Compacting Stiff and Tender Mixes

Due to aggregate size considerations and the need for smoothness, the total thickness for the runway mix was divided up into three equal layers of 3 inches. The base course layer contained a maximum aggregate size of 1 inch while the upper two layers used an aggregate with a maximum size of 3/4 inch.

The base course, which was designed using the Marshall method, contained a PG 64-34 polymer modified asphalt binder. Due to the aggregate gradation limits in the specifications, it was impossible to develop a stable mix even though the coarse aggregates were highly crushed and the fine aggregates had a fine aggregate angularity value over 49.

Strata Corp. was required to construct a test section to show that the required level of density could be obtained before paving was permitted to start on the main runway area. It was quickly determined that this mix, even though it contained poly mer modified binder, was extremely tender. The mix had a tendency to move and shove excessively when a double drum vibratory roller was used in the breakdown position, even when this roller was only used to make one or two passes over the mix. The mix checked badly (short, transverse cracks appeared in the surface of the mix) if additional passes with the vibratory roller were made.

When surface temperatures were measured with an infrared thermometer, it was found that the tender zone extended from about 240 degrees F down to less than 140 degrees F. To keep the mix from moving under the double drum steel wheel finish roller, operated in the static mode, it was necessary to roll when the mix surface temperature was under 120 degrees F. The tenderness of the mix made it impossible to obtain the desired level of density using a normal rolling pattern.

The mix was produced at a rate which typically exceeded 400 tons per hour. The mix delivery temperature was increased to enlarge the range of the upper temperature zone. The mix was placed using a Roadtec SB 2500 material transfer vehicle to deliver the HMA material into a Blaw-Knox PF 3172 paver.

A revised compaction train was then used to compact the tender mix. Breakdown rolling was done using two double drum vibratory rollers operated in echelon. A Dynapac CC 501 roller was used on one side of the 12-foot-wide lane and a Cat CB 634 B roller was employed on the other side. The rollers were wide enough to compact the full width of the lane and also overhang the edge of the lane by 6 inches on each side, as well as overlap by 6 inches in the center of the lane. These rollers each made five passes over the mat, all in the upper temperature zone, directly behind the paver screed.

Intermediate rolling was carried out using two pneumatic tire rollers. One of these two rollers was a Cat PS 360B. The other was a vibratory pneumatic-tire roller — a Sakai GW750. Both were operated immediately behind the two vibratory rollers — within 300 feet of the paver. Due to the difference in the size of the two pneumatic tire rollers, they were operated with one directly behind the other.

No finish roller was used for the compaction of the base course mix. Due to the tenderness and temperature of the mix when compacted with the two pneumatic tire rollers, the pneumatic tire rollers were able to remove their own roller marks and produce a smooth pavement surface. With the change in the rolling pattern, the required level of density was then consistently achieved.