The National Asphalt Pavement Association (NAPA) has published some general rules Based for HMA mix type use, which are summarized in Table 1. Notice that dense-graded HMA is generally appropriate for all uses, SMA and OGFC are typically used as surface courses on high volume roads and ATPB is usually used for base courses on high volume roads. Keep in mind that Table 1 is just a summary of general guidance and that there are, as always, case specific exceptions.
|Low Traffic||Medium Traffic||High Traffic|
|< 300,000 ESALs||300,000 - 10 million ESALs||> 10 million ESALs|
|= Moderately Appropriate|
|empty||= Not Appropriate|
Note: Before deciding to use ATPB, the Pavement Research Center’s research results should be carefully considered.
Determining Appropriate Mix Types
Most of this process is taken directly from the NAPA HMA Pavement Mix Type Selection Guide (2001).
- Determine the total thickness of HMA required. This is accomplished using an appropriate structural design procedure.
- Determine the types of mixtures appropriate for the surface course based on traffic and cost.
- From Table 2, identify the general traffic category for the pavement in question then select those mix types that are appropriate for the surface course.
- Determine what aggregate size to use for a mix. In general, the higher the traffic loads, the higher the nominal maximum aggregate size should be.
- Consider appearance. Mixes with larger aggregates often have a coarser surface texture and may be more susceptible to segregation during placement. Therefore, for a city street where appearance is an issue, a finer mix such as a 9.5 or 12.5-mm (0.375 or 0.5-inch) dense-graded mix may be appropriate. Conversely, for a heavy industrial area where load resistance is much more important that aesthetic appearance, a 19.0-mm (0.75-inch) mix may be more appropriate. However, never sacrifice performance for appearance.
- Consider traffic flow. Maximum aggregate size can also affect traffic flow during rehabilitation of existing roadways. In many urban areas off-peak construction is used to minimize traffic impacts. However, for a road to be released to traffic during peak hours, either the lane drop-off (elevation difference between adjacent lanes) must be kept below a specified minimum value (typically less than 37.5 mm (1.5 inches) with proper signage) or all lanes must be brought to the same elevation. Bringing all lanes to the same elevation at the end of each paving day may require changing traffic control and moving paving equipment, which can increase construction costs and decrease safety. Therefore it is often better to satisfy the lane drop-off requirement. However, with larger aggregate mixes the minimum lift thickness may exceed the maximum lane drop-off allowed. As a result, using a finer gradation may allow paving one lane, then releasing the road to traffic, then paving the other lane. Again, do not sacrifice performance.
- Subtract the surface course thickness from the total thickness and determine what mix or mixes are appropriate for the intermediate and/or base courses using Table 2.
- Continue to subtract intermediate/base course thicknesses from the total thickness until mixes and layer thicknesses have been selected for the required pavement section.
Footnotes (↵ returns to text)
- National Asphalt Pavement Association (NAPA). (2001). HMA Pavement Mix Type Selection Guide, Information Series 128. National Asphalt Pavement Association. Landham, MD.↵