The Basics of Successful Asphalt Compaction

Asphalt compaction is an essential part of the flexible paving process. The production, laydown and compaction of hot mix asphalt (HMA) or warm mix asphalt (WMA) are time-bound — that’s why delays really can’t be permitted in this construction process. Even moderate delays may interfere with the ability to achieve the ultimate objective of long-life pavements.

Knowing how to perform each phase of the asphalt compaction process is important — following best practices is critical. Here are the most important things to know about each step.

THREE PHASES OF ASPHALT COMPACTION

Historically speaking, the paving industry has recognized that there are multiple activities involved in the removal of air voids from paving materials. This is accomplished during the three phases of asphalt compaction:

1. The paver screed is the first tool used to improve the load carrying capacity of pavement materials during the laydown process.
2. Roller compaction is the next phase. Depending on the application, material specifications, type of paver and other variables, one or more compactors are employed to further increase the density of the pavement.
3. The final phase is traffic compaction via on-road vehicles. The industry acknowledges that high volumes of heavy truck traffic will further reduce pavement air voids. This is true even when the pavement has been properly compacted during the construction phases of laydown and roller compaction.

Let’s take a deeper look at the procedures of each phase.

PAVER SCREED COMPACTION PROCESS

Many variables influence the ability of the paver screed to remove air voids from the asphalt paving mixture. Depending on the type of screed, its weight, paving speed and other variables, material density following laydown can range from as low as 70 percent relative density to as high as 90 percent relative density — even higher in some cases. Screeds, which utilize the influence of vibration during laydown, generally produce somewhat lower overall density, but they’re capable of faster paving speeds for applications where productivity is more important. Vibratory screeds permit paving speed as fast as 100 feet (30 meters) per minute.

Tamping screeds equipped with either single or dual tamper bars will typically produce higher density, but they’ll accomplish this at slower paving speeds. The forward speed of tamping screeds is limited by the frequency of the tampers and by the width of the tamper bar, which relates to the contact surface. Paving speeds using a tamper bar screed are rarely faster than 33 feet (10 meters) per minute. This is generally considered too slow for the type of asphalt paving typically done in North America.

The temperature at which pavement is laid by the paver finisher is important to successful compaction, but it’s material dependent. Stone mastic asphalt (SMA) mixes are produced with binders that are relatively stiff and aggregates that are often blocky and coarse-graded. SMA is regularly produced at temperatures higher than 350° Fahrenheit (176° Celsius) and laid before the mix cools below 300° Fahrenheit (149° Celsius).

Superpave mixes that contain polymer-modified binders are produced and processed at similar high temperatures. Marshall mixes with neat asphalt binders are made at somewhat lower temperatures to prevent binder damage through overheating. These mixes are typically placed at temperatures between 250° to 300° Fahrenheit (121° to 149° Celsius). Newer technology mixes, called warm mix asphalt (WMA), permit mixing temperatures to be reduced as much as 100° Fahrenheit (38° Celsius) for considerable energy savings during production.

Processing time for all of these mixes is approximately the same duration. Uniformity of temperature during the paving process is paramount to achieve uniform pavement air void content and bearing capacity.

ROLLER COMPACTION PROCESS

The process of roller compaction is integral to the construction of high-quality flexible pavements. When following pavers with vibratory screeds, compactors need to increase material density by 20 percent on average. This needs to be completed before the asphalt pavement mix temperature drops below its limit of workability. Depending on mix properties and other variables, this low temperature limit can be as warm as 175° Fahrenheit (80° Celsius) or as cool as 120°Fahrenheit (50° Celsius), depending on the mix design and asphalt cement binder properties.

Breakdown rolling is the most important part of the asphalt compaction process and results in most air voids being removed from the pavement structure. It’s important to select the proper type and size of compaction equipment for breakdown rolling and to uniformly cover the entire pavement surface.

Selecting a breakdown compactor is often done based on its rolling width and optimizing uniform coverage of the paved panel. For example, if the laydown width is 12 feet (3.66 meters), a breakdown compactor with a 79-inch (2-meter) drum width can cover the panel width in two passes side by side with sufficient overhang of drum edges and overlap in the center.

Selecting a breakdown compactor with a narrower drum width will reduce productivity since three passes side by side will be necessary to properly cover the panel. Selecting a breakdown compactor with a wider drum won’t increase productivity. In fact, this may cause non-uniform panel density when excessive overlap in the rolling pattern occurs.

Compactors that follow the breakdown compactor should also be selected so that the compactor train accomplishes both objectives of density and smoothness while maintaining the balance of productivity set by the paving train.

TRAFFIC COMPACTION PROCESS

Agencies and research groups inside the flexible pavement construction industry have proven that pavements receive further air void reduction through the weight effects of traffic on the pavement. The AASHO Road Test was a series of experiments conducted by the American Association of State Highway Officials in the late 1950s to determine the performance of pavement structures under load. The Special Report issued following these tests showed that the greatest pavement damage was caused by on-highway truck traffic. The legal load for tandem axle trucks was 48,000 pounds (21,770 kg) on the trailer axles and 12,000 pounds (5440 kg) on the tractor axles. One semi, at the time fitted with bias-ply tires inflated to 70-80 psi (4.8-5.5 bar), was found to cause more pavement distress than over 50,000 passenger cars and pickup trucks.

The ability to properly design and construct flexible pavement structures allows for traffic compaction to occur without creating early pavement failure or excessive pavement surface distress. Attention to detail during the material selection process, pavement design and field construction activities will result in long-life flexible pavements requiring only routine maintenance. Successful asphalt compaction paves the way to fiscal responsibility, keeps the motoring public happy, plus it helps avoids traffic delays in construction zones.