Introducing new roller technology to the asphalt paving industry is ongoing, and with good reason. Much of the technology is centered on achieving an improved pavement quality and is often combined with lower application costs. Naturally, forward-thinking contractors are willing to try any cutting-edge applications technology that potentially can be more profitable for them. Wider margins that result from lower paving production costs enable the contractor to realize greater profits and be in position to submit more competitive bids.
A contractor known for evaluating new technologies by trying them out on its construction projects is Rea Contracting Ltd., Charlotte, N.C. Rea Contracting is a wholly owned subsidiary of the Lane Construction Corp. and has 70 years of experience as a leading highway, paving and bridge contractor. Today, it is one of the biggest contractors of its kind in North Carolina.
Hiring a new manager
Rea Contracting is using a high-tech BOMAG vibratory roller on the biggest highway construction project now under way in North Carolina. Actually, there are two contracts associated with the construction project. Contract C200893 is held by Rea Contracting at a value of $99.5 million. Contract C200970 is held by R.E. Goodson Construction Co. Inc. for $76.3 million. Both contracts involve the relocation of a section of U.S. 74 to build I-74, which is considered a Strategic Highway Corridor by the North Carolina Board of Transportation. I-74 is a partially completed interstate that, when completed, will link Iowa to South Carolina.
When this project is finished, the new highway section will be 20.23 miles long with four travel lanes (2+2) divided by a variable median 46 to 70 ft wide. Each travel lane will be 12 ft wide with paved inside shoulders 4 ft wide and paved outside shoulders 12 ft wide. Rea’s contract is for constructing 11.3 miles of the highway and a cloverleaf. Highway construction stipulations call for bringing the highway alignment to the specified grade, building a drainage system, paving the travel lanes and shoulders and installing guardrails and other safety structures.
Rea Contracting started its section of the project at the beginning of December 2004, and by the end of 2006, 40% of the work has been completed. It is projected that the job will be completed at or before the end of 2008.
The high-tech roller being used on this project is a BOMAG BW190AD-4 AM (Asphalt Manager) tandem-drum vibratory roller. Central to an AM roller is the Variomatic system that takes the guess work out of achieving the wanted degree of compaction with preciseness and efficiency. Generally, the AM roller is positioned as the breakdown (lead) roller in a paving compaction train.
Here is how the Variomatic system functions on the roller: In the breakdown position and at the start of the compaction process, all of the roller’s vibratory drum energy is directed into the freshly laid “soft” asphalt. As compaction progresses throughout several passes by the roller, the vibratory drum energy increasingly is reflected back to the drums. The rate of the energy reflection correlates directly to the degree of material stiffness. As the material stiffness increases, less and less energy is needed to reach the desired material density. Essentially, a changing response to the increased material stiffness is created that is read by accelerometers installed in the roller’s front drum. A microprocessor installed in the operator’s cab in turn receives this reading from the accelerometers, computes the input data and redirects or vectors the front drum’s vibratory energy. The result of this precise density monitoring and generated energy placement control ensures no over- or under-compaction of the material.
Achieving precise compaction densities with the least number of passes is very difficult for even an experienced operator to achieve using a conventional roller.
Just-right compaction
Leading Rea’s people on the project is Bill Moyers, P.E., and project manager. There are 75 people under his management for excavating, grading and paving. Despite a very wet autumn in 2006, Moyers said, the project is on schedule.
A total of 5.9 million cu yd of earth is being excavated to bring the entire roadway to specified grade. Following fine grading, a 254-mm-thick stone sub-base is laid and compacted. Next is the asphalt paving base, which is compacted to 70 mm, followed by an intermediate course 60 mm thick after compaction. Lastly, the surface course is laid in two 50-mm compacted lifts. All of the asphalt paving for this project is Superpave, with the Rea Contracting part of the project totaling 500,000 tons of hot-mix asphalt (HMA). This HMA quantity includes what is needed to pave the four shoulders, which consists of two 50-mm lifts.
NCDOT specifies a minimum density of 92%. Clarence Douglas, quality control supervisor for Rea Contracting, said the result recorded through nuclear testing on the first 10,000 tons of compacted surface course paving has not deviated significantly from 93.3%. Dominick Barilla, superintendent of public contracts for Rea Contracting, supervises seven of the company’s paving crews, including the crew on this project. He said optimizing the density with a BOMAG AM system can help reduce production time and labor costs associated with it, yet experience no under- or over-compaction results. Simply, the optimized density is easier achieved using the new BOMAG technology than if a conventional BOMAG, or another make of tandem-drum vibratory roller, were used, he said.
A major benefit to quality, uniform compaction, according to Lawrence Burke, quality assurance superintendent for NCDOT, is it will afford the paving greater longevity.
“The more uniform the density is and the more consistent the specified percent of air voids there are, the longer the road’s usable life will be. Plastic flow is another concern connected to the quantity of air voids. We [NCDOT] want air voids in the paving to be about 6% and that the air voids are contained in a paving that is not permeable to surface water. The paving needs to contain just enough air voids to enable it to expand and contract according to temperature changes, but not so much as to make the paving permeable,” he said.
“Long year” feels positive
Here is a lineup of equipment used for paving the two-lift surface course on this project: A Roadtec SB-2500 material transfer vehicle leads, followed by a new Blaw-Knox PF-5510 paver. For the inside lane and shoulder pass, the paver is set up to make a nominal 16-ft-wide strip, followed by the outer lane and outer shoulder passes, both that are nominally 12 ft wide. Each seam between the lanes is overlapped by 6 in. so the two surface course lifts’ seams do not correspond. For example, if the inside lane lift is 16 ft 3 in. wide, the second (top) lift is 15 ft 9 in.
The paving train continues on with the three new BOMAG rollers in line. It starts with the BW190AD-4 AM tandem vibratory roller used in the breakdown position. Its operating weight is 26,896 lb. The working width is 79 in. with the front drum featuring the Variomatic system. Douglas said it requires only three passes with this roller in the vibratory mode to bring the material to specified density.
The intermediate roller is a BOMAG BW11RH static pneumatic-tired roller. Its operating weight, including water ballast, is 27,000 lb. It features nine (five on the front axle, four on the rear axle) overlapping pneumatic tires, which brings the working compaction width to 68 in. Two compaction passes are made when the asphalt material is about 190°F.
Bringing up the rear as the finish roller is a BOMAG BW278 tandem vibratory roller. This roller has an operating weight of 20,600 lb and a working width of 78 in. It commences rolling once the asphalt material has reached 150°F. Two passes are required with the drums set in the static mode.
In order to ensure exceptional ride smoothness, Barilla measures the first surface course lift rather than wait for measuring only the final lift. Measuring for smoothness is accomplished by using a straight-edge measuring device. “For achieving the optimum riding characteristics on the roadway, I evaluate the surface smoothness results starting with the first 50-mm lift. If there are any corrections to be made to this lift, they can be made immediately. If we wait and make the paving corrections after the final [second] lift is put down, it will compound our correction problems. It is both very time consuming and more costly,” said Barilla.
Unlike some smooth ride test systems that are either installed on or pulled by a motor vehicle, Barilla uses a Hearn walk-along straight edge. Riding on large-diameter bicycle-like wheels, the straight edge can easily be either pushed or pulled by one person. Barilla has no problem staying up with the paving train. The measurement results are automatically recorded on a roll of graph paper. Any dramatic road-smoothness deviations recorded can be corrected straight away. Barilla said that smooth paving results using the present paving train have been outstanding, and with the second lift in place will result in receiving bonuses from NCDOT.
So far, both Moyers and Joe Bailey, NCDOT resident engineer, are very satisfied with the results of the paving. “It is 2007 that will be the ‘long year’ for paving on this project, but I am confident we will continue to achieve very good results. We have put down enough paving in 2006 with quality results that I see no reason why it will not continue,” said Moyers.
Bailey agreed with Moyers and sums it up as follows, “This project is going well, which includes the paving results, and a major reason is we [NCDOT] have two outstanding contractors [Goodson and Rea]. Both companies have a good reputation for doing quality work.”
