By John Greaney
Early in my career, I was told a truth that has rung true for years. An experienced contactor said, “A good roller man can make a bad paving job look good, but a bad roller man can really mess up the best paving job.” There have been many innovations in compaction over the years, but the basics always ring true, achieve density and make the water run the right way. In this article I’m going to discuss a few tips and actors that are important to compaction basics.
Compaction starts behind the screed both tamping and vibratory. In its most basic form you are getting the air out of your asphalt mix in order to reach maximum performance of your mix. Operators have a short amount of time to maximize density from our asphalt mix and road smoothness and is directly impacted by your compaction efforts.
The more air voids your asphalt mix has, the more it is compromised in terms of strength, durability, rutting, and moisture damage, all leading to a short life cycle for your pavement. The asphalt paving industry relies on compaction to increase the pavement density and extend the life of your pavement. The binder and aggregate particles mix, and are forced together through compaction thus increasing aggregate interlock and inter particle friction reducing the air voids with in your pavement. Vibration helps in this effort, but over vibrating can cause rideability issues.
Temperature, Temperature, Temperature
Asphalt mixtures cool quickly, so it is highly important to compact while the mix is hot. Typically asphalt comes from the plant around 300°, so you have a short window of opportunity before the temperature drops below 240°. During this time, the mix should not shove or check and should remain stable for compaction process. As it cools and firms up, the compaction becomes increasingly difficult. Also, you must keep in mind to consider the ambient air temperature and surface temperature, all factors that affect your window of compaction opportunity. In addition, there is a tender zone. Depending on the kind of aggregate and mix properties, this zone ranges from 240° to 190°. This is what I call the “no go” zone, meaning stay off the mat. During this time the mat shoves, checks and can create that bulge in front of the roller that we all have seen. Once the temperature drops below 190° you have another opportunity and the mix can support again the weight of a roller. But take note, this last opportunity to achieve desired compaction is difficult and the operator needs to watch for stress cracks in the mat.
The thickness of the lift will determine how your roller operators proceed. During vibration, the weight with centrifugal force lifts and drops the heavy steel drum as it moves. Generally, the operator can control the settings of amplitude or “beats” allowing for fine tuning in regards to how thick your mat or lift is. In my experience, the roller operator needs to take note not to get into a hurry as traveling to fast will cause gaps or ripples in your mat. If the operator isn’t running the correct settings, or traveling at the correct speeds, it can affect the quality of your mat and overall life of the pavement.
The roller operator starts compaction at the tie-in points and follows the edge from there mainly using dynamic vibration, thus increasing the effect of compaction. Best practices dictate that the intermediate pass should be done with a pneumatic roller as the tires compact in a different “kneading” motion, adding to the density numbers. The final roller pass is generally completed with a steel drum under static in an attempt to “iron out” any remaining irregularities such as lines left by the drum, or tire marks by the pneumatic roller, etc.
While the industry is shifting on different mix designs to increase life expectancy or decrease air voids, one important factor that will remain is the compaction you have on your mat directly impacts the quality and life of your pavement. These small steps can make the difference in a good job versus a great job. And great jobs get referrals.
We would be remiss if we didn’t acknowledge the stress we all have been going through the last couple of months. On behalf of the entire industry, we salute those who have been deemed “essential” and look forward to getting back to normal.
John Greaney is a Territory Manager for LeeBoy and can be contacted at email@example.com
Rick Smith is a territory manager at ST Engineering LeeBoy, Inc. You can contact him at firstname.lastname@example.org.