A new tool for improving asphalt pavement compaction
By Bob Horan, P.E.
How important is the compaction process in obtaining a quality, long-lasting asphalt pavement? Virtually everyone agrees that it is more than important—it is critical. Poorly compacted pavements just do not perform up to expectations. That poor performance is unacceptable in an environment where the cost of materials and construction are at all-time highs.
A new technology is coming on strong in the United States that has the potential to dramatically improve the compaction process. The same technology also looks promising to provide exciting new tools for compaction acceptance. It is called Intelligent Compaction or IC for short.
While experience with IC overseas and in this country shows that applying this technology to granular subgrade and aggregate base compaction is a slam dunk, the jury is still out on whether it can be used effectively to improve the compaction process of asphalt pavement materials. But, things are looking very promising.
What is Intelligent Compaction?
Intelligent compaction is a relatively new technology in the United States, but has been used for quite some time overseas. IC was observed by government, industry and academia personnel during a European Scan trip to identify improved compaction technologies. According to the Federal Highway Administration (FHWA), “Europeans have over 10 years experience with Intelligent Compaction for soils and asphalt. Once our Scan Team saw it, we knew we wanted it in the US. It was a matter of making our case to DOTs and industry…”
IC technology is most applicable to vibratory rollers, although some manufacturers are studying ways to use the technology on static drum rollers. The principle behind vibratory rollers is that compactive effort is applied using the dynamic force created by the weight of the roller and rotating eccentric masses in the roller drum. Most commonly, IC technology is based on the use of accelerometers to measure the materials’ response to the vibratory impulses being applied by the vibratory rollers to the pavement.
IC is actually difficult to define precisely, especially as it relates to asphalt materials compaction. This is because manufacturers are taking different approaches to the development of the IC concept. When defining IC, one must remember that it is not just one thing—it is a number of things that all work together as a system. A general definition of IC is that it is a system of hardware, software and analysis equipment that are installed on rollers with the purpose of improving the compaction process.
IC rollers provide the capability to continuously determine a measured value of the material during the compaction process, to adjust compactive effort on-the-fly and to develop color-coded mapping of the roadway being compacted. Those capabilities have the potential to revolutionize the compaction equipment industry and to eventually allow for fundamental changes in the compaction process and also in the compaction acceptance procedures.
How Does IC Work?
In its most basic form, the main function of an IC roller is to improve the effectiveness and efficiency of the compaction process. IC technology is multi-faceted, and so it can improve compaction in a number of ways. First, it can provide real-time, continuous feedback to the operator or other project personnel during the compaction process. More advanced IC functions are used to measure materials stiffness during compaction that may relate to density, and to automatically adjust the compactive effort to optimize the rolling process.
Ultimately, in order to be truly intelligent, a roller must be able to make and execute a “decision.” The FHWA has suggested a broad definition of IC. This definition is meant to provide a long term goal for the development of IC technology that FHWA would ultimately want to achieve in the United States. This definition is based on three basic capabilities that must be present to make a roller “intelligent:”
- A stiffness measuring and recording system
- A GPS-based documentation system that is capable of producing color-coded mapping of pertinent information during the compaction process
- An automatic feedback system that is capable of varying compactive effort based on measured stiffness.
The system where all three of these components work together will make it possible, in theory, to continuously optimize the effectiveness and efficiency of the compaction operation for contractors and provide new acceptance methods for agencies. However, as the technology develops in this country, roller manufacturers have chosen to introduce IC rollers that do not have all three of these features. Early trials has been shown that rollers that have only one or two of these IC capabilities are still allowing for major improvements in monitoring and executing compaction of asphalt mixtures.
Is IC Technology Available?
Although IC technology on single drum soils rollers is more advanced in the United States, the technology to apply IC capabilities to tandem drum rollers that are used for asphalt compaction is coming on fast.
Four manufacturers are currently offering IC technology for tandem drum asphalt rollers in the United States or are rapidly developing the technology. The four manufacturers are BOMAG, Caterpillar, Dynapac and Sakai America. While Caterpillar, Dynapac and Sakai are still developing their tandem drum technology, BOMAG has been offering IC asphalt rollers for sale to contractors for several years. A fifth manufacturer, Case, plans to introduce tandem drum rollers with IC technology into the United States in 2008. As can be expected, the technology varies considerably from one manufacturer to another.
What Does an IC Roller Look Like?
As always, asphalt compaction is accomplished using tandem drum rollers. But, whether single drum rollers used for soils and aggregate compaction or tandem drum rollers used for asphalt, rollers that are equipped with IC technology look very much like conventional rollers. That is until you get up in the cab and take a closer look. That is because manufacturers typically modify their standard rollers with IC hardware and software. In the photo of the BOMAG BW 190 AM tandem drum roller, you may not even notice the difference until you get up in the operator’s seat. Then you will notice that a visual display, a documentation system and other IC related hardware have been added.
Special Issues for Using IC for Asphalt Compaction
Some experts say that the same IC technology used effectively for soils and aggregate base compaction will not work for asphalt pavement. In fact, some major roller manufacturers have come to that conclusion. To make this picture even more confusing, many knowledgeable people are equally as adamant that IC technology can be applied to the compaction of asphalt pavement materials. If IC technology has been shown to improve soil compaction, why is there uncertainty about applying the technology to asphalt?
Everyone can agree that there are some special concerns with applying IC to asphalt. The concerns are as follows:
- Thin lift applications. With the exception of some asphalt base courses, asphalt materials are constructed in relatively thin lifts. Because of this, it is speculated that the stiffness values measured during the compaction process may not always be just be a function of the density state of the asphalt mat but may also be influenced by the underlying materials.
- Temperature versus binder viscosity. Thin lift asphalt applications cool rapidly during the compaction process, which results in related increases in binder viscosity as the mat cools. Since increasing binder viscosity will translate into increased measured mix stiffness values, increases in stiffness will occur strictly based on lower mat temperatures. Because the aim is to measure stiffness increase as it relates to density increase, mat temperature needs to be considered in asphalt IC applications.
- Varying subgrade support. A high percentage of asphalt applications are overlays on existing roadways, which include many pavements with poor subgrade support. In order for IC to work effectively, changes in stiffness measured must be totally a function of the density of the asphalt mat. Therefore, it is best if the underlying material provides a uniformly stable platform for compaction.
- Mat temperature measurement. Currently, IC rollers measure the surface temperature of the asphalt mat. Due to many factors, surface and internal temperatures can be significantly different. The internal temperature of the mat is the one that needs to be measured.
Is IC needed?
Do current compaction and acceptance procedures need to be improved? That is a rather broad question but the short answer is “yes.” The consensus among knowledgeable government and industry people is that the desired density is not achieved consistently enough in asphalt pavements as they are constructed. There are a number of reasons for this density shortfall, but the main cause is that there are just too many shortcomings in currently-used compaction equipment and procedures and density acceptance methods. IC technology may eventually address these concerns.
Another group who thinks the compaction equipment and process can be improved is the roller manufacturers. This is evidenced by the fact that at least five major manufacturers of compaction equipment are now developing IC technology. They would not be doing that if they did not think that there was a need and, more importantly, they did not think their customers would buy their Intelligent Compaction technology. See table (below) for a list of roller manufacturers that are known to be working with roller IC technology for tandem drum asphalt rollers.
|
Manufacturer
|
Tandem Drum IC Status
|
Measuring System
|
Auto Feedback Control (y/n)
|
Global Positioning System
|
Documentation Type
|
|
BOMAG Americas
|
Available*
|
Vibration Modulus (Evib)
|
Y
|
N
|
Strip Chart
|
|
Case
|
Planned***
|
Stiffness (k)
|
?
|
Y
|
Mapping
(Temp., Roller Passes, k) |
|
Caterpillar
|
Under Development (Avail. 2008)**
|
none
|
N
|
Y
|
Mapping
(Temp., Roller Passes) |
|
Dynapac US
|
Under Development (Avail. 2008)**
|
none
|
N
|
Y
|
Mapping
(Temp., Roller Passes) |
|
Sakai America
|
Under Development (Avail. ?)**
|
Compaction Control Value (CCV)
|
N
|
Y
|
Mapping
(Temp., Roller Passes, CCV) |
| *Available status means that technology has been developed and is available for purchase from the manufacturer in the UnitedStates; i.e. BOMAG BW 190AM, which is a tandem drum vibratory roller that is equipped with BOMAG Asphalt Manager Intelligent Compaction (IC) technology, is readily available for purchase. As on 9/12/2007, approximately 39 had been shipped in the United States. |
**Under Development status means that technology is currently being developed in the United States. Rollers are available for research but are not available for sale; i.e. Sakai America has conducted two demonstrations/tests of their tandem drum roller and has another test planned. | ***Planned status means that the manufacturer has indicated that they plan to develop IC technology for asphalt compaction in the future in the United States. But, a roller is not currently available; i.e. Case has told FHWA that they are planning to begin importing Case/Ammann tandem drum rollers in 2008 and that IC technology will be included in that effort. |
Another indication that IC technology is promising is that federal and state government agencies are showing a lot of interest in learning more about this technology and implementing the use of IC around the country. Thirteen states have agreed to participate in a pooled-fund study (along with the FHWA) to study the application of the technology on a number of different paving materials, including asphalt pavement. That study is now getting started and will have the mission of accelerating the study and implementation of IC technology in the United States.
What are the Benefits of IC?
Improvements are needed in conventional compaction equipment, processes and acceptance procedures with the ultimate goal to improve density. Based on what is being learned from work with IC technology overseas and in this country, IC appears to have the capability of providing substantial benefits in each of these areas. Specifically, IC has been shown to provide the following benefits:
- Improved density of pavement materials including reduction in variability of density
- Enhanced productivity of the compaction process based on the possibility of reducing the number of necessary roller passes and the time of compaction
- Continuous measurement and recordation of materials stiffness values that likely relate directly or indirectly to density
- GPS-based color-coded mapping of materials stiffness, mat temperature and roller passes, to provide real-time feedback to the operator and other project personnel during the compaction process
- Innovative acceptance procedures based on measurement of stiffness and mapping capabilities
- Identification of areas requiring additional compaction
- Improved depth of compaction
- Ultimately, improved asphalt pavement performance and reduced highway repair costs.
Bob Horan is a Regional Engineer with the Asphalt Institute.