By John Duval, P.E. • Scott Barnes, P.E. • Bob Weber • Geno Liva, P.E.
In order to improve the quality of hot mix asphalt (HMA), the Montana Department of Transportation (MDT) has moved to the exclusive use of volumetric acceptance criteria for Superpave mixes. MDT initiated the volumetric specification in 2003 and has successfully used the system on all major projects since.
Up until 2003, MDT accepted HMA mix on its highway projects based on conformance to the job mix formula (JMF) with incentives/disincentives based upon contractor-established aggregate gradation targets. Under the old system, MDT established the asphalt content in the field and paid the contractor separately for the asphalt binder used on the project.
MDT has been monitoring volumetric properties on field-produced HMA using portable laboratory trailers since 1984, giving the Department a comfortable understanding of volumetric properties and how they affect HMA. With the advent of Superpave, MDT took steps to improve the quality of raw materials in their mixes to include crushed, clean, more cubical aggregates and modified asphalt binders.
These improvements came at significant cost to the Department and they were viewed as a critical step to meeting targeted volumetric properties during design. But when production started on Superpave projects, MDT was still accepting these mixes on the basis of aggregate gradation alone. MDT Testing Engineer Scott Barnes sums it up in saying, “We had good success with Superpave, but we knew it was capable of much better.”
From the contractors’ point-of-view, once production started, there was little incentive to produce mix that met the state’s volumetric targets. In fact, HMA producers complained that to stay competitive, they could not afford to slow production or halt work altogether to troubleshoot the plant when volumetric targets were not being met. As long as the mix that was loaded in the truck conformed to the established JMF, contractors generally did not alter plant settings.
Time for a Change
In 2000, MDT began actively working toward end-result volumetric acceptance criteria for Superpave mixes. Barnes recalls, “After years of reviewing the data, we recognized that a volumetric end result specification, with adequate incentives, would level the playing field and allow quality to compete with quantity.”
Over the next three years, Barnes and a dedicated group of MDT engineers studied the issue, held discussions with HMA producers in the state, and continued to monitor volumetric properties on MDT Superpave projects throughout the state. With the help of technical and industry experts, MDT established that Superpave mixes would be accepted based on four criteria: air voids (Va), voids in the mineral aggregate (VMA), voids filled with asphalt (VFA), and the dust/binder ratio (D/Pb). Together with the Hamburg Wheel Track test, these four factors are used exclusively as acceptance criteria for Superpave mixes in Montana.
Barnes also knew that simply changing the way quality was measured would not alone affect the quality of the HMA mixes being produced in Montana. He needed the buy-in of the HMA producers and contractors in the state. The HMA producers in Montana had reviewed the Department’s historical data on volumetrics and saw potential risk in the variability of the data. “We were tentative supporters of volumetric criteria,” explains Dwayne Rehbein, General Manager of Riverside Contracting, Inc. of Missoula. “We knew it was the right thing to do, but we simply did not know how these changes would affect the bottom line.”
Controlling the Binder Content
According to Rehbein, he and other producers supported the effort as an alternative to the way MDT controlled asphalt content on Montana highway projects. HMA producers were responsible for crushing, sizing, and blending aggregates. They had control over mix temperature and the overall rate of production. Contractors determined how trucking, paving, compaction, and other details of construction were handled. Yet, while they controlled nearly every other facet of production, they did not control asphalt content, which remained under the authority of MDT.
Clearly, asphalt content has a direct effect upon the volumetric properties of the mix, including the air voids of the compacted mat. With MDT controlling asphalt content, Montana’s HMA producers’ hands were tied when attempting to meet target volumetric properties and density criteria for the compacted mix. “We were motivated to support this system because we wanted to get MDT out of our control shacks,” explains Rehbein. “We felt that we could better control the final outcome of the mix if we also controlled the asphalt content of the mix.”
As they continued to move forward in developing the specification, MDT established a system of pay factors that provided contractors a maximum incentive of 12 percent for achieving all four volumetric targets. Table 1 shows the pay factors used for volumetric acceptance of plant-mix. Each of the four volumetric criteria can earn a maximum incentive of 2 percent. An additional 4 percent incentive is applied to the lot payment if the sum of the individual pay factors for the lot is 6 percent or higher.
In 2003, MDT began including a special provision into contracts that detailed the volumetric acceptance criteria and pay factors. For that first season, the new volumetric special provision was added to two projects by change order. In the years 2004 through 2006, all the larger paving projects have been constructed using volumetric acceptance criteria. To date approximately 136 paving project have been built using end result volumetric specifications.
Table 1 – MDT Incentive Pay Factors
MAXIMUN PAY FACTOR
|Voids in Mineral Aggregate (VMA)||1.02|
|Voids Filled with Asphalt (VFA)||1.02|
|Voids in Total Mix (VTM)||1.02|
|Dust/Binder Ratio (D/Pb)||1.02|
One of those 136 projects was paved in August 2006 twenty-five miles north of Missoula where US Highway 93 snakes its way across the Flathead Indian Reservation. Near the town of Arlee, Montana, MDT used volumetric acceptance criteria on an overlay project constructed by Riverside Contracting, Inc. Except for the way the mix was tested and accepted, this project looked similar to many others across the country. Windrows of Superpave mix were laid on the grade of this four-lane highway and fed into the hopper of the paver.
Bulk samples of mix were taken from behind the screed for testing at the MDT field laboratory parked near the jobsite. The plant-mix was compacted in the field laboratory. At this point, the four volumetric properties of the mix were calculated and entered into the MDT Quality Assurance computer program. Lot pay factors were then calculated based entirely on the volumetric criteria measured in field lab.
MDT test results on this project showed that the mix met all acceptance criteria on each of the 14 days of paving. The average HMA mix incentive paid to Riverside Contracting on this project was 8.3 percent.
After three seasons, it is clear that one of the dividends of the new system is an intense focus by HMA producers on the quality of their mixes. “We have learned more about our asphalt plants and how to control them than we ever thought possible,” says Riverside’s Rehbein. Confirming a shift from focusing entirely on meeting high production rates under the old system, Rehbein continues, “Now, we’ll halt production to troubleshoot our system if the mix is substantially out of specification.”
It may be several more years before the Montana asphalt community knows whether the focus on quality during construction will improve long-term performance. In the meantime, MDT has charted a new course toward total reliance on volumetric properties for acceptance of Superpave mixes and has no intentions of turning back. Full control of HMA production has been handed to the contractor, who is in the best position to adjust plant settings to achieve volumetric targets. MDT is confident it can rely on the volumetric properties of the mix to determine acceptance, incentives and disincentives. This effort truly represents a major shift in priorities for both the contractor and MDT, producing favorable results on both sides. All indications are that HMA mix quality is high and that Montana’s step toward volumetric acceptance criteria is paying dividends to both the motoring public and the plant mix producers in the state.
|John Duval is the Asphalt Institute Field Engineer for the Pacific Northwest and Rocky Mountain Region.
Scott Barnes is the Testing Engineer for the Montana Department of Transportation.
Bob Weber is a Construction Reviewer for the Montana Department of Transportation.
Geno Liva is the State Bituminous Engineer for the Montana Department of Transportation.