This was presented at the Utah Department of Transportation
(UDOT) Engineer's Conference held November 20-22, 1997 at Snowbird,
UT. The conference was attended by representatives from UDOT,
Industry, and Academia.
Today's traffic volumes and on-time delivery schedules have caused highway user costs to soar. Each mix design for high traffic pavements must endure a longer life so that the public can move from point A to B safely and with little delay. We now have a great tool, Superpave, to achieve these higher goals of pavement performance.
The most important point is that Superpave mixes are still blacktop. The improvements to current practices include: 1) aggregate and asphalt criteria and selection, 2) mix design criteria and tools, and 3) performance criteria and tools. Similar to the Marshall hammer, the Superpave Gyratory Compactor (SGC) allows us to simulate traffic densification for low and high traffic environments. With its kneading action, the SGC can simulate much higher traffic loads than the Marshall hammer.
Similar to Utah, Indiana has been very aggressive in implementing Superpave and gained much experience with the Superpave mix design procedure in 1996. From an industry perspective, the greatest improvements that Superpave provided Indiana were increased film thickness and stronger aggregate skeletons.
There are some quirks in the Superpave design process that need to be revised for smoother Superpave implementation. For example, the Voids Filled with Asphalt (VFA) requirement for 9.5mm mixes and the moisture damage test (AASHTO T-283) gave some unexpected results.
The Voids Filled with Asphalt (VFA) criteria of 75 percent for 9.5mm nominal size mixes created a maximum of 16 percent in the Voids in the Mineral Aggregate (VMA). This gave contractors the narrow range of 15 to 16 percent VMA. As long as users are aware of this unexpected limit, the details can be resolved by allowing a slightly higher VFA limit for these mixes.
None of the Superpave mixes that we, Koch Materials Company, designed for Indiana required anti-strip based on an 80 percent minimum Tensile Strength Ratio (TSR). This seemed unusual because some Indiana mixes required anti-strip in the past. Some TSR results on specimens fabricated in the gyratory compactor appeared unreasonably high. Could the increase in TSR be attributed to the change from Marshall specimens to Superpave Gyratory specimens? According to a Transportation Research Report, the percent Tensile Strength Ratio's (TSR's) calculated in AASHTO T-283 produce higher TSR's for mixes that are compacted in the SGC than in the Marshall hammer. It is the author's opinion that the increased TSR's are most likely caused by the different aggregate orientation of the specimen in the SGC and the difficulty of specimen saturation. The TSR test should be validated with field experience.
In contrast to TSR results, some mixtures were susceptible to moisture damage based upon the Hamburg Wet Wheel Tracking (HWT) results. Colorado DOT rated the HWT the most successful in predicting moisture damage of bituminous pavements.
Two of the limiting criteria in Superpave material selection are the Fine Aggregate Angularity (FAA) and VMA. Contractors should allow extra mixture design time since the FAA and VMA can be difficult to meet with many current asphalt mixes. FAA can be achieved more easily with crushed materials, better aggregate quality control and less natural sand. FAA should also be measured on the combined fine aggregate of the Job Mix Formula (JMF) for true mixture representation. VMA can be achieved more easily with crushed materials, semi-gap graded mixes, decreased minus 0.75 material (dust) and re-screening stockpiles.
Since the Superpave mixes are typically coarser than current mixes, more field compactive effort may be required. The Superpave Gyratory Compactor is a great tool to understand asphalt mixture densification. If the mix density does not exceed the Superpave NInitial criteria of 89 percent, the break down roller should be able to stay near the hot-mix paver. Most Superpave mixes are very stable and can easily support the weight of the roller.
Field quality control (QC) of any material is essential to ensure that the designed parameters are achieved in the field. Superpave offers a great tool to assist contractors with QC. The SGC provides real-time compaction (density) data that allows the contractor to measure asphalt mixture properties (density, VMF, VFA) after just seven minutes of placing the mixture in the compactor. Although the two to four hour oven cure and 150mm specimen size are inconvenient, they provide improved parameters for volumetric mix design and mixture representation.
The future looks bright. Proof tests, such as the Hamburg Wheel
Tracking device and the Asphalt Pavement Analyzer have gained
popularity in understanding mixture durability and rutting. More
importantly, the need for predicting pavement performance is increasing
rapidly. After the refinement of the Superpave Shear Tester and
Indirect Tensile Tester for predicting rut depth, fatigue cracking,
and low temperature cracking, we will have many tools to predict
pavement life. Some states are specifying asphalt mixes that
must meet 10mm maximum rut depth, 15 percent maximum fatigue cracking,
and 12 full length cracks per 500 foot section maximum low temperature
cracking. These new specifications, pavement warranties and increasing
highway user costs are driving the need for more accurate asphalt
pavement performance predictions that we, as an industry, must
provide.
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