Relationships of HMA In-Place Air Voids, Lift Thickness, and Permeability

TRB’s National Cooperative Highway Research Program (NCHRP) Web Document 68: Relationships of HMA In-Place Air Voids, Lift Thickness, and Permeability is a four-volume report that evaluates the minimum hot-mix asphalt (HMA) lift thicknesses and minimum in-place density necessary to achieve an impermeable, durable pavement.  The report also evaluates enhancements to AASHTO T166 (Bulk Specific Gravity of Compacted Bituminous Mixtures Using Saturated Surface-Dry Specimens) designed to achieve a precise, uniform, and more accurate determination of the bulk specific gravity of compacted HMA specimens. A summary of this report was published as NCHRP Report 531.

• Volume one examines  the minimum thickness to nominal maximum aggregate size (t/NMAS) ratio needed for desirable hot-mix asphalt (HMA) pavement density levels to be achievable; evaluates the permeability characteristics of compacted samples at different thicknesses; and explores the factors affecting the relationship between in-place air voids, permeability, and lift thickness.

• Volume two compares the American Association of State Highway Officials (AASHTO) T166 with other methods of measuring bulk specific gravity to determine under what conditions AASHTO T166 is accurate, evaluates whether improvements to AASHTO T166 are necessary to achieve a more accurate measure of bulk specific gravity, and examines other methods of measuring bulk specific gravity.

• Volume three evaluates the  relationships between permeability, lift thickness, and in-place density in the field.

• Volume four is the appendix of the report.