3  Flexible Pavement Types

There are many different types of flexible pavements.  This section covers three of the more common types of HMA mix types used in the U.S.  Other flexible pavements such as bituminous surface treatments (BSTs) are considered by most agencies to be a form of maintenance and are thus covered under Module 10, Maintenance & Rehabilitation.   HMA mix types differ from each other mainly in maximum aggregate size, aggregate gradation and asphalt binder content/type.  This Guide focuses on dense-graded HMA in most flexible pavement sections because it is the most common HMA pavement material in the U.S.  This section provides a brief exposure to:

• Dense-graded HMA.  Flexible pavement information in this Guide is generally concerned with dense-graded HMA.  Dense-graded HMA is a versatile, all-around mix making it the most common and well-understood mix type in the U.S.
• Stone matrix asphalt (SMA).  SMA, although relatively new in the U.S., has been used in Europe as a surface course for years to support heavy traffic loads and resist studded tire wear.
• Open-graded HMA.  This includes both open-graded friction course (OGFC) and asphalt treated permeable materials (ATPM).  Open-graded mixes are typically used as wearing courses (OGFC) or underlying drainage layers (ATPM) because of the special advantages offered by their porosity. 

This section is taken largely from the NAPA's HMA Pavement Mix Type Selection Guide (2001).  In addition to the general information presented here, the HMA Pavement Mix Type Selection Guide provides specific information on minimum lift thicknesses, mix selection criteria, mix materials as well as several informative examples.

3.1  Dense-Graded Mixes

A dense-graded mix is a well-graded HMA mixture intended for general use.   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.  They can further be classified as either fine-graded or coarse-graded.  Fine-graded mixes have more fine and sand sized particles than coarse-graded mixes (see Table 2.1 for definitions of fine- and coarse-graded mixes). 

Table 2.1: Fine- and Course-Graded Definitions for Dense-Graded HMA (from NAPA, 2001)

3.2  Stone Matrix Asphalt (SMA) Mixes

Stone matrix asphalt (SMA) is a gap-graded HMA (see Figure 2.7) that is designed to maximize deformation (rutting) resistance and durability by using a structural basis of stone-on-stone contact (see Figures 2.8, through 2.12).  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 (about 20 - 25 percent) because it requires more durable aggregates, higher asphalt content and, typically, a modified asphalt binder and fibers.  In the right situations it should be 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 (NAPA, 1999).

Figure 2.7: Typical SMA and Dense-Graded HMA Aggregate Gradations

Figure 2.8: SMA Structure

Figure 2.9: SMA Aggregate Structure.  Notice the stone-on-stone contact of the larger aggregate particles.	Figure 2.10: Dense-Graded HMA (left) vs. SMA (right).  (it is a bit more shiny from the extra asphalt binder)
Figure 2.11: Dense-Graded HMA (left) vs. SMA (right).  Notice the SMA has a better-defined large aggregate skeleton (from NAPA, 2001)	Figure 2.12: SMA Pavement Surface

3.3  Open-Graded Mixes

An open-graded HMA mixture is designed to be water permeable (dense-graded and SMA mixes usually are not permeable).  Open-graded mixes use only crushed stone (or gravel) and a small percentage of manufactured sands.  There are three types of open-graded mixes typically used in the U.S.:

1. Open-graded friction course (OGFC).  Typically 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.  See Figure 2.13.
3. Asphalt treated permeable bases (ATPB).  Less stringent specifications than OGFC or PEM since it is used only under dense-graded HMA, SMA or PCC for drainage.  See Figure 2.14.

Figure 2.13: Core from a Pavement Using PEM as the Wearing Course (from NAPA, 2001)	Figure 2.14: Asphalt Treated Permeable Base

3.4  Mix Selection Guidance

Based on the previous information, there are some general rules for HMA mix type use, which are summarized in Table 2.2.  Notice that, as discussed, dense-graded HMA is generally appropriate for all uses, SMA and OGFC (and PEM) 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 2.2 is just a summary of general guidance and that there are, as always, case specific exceptions.

Table 2.2: General Appropriateness of Mix Types For Each HMA Layer (NAPA, 2001)

Course

Low Traffic

Medium Traffic

High Traffic

(< 300,000 ESALs)

(300,000 - 10 million ESALs)

(> 10 million ESALs)

Dense

SMA

OGFC

ATPB

Dense

SMA

OGFC

ATPB

Dense

SMA

OGFC

ATPB

Surface

Intermediate

Base

= Appropriate

Note: Before deciding to use ATPB, the Pavement Research Center's research results should be carefully considered.

= Moderately Appropriate

empty

= Not Appropriate

3.4.1  Determining Appropriate Mix Types

Most of this process is taken directly from the NAPA HMA Pavement Mix Type Selection Guide (2001).

1. Determine the total thickness of HMA required.  This is accomplished using an appropriate structural design procedure.
2. Determine the types of mixtures appropriate for the surface course based on traffic and cost.
   • From Table 2.2, 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. 
3. 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 2.2.
4. Continue to subtract intermediate/base course thicknesses from the total thickness until mixes and layer thicknesses have been selected for the required pavement section.