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 constructing a hot mix asphalt (HMA) 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 will result in pavement with decreased stiffness that can encounter accelerated aging, reduced durability, or increased rutting, raveling and moisture damage – all things that will make your company look bad and inevitably cause you to lose business.
According to the Asphalt Institute Handbook, compaction is defined as “the process used to densify, or reduce, the volume of a mass of material.” For 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.
When most contractors think of compaction, they think of 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. When air or water penetrates through these voids the mat will encounter premature pavement distress. Air will oxidize the asphalt binder faster, making it more brittle, 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, resulting in more rapid pavement deterioration.
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 mix is transferred from the paver to the surface beneath the screed.
Scherocman says that 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 the 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 can simply make it more likely a crew will achieve the desired level of density.
“The problem is that many screed operators turn the vibration off,” Scherocman says. “Why? Because it’s less comfortable to stand on a vibrating screed than a static one. The single biggest improvement and the easiest improvement contractors can make in obtaining density is to operate their screed in vibratory mode.”
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 (rubber-tire), and static — come into play. In a conventional three-roller operation, a double-drum vibratory steel roller is the first roller on the mat in what is termed the “breakdown” phase. That’s often followed by a rubber-tire roller in the “intermediate” phase (though more and more the rubber-tire roller is used 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.