RoadScience Tutorial: Want Optimum Density? Get Smart
Open-graded hot mix asphalt friction courses (OGFCs) and stone matrix asphalt (SMA) lifts are gaining popularity, but they have different compaction demands. Modern liquid asphalt binders likely will incorporate polymer modifiers or rubber to enhance performance, but such binders can be sticky at certain temperatures and foul the drum, especially if the water system is not doing what it should.
Pave Me Tender
Superpave mixes can be stiff and hard to compact due to their large amount of crushed fine aggregate. In other cases, coarse-graded Superpave mixes can be “tender” and move or shove under both static and vibratory compaction. At certain temperatures, those tender mixes will move forward as the steel drums approach, causing a wave to form. Small transverse cracks will form at the top of the mix, making it harder for the operator to get density.
Superpave mixes have three zones, based on temperatures, which an operator has to take into account. They are the upper zone, from 302 to 239 degrees Fahrenheit, in which compaction can be achieved with steel wheel rollers without shoving; the middle zone, from 240 to 194 degrees Fahrenheit, in which the tender mix actually moves under the drums; and the end zone, below 194 degrees Fahrenheit, where the mix again becomes stable and no shoving will occur.
The challenge for the operator is to identify the tender zone, and avoid it until the temperature drops to the point at which compaction can resume. As much as 30 to 40 percent of compaction may be achieved before the mix gets tender and begins to push. Use of a roller with an onboard temperature gauge helps an operator identify the tender zone.
The next generation of compactors is providing ultra-high vibration, and can vary amplitude (force) with vibration (frequency) according to the type of asphalt and aggregate being placed, hopefully ruling out overcompaction that can shatter aggregate in-place, leaving white streaks on black pavement and a compromised aggregate structure in the lift.
Yet another development is oscillation technology, which supplants an up-and-down compactive force for a side-to-side force, working the drum and asphalt lift sideways. Proponents say by “massaging” the asphalt side-to-side with oscillation, density can be obtained faster on warm mats, tender zones can be compacted, and density can be attained on cooling or cold mats, including longitudinal joints.
The density number, or percent density, balances the volume of air voids against the volume of solid material in the asphalt lift.
If the percentage of air voids is too low, a pavement may have reduced flexibility, but too high an amount of air voids may contribute to the pavement’s disintegration. In any case, the conventional (non-Superpave) HMA mat must be compacted before its temperature drops to 176 degrees Fahrenheit, at which time viscosity or stiffness of the asphalt cement binder increases to the point that it is no longer possible to move the constituent aggregate with repeated passes of the roller. If compaction is not achieved by then, the operator runs the risk of shattering aggregate as he or she vibrates the mat over and over again.
For consistent, optimum density, the best compactor operators will achieve a rolling pattern early on in the day, and maintain it. Working with density readings, they will observe how density increases with each pass, and once they reach their percent density target of their density target of 92 percent or higher, the operator will know how many passes it will take to get there, and can gauge the needed rolling pattern and number of passes at that point. When the number of required passes is exceeded, the operator will be able to feel and hear overcompaction starting.
The need for a skilled operator notwithstanding, nearly all current research in both soil and asphalt compaction quality and productivity is addressing intelligent compaction.
For example, the Federal Highway Administration is exploring intelligent compaction technology and is working with 12 state pooled-fund research partners of Texas, Kansas, Iowa, Minnesota, North Dakota, Alabama, Indiana, Georgia, Virginia, Maryland, Pennsylvania and New York State. This Pooled Fund 954 is looking at the complete pavement structure, with an emphasis on technology transfer. (The pooled fund Website — http://www.intelligenctcompaction.com— is a treasure trove of information.
At the same time, National Cooperative Highway Research Project (NCHRP) 21-09 is developing specifications using detailed research on fewer projects, with an emphasis on subgrade soils and aggregate base.
“Why do we need intelligent compaction?” asks Robert D. Horan, P.E., district field engineer, The Asphalt Institute, and facilitator, Georgia Department of Transportation Intelligent Compaction Pooled Fund Project, at an intelligent compaction workshop in Georgia in September. “Proper in-place density is vital to good performance. Conventional compaction equipment and procedures have shortcomings and too often produce poor results. Intelligent compaction technology appears to offer a better way.”
These shortcomings include limited “on-the-fly” feedback to the operator, and the dangers of over- or under-compaction, he said. Conventional density testing takes place in a limited number of places, that is, only where cores or readings are taken, and that takes place only after compaction is complete.
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