Everyone familiar with asphalt construction recognizes the importance of compaction to pavement performance. Compaction equipment manufacturers are refining their product lines to offer machines that more efficiently achieve asphalt compaction.
The manufacturers are using a variety of approaches to achieve optimum results. Some manufacturers are developing equipment using the “Intelligent Compaction” (IC) approach. Other manufacturers are looking at a different means of applying compactive energy while some companies prefer to supply compactors with more conventional operational features.
The IC concept uses the sensing capabilities of the roller to make adjustments to the machine’s operating characteristics. Generally, in order to meet the Federal Highway Administration’s (FHWA) goal of being capable of “intelligent” compaction, a roller should have:
- A means of measuring and documenting mixture stiffness
- A GPS-based, color-coded map of the roller’s coverage of the mat during compaction
- An automated system of adjusting the compactive effort based on measured stiffness.
(At this time, few, if any, manufacturers are providing rollers with all three IC capabilities.)
Asphalt magazine contacted several compaction equipment companies to learn about their approach to improving compaction of asphalt pavements. A summary of their comments follows.
BOMAG Americas, Inc. is one of the companies that promotes the IC approach. BOMAG calls its IC system “Asphalt Manager.” According to Chuck Deahl, National Accounts Manager, their system works by monitoring the vertical movement of the drum as measured by an accelerometer. Initially, the roller vibrates the drum vertically, as the mat stiffens, the roller shifts to an angled impact and finishes with horizontally applied vibrations. This shift in impact results in less chatter and less broken aggregate on the surface of the mat.
BOMAG introduced their first IC asphalt compactor on a project in western Colorado four years ago. They now have three full paving seasons of experience with IC on asphalt. BOMAG offers IC on their BW190AD-4AM double drum asphalt roller. (Currently, for all manufacturers, the IC approach applies to only one drum of an asphalt roller. The second drum operates as a standard vibratory roller.)
Deahl believes that IC offers several advantages. He describes IC as a tool to accomplish three goals: density, smoothness and balancing production among the three rolling phases (breakdown, intermediate and finish). Deahl says that IC-equipped rollers measure stiffness, not density. But he says that stiffness can be correlated to mat density by monitoring the stiffness readings and density gauge readings for the roller passes. He also explains that an IC roller can function as a proof roller. The read-outs of passes over the subgrade or aggregate base can be reviewed for particularly low readings. Deahl predicts that some type of IC will be used on most rollers in five years.
The Hamm Compaction Division of Wirtgen America, Inc. has elected to incorporate another type of compactive effort into their rollers rather than develop an IC compactor at this time. According to Bruce Monical, Marketing Manager, Hamm’s top-of-the-line rollers use an oscillating force for compaction rather than the common vibrating type of compaction.
Oscillation compaction works by “massaging” the asphalt mix from side to side, rather than by forcing it downwards by an up-and-down impact. With oscillation compaction, the drum never leaves the mat. Unlike traditional vibratory compactors that achieve compaction by “bouncing” the drum on the asphalt mat, Hamm’s oscillation technology ensures that the roller’s drums maintain constant contact with the ground for faster, more effective compaction.
In the oscillation drum, two eccentric masses turning in the same direction cause a movement around the drum axle. The movement changes its direction of effect during one turn so it generates an oscillating or rocking movement of the drum. Horizontal forces are transmitted from the drum into the pavement. The result is better compaction in fewer passes, with less vibration-related wear and tear on operators and surroundings.
According to Monical, experience has shown that oscillatory compaction is suited for specific applications. It has been shown to be particularly effective in the intermediate rolling position, for rolling longitudinal joints and in urban settings.
Hamm’s oscillatory rollers are available in three drum widths, 59, 66 and 78 inches.
According to Todd Mansell, Product Applications Manager for Sakai America Inc., Sakai is developing Intelligent Compaction capability for their asphalt rollers. They have been conducting field tests since May 2006 and will have demonstration rollers in the United States in the spring of 2008 to participate in various states’ field trials.
Sakai describes intelligent compaction as “… a developing technology for real-time measurement that allows greater control of the compaction process.” They see IC as a means of measuring, real-time, whether compaction goals are being achieved.
Mansell states that IC provides a density map of the entire project and the information that is displayed and recorded can help train operators to improve rolling patterns. He says that IC can contribute to a more uniform quality of pavement and thus is a particularly good choice for projects operating under Percent-Within-Limits (PWL) specifications and for warranty work. IC-equipped rollers can provide real-time display documentation of stiffness values that are correlated to density, number of roller passes and the temperature measurements on the job.
Sakai is working to develop their second level of intelligent compaction system. In addition to the rolling pattern, mixture stiffness, and temperature monitoring capabilities of IC, they are developing a more sophisticated system that will include integration software that is compatible with AutoCad™ or similar site development software for greater job management capability.
Sakai will display IC on their asphalt machines at CONEXPO in March 2008.
According to Dale Starry, Director of Strategic Technologies for Volvo Road Machinery, Volvo (formerly Ingersoll-Rand) will continue to focus on their successful line of vibratory rollers. The Volvo double drum vibratory roller line is split into two parts.
The smaller Volvo units are designed for commercial applications and smaller projects where maneuverability is important and productivity is less important. There are small double drum vibratory compactor models weighing from 1.7 to 4.2 tons with drum widths from 36 to 54 inches. Larger units are designed for highway applications and larger commercial projects where productivity is more important than maneuverability. This portion of the product line spans weights from 7.5 to 16.9 tons with drum widths from 57 to 84 inches.
In addition, the Volvo asphalt compactor product line includes three models of combination rollers (vibratory front drum with four rear pneumatic tires), two models of single drum vibratory asphalt compactors (vibratory front drum with two rear pneumatic tires) and two models of all-wheel pneumatic compactors.
Volvo continues to support the technologies developed by Ingersoll Rand, including SMART drums for the highway-class single and double drum vibratory compactors, patented drum-wetting systems, sequential drum vibration for its large DD models and patented operator platform with rotating seat and console. A unique sliding and rotating enclosed cabin is also in development and field testing.
Starry described his position on the state-of-the-business as being a supporter of Intelligent Compaction for soils work. He also said that he is withholding judgment on IC’s application to hot mix until a reasonable solution can be discovered to take into account variations in stiffness of asphalt due to temperature.
Regardless of the approach to achieving efficient compaction, it is obvious that the compaction equipment manufacturers are committed to furnishing equipment suited to meeting the contractors’ needs. The future of asphalt compaction appears to be very promising.