HMA Mixture Types

Most HMA mixtures can be broadly categorized as one of three types: dense-graded, gap-graded or open-graded (Figure 3). While the names are drawn from the aggregate gradation used, their purpose, materials and design methods also tend to differ.

In general, when strictly followed the Superpave mix design procedure produces dense-graded HMA. However, aspects of the Superpave mix design procedure as well as the PG asphalt binder grading system are frequency used in gap-graded and open-graded mix design. This section provides a brief overview of each mixture type.

Dense-Graded

A dense-graded mix is a well-graded HMA mixture intended for general use (Figure 4).   When properly designed and constructed, a dense graded mix is relatively impermeable.  Dense-graded mixes are generally referred to by their nominal maximum aggregate size. 

Purpose:		Dense-graded mixes are suitable for most pavement uses and for all traffic conditions.
Materials:		Well-graded aggregate, asphalt binder (with or without modifiers), RAP
Other Info:		Dense-graded HMA is by far the most common type of HMA.

Stone Matrix Asphalt (SMA) (Gap-Graded)

SMA is a gap-graded HMA that is designed to maximize deformation (rutting) resistance and durability by using a structural basis of stone-on-stone contact (Figure 5).  Because the aggregates are all in contact, rut resistance relies on aggregate properties rather than asphalt binder properties.  Since aggregates do not deform as much as asphalt binder under load, this stone-on-stone contact greatly reduces rutting.  SMA is generally more expensive than a typical dense-graded HMA because it requires more durable aggregates, higher asphalt content and, typically, a modified asphalt binder and fibers.  In the right situations it is cost-effective because of its increased rut resistance and improved durability.  SMA, originally developed in Europe to resist rutting and studded tire wear, has been used in the U.S. since about 1990.

Purpose:		Improved rut resistance and durability.  Therefore, SMA is almost exclusively used for surface courses on high volume interstates and U.S. roads.
Materials:		Gap-graded aggregate (usually from coarse aggregate, manufactured sands, fibers and mineral filler all combined into a final gradation), asphalt binder (typically with a modifier)
Other Info:		It is important to avoid the tendency for the asphalt binder to to drain off the aggregate and down to the bottom - a phenomenon known as "mix draindown".  Mix draindown is usually avoided by adding cellulose or mineral fibers to keep the asphalt binder in place.  Cellulose fibers are typically shredded newspapers and magazines, while mineral fibers are spun from molten rock.  A laboratory test is run during mix design to ensure the mix is not subject to excessive draindown. In mix design a test for voids in the coarse aggregate (AASHTO T 19) is used to ensure there is stone-on-stone contact. Other reported SMA benefits include wet weather friction (due to a coarser surface texture), lower tire noise and less severe reflective cracking.  Mineral fillers and additives are usually added to minimize asphalt binder drain-down during construction, increase the amount of asphalt binder used in the mix and to improve mix durability.

Open-Graded

An open-graded HMA mixture uses open-graded aggregate and is designed to be water permeable (dense-graded and SMA mixes usually are not permeable) (Figure 6).  There are three types of open-graded mixes typically used in the U.S.:

1. Open-graded friction course (OGFC).  Typically more than 15 percent air voids, no minimum air voids specified, lower aggregate standards than PEM.
2. Porous European mixes (PEM).  Typically 18 - 22 percent air voids, specified minimum air voids, higher aggregate standards than OGFC and requires the use of asphalt binder modifiers. 
3. Asphalt treated permeable bases (ATPB) (Figure 7) .  Used in HMA pavement as a drainage layer. Less stringent specifications than OGFC or PEM.

Purpose:		OGFC and PEM are used as surface courses only.  They reduce tire splash/spray in wet weather and typically result in smoother surfaces than dense-graded HMA.  Their high air voids trap road noise and significantly reduce tire-road noise. ATPB is used as a drainage base course.
Materials:		Aggregate (crushed stone or gravel and manufactured sands), asphalt binder (with modifiers)
Other Info:		Both OGFC and PEM are more expensive per ton than dense-graded HMA, but the unit weight of the mix when in-place is lower, which partially offsets the higher per-ton cost.  The open gradation creates pores in the mix, which are essential to the mix's proper function.  Therefore anything that tends to clog these pores, such as low-speed traffic, excessive dirt on the roadway or deicing sand, should be avoided.