The quick — but not exactly complete — definition of intelligent compaction (IC) applies to vibratory rollers that automatically adjust their energy output, so as neither to under-compact nor over-compact materials. Thus, theoretically, IC yields precise and consistent results across the jobsite, while providing detailed documentation of compaction quality.
Are these not answers to the very petitions included in the prayers of all those responsible for compaction? Further, we're assured that IC is accepted as established technology in a number of western European markets. Might we reasonably expect, then, that roller users in North America are lining up to buy such machines?
So far, lines have been short on this side of the Atlantic. We hasten to add, however, that short lines at present indicate nothing negative, either about the technology or the machines available — or soon to be so — for North American jobsites.
At this writing, Ammann, Bomag and Dynapac all have IC machines ready for work in soil and aggregate, and Ammann and Bomag have asphalt models as well. Caterpillar plans to officially introduce an IC soil machine next year, and an asphalt model a year or so after that. Sakai's intentions are to be in the North American market with both types of IC rollers sometime in 2008, and Dynapac is developing an asphalt machine that will be available soon, says the company, first in Europe and then globally.
(Industry speculation is that Ammann will no longer market soil compactors in North America under its own name, but will supply these machines to a major U.S. manufacturer for marketing under that company's brand. Unclear at this point is whether this transfer will include IC soil machines. Our best intelligence says that Ammann will continue to market its asphalt machines in North America under its own name and, if so, Ammann's IC technology will remain available here.)
Although the estimated 20- to 30-percent price premium for IC models versus their conventional counterparts may be an issue for some users, slow sales at present seem more the result of the North American market still warming to the technology. Perhaps what's happening here is that the potential benefits of IC are so compelling, that those who stand to benefit from more precise, more consistent compaction want verification that the process can deliver what it promises.
Among those looking for the facts are the Federal Highway Administration (FHWA) and a growing number of state departments of transportation, Minnesota in particular. The FHWA now has in its hands a 160-page study — "Intelligent Compaction: Strategic Plan" — which details the technology and its potential benefits, identifies research needs and presents a national implementation plan.
The Minnesota DOT, in 2004 and 2005, extensively evaluated IC soil rollers from Bomag, Ammann and Caterpillar on state test sites, and continues to research the subject. According to John Siekmeier, P.E., senior research engineer in the Minnesota DOT's Office of Materials, certain of the state's roadway contracts in 2006 will require the use of IC machines on granular soils.
Also this year, both the National Cooperative Highway Research Program and the Transportation Pooled Fund Program will be starting studies regarding the use of IC rollers on soil structures, and the latter group's study also will encompass the roller's application in aggregate and on asphalt pavement.
For all of their sheer brute mass and power, vibratory soil and asphalt rollers have been getting smarter for decades.
In the mid-1970s, for example, Swedish manufacturer Geodynamik developed its Compactometer, a device (still being marketed in increasingly "smarter" versions) that measures the drum's movement and processes the resultant signals to provide a continuous relative value for a material's level of compaction or "stiffness." (Stiffness is loosely defined as a measure of a material's ability to resist deformation under load). This process has become known in Europe as "continuous compaction control" (CCC), and is widely used, along with a recording and documentation system that logs the measured values.
The Geodynamik system also reports drum frequency and monitors the "double-jump" condition, which occurs when the drum acquires so much energy that it begins to move upward during a vibratory cycle before hitting the ground, thus skipping every other impact. A machine operating in this manner may damage both itself and the material it's compacting.
The Geodynamik system and similar proprietary CCC systems, such as Dynapac's Compaction Analyzer (DCA), have gone a long way toward keeping the roller operator informed about the compaction process. And in some instances, these computerized systems are available with GPS assistance and can be retrofitted to existing units.
"Intelligent" innovations might also include, for example, the oscillatory-vibration system, which moves the drum in a rocking motion (versus conventional vertical vibration). According to Carl Pettersson, Geodynamik's managing director, the company initially developed the idea in the 1980s. More recently, Hamm has developed a proprietary oscillation-compaction system, which, says the company, prevents over-compaction by changing the drum's effort when material begins to firm. The system does this, says Hamm, by virtue of the physics designed into the drum's eccentric-weight system.
Also, in yet another display of intelligence, Ingersoll Rand's large DD-158HFA asphalt compactor allows the operator to select from a range of eight amplitude settings. Then, the machine automatically adjusts frequency "to the optimum performance setting."
As smart as the machines have become through the use of these and numerous other innovations, however, the "intelligent-compaction" process goes beyond preceding developments. Here's a definition synthesized from several sources:
An intelligent-compaction roller is a vibratory model that continuously measures and reports the stiffness of material, while simultaneously and automatically adjusting its compaction effort based on those measurements, imparting more energy to soft areas and less (or no) energy to hard areas. The roller also is equipped with a documentation system that allows real-time correction of the compaction process, while providing a permanent record of compaction results, including stiffness values for virtually every point in every lift.
Among the potential benefits of IC rollers, says the FHWA study, is increased productivity, "because compaction energy is customized, based on measured stiffness. The result, says the study, "is a more rapid increase in compaction during initial passes, which may mean fewer passes to reach target values." The IC roller also may have the capability to compact deeper lifts, says the study, "because the magnitude of maximum amplitude that is used during the initial roller passes is significantly increased when compared with conventional rollers."
That last statement squares with what we heard from Bomag's Steve Wilson, marketing services/product manager, who told us that the Bomag BW213-4 soil compactor equipped with VarioControl, the company's IC system for soil machines, produces 82,000 pounds of centrifugal force at a 0.094-inch amplitude. This compares with the conventional 213's rating of 67,000 pounds at a 0.079-inch amplitude.
The results of potentially having to make fewer passes may include savings in time, fuel and machine maintenance. Some make the point, too, that because the IC roller's compaction forces diminish as material approaches target values, less energy (and less resultant stress) is reflected back into the machine's structures and drive train, thus potentially reducing wear and tear.
Hand-in-hand with increased productivity, says a Minnesota DOT study, is the prospect of improved compaction quality — on two fronts. First, because IC rollers have the potential to eliminate over-compaction and under-compaction, applying additional effort only if necessary, they actually are exercising a form of process control, similar, say, to a factory's computerized machining center, which constantly checks the quality of its own work. This control produces more uniform compaction, says the study, provided soils are within the moisture-content range necessary to achieve the target compaction.
"Second," says the study, "several demonstration projects have illustrated that compactors equipped with IC capability, like conventional compactors, cannot compact all soils under all conditions. Soils with moisture contents far from optimum, with soft or wet underlying materials — or other problems — cannot be compacted to target levels. Because of the surface-covering documentation that the IC roller provides, however, these problem areas may be identified and corrected before being covered by additional lifts." (Of course, a standard roller with a documentation system could also provide this benefit.)
Documentation provided by the IC process also offers the prospect of streamlining quality control. Since the IC roller automatically checks virtually every spot on the jobsite for compaction results, just a handful of manual spot checks may be required for confirmation.
Without this extensive verification, however, considerably more quality-control checks must be made (but still relatively few when compared with the IC roller's thorough coverage). Conducting numerous manual checks — whether by soil-replacement tests, asphalt coring or quick-reading gauges — can potentially be time-consuming, expensive and perhaps even risky on busy jobsites with a fleet of fast-moving construction machines.
Full documentation of compaction results also may serve as the contractor's proof of performance if pavement-warranty issues arise, and these documented results may be the basis, too, on which states potentially may award bonuses or assess penalties for compaction-work quality.
In addition, says Minnesota's Siekmeier, "intelligent compaction provides comprehensive data on the mechanistic properties of all materials compacted, permitting links between design, construction and performance. For example, the data record produced by the compactor, which covers all areas and all lifts, will be essential to the pavement-management process. Long-term performance may be correlated with the properties produced during construction."
Among the ultimate payoffs of the IC process, though, is the potential for creating structures that have longer useful lives. European experience, says the Minnesota DOT's research, "clearly demonstrates that greater compaction uniformity increases the useable life of pavement systems, and similar benefits occur with embankment compaction and buried-structure backfill."
Probably safe to say in regard to IC rollers generally is that most (but not all) use accelerometers (force/motion sensors) to measure drum movement relative to the machine frame, then employ proprietary software to calculate a stiffness value from these signals. Except for Sakai's proposed IC asphalt system, double-drum IC rollers use (will use) force-control for only one drum.
Although you'd have to take up the specifics with individual manufacturers, most would likely say that their measured stiffness values correlate well with accepted soil compaction tests, such as the Proctor method, provided soils are relatively homogeneous, granular in nature, and within acceptable moisture-content parameters. Some say also that their stiffness values for asphalt correlate well with commonly used density-measuring methods for this material, such as the Marshall test. But others, in truth, are still working on more closely determining the correlation between stiffness values reported for asphalt with those derived from conventional methods.
The IC system's software uses stiffness measurements (determined by the vibratory drum's reaction with the material beneath) as a reference for controlling the drum's energy imparted to the material. At least one manufacturer, however, is experimenting with an IC system that will not be based on drum vibration, but will instead correlate the drum's rolling resistance with material stiffness. If the system is found viable, it may allow the IC concept to extend to non-vibratory machines and may provide an alternative method for padfoot machines to determine stiffness values when working in cohesive soils.
In most instances, the recording and documentation systems that form an integral part of the IC package can be either the conventional type (requiring the operator to manually start and stop the recording process along a measured length of the jobsite), or the GPS-based or robotic-total-station-based type, which automatically provide centimeter-level positioning. As the IC process becomes more widely used, GPS-based systems likely will predominate, but conventional systems (or the ability to revert to a conventional system) will still be viable in areas not conducive to good GPS signals.
Intelligent compaction is not without its critics, and much of that criticism is directed at the questioned effectiveness of IC systems working on asphalt pavements. For example, some contend that for compacting hot-mix asphalt, IC technology is not sufficiently developed to recognize the difference between stiffness created by an increase in the material's level of compaction, and that created by the cooling of the mix and the subsequent loss of asphalt-cement fluidity.
Others question the IC system's ability, when working on thin lifts, to distinguish between surface stiffness and that of underlying layers, whether base materials or previously placed mats. And, there are those who maintain that accelerometers, the basic sensors in IC systems, do not work effectively at measuring asphalt stiffness. Of interest here is the Geodynamik Asphalt Compaction Documentation (ACD) system, which uses proprietary algorithms, not accelerometers, to assess the compaction level of asphalt. Also, the Dynapac IC system for asphalt (currently being developed) will not use accelerometers.
Another criticism leveled at IC rollers is that they are unable to provide reliable stiffness measurements when the drum is in the process of adjusting its force. Proponents of the IC approach, however, counter that systems (which vary amplitude to vary force) are designed to select a "fixed" amplitude during the adjusting process and, thus, can be trusted.
Criticism aside, the combination of academic research, in-field experimentation and actual experience of contractors using IC machines — both on soils and asphalt — will surely, meticulously and objectively, help sort out the true capabilities of the IC process.