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Of all the aspects of the hot mix asphalt paving process, compaction plays the most important role in determining the useful life of the finished pavement, even more than the mix itself. Whether you’re constructing an asphalt pavement from the ground up or placing an overlay, inadequate compaction will result in pavement that will deteriorate more quickly than a properly compacted pavement, causing your clients to spend more money sooner to repair or replace it.

Improper compaction results in pavement with decreased stiffness that can encounter accelerated aging, reduced durability or increased rutting, raveling and moisture damage — all of which will make your company look bad and inevitably cause you to lose business.

According to the Asphalt Institute Handbook, compaction is “the process used to densify, or reduce the volume of, a mass of material.” With hot mix asphalt (HMA), compaction locks aggregate particles together to provide stability and resistance by using force to compress the particles into a tighter space, thus increasing the density. As air void content is reduced, density increases, the mix becomes more stable and pavement strength is increased.

For most contractors, the focus of compaction is reducing air voids. That’s partly because the higher the percentage of air voids in the finished mat, the greater the likelihood the pavement performance will suffer in the long run. As air or water penetrates through these voids, the mat will encounter premature pavement distress. Air will oxidize the asphalt binder faster, making it brittle and causing cracks sooner. Also, if a pavement isn’t properly compacted, it’s easier for water to enter the pavement structure, stripping the asphalt binder from the aggregate and resulting in even quicker pavement deterioration.

Vibration Speeds Compaction

Jim Scherocman, consulting engineer and National Pavement Expo speaker, says the two most important factors in compaction are use of the vibratory screed on the paver in vibrating mode and mix temperature during compaction.

HMA arrives at most jobsites at temperatures ranging from 275° F to 300° F and begins to cool as soon as it’s transferred from the haul truck to the paver. It’s at its hottest when it moves beneath the screed, so it’s at that point that most compaction should take place.

The basic pieces of equipment used during the compactive effort are the paver’s vibrating screed and various types of rollers. The vibratory screed provides primary compaction when the mix is at its hottest as the mix is transferred from the paver to the surface beneath the screed.

According to Scherocman, when mix is placed using a vibratory screed in vibrating mode, it will contain roughly 20% in-place air voids. If the vibration in the screed is turned off, the air void content will likely be more than 30% on mix immediately behind the paver.

Proper compaction can reduce air voids to about 8% of the mat, which is generally an acceptable target (unless job specs are more restrictive). Using a vibratory screed in vibrating mode can make the roller operator’s job easier, and more likely that a crew will achieve target density.

“The problem is that many screed operators turn the vibration off. Why? Because it’s less comfortable to stand on a vibrating screed than a static one,” says Scherocman. “The single biggest and easiest improvement contractors can make in obtaining density is to operate their screed in vibratory mode.”

Get It While It’s Hot

Once the mix has been compacted by the screed, it still requires additional compaction to reduce air voids to 8%. This is where various types of rollers (vibratory, pneumatic and static) come into play.

In a conventional three-roller operation, a double-drum vibratory steel roller is used in the breakdown phase. It’s often followed by a rubber-tire roller in the intermediate phase — though rubber-tire rollers are being used more and more in breakdown rolling, as well.

In some cases, a static steel wheel roller is used as a finish roller. Scherocman says it’s becoming more common to see two vibrating double-drum rollers operating in echelon (side by side) on larger paving projects involving thicker mats.

Regardless of the types of rollers applied or the order in which they’re used, the key to compaction success is to roll the mat while it’s still hot, since the binder will stiffen as the mix cools and begin to resist further densification. Scherocman recommends rollers follow closely behind the paver, and indicates a good rule of thumb is to complete the breakdown phase before the surface temperature of the mix falls below 250° F. He suggests using an infrared gun to help track the surface temperature of the mix.

Compaction must be completed while the mix is hot and workable — generally at temperatures above 175° F. The cooler the air temperature and ground (base) temperature, the less time there is available for successful compaction, and the more difficult it can be to obtain the required density. This is the main reason there’s little asphalt paving during cold weather.

“The most important thing you can do as a contractor if you want to give your customer a durable, long-lasting pavement is to compact it properly,” Scherocman states. ET

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