2  Flexible Pavement Basics

Flexible pavements are so named because the total pavement structure deflects, or flexes, under loading.  A flexible pavement structure is typically composed of several layers of material.  Each layer receives the loads from the above layer, spreads them out, then passes on these loads to the next layer below.  Thus, the further down in the pavement structure a particular layer is, the less load (in terms of force per area) it must carry (see Figure 2.2).  

Figure 2.2: Flexible Pavement Load Distribution

In order to take maximum advantage of this property, material layers are usually arranged in order of descending load bearing capacity with the highest load bearing capacity material (and most expensive) on the top and the lowest load bearing capacity material (and least expensive) on the bottom.  This section describes the typical flexible pavement structure consisting of:

• Surface course.  This is the top layer and the layer that comes in contact with traffic.  It may be composed of one or several different HMA sublayers.
• Base course.  This is the layer directly below the HMA layer and generally consists of aggregate (either stabilized or unstabilized).
• Subbase course.  This is the layer (or layers) under the base layer.  A subbase is not always needed.

After describing these basic elements, this section then discusses subsurface drainage and perpetual pavements. 

2.1  Basic Structural Elements

A typical flexible pavement structure (see Figure 2.3) consists of the surface course and the underlying base and subbase courses.  Each of these layers contributes to structural support and drainage.  The surface course (typically an HMA layer) is the stiffest (as measured by resilient modulus) and contributes the most to pavement strength.  The underlying layers are less stiff but are still important to pavement strength as well as drainage and frost protection.  A typical structural design results in a series of layers that gradually decrease in material quality with depth.

Figure 2.3: Basic Flexible Pavement Structure

As seen in Figure 2.4, a flexible pavement structure can vary greatly in thickness. The signs on top of the pictured cores indicate the State Route (SR) and the Mile Post (MP) where the core was taken.  The scale at the right edge of the photo is in inches.

Figure 2.4: Various Flexible Pavement Cores from Washington State

2.1.1  Surface Course

The surface course is the layer in contact with traffic loads and normally contains the highest quality materials.  It provides characteristics such as friction, smoothness, noise control, rut and shoving resistance and drainage.  In addition, it serves to prevent the entrance of excessive quantities of surface water into the underlying base, subbase and subgrade (NAPA, 2001).  This top structural layer of material is sometimes subdivided into two layers (NAPA, 2001):

1. Wearing Course. This is the layer in direct contact with traffic loads.  It is meant to take the brunt of traffic wear and can be removed and replaced as it becomes worn.  A properly designed (and funded) preservation program should be able to identify pavement surface distress while it is still confined to the wearing course.  This way, the wearing course can be rehabilitated before distress propagates into the underlying intermediate/binder course.
2. Intermediate/Binder Course. This layer provides the bulk of the HMA structure.  It's chief purpose is to distribute load.

2.1.2 Base Course

The base course is immediately beneath the surface course.  It provides additional load distribution and contributes to drainage and frost resistance.  Base courses are usually constructed out of:

1. Aggregate.  Base courses are most typically constructed from durable aggregates (see Figure 2.5) that will not be damaged by moisture or frost action.  Aggregates can be either stabilized or unstabilized. 
2. HMA.  In certain situations where high base stiffness is desired, base courses can be constructed using a variety of HMA mixes.  In relation to surface course HMA mixes, base course mixes usually contain larger maximum aggregate sizes, are more open graded and are subject to more lenient specifications.

Figure 2.5: Limerock Base Course Undergoing Final Grading

2.1.3  Subbase Course

The subbase course is between the base course and the subgrade.  It functions primarily as structural support but it can also:

1. Minimize the intrusion of fines from the subgrade into the pavement structure.
2. Improve drainage.
3. Minimize frost action damage.
4. Provide a working platform for construction. 

The subbase generally consists of lower quality materials than the base course but better than the subgrade soils.  A subbase course is not always needed or used.  For example, a pavement constructed over a high quality, stiff subgrade may not need the additional features offered by a subbase course so it may be omitted from design.  However, a pavement constructed over a low quality soil such as a swelling clay may require the additional load distribution characteristic that a subbase course can offer.  In this scenario the subbase course may consist of high quality fill used to replace poor quality subgrade (over excavation).   

2.2  Perpetual Pavements

"Perpetual Pavement" is a term used to describe a long-lasting structural design, construction and maintenance concept.  A perpetual pavement can last 50 years or more if properly maintained and rehabilitated.  As Michael Nunn pointed out in 1998, flexible pavements over a minimum strength are not likely to exhibit structural damage even when subjected to very high traffic flows over long periods of time.  He noted that existing pavements over about 370 mm (14.5 inches) should be able to withstand an almost infinite number of axle loads without structural deterioration due to either fatigue cracking or rutting of the subgrade.  Deterioration in these thick, strong pavements was observed to initiate in the pavement surface as either top-down cracking or rutting.  Further, Uhlmeyer et al. (2000) found that most HMA pavements thicker than about 160 mm (6.3 inches) exhibit only surface-initiated top-down cracking.  Therefore, if surface-initiated cracking and rutting can be accounted for before they impact the structural integrity of the pavement, the pavement life could be greatly increased.

Researchers have used this idea as well as pavement materials research to develop a basic perpetual pavement structural concept.  This concept uses a thick asphalt over a strong foundation design with three HMA layers, each one tailored to resist specific stresses (TRB, 2001):

1. HMA base layer.  This is the bottom layer designed specifically to resist fatigue cracking.  Two approaches can be used to resist fatigue cracking in the base layer.  First, the total pavement thickness can be made great enough such that the tensile strain at the bottom of the base layer is insignificant.  Alternatively, the HMA base layer could be made using an extra-flexible HMA.  This can be most easily accomplished by increasing the asphalt content.  Combinations of the previous two approaches also work.
2. Intermediate layer.  This is the middle layer designed specifically to carry most of the traffic load.  Therefore it must be stable (able to resist rutting) as well as durable.  Stability can best be provided by using stone-on-stone contact in the coarse aggregate and using a binder with the appropriate high-temperature grading.
3. Wearing surface.  This is the top layer designed specifically to resist surface-initiated distresses such as top-down cracking and rutting.  Other specific distresses of concern would depend upon local experience. 

In order to work, the above pavement structure must be built on a solid foundation.  Nunn (1998) notes that rutting on roads built on subgrade with a CBR greater than 5 percent originates almost solely in the HMA layers, which suggests that a subgrade with a CBR greater than 5 percent (resilient modulus greater than about 7,000 psi (50 MPa)) should be considered adequate.  As always, proper construction techniques are essential to a perpetual pavement's performance.  Figure 2.6 shows an example cross-section of a perpetual pavement design to be used in California on I-710 (the Long Beach Freeway) in Los Angeles County.

Figure 2.6: Example I-710 Long Beach Freeway Perpetual Pavement Design
(from Monismith and Long, 1999)

Finally, the most important point in this brief perpetual pavement discussion is that it is possible to design and build HMA pavements with extremely long design lives.  In fact, some HMA pavements in service today are living examples of perpetual pavements.  For instance, two sections of Interstate 40 in downtown Oklahoma City are now more than 33 years old (built in 1967) and are still in excellent condition.  These sections, which support 3 to 3.5 million ESALs per year, have been overlaid but the base and intermediate courses have lasted since construction without any additional work (APA, no date given).