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