Eight Things to Consider When Specifying Rollers for Compaction
There are a number of variables that determine the right roller for each compaction job. The most obvious is whether you’re compacting soil or asphalt. Other factors are the width of the area to be compacted, the final density required, the grade of the area to be compacted, the type of asphalt mix, soil type, as well as the type of materials that lay below the lift being compacted. There are also considerations with each machine related to frequency, amplitude and centrifugal force that will have an effect on final density.
Compaction helps ensure the integrity of whatever structure or passageway is being built near it, from foundations to utility lines and roadways. Here are a few things to consider when renting or buying a machine for compaction:
Soil Compaction – Smooth Drum or Pad-Foot Drum?
When looking at the type of soils being compacted, more granular materials such as sand and gravel typically require a smooth drum. The smooth drum will allow the material to lock together similar to a puzzle piece. Having optimum moisture in the soil is critical to allow for movement without friction.
More cohesive materials, such as clays, require a pad-foot drum. Cohesive soils tend to stick together and slip over each other. For these types of soils, it is critical to shear the soil with the pad-foot drum in order to achieve compaction.
It’s also important to note that contractors have another option with the use of pneumatic tire compactors. These machines feature two rows of tires (one in front, one in back) that overlap and provide static compaction through simple weight and kneading. Ballasts can be added to increase weight to meet compaction requirements.
The contractor needs to look at meeting the required compaction density in the least amount of passes, but that doesn’t always mean a bigger drum is better. Some contractors look at a large road job and immediately think an
84-inch drum would get the job done faster than a 66-inch drum, but that 66- inch drum may ultimately be the
optimal configuration for getting the job done in a comparable number of passes while minimizing overlap, which helps reduce over-compaction (the disintegration of an area that’s
already reached its required density).
Weight is an important consideration when taking into account the size of lift you are doing and the desired final compaction density. The static weight of the machine plus the force generated by its eccentric motor creates centrifugal force, which helps determine the compaction depth you can get. Too heavy of a machine used on a thinner lift can cause over-compaction and too light of a machine on thicker lifts means not compacting to the base of the lift. In some cases a light machine may have a higher centrifugal force due to higher amplitude.
Soil compaction with significantly deeper lifts will require a heavier machine with greater centrifugal force in order to reach those deeper depths. With asphalt, where the lifts are considerably thinner, frequency is more important than centrifugal force. The thinner the layer = less force, therefore lighter machines may have an advantage.
Weight distribution is also important. By way of example, there are models in the CASE fleet where the engine is mounted in the rear and the water tank is mounted to the front (feeding water to both front and rear rollers). This nearly equal weight distribution ensures the same compaction weight going down on the front and rear drum.
One of the common mistakes we see, specific to soil compaction, is using too light of a machine, or laying lifts that are too deep. Contractors build deep lifts
in the hopes of getting the job done quicker, but the likelihood of soft spots increases if that machine doesn’t have the proper centrifugal forces to compact the entire lift. The contractor then either has to rent or switch out for a larger machine to hit those depths, which ultimately sets job completion further back. It is important to know the realistic capabilities of the equipment you have.
Water Tank Size
Water plays an important role in preventing asphalt from sticking to the rollers — get a water tank that allows compaction equipment to run all day without having to stop and fill up, which helps to maximize uptime and productivity.
Look for Crab Offset and Curb Clearance
The ability to offset drums is important for a couple of reasons. When compacting asphalt on a roadway or another large-scale project, it’s important to bind each pass together. By having a
compaction machine that allows you to have the rollers either in-line or offset, you’re able to have an overlap of up to about six inches to bind the two passes together. It also helps give you tight turning radiuses for added
maneuverability when compacting in confined areas such as cul-du-sacs.
A roller with high curb clearance also helps in terms of maneuverability and is excellent for work in residential or commercial areas by allowing the operator to work right up to existing curbs and other low structures.
It is now more common to see compaction meters made available from the machine manufacturer. These meters measure the amount of rebound at each drum as they go over an area. If it passes over an area and does not register any rebound during soil compaction, for instance, that lets the operator know they’ve hit a bad area: maybe there’s an old stump or a large rock there, or the moisture content is such that it’s not allowing for compaction. On the contrary, if the meter is registering full rebound, the operator will know to avoid over-compaction of that area. The compaction meter helps identify areas that will require added attention prior to testing.
Look for compaction equipment that has scheduled maintenance and service items in easily accessible locations. This is not only important in terms of convenience, but is also critical should the machine need servicing during the course of a job. When compacting asphalt, it must be compacted while it is at a certain temperature. If the temperature drops too low, the asphalt could be ruined. So if a machine goes down and you can’t service it quickly or get a backup, your job deadline may be at risk.
Easy Operation and Comfort
All construction equipment is becoming more operator- and user-friendly in terms of comfort – important when sitting all day on a compaction machine. Things to look for while sitting in the cab of that machine are visibility to the drums (sloped hoods to the rear are an asset on soil compactors), seat maneuverability (sliding and rotation) within that cab for both forward and backward operation, and intuitive controls at your fingertips for adjusting amplitude and frequencies on the fly.