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Intelligent compaction: The next big thing?
Caterpillar, Bomag, and Ammann demonstrated their intelligent compaction equipment at an open house on July 17, 2006, at the MnROAD test facility in Albertville, Minnesota.
Photo courtesy Mn/DOT
Intelligent compaction may be the next big thing in pavement construction. By combining several advanced technologies, manufacturers are producing compactors that show operators real-time information about the degree of compaction achieved throughout an entire construction project.
This leading-edge concept was described in two presentations at the 2006 Minnesota Pavement Conference. John Siekmeier, a senior research engineer at Mn/DOT’s Office of Materials, and Dean Potts, Advanced Design Group Engineering manager at Caterpillar, discussed the promise and the challenges ahead for intelligent compaction.
How do you measure strength?
Siekmeier began by reviewing the history of soil strength measurement. He described the currently accepted approach as a “three-legged stool”: By controlling moisture content and density, we are able to estimate and control material strength.
Intelligent compaction defined
Potts defined an intelligent compactor as a piece of equipment that measures soil/asphalt compaction, displays the measurements to the operator, records and maps the compaction results using global positioning system (GPS) coordinates, and controls (or guides) the machine compaction effort in response to the measurement system.
The equipment creates a map showing the quality of compaction across the entire area of each lift of material in a project. It also allows the operator to target problem areas, increase compaction where needed, and prevent over-compaction. And last, the speakers said, it archives all collected data for use if problems arise and to define requirements for later projects.
Mn/DOT and many other agencies across the United States are working on pilot projects with the goal of redefining compaction acceptance criteria and quality control/quality assurance procedures for subgrades and bases. Since moisture control is one of the “legs” of the stool, these projects are evaluating not only intelligent compactors but also various moisture measurement devices.
How they do it
Dean Potts described three different approaches to compactor-based compaction measurement:
- Compaction meter value method. A drum-mounted accelerometer measures G-force at the vibratory drum’s fundamental frequency (typically vertical accelerations only) and at harmonic frequencies. This information is used to control the drum’s amplitude.
- Force vs. displacement method. This is more of a direct measurement technique in which drum-mounted accelerometers and position sensors measure stiffness and send signals that control drum amplitude and frequency.
- Energy or power method. The system measures the driveline power needed to roll over soil or asphalt and corrects for grade and machine acceleration. This is the only approach of the three that works for both vibratory and non-vibratory compactors.
Asphalt compaction
From research on the use of intelligent compactors in asphalt projects, Potts drew the following conclusions:
- The condition of underlying material is critical to the successful compaction of asphalt surface layers.
- Measuring asphalt compaction also measures the compaction of the base and subgrade materials. To what degree or depth depends upon the asphalt thickness and which of the three measurement-feedback systems is employed.
- Changes in asphalt temperature make significant changes in asphalt stiffness. Measurements need to be corrected for temperature.
- Changing amplitude and vibration frequency from first to last passes can significantly reduce the number of passes required to reach full density; this is more significant in asphalt than in soil compaction.
The speakers reached the following conclusions:
- Intelligent compaction produces better and more consistent compaction, which results in fewer road failures. This reduces cost and disruption of service—and increases safety.
- Intelligent compaction records will be useful in analyzing road performance so future roads can be designed to be even more durable and provide greater value.
- Intelligent compaction is now possible at reasonable costs due to innovations in GPS, machine controllers, and sensors.
- Intelligent compactors guide crews during construction and allow precise and efficient inspection via proof rolling.
- Equal numbers of passes do not create equal compaction when base and sub-base are not uniform in soil type, gradation, moisture, or when there are problems in the deep structure below fill.
- Increasing the compaction effort on a soft spot does not always bring it up to the same level as the surrounding material, in which remediation of some sort is needed. Moisture content is an important variable.
- The operator can determine when an area has reached the desired compaction level, or if the area will not reach the desired compaction without remediation.
- Over-compaction can be controlled by automatic amplitude adjustment (machine input) and by reducing the number of passes (operator input).
- The operator of an intelligent compactor gets immediate quantitative information, which results in a better job with less wasted effort. Therefore, the importance of the operator is enhanced. Thus, the word intelligent in intelligent compaction refers to both the mechanical system and the operator.
For more information: http://mnroad.dot.state.mn.us.
—Dick Kronick, LTAP freelance writer
This article is reprinted from the 18-page Minnesota Pavement Conference Session Summaries, produced by Minnesota LTAP and online at www.mnltap.umn.edu/publications.