Compacting for Superpave Success

By: James A. Scherocman, P.E.

In other cases, however, use of the Superpave mix designs has resulted in a "tender mix," one that is difficult to properly compact. These mixes move and check or crack under the applied compactive effort of the rollers. The desired level of density is hard to obtain because the rollers have to attempt to catch up with the mix that is moving both longitudinally and transversely in front of them.

Superpave mix designs

In most cases, the requirements for a Superpave mix are very similar to the requirements for a Marshall mix design. The primary exception, for what has sometimes been called Superpave Level 1 or Volumetric design, is the use of the gyratory compactor instead of the Marshall hammer to compact the laboratory prepared specimens. For high levels of traffic, use of the Superpave mix method typically results in a lower asphalt content in the mix for a given type and gradation of aggregate used in the mix.

Superpave mix design methods require the use of volumetric calculations to determine the properties of the mix. Similar to the Marshall mix design method, the air-void content and the voids in mineral aggregate (VMA) content of the mix are very important properties. The mix is typically designed for a given air-void content and the VMA content is used along with other mix characteristics to determine the optimum asphalt or binder content for a given mix.

Because of the greater compactive effort imparted to the laboratory mix by the gyratory compactor compared to the Marshall hammer, the VMA value for the Superpave mix is normally somewhat lower than the HMA mix using the same aggregate type and gradation but compacted using the Marshall hammer. For this reason, it is sometimes more difficult to meet a minimum VMA value for a Superpave mix design compared to a Marshall mix design. (VMA is not usually a specification requirement for a HMA mix designed using the Hveem method.)

The original Superpave specification for aggregate gradation set up a so-called "restricted zone" through which the combined coarse and fine aggregate gradation was not allowed to pass. It was thought that by keeping the aggregate gradation out of the restricted zone, when plotted on 0.45 power gradation graph paper, production of a tender mix would be avoided. The location and the size of the restricted zone changes as the nominal top size of the aggregate incorporated into the HMA mix changes.

Further, it was originally recommended that the aggregate gradation be kept on the coarse (bottom) side of the restricted zone. The reason for this suggestion is to increase the amount of coarse aggregate in the mix and thereby increase the internal strength of the HMA mix. A mix that has a gradation above the restricted zone contains a greater amount of fine aggregate and, thus, is theoretically less strong than the mix, which contains more coarse aggregate.

Various state highway departments investigated the aggregate gradations used in their HMA mixes over the years. It was found that some mixtures that have an excellent performance history under heavy traffic contain aggregate gradations that would pass through the restricted zone. Based on this information, the requirement that the aggregate gradation shall not pass through the restricted zone has been modified to a recommendation that the gradation should not pass through the zone. This change means that HMA mixtures that have a good performance history but have an aggregate gradation that passes through the restricted zone can still be used, which makes good common sense.

The Superpave mix

The Superpave mix design gradation method provides a method to determine a so-called maximum density line. The maximum density line in the Superpave methods is drawn from a point of 100% passing on the first size larger than the nominal maximum size sieve to the zero point.

Theoretically, an aggregate gradation that approximates the maximum density line would result in a HMA mix that has both a minimum VMA content and also a minimum binder content. Such a mix would be very sensitive to the amount of asphalt cement added to the mix. In addition, such a mix would be very sensitive to any moisture remaining in the aggregate during mix production at the asphalt plant. Thus, it is recommended that a mix not be produced that has a combined coarse and fine aggregate gradation that approaches a straight line similar to the maximum density line.

It has been found, however, that as the gradation of the combined coarse and fine aggregate is moved away from the so-called maximum density line on the coarse (bottom) side, that the VMA content of the mix is actually reduced instead of increased. In turn, the density of the mix is actually increased instead of decreased.

The question has been raised, therefore, as to whether or not the line called the maximum density line in the Superpave system is really the true maximum density line. Indeed, it has been found for many HMA mixes the minimum VMA value, the minimum asphalt content, and the maximum density occurs with an aggregate gradation that is just below the restricted zone-just where the best mix is supposed to be designed.

Mixtures that are at a minimum VMA content have a distinct possibility to be tender during compaction. If the binder content of the plant produced mix is slightly high, the mix may become unstable under the rollers.

If the mix contains some residual moisture, again the mix might be tender and move under the compaction equipment. Even though the mix might meet all of the Superpave mix design requirements, that mix might not be able to be properly compacted on the roadway, in part because of the sensitivity of the mix to an excess of fluids content-asphalt cement and/or moisture.

It must be pointed out that tender mixes have existed for years. Mixes that are produced at the minimum VMA limit will be prone to tenderness whether designed by the Superpave method, the Marshall method, or the Hveem method. While most Superpave HMA mixtures are quite stable and stiff, some of these mixtures, particularly those with gradations just on the coarse side of the restricted zone, may be internally unstable and very difficult to compact.

Compaction of stiff mixtures

Superpave mixtures that are properly designed will be reasonably stiff and stable and will require a considerable amount of compactive effort in order to attain the required degree of density. The mix will support the weight of the compaction equipment directly behind the paver. If the mix is placed at a temperature of 280 deg F or greater, the rollers will typically be able to properly compact the mix before it reaches a temperature of 175 deg F.

Most often, three rollers are used-a breakdown or initial roller, an intermediate roller, and a finish roller. For breakdown rolling, a vibratory steel-wheel roller is most often used.

For intermediate rolling, a pneumatic tire (rubber-tire) roller is generally employed although sometimes a second vibratory roller is used. Finish rolling is normally done with a static steel-wheel roller.

The breakdown and intermediate rollers should stay close to the paver. If the mix is stable, a bow wave will not occur in front of the vibratory roller drum and the mix will not exhibit any cracking or checking. With a relatively stiff mix, the finish roller should also be close to the paver because there will be minimal marks to remove from the breakdown and intermediate rollers.

For very stiff mixes, or when a high degree of density is desired, a pneumatic tire roller should be employed for breakdown rolling. In this case, for intermediate rolling, a vibratory steel-wheel roller follows directly behind the pneumatic roller and finish rolling is again performed with the static steel-wheel roller.

Because of the internal stability and strength of the stiff mix, more compactive effort may need to be applied in order to obtain a given level of density (percent of the theoretical maximum density) but the mix will not move under the compaction equipment during the rolling process.