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How Temperature Impacts Compaction
Paving crews can improve their finished pavement by keeping the rollers close to the paver —and to each other
The proper position of the roller train behind the paver aids compaction efforts. The paver needs to place the mix while the temperature is between 350˚F and 290˚F. To the right of the paver are two "breakdown rollers" working in tandem in a temperature range between 320˚F to 250˚F. The arrow indicates where "density tests" are best conducted (after the breakdown rollers) and the furthest right section shows where finish rolling is done (temperature range 150˚F to 100˚F.)
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Figuring it out
By Dale Starry
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.
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