become permeable when ated based on NMAS, gradation, and mix type (Superpave air voids are above approximately 6 percent. and SMA).