Pavement performance depends on many factors. Mix designs, ambient and mix temperatures, rolling methods, joints, and types of equipment used can all be analyzed and tweaked to deliver a better surface. Although contemporary mix designs and proper asphalt rolling techniques can enhance the end result, major improvement of the surface can begin before the first layer of asphalt is laid.
A book by J.T Pauls and J.F. Goode called Public Roads, published in 1939, speaks of the relationship between the top and subbase. It states, "Recent laboratory and field investigation have repeatedly emphasized the value of thorough consolidation in both the base and surfacing courses. The importance of compaction in highway construction has long been recognized." If proper compaction techniques were recognized in 1939 as the critical first step in highway construction, what progress has been made in almost 70 years?
A uniform subbase remains essential to maximizing pavement performance. A well-laid base decreases permeability, increases strength and reduces settlement. Paving contractors on federal-aid highway projects are now being held accountable for these characteristics. In 35 states, inferior or superior work is awarded penalties or bonuses, respectively. Although road warranties are just taking hold in the United States, many European countries have a long history of warranties for pavement construction and performance. Developed independently, through government or private industry, it is agreed that these warranty programs have collectively improved the quality of their respective highway systems.
Apparently more effective, how do the stringent European construction methods differ from domestic standards? Intelligent Compaction (IC) is a technology that improves the accuracy and efficiency of subbase, lift and surface compaction. Developed in Europe in the 1970s, IC is now gaining momentum in the United States as its benefits become move evident. It is being recognized and endorsed by leading government agencies as a positive catalyst for the entire paving process.
A Technology Whose Time Has Come
At the basic level, the vibrating drum of a roller exerts centrifugal force. This is expressed in terms of frequency, which is vibrations per minute, and amplitude or vertical distance covered. Devices then measure how the force is deflected by the subbase and display a stiffness value for the surface being compacted. The operator must monitor this value on a gauge or screen then compare it against the established ideal determined by a series of on-site density readings. If the compaction values are higher or lower than what is desired, rolling must stop or be repeated if necessary.
Intelligent Compaction (IC) technology was explained by the FHWA in a 2005 report. They cite vibratory rollers equipped with a measurement/control system that automatically controls compaction parameters in response to the materials' stiffness measured during the compaction process. Required equipment includes a documentation system for roller passes and material stiffness readings, plus a positioning system.
By using GPS, the location of compaction value and roller passes can now be mapped and recorded. But, the operator still must react to the information and exercise control. True Intelligent Compaction is achieved when amplitude and frequency are adjusted automatically in response to the stiffness feedback from the vibrating drum — without operator intervention. An IC system can collect data, analyze data and then make a decision to execute — at the rate of 1,700 to 4,000 times per minute. The potential to collect and dynamically react to accurate real time test data is a compelling metric.
Mapping Uniformity
Uniformity of construction helps avoid costly pavement failures and improves performance. Aided by GPS, IC density and roller pass readings can be printed out as documentation and archived. This eliminates guesswork, avoiding over- or under-compacting subgrade materials. A real time, constant data stream proves much more accurate than random point data sampling within a project. These roadmaps can literally be used to support warranty data or be incorporated into a pavement management system. As small areas that go bad require replacing large areas, finding them first is cost effective. Tracking, comparing and analyzing the relationship of the subbase and aggregate will allow faults to be predicted and prevented. Using IC technology not only saves time and money in the short term, higher quality and uniformity are even greater long-term benefits.
As with any new technology, the distinction between being a trend and a true innovation is defined by the long-term benefits it delivers. Intelligent Compaction is an important improvement to an existing method of work. Automating the compaction process is cost effective plus maximizes accuracy by eliminating human error. Continuous data sampling ensures uniformity, improves long-term performance and reduces risk making it invaluable for warranty work. The data collected also become design tools for future projects.
Terminology Shift From Density to Modulus
Testing for density has been the standard method to confirm that proper compaction levels have been achieved. Because more precise tests have been developed, new terms are now being used when evaluating compaction. Stiffness is the measure of a material's ability to resist deformation under load. Stiffness and density have a cause/effect relationship. The higher the stiffness, the denser the material becomes. Density is the absence of air, the ratio of mass to volume. Water content is not factored in.
Another relatively new term is modulus. Sometimes used interchangeably with stiffness, it is defined as expressing the measure of elasticity or resistance in relation to some other factor or factors. Modulus may be a better indication of the ability to resist deformation under load rather than just density. Unique factors affect modulus, not just the absence of air. Subbase characteristics can change in a matter of feet. These cannot be predicted but must be tested for repeatedly. Density should not be taken alone as the only parameter to determine proper compaction. Other variables affect the relative strength of a subbase to support weight.
The current goal of compaction is uniformity, based on relative density and moisture limits. However, many more factors contribute to overall behavior during compaction. Moisture, stress level and history, loading rate, and age are all variables. Evaluating and establishing target values by a few random samples loses credibility when all the elements involved are considered.
In the medical world, the X-ray was an established, accurate way to scan the body and examine bone structure, the densest element of the body. The CAT scan (computed axial tomography or CT) was then invented and provides a much clearer picture of the bones plus surrounding tissue. Similarly, testing for modulus with IC interprets the reaction of the subbase to the G-force of the vibrating roller, taking into consideration how all the elements contribute to the reaction. Just like CAT scans revolutionized medicine, Intelligent Compaction technology will modernize test methods and open doors for a wide range of applications.
Different Manufacturers, Different Methodologies
Like comparing apples and oranges, the relative accuracy of current intelligent vibratory compaction equipment differs by manufacturer. Each engages various technologies and compaction formulae, a wide range of algorithms with an inconsistent set of factors. The effect of compactor speed, vibration direction, amplitudes, and frequency is determined in dissimilar fashion.
Bomag's compaction management system (BCM) measures stiffness based on plate-loading test parameters. They correlate force to ground deformation or subsidence, accounting for both the depth and breadth of the area involved. The relationship between the soil contact force and the deflection of the roller drum is the value measured.
Compaction Meter Value (CMV) was pioneered by Geodynamik. This technology has a drum-mounted accelerometer sensor that monitors stiffness by measuring G-force at the drum's fundamental and predictable frequencies. The amplitude variability is then adjusted up or down. CMV is used by Dynapac, CAT, HAMM, and Geodynamik.
Compaction Control Value (CCV) is the index used by Sakai. As the stiffness under a vibrating drum increases, the centrifugal force is deflected or returned. The acceleration waves of the drum then become more random as the soil is compacted. CCV calculates this relationship in non-linear equations based on levels of compaction. Dave Brown, VP of Sales and Marketing, Sakai America, says, "It's not always a good thing to be the first one on the block to market a new technology. Sakai always prefers to do homework first to ensure we design in the best features to benefit our customers. Intelligent Compaction is no exception. This technology demands that we dot our i's and cross our t's."
Technologies that support GPS, wireless communications and data archival will remain viable through many product generations. In the near future, QA might specify modulus and strength rather than density; IC equipment could then perform as a QC device. In this broader role, IC will improve quality by integrating design, construction and performance objectives.
Intelligent Compaction At Work
Headquartered in Waukesha, Wisconsin, Payne & Dolan was recently awarded over half of the contracts for the $810-million Marquette Interchange project, the most extensive freeway project in Wisconsin state history. Payne & Dolan will be responsible for all the asphalt, an innovative "perpetual pavement" that utilizes a long-lasting, all-asphalt recipe over a base of crushed stone. Payne & Dolan recently completed paving on I-75 near West Branch. This was the largest asphalt job in Michigan, laying 426,000 tons over both aggregate base and rubblized concrete. John Bartoszek is their corporate manager of Technical Services and Engineering. He had evaluated comparable compactors from different manufacturers for the I-75 job.
Because Michigan regulations dictate a required density for aggregate courses used in pavement construction, the ability to measure density correctly was critical. After extensive in-house testing, Bartoszek chose Sakai SV510D compactors because their CCV (compaction control value) correlates more closely with nuclear gauge density readings and repeatability made it the most accurate intelligent compaction system. He also found Sakai's engineers very proactive to work with in addressing Payne & Dolan's needs.
The SV510D was designed for high-yardage compaction jobs on a wide range of soil types and rockfill. This includes highway and airport subbases, embankments, and large commercial tracts plus high-risk areas such as dams or reservoirs. This machine delivers 50,710 pounds of centrifugal force over its 84-inch-wide drum. Dual frequency and dual amplitude allow for the optimum vibration setting in any application; this setting is controlled by a switch in the operator's station.
The SV510 comes in smooth or padfoot as well as a combination drum configuration unique to Sakai. Typically, a padfoot shell is removed to expose a smooth base and ends up reducing the diameter by 4 inches to 6 inches. This dramatically changes the geometry between the vibrating drum and the rear wheels. Instead, Sakai designed a smooth drum shell that is mounted over a padfoot base. The roller only changes diameter by 2 inches. "This design element alone solves the traction problems caused by other conversion methods," explained Stan Rakowski, Technical Service manager, Sakai America. "It only takes two hours to change the drum and can be done on the job site."
Payne & Dolan realized other benefits on-site. Bartoszek observed, "Intelligent Compaction allows an entire paving operation to operate more efficiently. Over-rolling of unbound aggregates is limited or eliminated and densities are more uniform. Inspections are minimized plus we had better communication with the MDOT inspector." Road warranties are also in effect in Michigan, one of 35 states with some sort of warranty provisions on federal-aid highway projects.
These are positive stipulations to warranties, according to Bartoszek. He believes, "A warranty challenges a contractor to use value engineering and best management practices when building a project. Doing your best creates a win-win situation for the state, the contractor and the end users — the drivers."
Top-Down Endorsement of IC
The FHWA, DOT and TRB all have initiatives to promote IC technology. They are driven by the same ambition: to automate the compaction process, eliminate over- or under-compaction, and reduce risk by obtaining continuous real time assessment of job quality during construction work.
Among the state DOTs that are pro-actively promoting IC, MnDOT held an open house and demonstration of IC at their Mn/ROAD Facility in 2006. They also recently completed a six-mile project on Highway 64. This project was one of the first in the United States to use IC on a full-scale project basis. The benefits realized were immediate. Guesswork was eliminated, avoiding over- or under-compacting subgrade materials. The constant data stream proved much more accurate than point data within the project. Time and cost savings were actualized; higher quality and uniformity was credited to the IC technology utilized.
The FHWA produced a report, "Intelligent Compaction Strategic Plan," in 2005 that establishes a five-year plan to study IC and implement the technology. An offshoot of this is the Transportation Pooled Fund Program TPF-5(128), a FHWA study led by 12 states to accelerate development of IC QA/QC specifications and create a knowledgeable IC expertise base within participating DOTs. IC equipment will be identified and researched as QA/QC testing devices; simplifying IC use, cost effectiveness and improving accuracies are the objectives. The TRB (Transportation Research Board) technical committees identify and research critical problems. One of high priority is "Intelligent Compaction for Embankments and Pavements: Needs and Challenges." It should be clear to any paving professional that IC is something they need to know about.
Pros and Pros
As a QA/QC tool, IC is literally ground-breaking technology. Championed by the federal government, the industry shift to using soil modulus instead of density as a key parameter is historic. Intelligent Compaction has opened the door to a new cause/effect relationship between QA and QC. Top down, quality can be specified and designed into roads. Bottom up, contractors can exercise quality control in ways never thought possible. Risk factors can be eliminated before they occur.
Initially, IC requires an investment in sophisticated equipment. But, the advantages of IC far outweigh the outlay of finances and effort necessary to change the way people work. It seems obvious Intelligent Compaction will deliver a positive return on both investments. IC is the solution to many problems no one knew needed to be solved, until the technology was available. That time is now.
| Acknowledgements |
| Information for this article courtesy of Sakai America |