Hot-mix asphalt pavement design

December 2005 » Feature Articles
During the pavement design phase, it is important to understand the purpose of the pavement and of each individual pavement layer. Therefore you can optimize the selection of materials and mix design to achieve the desired performance, while sparing the owner's or taxpayers' money.

Optimizing performance and cost effectiveness

BY DAVE NEWCOMB, PH.D., P.E.

During the pavement design phase, it is important to understand the purpose of the pavement and of each individual pavement layer. Therefore you can optimize the selection of materials and mix design to achieve the desired performance, while sparing the owner's or taxpayers' money. For example, specifying a high content of crushed aggregate and polymer-modified asphalt binder in a low-volume pavement, or in the lower layers of a medium- or high-volume pavement, may not be prudent in that the hot-mix asphalt (HMA) will cost more than is warranted by the application. On the other hand, specifying a high natural (rounded) sand content for an interstate highway surface mix could be a recipe for premature rutting.

Terminology

Before discussing how to select HMA for particular applications, it is best to define terms for pavement layers and traffic levels. Pavement layers are divided into surface (wearing), intermediate (binder), and base layers. The surface layer contains the highest-quality materials to provide skid resistance, noise reduction, rutting resistance, and surface drainage. The intermediate layer is next, and it is intended to transmit stresses into the lower pavement layers without deforming (rutting). The HMA base layer is located at the bottom.

While the structural requirements for this layer are not as critical as for the surface and intermediate courses, it must be durable, especially if the soil is wet for long periods of time.

The vast majority of HMA used in the United States is dense-graded. This means there is a continuous grading of material as the size progresses from fine to coarse.

Because of its gradation, the packing of the aggregate particles is denser than in other gradations. Open-graded friction courses (OGFC) and asphalt-treated permeable bases (ATPB) are usually one-size gradations that contain a minimal amount of fine material. Their purpose is to allow water to drain through them, whether they are on the surface, as with OGFC, or underneath the other HMA layers, as with ATPB. Stone matrix asphalt is a premium surface mix made with a gap-graded aggregate. Gap grading means that there is a fraction of the aggregate missing between the coarse and fine material.

Traffic loads for highways generally are categorized according to the number of 18,000-pound equivalent single axle loads (ESALs). Low-volume roads are those designed for less than 300,000 ESALs.

They generally are local and county roads, recreational roadways, or city streets with minimal truck traffic. Roads with moderate traffic have between 300,000 and 10 million ESALs. They may be two-lane or multilane facilities that are partially or completely controlled access. Highvolume roads are those designed for more than 10 million ESALs. They may be twolane or multilane roadways, and may have a high volume of truck traffic.

Matching the mix to the application

Although one could specify any mix for any application, the question a savvy designer asks is, “Which mix makes the most sense?” To help answer this question, designers should understand the relative appropriateness of any given mix type to specific levels of traffic and its location in the HMA pavement structure.

Dense-graded mixes are appropriate at all levels of traffic, but one must match the application to the binder and aggregates used in the mix. Stone matrix asphalt is a very effective surfacing material for hightraffic roadways, and its use on roads with medium traffic levels, or as an intermediate layer, could be questionable from an economic viewpoint. OGFC is a very good material for surfacing high-volume roads to reduce splash and spray, and may be used on medium-volume road surfaces as well. It is most effective on high-speed roads. ATPBs usually are found only in high-volume roads, if they are used at all.

On medium-volume roads, they may be used to drain water from areas where the water table or spring thaw may be problematic.

What materials should be used? In choosing materials for a specific type of mix, a designer needs to understand the application. Performance according to the traffic demands, environment, and available materials should be the driving force to optimize costs and benefits.

As stated above, surface mixes for high volume roadways should be designed for the best possible performance. Because they are subjected to the tires from heavy truck traffic, and because the surface is exposed to environmental elements such as water, sun, and temperature extremes, these mixes in particular must be made from hard, angular, polish-resistant aggregate and a binder that is resistant to thermal cracking and rutting.

High-quality aggregate is needed for two purposes: 1) the angularity gives it the ability to lock together, providing the mix with high internal friction to reduce rutting, and 2) the polish resistance helps to provide the needed surface friction to reduce skidding.

It is not uncommon on high-volume roads to find polymer-modified asphalt being used in the surface mix. Generally speaking, the spread between the hightemperature and low-temperature grades of the performance-graded binder are sufficiently wide to warrant the use of polymer modifiers.

The low-temperature grade is selected to resist cracking at cold temperatures while the high-temperature grade is selected to help resist rutting. When designing surface mixes for high-volume roads, it is important to use the best materials, mix design, and construction practices to maximize performance and surface life, thereby minimizing traffic disruptions.

For the surface layers of pavements with moderate traffic levels or mixes that are placed in the intermediate layer of a highvolume road, polymers and angular aggregates may still be needed to avoid certain performance problems. However, the risks usually are not as great as with highvolume surface mixes.

Moderate-traffic surface mixes should still contain polish-resistant aggregate to minimize skidding. Rutting resistance may still need to be considered carefully, especially if the pavement in question is located in an industrial area subjected to frequent heavy truck traffic.

For low-volume roads, it is still important to ensure safety, but the effects of the environment become more important than traffic loads. Although dense-graded mixtures are the most desirable for lowvolume road applications, there is a need to ensure the durability of the mixtures by designing them with an adequate amount of asphalt. The asphalt serves to help resist cracking and raveling of the surface.

Because of the low traffic levels, the amount of angular aggregate can be reduced and there is no need for a polymer- modified binder.

Pavement designers often debate the appropriate size aggregate to use in a given mix. In base mixes, it is not unusual to find a 19 mm, 25 mm, or even 37.5 mm nominal maximum aggregate size (NMAS). This decision can be based on economics, which favors larger aggregate in many cases. In the intermediate layer, aggregate sizes may range from 9.5 mm for leveling courses to 37.5 mm. Surface mixes are usually made from 4.75 mm to 19 mm NMAS. Generally speaking, the smaller the NMAS, the smoother and quieter the pavement surface becomes.

Ensuring performance

Once the mix type has been selected, the pavement must be designed properly to achieve the best results. One of the aspects of pavement design that directly affects good construction and performance is the thickness of the lifts in which the HMA is placed. If the lift is too thin for the NMAS, then the coarse aggregate particles can stack on top of one another during compaction, leaving large air voids that are permeable to air and water and resulting in decreased durability. On the other hand, if the lift is too thick, compaction may be difficult to achieve lower in the lift. Additionally, ride quality may suffer because there are fewer opportunities to achieve smoothness with fewer lifts.

The National Center for Asphalt Technology has performed a project under the National Cooperative Highway Research Program (NCHRP) to investigate the relationship between lift thickness and HMA density. This work is published in NCHRP Report 531, Relationships of HMA In-Place Air Voids, Lift Thickness, and Permeability. Researchers found that for a dense-graded asphalt mixture, the lift thickness should be no less than three times the NMAS for fine-graded mixtures and no less than four times the NMAS for coarse-graded mixtures. Thus, a 9.5-mm mix with a fine, dense aggregate gradation should have a minimum lift thickness of 28.5 mm (about 1-1/4 inches), whereas a 37.5-mm mix with a fine gradation should have a minimum lift thickness of 112.5 mm (about 4-1/2 inches). If the gradations are coarse, a 9.5-mm mix should have a minimum lift thickness of 38 mm (about 1-1/2 inches) and a 37.5-mm mix should have a minimum lift thickness of 150 mm (about 6 inches).

Choosing the right material for the right application is the key to high value in terms of cost, ease of construction, and performance.

Dave Newcomb, Ph.D., P.E., is vice president for research and technology of the National Asphalt Pavement Association. He can be reached at dnewcomb@hotmix.org.

 

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