A look at high-quality base construction. 

Just as any structure needs a strong foundation, so does a road. A road base serves as a structural component to provide strength; it can serve as a drainage layer to carry water out from under the pavement; and it produces a strong, level platform for the paving equipment.

But before the base goes down, many agencies stabilize the subgrade with a chemical such as lime, Portland cement, or fly ash. Depending on the soil type, the state of Kansas uses all three of those materials for Interstate highway subgrades.

“We use lime for the heavy clays, Type C fly ash for silty soils, and Portland cement for the sandy type soils,” says Andrew Gisi, a geotechnical engineer with the Kansas Department of Transportation. The chemical reaction of lime with clay causes the fine-grained clay particles to expand a bit and to form agglomerates that are less susceptible to moisture changes than is the more expansive clay. “The agglomerates won’t give up moisture and they won’t take on moisture,” says Gisi.

 

Above, from left to right: a panoramic view of a 24-inch-thick rock base fill on the Ozark Mountain Highroad in Missouri; stabilizing a grade with cement kiln dust in Oklahoma; and a Lafarge pugmill plant for making roller-compacted concrete.

If left untreated, many clay soils take on moisture and cause differential swelling in the subgrade. Such swells can cause undulations in asphalt pavements and random cracking in concrete.

Gisi says Type C fly ash contains some lime that reacts with the clay in Kansas’ silty soils, and causes the clay particles to expand to silt-size particles. Once the clay particles have reacted chemically, the pozzolans in the fly ash will cement the particles together and prevent them from absorbing or giving up moisture. In the case of sandy soils, the Portland cement binds the particles together and increases subgrade strength.

For all highways with treated subgrades, Kansas requires the contractor to cut the subgrade to a precise elevation using a trimmer controlled by a stringline. And when placing the base over the subgrade, the contractor can use the same stringline to control grade and cross-slope, Gisi says. 

For a 2.5-mile project on MacArthur Boulevard in Oklahoma City, contractor Haskell Lemon Construction is stabilizing the base with 5 to 6% cement kiln dust, says Bob Lemon, a vice president with the firm. CKD is a low-grade cement by-product of the cement manufacturing process.

“We use a road stabilizer to mix it in about 8-inches deep, and add water to get the chemical reaction going,” says Lemon. “Then when it’s mixed, we go in with the padfoot roller and make five or six passes. It’s fluffy behind the stabilizer, and we can get it compacted easily.

“We leave it about one-tenth (foot) high,” Lemon says. “Then we’re using a trimmer and stringline to cut it to the precise grade we need.”

On an Interstate 40 project in Oklahoma City, Haskell Lemon is placing base and paving with asphalt on two inside shoulders over what was a grass median strip. For the base, the contractor is placing two 4-inch lifts of 1.5-inch-minus limestone and compacting it with 12-ton double-drum rollers.


Missouri specifies asphalt-treated permeable base under Interstate pavements for drainage purposes.

Getting the precise grade is relatively easy, Lemon says, because crews can use the existing Interstate pavement as a benchmark. “We’re running a Topcon laser system off the existing pavement, and we place base up to 8 inches below the pavement,” says Lemon. “We use two small dozers to push aggregate away from the trucks, and our motor grader has a laser receiver on it. We have portable concrete barriers set up to separate our guys from traffic on the Interstate.”    

To bind, or not to bind?

Aggregate bases come in two types — bound and unbound. An unbound base, one with no binder as glue, is less expensive; but a bound base provides a stiffer, stronger layer, says John Donahue, a pavement engineer with the Missouri DOT. Often, a base that is bound with Portland cement or asphalt cement will permit the pavement designer to lessen the thickness of the pavement section itself.


A common 4-inch crushed stone base is placed by a spreader in Missouri.

Kansas, for example, lets the contractor decide whether to treat aggregate bases with cement or asphalt, depending on which one is more economical, Gisi says. Either base is typically 4- to 6-inches thick for heavy-duty pavements.

With an unbound base, the principle is to use larger aggregates that can transmit water, Donahue says. You want to avoid placing fine particles — that can draw in moisture through capillary action — against the bottom side of the pavement. The idea is to drain water away from the pavement. “If you get too many fines in a base, it will plug up the material and make it susceptible to frost heave and expansion,” says Donahue.    

For Interstate highways, Missouri will use a bound base directly under the pavement. This base consists of an open-graded aggregate, 4-inches thick, bound with 2.5% asphalt cement or 250 pounds/cubic yard of cement (two and a half sacks). “That aggregate is very unstable, so we treat it with a percentage of asphalt cement or Portland cement,” says Donahue. “Below that layer we place a 4-inch granular layer that acts as a filter to prevent the top layer from plugging up.” Typically, that bound base is placed with an asphalt paver or a belt placer.

For a road like that, Missouri specifies edge drains of perforated PVC pipe that run parallel to the edge of pavement. At intervals of say, 300 to 500 feet, 4- to 6-inch-diameter perforated lateral pipes connect to the edge drains and carry water directly to the ditch.  

Shot rock where available

The other base for heavy-duty pavements in Missouri consists of an 18-inch layer of 12-inch-minus shot rock, says Paul Corr, senior project manager for St. Louis-based Fred Weber Inc. Topping that will be a 2-inch-thick layer of 1-inch-minus aggregate — typically cut to grade with a trimmer.

The shot rock is typically placed with a bulldozer. Corr says that by the time the dozer and trucks run over it, the material is pretty well compacted to the maximum. With the shot rock — unlike the bound aggregate base — you don’t need edge drains. Missouri specifies that the shot rock layer run all the way to daylight at the slopes of the cross section.

“One of the beauties of the shot rock is that it will bridge over soft spots in the subgrade,” says Corr. “Sometimes, we mix in lime or fly ash to dry up the subgrade.” He says geogrids, such as a Mirafi product, can bridge over an unstable spot. “We used Mirafi’s BX1200 and it worked wonderfully to bridge over a soft spot at our project at the St. Louis airport,” says Corr. “We used 20,000 square yards of it there.”

Weber often installs the 2-inch topping material with a motor grader equipped with a Global Positioning System. “If we use a GPS motor grader, we’ll bring in a trimmer and stringline system to make the grade precisely correct,” says Corr. “Or the robotic motor grader, which runs off total stations in lieu of satellites, is a lot more accurate than a GPS system.” 

Corr says the shot rock base is often easier to build than the base with smaller aggregates because the thick shot rock is more weatherproof. “You’re driving on rock, so you don’t have to worry about rain,” says Corr. “That 4-inch layer of 1-inch material is pretty easy to lay out with the GPS grader, but rain can give you headaches if the dirt gets wet.”

Roller-compacted concrete in Canada

It’s not done very often, but Lafarge-Edmonton Construction has built road bases from thick layers of roller-compacted concrete. Lafarge makes RCC using a twin-shaft pugmill instead of a conventional concrete batch plant. RCC does not flow; it is much drier than conventional concrete; you add just enough water to hydrate the cement. RCC costs about 30% less than conventional ready-mixed concrete, say Lafarge officials. It is typically hauled in tandem-axle end dumps and is compacted with rollers.

An ABG dual tamping bar paver is used to place roller-compacted concrete on Yellowhead Freeway in Edmonton, Alberta, Canada.

In 1997, Lafarge used RCC to pave the approaches at five intersections on Yellowhead Freeway, a major arterial through Edmonton, Alberta. Lafarge would mill out 12 inches of existing asphalt starting Friday night, then start paving with 10 inches of RCC by Saturday afternoon or Sunday morning. Allowing for no cure time on the RCC, the contractor would place a 2-inch overlay of hot-mix asphalt and open the road for traffic Monday morning.

For the most part, Lafarge uses RCC to pave large equipment yards, log yards, and such heavy-hauling facilities. To pave with RCC, Lafarge has three ABG Titan asphalt pavers, which are dual tamping bar machines. “The ABG paver is very effective at compacting the RCC coming out of the paver,” says James Eustace, Lafarge marketing manager. “If you don’t get that compaction coming out of the paver, the uncompacted concrete will develop humps and dips under a roller.” (ABG is now owned by Volvo Construction Equipment following Volvo’s buyout of the Ingersoll Rand road-building equipment.)

A spreader places aggregate base in Kansas.

So how have the RCC intersections held up for the past 10 years? “They’ve done pretty well,” says Eustace. “Because it’s concrete and we get a lot of frost movement, there’s been some movement of the slab. There are no joints; we’ve tried that and it doesn’t work. The concrete has developed some random cracking, and it does reflect up through the asphalt. The city of Edmonton did seal the cracks once.”

Eustace says Edmonton officials are “happy as hell” with an RCC project on Meridian Street, this one paved in 1999. The project got 9 inches of RCC, followed by an asphalt overlay three years later. “It’s excellent,” says Eustace. “It’s held up very well.”  

The Painstaking Paving Process at a Speedway

A whirlwind of construction activity wrapped up in July to rebuild the Bristol Motor Speedway, a NASCAR racetrack in Bristol, Tennessee. Crews worked night and day to finish the project in just 15 weeks.

The high-banked oval track is 0.533 mile around, and it is one of only a few such tracks in the country to be paved with concrete. The slope of the track varies between 15% in the straight-aways to 57% on the turns. “It’s hard to walk on the track, let alone pave it with concrete,” says Jim Hosea, project manager for the contractor, Baker Concrete Construction, Monroe, Ohio.

To hold the track in position on its slope, designer Dan Zollinger specified a heavily reinforced lug, or footing, that is 4-feet deep by 1-foot wide and encircles the track at the base. The top of the lug is right at the top of the lean concrete base beneath the track’s 7-inch concrete pavement. “That lug acts like a compression ring to prevent the track from sliding down the hill,” says Rob Ford, Baker’s project coordinator.

Two other lugs form half-moons in aprons on the turns — 14 feet inside the large oval lug. The aprons between the half-moon lugs and the oval lug are heavily reinforced with steel so that together, the lug systems hold the track in place on the steeply-banked turns.

To place the lean concrete base and pave the track, Baker Concrete used a Gomaco SL-450X cylinder finisher that was specially designed just for the Bristol racetrack. The paver was 40-plus-feet wide, spanned the entire track, and ran on rails. The top rail was attached to brackets mounted on the upper concrete crash wall. And the bottom rail was mechanically fastened to a footer on the interior of the full-oval lug.

A Putzmeister Telebelt, 110-feet long, placed the lean concrete base in one pass and the concrete pavement in a second pass. The placer boom telescopes in and out hydraulically to place the material at varying locations. For the base, the contractor left the surface unfinished, to improve the bond with the concrete pavement surface. “The paver only advanced in 8- to 10-inch increments,” says Hosea. “It was a very slow moving operation.”

Hosea said it was a major challenge to pave the transitions from the gently sloped straight-aways to the steeply sloped turns. The upper end of the paver moved upward while the lower end moved downward — all the while maintaining a parabolic curve in the pavement surface.

“Achieving that transition was difficult because you’re trying to put a warp in the machine that it was not designed to accommodate,” says Hosea, “But we made it work.”