By Justin Zupanc, Compaction Product Manager, Volvo Construction Equipment
Anyone involved in roadbuilding or site preparation should understand the critical role a compactor plays in a project’s overall success.
For roads, proper soil compaction is critical to the strength and stability of everything under the pavement and helps prevent water from accumulating beneath the road. Similarly, accurate compaction of asphalt during the paving process makes it denser and more durable by eliminating air voids. These factors ultimately determine things like the smoothness and lifespan of the road.
When it comes to road and general site prep, soil compaction affects the stability and load-bearing capacity of the soil. This can prevent or prolong any settling or shifting and support proper water drainage to avoid erosion.
As compaction is such an important part of a project, selecting the right equipment to do it is a big decision. It should be obvious whether you need a soil or asphalt compactor, but deciding on the right machine from there takes a bit more consideration.
Here are some tips to help you select the right machine for the job. For this article, we’re talking about larger compaction projects like those mentioned above — not smaller ones that can be done with a tamper, rammer or plate compactor. The word “roller” is also used interchangeably with “compactor.”
If we’re talking soil
There are multiple kinds of soil compactors, and the first deciding factor is the makeup of the soil itself. Vibratory compactors excel in granular soils like sand and gravel but can also work for cohesive soils like clay and silt. Cohesive soils, however, may require the “kneading” action of a padfoot roller (also known as a sheepsfoot roller) to achieve higher density and stability.
With vibratory rollers, there are several configurations to choose from. A single-drum vibratory compactor has one smooth drum at the front and two tires at the back for the best combination of traction and compaction — but you may need to make several passes to get the soil as compressed as you’d like.
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A double-drum model, just like it sounds, has drums at the front and back for maximum productivity. These are also called tandem rollers. A double-drum compactor can be used for some soil applications, but it’s more commonly used for asphalt.
There are static single- and double-drum compactors too, which means they do not apply vibration and won’t compress deeper soil layers like a vibratory model can. A vibratory roller can be operated in static mode simply by not turning on the vibration function.
A pneumatic tire roller features multiple tires (typically four or five) across each axle and is best for thin, granular soils. Heavier models can work for fine silt or gravel, too. Some OEMs offer combination rollers, which have a drum at the front and pneumatic tires across the back axle.
A padfoot or sheepsfoot roller is essentially a single-drum model with lugs or feet protruding from the drum to knead more cohesive soils. The dimensions, weight and number of lugs will impact the quality of compaction. A single-drum roller can be equipped with a dedicated padfoot drum or have an optional bolt-on shell kit that can be added or removed per job requirements.
If we’re talking asphalt
In paving, the material being compacted is a human-made mixture, so you have more control over the consistency and particle size range. An asphalt-only top layer provides a smooth driving surface, better durability for high-traffic zones and a more aesthetically pleasing finish.
A chip seal application provides more traction and is usually more cost-effective but is not typically suited for higher traffic or speed rated roads. For this technique, the road is sprayed with a thin, even layer of heated liquid asphalt and then a crushed rock layer is sprinkled on top and compressed into the mix.
For a smooth asphalt finish, you’ll need double-drum vibratory compaction because it will remove air voids better and make the pavement last longer.
A pneumatic tire roller is more common for a chip seal application because the tires provide more of a kneading action to press the “chip” layer into the asphalt without fracturing the stones.
Determining compactor size
Once you know the kind of compactor you need, it’s time to find the proper machine size. For the most part, this comes down to the requirements of the job.
Road work typically involves strict specifications on density, smoothness and thickness, which are greatly affected by the number of passes done during compaction. A roller’s drum width is the top machine spec to consider here because it dictates how many passes it will take to sufficiently compress your material.
It’s possible to use drums that are too narrow or too wide for the lane being paved, but the objective is to achieve density in as few passes as possible. With a drum that is too narrow, additional passes will be required and that will lead to inconsistent densities across the lane. A drum that is too wide may create too much overlap in the centre, thus creating a significantly higher density in the centre of the lane.
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Most manufacturers of highway-class compactors offer three common drum widths: 1675, 1980 and 2135 mm. These widths allow you to compact the average lane of traffic in two passes (including an appropriate overlap of a few inches) without the machine being too wide for transportation regulations.
If you’re doing something other than road work, models with drums in the 711-mm to 1422-mm range may be just fine. These are great for parking lots, driveways, bike paths and sidewalks and in residential zones where a giant machine is impractical and probably unwelcome.
In addition to the drum’s width, its diameter can make a difference in your productivity as well. A larger drum has less tendency to displace material and cause ripples in the mat. It also has a greater area of contact, which reduces drum edge markings and material displacement.
As you can imagine, larger roller models are much heavier, which increases the level of compaction they provide. Some models allow you to add water or sand to sealed compartments to increase a machine’s weight if needed.
Other compactor specs that matter
A few other machine specs that play into your machine choice are amplitude, frequency and the drum’s eccentrics.
Amplitude is the total distance that the vibrating drum is vertically displaced from its neutral position when vibration is on. Higher amplitude is required for thicker layers or difficult-to-compact mixes, while lower amplitude is needed to compact thinner layers or mixes with softer aggregates.
Frequency is the number of cycles the vibrating mechanism makes per minute (VPM) or second (Hz). Higher frequencies allow faster rolling speeds without compromising compaction quality but will provide less force per impact.
Some manufacturers offer wider ranges of amplitudes and more frequency settings than others, all of which will affect what kind of compaction work you can take on and how fast you can complete your projects.
Tandem rollers use rotating eccentrics to create vibration. Models with auto-reversing eccentrics, however, will also reduce the bow wave in front of the drums, which minimizes material displacement and ensures more consistent compaction results. This feature helps deliver superior smoothness to the finished mat.
Don’t forget the support
Compactors have straightforward controls and are generally considered easy to learn how to operate. However, if you have newer operators, or want to better support your more experienced operators, consider the technology that may or may not be included with the machine.
Many OEMs include a basic level of assistance via a human-machine interface (HMI), which is like a touch screen tablet.
For asphalt compaction, this often includes pass mapping and temperature mapping so the operator can see where they’ve been and whether their material is still at an appropriate temperature. More advanced software may be able to track density and stiffness or the capability to record and share data about the job.
For soil compaction, assistive software may include the ability to identify buried utility lines or provide a map of the site’s load-bearing strength, also known as the compaction measurement value (CMV).
Regardless of a machine’s technological offerings, there is no substitute for solid training and a good relationship with your dealer in case questions come up.
Putting it all together
With a good understanding of your jobsite conditions and project specs, choosing the right compactor doesn’t have to be a complicated decision. Identify the type of machine you need, work with your sales or rental rep to dial in the specs, and make sure your operators have what they need to succeed. You’ll be building a solid foundation in no time.
