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1 The ratio of the lift thickness to nominal maximum aggregate size (NMAS) is critical to achieving adequate asphalt mixture compaction during construction, which in turn has a profound impact on asphalt pavement performance and durability. If the lift thickness is inadequate, there is not enough room for the aggregate particles to be reoriented and densi- fied; this can be exacerbated by rapid cooling of thin lifts, which shortens the time available for compaction. If the lift is too thick, the material at the bottom of the lift may be beyond the zone of influence of the compactor and thus may not be adequately densified. While the need for a balance of lift thickness to mix size had been recognized historically, the issue was subject to increased scrutiny during the early days of Superpave® implementa- tion, most notably by the Florida Department of Transportation (DOT). In 1996, the Florida DOT began implementing Superpave mix designs on a number of pilot projects. As in most other states, these mixtures were more coarsely graded than their conventional Marshall mixes. The Florida DOT and its contractors experienced difficulty in compacting these coarse mixes, resulting in higher permeability than with their fine graded mixes. To address this issue, the Florida DOT changed the density requirement for coarse graded Superpave mixtures and increased the minimum lift thickness, among other specification changes. Many states have now moved toward the use of finer gradations and in some cases smaller NMAS mixtures, which make it somewhat easier to obtain compaction, but the ratio of lift thickness to NMAS is still an important consideration. NCHRP Report 531, published in 2004, documented the key relationships between lift thickness, air voids or density, and permeability. The authors recommended, among other things, a ratio of lift thickness to NMAS (t/NMAS) of at least 3:1 for fine graded mixtures and at least 4:1 for coarse graded mixtures. This synthesis was conducted to summarize the state of the practice regarding the effects of lift thickness on ultimate pavement performance. The t/NMAS ratio has been shown in many research efforts, as well as in practice, to have a major effect on the ability to ade- quately compact an asphalt mat. This effect on density in turn affects permeability, moisture damage, cracking, and more. This synthesis includes a comprehensive review of relevant literature. In addition, U.S. and Canadian transportation agencies were surveyed to determine the state of their practices, policies, and experiences related to the effects of lift thickness. Paving con- tractors and paving associations were polled to assess the effects of agency policies on the contractorsâ operations and ability to achieve adequate compaction. The synthesis concludes with case examples of agencies and industry to illustrate various aspects of and approaches to dealing with the effects of lift thickness, density, and ultimate pavement performance. S U M M A R Y Impact of Asphalt Thickness on Pavement Quality
2 Impact of Asphalt Thickness on Pavement Quality Asphalt surfaced roadways comprise the vast majority of the nationâs roadway network. As states continue to deal with critical infrastructure needs and limited budgets, ensuring a long service life from asphalt pavements is critical. The most important factor affecting asphalt pavement life is density. Therefore, understanding how the lift thickness and nomi- nal maximum aggregate size in the asphalt mix affect the ability to adequately densify the pavement is crucial.
