Important Note: The information and techniques presented herein are general guidelines for the compaction of asphalt. Contractors are advised to select their compaction equipment and adapt their compaction strategies based on the specific requirements, site conditions, and material characteristics of every project to ensure the best results.

Highway Paving: Quality & the Role Compaction Plays

Quality asphalt paving isn’t just about the finished product—it’s about a precise and coordinated process.

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The Unique Challenges of Highway Paving

Highway paving applications are a complex and demanding type of work. They require exceptional quality because the finished road will host a high volume of vehicles traveling at high speeds. Operations are also highly time sensitive, often taking place at night with re-routed traffic, and require the entire team to be properly coordinated. The goal is to complete a target tonnage and reopen lanes as scheduled the next morning.

The process demands careful logistics, from the asphalt plant producing the mix, to on-road trucks transporting the material, and finally to the on-site team removing old material and placing new material in a consistent manner. A critical focus on quality is maintained throughout, with the material tested after placement and once the temperature has lowered to a stable state.

The Asphalt Rollers’ Role in Highway Paving

Effective asphalt compaction is the direct link between a well-coordinated operation and achieving a high-quality finished road. This process is performed by a specialized rolling train of asphalt rollers, where each machine is assigned a specific task.

Column Rolling Train
This is a sequential setup where rollers follow one another. It typically begins with a breakdown roller—usually a double drum vibratory or oscillatory roller—to perform the initial compaction pass. This is followed by an intermediate roller (commonly a pneumatic tire roller which is sometimes called a traffic roller), which uses static pressure and spaced out tires to provide a kneading effect that compacts from the bottom up. The final pass is made by a finish roller, another vibratory or oscillatory double drum, to remove any marks and smooth out the surface.

Echelon Rolling Train
This formation is used for wider roads. It consists of two similar double drum rollers working side-by-side. One leads slightly on the left side of the paving path, with the second slightly trailing on the right side, with their drums slightly overlapping in the middle to ensure a seamless bond between the lanes.

Defining Quality in Highway Paving

In highway paving, quality is primarily defined by two key measurements: density and ride quality. The ultimate goal of a paving project is to achieve both optimal density for durability and a smooth surface for comfort and safety.

A key measure of ride quality is the International Roughness Index (IRI). This index measures the smoothness of the finished pavement, with the goal being to achieve the smoothest ride possible. A lower IRI value indicates a smoother ride. The IRI is calculated by a tool that measures the vertical movement of a vehicle per mile of travel. Several factors can affect ride quality, including the condition of the subbase, the milled surface texture, material consistency, screed settings, and, critically, the chosen compaction methods.

The Roller’s Impact on Density

The primary role of the roller is to achieve a target density of the asphalt mat. Rollers achieve this by using a combination of static weight and dynamic force (through vibration or oscillation) to rearrange the asphalt’s aggregate particles and eliminate air voids. The result is a dense, stable mat that is resistant to rutting and cracking yet durable enough to withstand heavy traffic loads for years to come.

The Roller’s Impact on Ride Quality

Beyond simply compacting the asphalt to the correct density, the asphalt roller is the final piece of equipment with a direct impact on the pavement’s ultimate smoothness. The techniques and technology used during the rolling process are critical for achieving a low IRI. A skilled roller operator will make the correct number of passes at the appropriate speed to avoid creating imperfections. Technologies like oscillatory compaction can further enhance ride quality by using a back-and-forth kneading motion instead of vertical impact, which helps to eliminate surface ripples, creates negative texture (which improves sound, rolling resistance, and skid resistance), and provides a more consistent finish.

Maximizing Incentives with Quality Compaction

In many states and regions, highway paving projects include job incentives, or “bonuses,” which provide additional pay to contractors who exceed the standard project parameters. These bonuses are a significant motivation for achieving high-quality results. Common bonuses are awarded for:

  • Reaching a desired level of pavement smoothness (IRI) within a specific sublot.
  • Reaching a desired IRI for the entire job.
  • Reaching a desired density.
  • Reaching a desired joint density.
  • Completing the scope of work ahead of schedule.
  • A combination of the above, tying quality and efficiency to financial rewards.

IRI and Density Bonuses
State Departments of Transportation (DOTs) often offer tiered bonus structures to encourage superior results in both ride quality and density. For example, a project on I-90 in Montana offered attractive bonus incentives based on achieving favorable IRI scores. The bonus structure was tiered:

  • An IRI between 45 and 55 inches/mile was considered acceptable with no bonus or penalty.
  • An IRI between 35 and 44 inches/mile earned a bonus of up to 24%.
  • An IRI below 35 inches/mile earned a 25% bonus.

The paving contractor achieved an average IRI score of 29 and was awarded a $605,000 bonus for its efforts.

Additionally, many states provide incentives for achieving higher pavement density. These bonuses are typically structured as a tiered payment based on how closely the contractor’s results match the target density. For instance, a bonus might be awarded for achieving a density above 92% of the target, with a higher bonus for densities exceeding 94%. This tiered approach motivates contractors to aim for maximum compaction, which directly improves the pavement’s long-term durability and performance.

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