Here’s a few best practices for compaction of driveways, parking lots and patch work
By Brad Belvin and Dr. Yuki Tsukimoto from Sakai America
Compaction is a critical aspect of asphalt paving that directly impacts the long-term performance and durability of the finished surface. While many contractors understand the basics of compaction, optimizing this process requires a deep understanding of the factors involved and the potential consequences of mistakes.
Before you begin
Choosing the appropriate asphalt roller for a small-tonnage project like a residential driveway, small parking lot or patchwork is essential for achieving optimal compaction results.
For small-tonnage jobs, a lightweight, manoeuvrable roller is typically sufficient.
The type of asphalt mix will influence the required compaction effort and dense-graded mixes may require more aggressive compaction than open-graded mixes.
Also, consider the advantages and disadvantages of different equipment types, such as:
- Double-drum vibratory rollers: Vibratory action punches at a higher weight class than the equivalent ton static roller.
- Combination rollers: Combine the benefits of a front vibratory drum and static rear tires. Tires knead the mix reducing voids and boosting density. Trickier to learn on but reduces the need for multiple rollers for many applications.
- Pneumatic tire rollers: for finish rolling and chip seals.
- Walk-behind rollers with/out steering: easy to load/unload for quick patch jobs, manoeuvrable.
- Plate compactors: for around manholes, curbs and joints.
- Hand tools: for narrow and tight spaces.
RELATED: The importance of proper longitudinal joint compaction in HMA paving
The drum size and weight of the roller are crucial factors in achieving proper compaction. For residential driveways and small parking lots, a vibratory roller with a drum width of 90 to 130 cm and a weight of 1.5 to 5 tons is typically sufficient. For larger parking lots a double-drum roller with a drum width of 135 to 150 cm and a weight of 4 to 8 tons may be necessary.
Consider these general roller weight guidelines for common lift thicknesses:
- Residential driveways: 1.5 to 3 tons for 25 to 50 mm lifts, 3 to 5 tons for 50 to 75 mm lifts
- Small-to-medium parking lots: 3 to 5 tons for 50 to 75 mm lifts, 8 tons for 100 mm lifts
- Patchwork: 1.5 to 3 tons for 25 to 50 mm lifts
Note these are general recommendations, and the optimal drum size and weight may vary depending on the specific project requirements, such as asphalt mix density and compaction conditions.
Foundation for success
Before compaction of the asphalt mat itself comes into play, the subbase/base must be dry and properly compacted with no visible water or movement under repeated pressure of equipment. An asphalt mat is only as good as what lies beneath it. Base material is the support structure for the asphalt surface, distributing loads and providing stability. Poorly nonuniform compacted base material can permit rutting, crack propagation, uneven wear, potholes, frost heave from water intrusion and, at worst, base failure resulting in the need for complete tear out.
Mix moisture is under the plant’s control but contractors should control moisture in the base before laying the mat as well as ensuring that mix isn’t subjected to rain before application. Ensure the base design accounts for the anticipated loads. For example: no amount of compaction will prevent pavement failure under a heavy truck for which the base is not prepared.
Beyond the basics
Beyond the standard practices of selecting the right roller and following recommended pass patterns, experienced contractors should consider not only the temperature of the mix before beginning but during compaction. Monitoring asphalt temperature is crucial for achieving optimal compaction.
Rolling at too low or too high temperatures can affect compaction quality and the long-term performance of the pavement. Rolling should cease in the “tender zone” and finish before the mix cools.
Rolling in the “tender zone” can cause shoving of the mat and checking. Rolling at a temperature that is too cool will be ineffective and damage the mat. Temperature ranges will vary with your local plant mix.
A general guideline for the tender zone of HMA would be 60C to 121C. Don’t rely solely on a temp gun, use a probe to check the internal temperature of the mat.
Assessing density
While formal density testing may not be common for smaller paving projects, experienced contractors can use observations and practical techniques to assess compaction quality.
For example, a visual inspection will show signs of aggregate segregation, unevenness (smoothness), depressions around stop-and-start areas, roller edge marks, shoving, checking and cracking. Ensure joints are smoothly compacted and minimally detectable.
As well, after the fifth to seventh pass, an operator may detect the vibrating drum changing from bounces to random vibrations as compaction progresses. However, the easiest way to achieve uniform compaction from the operator perspective is to accurately count the number of passes, equally covering the entire job.
Where a density target is required, electromagnetic density or non-nuclear gauges (i.e. Transtech) are simpler, less expensive and require no licensing compared to nuke gauges. Intelligent compaction is usually reserved for larger projects and larger rollers due to the investment expense. Core samples and nuke gauges are more commonly used on larger projects.
Prevent common issues
The following are symptoms of improper compaction. To deliver a long-lasting finished product, contractors should become familiar with these and take preventative measures for the next job:
Washboarding: This is caused by over speeding a vibratory roller (space between impacts). To prevent washboarding, slow the roller, turn up the vibratory frequency, or a mix of both.
Rutting, depressions and potholes: These are the results of improper base compaction, incomplete mat compaction and a too thin lift. The contractor should ensure an even, compacted base for proper support of the mat. Ensure proper compaction of the mat by utilizing temperature-based rolling with a sufficiently weight roller for the lift thickness.
Alligator cracks: This is the result of improper or non-uniform base compaction or insufficient lift thickness. To prevent, ensure thorough and uniform compaction of the entire base, select a mat thickness appropriate for the intended load.
Ravelling: Aggregate that is not bonded well with the binder can cause ravelling. To prevent ravelling, compaction should begin at hot temperatures (above 143 C but depends on the mix type) to ensure proper binding.
Crushed surface aggregate (white capping): This occurs when the vibratory force is too high. Reduce the roller’s amplitude setting to prevent white capping.
Regular maintenance of compaction equipment is also essential for ensuring optimal performance and longevity. Follow the manufacturer’s recommended maintenance schedule and address any issues promptly.
Training is also important. Ensure operators are properly trained on the use of compaction equipment and the importance of proper rolling techniques. Well-trained operators can help prevent mistakes and optimize compaction results.
By understanding the principles of compaction, implementing best practices and addressing common challenges, experienced contractors can deliver high-quality work that meets or exceeds client expectations. Delivering projects with proper compaction will ensure long-term performance of the mat which is crucial to the reputation of an asphalt paving contractor.
