The Problem…

Pavements rarely last as for as long as they are designed. Although substantial effort has been applied to mix design improvement such as the new Superpave mix design system, few resources have been allocated to improved construction/compaction techniques in spite of the well known fact that different construction practices substantially affect pavement performance. Indeed, asphalt pavements have been compacted using the same steel drum rollers for almost one hundred years. While conventional compaction equipment gets the job done, there is room for improvement.

The need for improvement has become evident with the new Superpave mixes. The larger, more coarse aggregate structure is proving too much for conventional equipment. Also, Superpave mixes have been displaying a “tender zone” where the mix becomes unstable under the load of steel drum rollers.

The Theory…

Asphalt concrete properties significantly change with mix temperature. For normal operating temperatures, asphalt generally reacts elastically to loads – instantly deforming as load is applied and instantly rebounding after the load has been removed. Loading duration has little effect in this case. At elevated temperatures, the mix is much more visco-plastic, and reaction to load is much more critical. For short, sharp loads, the mix continues to react elastically, much like a viscous dashpot (“shock absorber”). Efficient compaction may only occur under mild constant loading.

Conventional compaction equipment actually works against compaction because the short, sharp load applied causes the mix to react elastically with little plastic deformation (required for compaction). On the other hand, the AMIR and HIPAC compactors apply a much smaller load over a longer period of time, allowing the asphalt to flow and therefore, promoting compaction. Comparison of how asphalt cement reacts to applied loading magnitude and rate is shown below:

Asphalt Stiffness versus Temperature for Various Compactors

As shown, conventional compaction equipment causes an initial increase in bitumen stiffness because of the rapidly applied loading. The static steel roller causes a 100 fold increase while the vibratory roller causes a 1000 fold increase in stiffness. Most of the conventional compactive effort is used to overcome this initial elastic response – not to actually compact the asphalt concrete. The AMIR however, does not cause the initial elastic response, so that all of its applied effort is directed toward compaction.

The Consolidation-Fluid Flow model of asphalt compaction was proposed by Ian Rickards of Pioneer Road Services in Australia, Steve Goodman and Dr. Abd El Halim from Carleton University and Dr. Ralph Haas from the University of Waterloo to explain the compaction phenomenon. The model contends that asphalt compaction occurs more like the consolidation of a fine-grained soil, requiring a longer load duration at lower pressure than the granular base material for which conventional compaction equipment was developed.

Solutions to Superpave Compaction Problems…

Perhaps the most important feature of the CFF model is that it is able to explain the problems currently experienced during the compaction of Superpave mixes.

The first Superpave compaction problem may be referred to as the “Super-Stiff” condition. Such mixes are composed of coarse aggregates with a high percentage of crushed faces to resist rutting. Conventional compaction equipment is having difficulty compacting these high strength mixes because the short, sharp loads applied are being resisted by the mix. A recent NAPA contractor survey noted that 15% of respondents could not achieve specified density with Superpave mixes. Furthermore, almost all respondents noted that additional compactive effort was required to compact Superpave mixes. According to the CFF model of asphalt compaction, a long load duration at lower applied stress would better facilitate plastic flow of these mixes, allowing more efficient compaction without overstressing the asphalt concrete.

The second, and more problematic Superpave compaction problem is called the “Tender Zone”. The tender zone occurs in some Superpave mixes within a temperature range of 105C to 85C (240F to 200 F) where uncompacted asphalt concrete excessively shoves under conventional compaction equipment. 66% of NAPA survey respondents experienced the Tender Zone with Superpave mixes. The mechanisms behind the tender zone are not fully known at this time, however, it is most likely caused by the gap grading of many Superpave mixes. Regardless of the cause, the Tender Zone may be mitigated by utilising the large confining contact area of the HIPAC.

The first paper concerning the CFF model was presented at the 1998 Canadian Technical Asphalt Association (CTAA) conference in Vancouver, BC by Mr. Stephen Goodman. Another paper concerning the CFF model was presented at the 1999 Transportation Research Board Annual Meeting in Washington, DC by Mr. Ian Rickards.

Contact CAART for more information concerning the CFF compaction model.