A Base Stacked for Strength
by Daniel C. Brown, Contributing Editor
Better Roads Staff | April 3, 2013
If it’s a super-stout pavement base model you’re looking for, look no further than the Des Moines (Ia.) International Airport. Last summer, Manatts Inc. built a subgrade, base and taxiway there that would prevail against the fury of the Devil himself.
The taxiway is more than a mile long. It is 75 feet wide with 25-foot shoulders on each side. Aided by several subcontractors, Manatts put together a $16.5 million project – using specifications from the Federal Aviation Administration – that has the following elements, taken from the bottom up:
• a 9-inch-thick layer of FAA-specified P152 subgrade – very densely compacted soil;
• one layer of geogrid for stability and strength;
• a 9-inch-thick layer of FAA-specified P-219 crushed, recycled concrete base course;
• a 6-inch layer of FAA-specified P-307 drainable cement-treated base;
• an engineering fabric bond breaker;
• all topped by 16 inches of P-501 concrete.
The existing taxiway was concrete pavement with an asphalt overlay. Manatts, which is based in Brooklyn, Ia., began by milling off the asphalt. Next Antigo Construction, a subcontractor, broke the concrete with a guillotine breaker. Another sub, Reilly Construction, used Caterpillar Model 330 and 345 excavators to load the broken concrete into Cat 740 articulated trucks, which hauled it to the on-site crushing plant.
“At that point, once the pavement removal began, Reilly followed up with dirt excavation,” says Chris Sawin, project manager/estimator for Manatts. “Much of the dirt was hauled away to a waste area. And some of the dirt was carried to a fill area, because we adjusted the alignment of the taxiway. They shaped the new taxiway with Caterpillar D6 dozers and a variety of motor graders.
“We had to meet compaction, moisture and density specifications on all the subgrades,” says Sawin.“That is what the FAA’s P152 spec calls for. You have to get the dirt to optimum moisture content and compact it to 95 percent of modified Proctor density. You really have to put a lot of effort into vibrating and compacting the dirt before it’s actually accepted and ready for rock.”
For subgrade compaction, Reilly used Caterpillar 563 sheepsfoot rollers, as well as a pull-type roller towed behind a John Deere wheeled tractor. “On the areas where there were cuts, they had to disk down 9 inches,” says Sawin. “The engineering firm would test density down to 9 inches.” The one-ply geogrid came next.
Next came Caterpillar 740 trucks hauling 1.5-inch top size crushed concrete back onto the grade. Cat D6 dozers spread and shaped the concrete in two 4.5-inch lifts. “It was compacted with steel drum rollers and a lot of water,” Sawin says. “Again, that base stone had to meet a density specification. They packed it in there tight with Caterpillar steel drum rollers. They had to roll in static mode because they could not break down the aggregate. In-place gradation tests verified that the crushed rock would maintain drainability.”
Then Manatts fine-graded the rock that Reilly had placed. “We trimmed it with a GOMACO 9500 trimmer,” says Sawin. To control line and grade on the trimmer, Manatts used a Leica PaveSmart 3D system with robotic total stations. The system requires no stringline, and it cuts the grade to an accuracy of 3 millimeters.
The 6-inch cement-treated base, laid by a Caterpillar 1055 asphalt paver, came next. “The cement-treated base is kind of like a road stone mixed with cement, and it’s placed with an asphalt paver,” says Sawin. “That’s really the only way to place this stuff, is with an asphalt paver, because it had to be drainable and it sets up hard – almost as hard as concrete. The top size aggregate in that material is 3/4 inch (0.75).”
Manatts mixed the cement-treated base with their nearby concrete plant – a CON-E-CO plant with a 12-cubic-yard drum. “We had very strict guidelines on the compaction of the cement-treated base,” says Sawin. “They wanted it strong and compact, but they didn’t want it too dense because they wanted it to be drainable. We followed it with three roller passes, using a BOMAG roller and a double-drum Hamm roller.
Next Manatts placed the engineering fabric. It serves as a bond breaker, to assure that the concrete pavement does not bond with the cement-treated base below it. And finally, the contractor paved the taxiway using a GOMACO GHP-2800 concrete paver working 18.75 feet wide.
Taking it to the Bank
Faster production brings more dollars to the bottom line. That’s why contractor Tim Hokanson is so pleased that he recently bought a Topcon grade control system for his John Deere 750K dozer.
Traffic control was the major challenge of the 3.1-mile Four Corners North project, says Hokanson, a vice president with A.M. Welles Inc., Norris, Mont.
The $11.5-million project called for widening U.S. 191, just south of Bozeman, Mont., from two lanes to five, including a turning lane. And the state of Montana required that Welles maintain two-way traffic through the project during construction.
“We couldn’t use a pilot car or switch over to one-lane traffic,” says Hokanson. “We always had to be building ahead of ourselves – giving two-way traffic a place to go on one side while we worked on the other side.
“That’s one of the main reasons we decided to buy a John Deere 750K dozer with another Topcon grade control system on it.”
Hokanson already had one Topcon grade control system mounted on a Caterpillar dozer. By adding the John Deere 750K with automated grade control, the contractor could excavate subgrade in one place while filling with base course material and getting ready for traffic in another.
The Topcon automated grade control system sped up his production by about 20 percent, Hokanson estimates. Welles nearly completed the project before winter in 2012. He says the Topcon system slashed about a month from the time it would have taken without one. In just 100 days, the contractor moved 350,000 to 400,000 cubic yards of material for the project.
“We found that the Topcon system is really good,” says Hokanson. “It saves a lot of time. When we saw how well the first one was working, you could tell that there was a huge amount of savings to be made with another one. That’s when we decided to purchase another grade control system and another dozer to put it on, which was the 750K.”
Plug and play
Installing and using the grade control system is a relatively simple matter, Hokanson said. When he was shopping for a new dozer, he found that a major benefit of the Deere-Topcon combination was the speed of installation. “Probably the best feature that I liked, right off the bat, was the ability to plug and play the Topcon system on the dozer,” says Hokanson. “Everything is all set up in the cab and ready to go. You just install it, plug it in, and turn in on. It only needs minimal calibration. I think within two hours it was ready to go.”
Plus, the grade control system eliminates the need for stakes and blue-topping. “The Topcon system probably saved us, on this project alone, with the work that the 750K did, in the neighborhood of $40,000 to $50,000 worth of staking and blue-topping,” Hokanson says.
“That doesn’t count the grading time,” he continues. “It also saved a lot in the amount of equipment being used during that whole time. We used the motor grader very little. We used it to finish off, but we didn’t need stakers, and we didn’t need another dozer to clean up behind the 750K, or the grader to clean up behind it. The Topcon is very effective in limiting the number of pieces that you need. By having the Topcon system we probably saved 80 percent of our motor grader time.”
Simple to use
Hokanson says using the grade control system starts by building a digital model of the project, which can be done off-line, in the winter. “Once the model is built and can be plugged into it, it will control the machine to the elevations that you want to cut or fill to,” he says.
“It does an extremely good job of carrying the grade through and being extremely accurate. We get an accuracy of plus-or-minus three-hundredths of a foot.
A monitor that “almost looks like a video game.”
“There’s a computer in the machine that helps control your elevations,” Hokanson says. “In the driver’s seat there’s a monitor, and it almost looks like a video game, that shows you the actual road.
“You can see where your shoulders are, where the ditches are, how your slopes sit, and the crown of your road. The operator can basically line up the machine with what he’s trying to do, and cut or fill a pass right to grade that can be accepted by the state.
Welles operator Chad Kent says the grade control system is awesome. “It saves a lot of time,” he notes. “You can lay an 8-inch layer of base stone in one pass, and there’s no blue-topping or any of that.”
Kent says the Topcon 3D-MC-squared system is super-fast to adjust the blade when it’s under automatic control. “I can probably run the dozer twice as fast as I could without the grade control,” he says.
Hokanson estimates that the Topcon grade control system will pay for itself in two years. He concludes: “With the size of the project that we have this year, I feel that the Topcon system would pay for itself with two of these projects.”
When Smoothness Pays Dividends
Nick Hilton, of contractor Fred Weber Inc., is very pleased with his new Leica stringless machine control system for guiding a base trimmer or concrete paver. So happy that Hilton and his company aren’t satisfied unless they get smoothness bonuses.
Weber, based in Maryland Heights, Mo., earned 5-percent bonuses – the maximum allowed by the state of Missouri – over a significant share of paving sections on two Highway 364 projects near St. Charles, Mo. On the majority of Highway 364 sections, Hilton says Weber consistently earned a minimum of 3-percent bonuses.
And last year Weber built a new 10-mile section of Interstate 69 near Newberry, Ind. It’s a four-lane divided highway for which the contractor is placing 11 inches of new concrete pavement. Profiler readings indicate that Weber earned an average of three percent bonuses on the project. Average profiler readings for the project ran at a Profile Index of 2 inches deviation per 0.1 mile. For anything less than 2 inches per 0.1 mile, bonus payments graduated upward to 106 percent of full pay.
The pavement base consists of 6 inches of 1-inch minus base stone topped by 3 inches of clean crushed stone. To place and trim the 3-inch layer, the contractor is using a GOMACO PS-2600, with a 7500 series rock spreader attachment, controlled by a Leica PaveSmart 3D (LMGS-S) machine control system. The GOMACO PS-2600 works 30 feet wide, and Hilton attributes a share of his smoothness results to the stringless control system on the PS-2600.
The Leica system regulates the steering and grade of the trimmer with no need to retrofit complex hydraulics. The system guides the trimmer in relation to a digitized 3D model of the highway, running on the Leica computer onboard the paver.
“When we would pave with a stringline, we were just happy to get 100 percent of our pay,” says Hilton, Weber’s manager of concrete operations. “Now, with the Leica PaveSmart 3D stringless system, we’re not happy unless we’re getting a bonus for smoothness.”
The GOMACO PS-2600 trimmer is equipped with two prisms, mounted above the machine, that serve as targets for the Leica robotic total stations. When setting up the robotic total stations, a technician back-sights each of them to three known control points. That fixes the location of the total stations relative to the digital model. The two total stations then follow the movement of the two prisms on the paver and communicate to the machine computer the trimmer’s precise location via radio link. The machine computer then computes the differences between the trimmer’s actual location and the digital terrain model. Knowing those differences, the Leica computer then instructs the onboard controller to regulate the machine’s steering and grade fully automatically.
For the mainline concrete paving stage, in this case using Weber’s GOMACO GHP-2800 paver, two of four robotic total stations are set out 400 to 500 feet ahead of the GOMACO, and another two stations are set at 800 to 1,000 feet out. As the machine passes the two total stations in front of it, the crew switches control to the two furthest total stations and moves the near ones out further. That way the crew leapfrogs the total stations down the highway – and the GOMACO paver never stops – a capability unique to Leica’s technology.
Less Time, More Bonus Money
Try on this challenge for size: You’ve got from March 6 until November 21 to remove 2.5 miles of six-lane Interstate highway, build 12 bridges, do the excavation and sewers and pave mostly five lanes in each direction with concrete. That’s the job awarded late in 2009 to Milestone Contractors of Indianapolis, for $72.8 million. The project was located on I-465 near Indianapolis.
So how did Milestone do? The contract with the Indiana Department of Transportation carries an incentive/disincentive clause of $60,000 per day. The maximum incentive could go to 15 days of payments. In fact, Milestone and its subcontractors reached substantial completion 16 days early and earned the entire incentive payment of $900,000.
“The biggest challenge was the time frame,” says Scott Cornelius, the project manager for Milestone. “We did some preliminary work through the winter months, but the main construction started on March 6 and had to be completed by November 21. It was a very aggressive schedule. We used multiple crews and did some double-shifting.
“Some night-time lane closures were allowed, but at all other times we had to maintain three lanes of traffic in each direction,” Cornelius says. “We used three main phases of construction. We pushed East-bound traffic towards the middle in the first main phase, and reconstructed the Eastbound outside lanes. In the second phase we pushed that East-bound traffic over onto the new lanes and completed the remaining construction for the East-bound lanes. And then in the last main phase, we moved all of the West-bound traffic over to the East-bound lanes and reconstructed all of the West-bound side.”
Antigo Construction, a subcontractor, broke up the existing concrete pavement with a multi-head breaker. Once an all-Caterpillar fleet of excavators and a dozer had removed the broken pavement, the earthwork began.
Milestone and Berns Construction, a subcontractor, split up the excavation work. “We split the project into East and West halves,” says Cornelius. “Berns performed work on the West half with excavation and sewer work. Milestone performed the work on the East half. We also had 12 bridges and we performed all but two of the bridges. A subcontractor, His Construction, LLC, built the other two.
Earthwork equipment included Caterpillar equipment as follows: a D6NXL dozer, a D5KXL dozer with a GPS system, a D6N dozer with a GPS system, a 330DL excavator, a Model 735 haul truck, a CS563C vibratory dirt roller, a CP563 sheepsfoot roller and a Komatsu PC-308 excavator.
The contract called for 240,000 cubic yards of excavation, some hauled to disposal, the remainder used for fills on site.
When the subgrade was brought to its proper elevation, another subcontractor, Mt. Carmel Sand and Gravel, moved in with a fleet of equipment to accomplish lime stabilization to a depth of 16 inches. Two Mack custom spreader trucks spread the lime and two Mack water trucks added water. One Wirtgen 2400 soil stabilizer mixed the lime into the subgrade. Three Caterpillar machines shaped and compacted the grade: a Model 163H motor grader, a Model 753 vibratory sheepsfoot roller, and a Model 753 vibratory steel wheeled roller.
“Then we placed subbase for the PCC,” says Cornelius. That consisted of a total of 9 inches of crushed limestone. The bottom 6 inches was 3/4-inch dense-graded stone, and the top 3 inches was open-graded material to make it drainable.” For fine grading the subgrade and placing the subbase, a Caterpillar 14H motor grader equipped with Trimble grade control swung into action.
“We worked a six-day-a-week schedule with some work on Sundays,” Cornelius says. “We did some double-shifting, but that was not constant. The pavement removal phase was double shifted, and so was placement of barrier walls for maintenance of traffic. Some miscellaneous bridge activities were double shifted. Concrete paving was not double-shifted.”
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