Concrete Thinking

Jan. 1, 2009

A value-engineering proposal by a Georgia contractor saved the state $11 million from an original $82 million interstate highway construction contract. The state split the savings evenly with the contractor, The Scruggs Co. of Hahira, GA.

The project involved the complete reconstruction of a 9-mile stretch of Interstate 75 in Cook County, GA. The existing concrete pavement consisted of two lanes northbound and two lanes southbound, each with aging asphalt shoulders.

“We changed the phasing and the traffic control to allow for two-lane contra-flow traffic, first on one side, then the other,” says John Romaine, concrete division manager for Scruggs. “That way we could work on one side while traffic used the other side. In the reconstruction process, we’re widening it to three lanes in each direction.” The original traffic phasing called for a complex plan to maintain traffic on both sides during construction, he says.

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Temporary Concrete
The state’s original phasing and design called for a five-lift construction of a temporary one-lane pavement on the median side: two lifts of graded aggregate base and three lifts of asphalt. Instead, Scruggs proposed and built temporary concrete pavements, 12 inches thick, down both sides of the pavement on the northbound and southbound sides alike. That concrete pavement was then torn out, crushed, and recycled for subbase construction under the new concrete pavement.

Photo: Shafer Contracting
A Gomaco 2800 four-track places the pavement on the I-75 project.

In other words, the original plan called for 22 inches of asphalt on both sides of the pavement to be used for shoulders permanently. “Both shoulders would have been 22-inch-thick asphalt, and that was not necessary. By changing the phasing and using temporary concrete, we eliminated the need for 150,000 tons of asphalt,” Romaine says. Instead of the thick asphalt shoulders, Scruggs built shoulders with 6 inches of aggregate base and 4.5 inches of asphalt.

What’s more, the new phasing plan slashed in half the amount of temporary concrete barrier needed on the project. “Originally we would have needed 180,000 feet of temporary barrier, and we reduced that to 90,000 feet,” he says.

And the contractor convinced the state to change the specifications on its temporary concrete barrier to allow for any barrier that meets NCHRP 350, Test Level 3. Originally the state had specified a Georgia standard barrier.

All of those measures helped save the $11 million. A major savings stemmed from the fact that Scruggs only needed a one-lift process to place the temporary concrete—not a five-lift process to place the base and asphalt.

Before paving the temporary concrete, Scruggs used a Wirtgen milling machine to remove the existing asphalt shoulder and cut the grade down to make room for the 12-inch-thick temporary concrete. The temporary concrete was placed with a Gomaco 9500 material transfer machine and paved with a Gomaco Commander III.

When the temporary concrete had been paved on both sides of an existing pavement, traffic could be moved from one side to the other and placed in the two-lane contra-flow configuration. That cleared the way for Scruggs to start work on the opposite side.

The original pavement was 9 inches of concrete on a stabilized subbase. “First we removed the existing concrete and the original asphalt shoulders,” says Romaine. “Then we shaped the subgrade. Some of it was moving cuts to fills and we imported some dirt.”

Scruggs hired a subcontractor, Mullinicks Recycling of Jacksonville, FL, to break, remove, and crush both existing and temporary concrete. It was all recycled as pavement base material.

“We placed the 12 inches of graded aggregate base in two layers across the 50-foot-wide section,” Romaine explains. “And we trimmed that with a CMI-made TR 4503 trimmer. On that subbase we paved a 3-inch inner layer of asphalt—fully across the three lanes and the shoulder.”

Next Scruggs could pave the new 12-inch thick concrete. First the contractor paved a 24-foot-wide slab with a Gomaco PS 2600 belt placer and a Gomaco 2800 four-track paver. After that, the contractor added the 12-foot inside lane and the 14-foot concrete shoulder. That 14-foot lane is striped as a shoulder for now but will permit expansion of the pavement to four lanes later. Concrete was batched with an Erie-Strayer MG-12 batch plant.

Scruggs achieved excellent smoothness on the project. “We paved 72 lane miles of concrete without a single area to grind,” said Romaine. “Our average was under 2 inches of deviation per mile from a 0.1-inch blanking band.” The Georgia specification calls for the contractor to grind areas with 7 inches or more of deviation per mile from the blanking band.

How was that smoothness achieved? “We made sure there was adequate track line with a solid subgrade,” says Romaine. “The stringline was checked daily, and we didn’t touch the slab behind the paver except to shape the edges where necessary.” Slump of the concrete was about 1.5 inches, and the water-to-cement ratio was held at about 0.45.

“It’s been a very successful project,” says Romaine. “The state took our value-engineering proposal and applied it to other projects on I-75. That has resulted in a lot of work for the concrete industry in Georgia.”

Surface Detail
Also in Georgia, Archer Western Contractors is widening a 14-mile stretch of Interstate 85 by two lanes on each side and is placing an 11-inch-thick, unbonded concrete overlay on the existing four lanes. Both the overlay and the full-depth sections are continuously reinforced concrete (CRC) pavements. The Georgia Department of Transportation is the owner for this $218-million project, located on I-85 about 30 miles northeast of the Georgia-Alabama line.

Because the overlay increases the elevation of the new pavement, all three of the bridges crossing the interstate had to be jacked up 15 to 20 inches, says Don Cowan, paving project manager for Archer Western, a division of the Chicago-based Walsh Group. Plus, adding the two additional lanes—on the insides of the northbound and southbound lanes—meant that all seven of the in-line bridges needed to be widened.

Archer Western is achieving superb smoothness numbers on the project. The average profilograph readings are 2–4 inches of deviation per mile from a one-tenth-inch blanking band. By comparison, the Georgia DOT specification calls for a maximum of 7 inches of deviation, or grinding is required. “We’ve only had to do minor bump grinding,” Cowan says.

Much of the credit for those smoothness numbers goes to paving superintendent Duke Vannoy, who is an experienced concrete paving man, according to Cowan. Vannoy pays super-careful attention to the grades of the asphalt base, to the stringline, and to the setup of the paver before it ever starts work.

“We measure our ride daily after we pour to make sure we have no smoothness issues,” says Cowan. “And we check the grade of the asphalt base carefully before we pave. If that asphalt is high or low, we get the asphalt subcontractor back in here to make it right.”

Phase 1 of the project called for construction of a 14-foot-wide, 10-inch-thick concrete pavement to help carry traffic while the new inside lanes were being constructed. Later, that temporary pavement became the base structure for a new 12-foot-wide tied concrete shoulder. The base concrete extends 2 feet beyond its surface pavement and will be covered with soil.

Phase 2 of construction consists of widening the in-line bridges, grading, drainage, and concrete pavement for the new inside lanes. The new full-depth lanes require a 12-inch-thick graded aggregate layer and a minimum of 3 inches of asphalt as a base structure. At completion of Phase 2, in August 2008, traffic was traveling on the inside two lanes on the southbound side as crews began to pave the overlay there.

“Last November we started paving the 11-inch CRC section in the median,” says Cowan. “We have completed all that concrete paving and slip-formed the barrier between the northbound and southbound sides.” The overlay required a 3-inch minimum thickness of hot mix asphalt to be placed atop the existing concrete. In some places the asphalt is thicker than 3 inches because of superelevations in curves and the like.

For mainline paving, Archer Western is using a Gomaco 2800 paver. A Gomaco PS 2600 placer-spreader strikes off the concrete ahead of the paver. Single lanes and ramp lanes are being paved with a Gomaco Commander III. For ramps, the contractor places concrete ahead of the Commander III with a Gomaco 9500 belt placer.

Archer Western is paving the overlay with a 24-foot-wide pass of the two-track paver. That machine will be followed by the Commander III paving 12 feet wide for the outside shoulder.

The new inside lanes carry traffic at a higher elevation than the existing lanes—while the overlay is being paved—so moving traffic through the interchanges up and down (across the existing lanes) proved to be a difficult proposition, Cowan says. Detours alongside the pavement had to be built to give traffic space to accelerate. It required careful engineering—and considerable thought—to locate and build the temporary asphalt ramps at the interchanges that carried traffic across the existing lanes.

The completion of construction is scheduled for the end of August 2009, Cowan says.

The “Unweave the Weave” Project
In terms of complexity, the interchange of Interstate 694 and Interstate 35E ranks right up there in the nation. Located north of St. Paul, MN, the interchange featured a “commons area” that crossed or weaved I-35E traffic with I-694. Because so many commuters had to cross lanes in a very short space, the area created a bottleneck at rush hour, says Frank Weiss, vice president with Shafer Contracting Co. Inc. in Shafer, MN. But now, under a $102.8 million construction project called “Unweave the Weave,” Shafer has greatly increased the capacity of the interchange.

The commons area formerly featured three lanes north/eastbound and three lanes south/westbound. But now the same commons area has six lanes north/eastbound and six lanes south/westbound. Plus, Shafer added lanes to many other ramps, entrances, and exits. “We’ve increased capacity by adding lanes and eliminating the ‘weave,’” says Weiss.

All Concrete
And Shafer paved it all with concrete—308,821 square yards, to be exact, not counting bridges. The depths of pavement are 9, 10, and 12.5 inches, with the 12.5-inch thickness being typical. It took more than 60 bypasses—temporary roadways to carry traffic when the permanent roadways were being built—to complete the project.

“We have had more positive comments from the traveling public in terms of maintaining traffic than for any other project I’ve been associated with,” says Weiss.

Construction began in December 2005 and was substantially completed in August 2008. After removal of the old pavement, Shafer excavated 2,113,000 cubic yards of earth and placed 1,112,000 cubic yards of select granular borrow material, most of it imported. Shafer also built 571,611 square feet of sound walls.

At the peak of excavation work, Shafer was running six excavators, 12–14 bulldozers, approximately 100 trucks, and various compactors and support equipment. Excavating equipment included Caterpillar 345 excavators, Caterpillar 330 excavators, and several Cat D6R dozers. Two cranes placed the sound walls. Shafer also placed 66,750 lineal feet of storm sewer.

The project included the construction of 10 bridges, eight permanent and two temporary. Shafer subcontracted bridge construction to Lunda Construction of Black River Falls, WI.

Many Short Pulls
To pave the concrete, Shafer used Rex and Gomaco pavers working in various widths, but the most common widths were 12, 16, and 24 feet. The contractor used a belt placer where possible; when paving ramps, it usually was necessary to dump concrete on the grade in front of the paver.

The complexity of the interchange meant working in tight quarters. “There were a lot of short pulls,” says Weiss. “We tried to maximize the length of pulls, but we had four stages with 11 phases, and all but one or two had concrete paving involved. It was pretty chopped up.”

That presented difficulties in maintaining pavement smoothness, because construction headers at the end of a pull have a tendency to create a bump in the pavement. “Construction headers create about 90% of the corrective grinding required by the state specifications,” says Greg Pelkey, who headed up concrete paving on the project.

What can be done about the headers? “We take great care when constructing our headers to maximize our ride incentives earned,” says Pelkey. “And we ask surveyors to shoot beyond the end of the concrete to ensure the best possible profile when tying into a previous phase.”

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The state smoothness specification awarded an incentive payment for 4.0 inches of deviation per mile from a 0.2-inch blanking band, using a lightweight inertial profiler. Shafer earned an incentive for much of the project. “We had several areas that averaged 0.0 inches of deviation, and the entire project average was 1.9,” says Pelkey.

“Smooth pavements start with a good grade,” says Pelkey. “We used a stringline, and we manufactured a specially designed finishing pan that we used behind the paver.”

The Minnesota DOT required the concrete to have less than a 0.40 water-to-cement ratio, and Shafer’s concrete averaged about 0.36. The pavement was placed as plain-jointed concrete with zinc-coated dowel bars spaced at 15-foot intervals. Most concrete was placed at a slump between 1 and 2 inches.

The Minnesota DOT says this project is designed to last more than 60 years, and it was featured in a National Geographic documentary called “Super Roads.”