There are three significant factors that influence the ability of a paving train to achieve specified or target density air void content on any paving project.
The first is mix design, especially with regard to content and aggregate gradation of asphalt cement. The second is the placement of the mix in a homogeneous and non-segregated manner, with sufficient thickness to permit particle rearrangement to eliminate voids. The final factor is the temperature of the hot mix asphalt (HMA) during the compaction process. This last factor is the one contractors most frequently miss on paving jobs.
HMA mix temperature and viscosity are correlated; in fact, their relationship is an inverse one. The stiffness, or viscosity, of asphalt cement is lower at higher temperatures and higher at lower temperatures. As viscosity changes, so does the ability to achieve material density.
Nearly all agencies have specifications that identify or suggest the maximum and minimum temperatures at which HMA mixes of various types can be processed. The term "processed" may include mixing at the plant, transportation from the plant to the paving project, mix laydown, and compaction.
Agencies also specify a minimum ambient temperature and/or the temperature of the base onto which the pavement is being placed. Often, this temperature can be no less than 40°F.
Two things must happen in order for a paving train to achieve a uniform final density of any mix. First, the mix must be delivered to the project in a homogeneous and non-segregated condition. Second, the compactors in the paving train must be close enough to the paver to work on the pavement while the mix is still hot and viscosity is low.
The industry has adopted many processes in an attempt to assure mix uniformity. The most common culprits in aggregate and temperature segregation are poor practice at the mix plant and improper truck loading, hauling, or mix delivery. While asphalt should always be properly loaded and hauled to the jobsite, mix segregation can be overcome through the use of a material transfer vehicle (MTV).
Assume that an HMA mix is delivered to the paver in excellent quality and that the paver is in excellent working order. The crew has properly made all screed adjustments and automatic controls are being utilized to achieve the required grade and slope of the pavement surface. Everything has progressed according to plan and the material placed on grade is ready for compaction to produce finished pavement.
If this mix is a Marshall mix, it is ideally delivered to the paver at a temperature between 260° and 280°F . If this mix is a Superpave mix, it should be delivered to the paver at a temperature between 300° and 330°F.
Naturally, environmental factors affect the rate of mix cooling during and following placement. Studies indicate that the majority (about two-thirds) of heat loss following laydown is into the base. Mix should be delivered to the paving site at a temperature of at least 250°F for a Marshall mix or 290°F for a Superpave mix immediately behind the paver screed.
As is true for much of life, learning often occurs as a consequence of negative, rather than positive, experience. Here is a prime example.
During the 2005 construction season, I visited a contractor's night paving project on three separate occasions. Each of these visits was made to provide practical assistance to this contractor to overcome failure and achieve a uniform and high density during the compaction process.
The contractor was not earning the available pavement density incentive from the agency even though the paving train on this jobsite incorporated an MTV to provide smoother paver operation and to help reduce segregation of the mix being placed.
During my first visit to this site in August, I noticed the breakdown compactor, or front compactor, was not following the paving train closely enough to satisfy best practices according to current industry definitions.