The multiple values of accelerated bridge replacement

A section of the I-15 Prairie Crossing superstructure is moved toward placement in Utah in October, 2009.

The cost of user delays in an era of unbridled traffic congestion is driving today’s fast-paced bridge erection technology, and it’s being encouraged by the Federal Highway Administration (FHWA) in partnership with active state DOTs.

Those state DOTs are accelerating bridge replacement via use of prefabricated bridge components that are either placed on site, or assembled on site into a superstructure and then installed in one swift action.

The fast-tracking of bridge replacement via prefab sections is only one of a series of major advancements happening in bridge technology. Others include:

* Fiber-reinforced polymer (FRP) composites continue their inroads into bridge design, at the expense of precast concrete and lightweight aggregate. Three new design themes emerged in 2011 as focus areas for the American Association of State Highway and Transportation Officials (AASHTO) Subcommittee on Bridges and Structures: rigidified FRP tube arches, hybrid composite beams, and reinforced thermoplastics technology.

* The growing acceptance of self-consolidating concrete (SCC) is making erection of conventional precast, post-tensioned structures – and those using FRP components – easier as they greatly reduce the need to vibrate concrete mixes into complex steel reinforcement, either in-plant for precast, or on site for poured-in-place.

* Both precast and cast-in-place concrete proponents look forward to concrete-grade coal fly ash escaping designation as a “hazardous waste” as sought by the Environmental Protection Agency (EPA). The classification of fly ash as hazardous waste could introduce chaos into the production of high-performance concrete for bridges.

Every Day Counts

Fast-paced bridge replacement using precast components is a high priority for FHWA and is a critical part of its Every Day Counts (EDC) initiative.

“Every Day Counts reflects a new sense of urgency we bring to our work,” said FHWA Deputy Administrator Greg Nadeau at the second International Warm Mix Conference in St. Louis in October (warm mix asphalt also is being promoted through the EDC program).

EDC aims to make highway and bridge building more efficient and effective, Nadeau said. “FHWA doesn’t deliver projects; we support our partners who support a more effective delivery of the federal-aid highway program.”

That includes fast replacement of bridges by use of what FHWA calls prefabricated bridge elements and systems (PBES) technology, he said. “As a result, bridges are built faster, and with much less disruption to the traveling public and, importantly, to commerce,” Nadeau said. “These techniques and technologies are going to have to be deployed, especially in areas that are experiencing significant congestion. We want to rapidly deploy technology that makes sense.”

With prefabricated bridge elements and systems, many time-consuming construction tasks no longer need to be done sequentially in work zones, FHWA says.

These PBES superstructures are assembled adjacent to or away from the jobsite to limit construction in the right-of-way, as is the conventional practice. “An old bridge can be demolished, while the new bridge elements are built at the same time offsite, under controlled conditions, then brought to the project location ready to erect,” FHWA says.

Benefits, FHWA says, include:

* Reduction of on-site construction time;

* Reduction of environmental impacts;

* Improved work zone and worker safety;

* Lowered initial and life-cycle costs; and

* Improved product quality via a better-controlled manufacturing or assembly environment and cure times, and easier access to components in a plant facility.

“Prefabricated bridge elements especially tend to reduce costs where use of sophisticated techniques would be needed for cast-in-place work, such as in long water crossings or higher structures like multi-level interchanges,” FHWA says.

But while precast, post-tensioned concrete I-beams and box girders have been used in repetitive bridge construction for decades — as in lengthy causeways along the Gulf Coast and in Florida, for example — what is new is the near-complete assembly of bridge superstructures from manufactured components on the jobsite but out of the right-of-way.

“The lion’s share of the construction work is done off-site, usually in a nearby staging area, and the new bridge superstructure is lifted or ‘rolled’ into place,” FHWA says. “This method takes advantage of precast elements to minimize the impact of the project on motorists by reducing the time needed for roadway work zones.”

Prefab elements for a superstructure will include: deck panels, both partial and full depth, precast or steel stay-in-place; I-beams with more efficient designs; and composite decks. Substructure prefab elements can include: pier caps, columns and footings; abutment walls, wing walls and footings; and bent caps.

“Increasingly, innovative bridge designers and builders are finding ways to prefabricate entire segments of the superstructure,” FHWA says. “A substructure system may consist of individual piers or prefabricated bent caps supported by prefabricated columns and/or prefabricated abutment elements. Total prefab bridge systems offer maximum advantages for rapid construction and depend on a range of prefabricated bridge elements that are transported to the work site and assembled in a rapid-construction process.”

An Early Adopter

Famously, the Utah DOT was an early adopter of prefabricated superstructure technology. In October, 2009 – as part of its Corridor Expansion (CORE) program – Utah used giant self-propelled modular transporters (SPMTs) to move bridge spans into place at Pioneer Crossing over I-15 at American Fork, south of Salt Lake City.

A south bridge span over I-15’s northbound lanes for a new diverging diamond interchange was moved into place with SPMTs on a Friday night, and a span over the southbound lanes was moved into place just two days later on Sunday night. Then, the existing four-span bridge was dismantled without reducing the Interstate’s three-lane capacity in each direction.

Then, on a weekend in June, 2010, the north bridge for the interchange was moved into place from a staging area in the northwest quadrant outside the interchange southbound ramp, over a quarter-mile from the bridge. The span over the southbound lanes of I-15 was moved into place on a Friday night, and the span over the northbound lanes was moved into place on the following Sunday night. These bridges over I-15 are the largest multi-girder spans moved with SPMTs in the United States.

The two spans of the north bridge had been constructed on temporary support piers in the staging area. Then, the SPMTs were moved under one 186-foot-long span, with nine 96-inch prestressed concrete Washington State bulb tee girders in the cross section. The span had a 45-degree skew and weighed 2,100 tons. Two lines of SPMTs had to be configured to support the massive span at each end.

Special tower stand jacks raised and lowered the span off the temporary supports and onto the new substructure elements, respectively. Chains were also used to help control the distance between the double lines of SPMTs. On the top of the bridge, piano-like wire was placed at the diagonals of the span to measure any span distortion. To avoid overstressing the deck concrete, only inches of distortion was allowed. The span superstructures were placed late Friday evening into early Saturday morning, and late the following Sunday night into early Monday morning, with minimal traffic restrictions and lane closures.

MassDOT: 14 in 10 Weekends

This summer, Massachusetts DOT achieved a remarkable bridge replacement record, with 14 bridges replaced in Medford, Mass. over 10 weekends from June to August with its I-93 “Fast 14” Rapid Bridge Replacement Project.

Because MassDOT used cutting-edge accelerated bridge construction techniques and materials to replace the bridges, all the bridge and associated work was completed over a five-month period.

“Using conventional methods, it would have taken at least four years to replace all 14 bridges, and during those four years drivers would have had to endure long-term lane closures,” MassDOT says. “MassDOT executed a traffic management plan and a comprehensive communications plan to minimize construction-related congestion and community impacts during construction, which was limited to off-peak hours.”

The innovations MassDOT used to accelerate the bridge replacements include design/build procurement, a prefabricated bridge elements system and a special rapid-setting concrete. “By replacing the bridges with modular superstructure units that were fabricated off-site, MassDOT eliminated years of work in the roadway,” the agency says.

This project was showcased by FHWA, receiving national attention for the innovation it used to get the bridges built so quickly and safely, and for limiting major impacts to road users to off-peak hours.

Prefab Speeds

Access Overpass

Even before Utah and Massachusetts, the Georgia DOT used extensive prefabricated bridge elements and systems to radically reduce the time and cost of a new bridge over I-85 in Troup County, as part of an improvement to provide access to a new Kia vehicle assembly plant there.

On the Utah I-15 project, a self-propelled modular transporter (SPMT) moves superstructure into position between bridge bents.

Georgia DOT Commissioner Gena Evans said at the project’s dedication in December, 2008 that the project was an enormous achievement, considering a tight, 18-month construction timetable that had to be met. Work was finished more than 30 days ahead of that schedule in the largest design-build construction project initiated by Georgia DOT.

“This effort proves that design-build can be successful when applied to the right projects,” Evans said. “Georgia DOT is proud to have played a role in helping to bring new jobs and improved mobility to the area.”

Though Kia was located near I-85, access to the highway was limited. Existing roads could not accommodate the estimated thousands of additional daily auto and truck trips, and a bridge was needed. To expedite construction, Georgia DOT chose prefabricated bridge elements and systems.

“With PBES, innovation could be incorporated into the design without increasing the user costs,” the DOT says. “Conventional bridge construction, using cast-in-place technology and traditional contracting methods, would have required 30 months. With PBES, the project was completed in only 16.5 months.”

The I-85 bridge was planned as a four-span concrete structure with eight columns per bent. Prefabricated elements were used for the substructure’s columns, pier caps and deck beams. The bridge components were cast off-site and shipped to the site on conventional semi-trailers. Each component was carefully cast to within a 0.25-inch tolerance so connections made in the field would fit precisely.

“We’re doing some innovative things, using precast, prestressed columns and caps on the bridge in order to expedite the work,” said then-Georgia DOT District 3 engineer Thomas Howell. “It’s a first in this district. The pieces were actually cast at a yard and brought out, instead of forming and pouring them on-site.”

Safety data sets were collected before, during and after construction to ensure that the innovations did not increase risks. With PBES, no worker injuries were reported. A single motorist incident involved minor vehicle damage with no personal injury. The cost savings with PBES were equally compelling, saving nearly $2 million, or 45 percent, of what the interchange would have cost if it had been built conventionally.

FRP Composites Refined

Even as concrete and steel bridge construction is accelerated via new technologies and techniques, fiber-reinforced polymer (FRP) bridge materials continue to make inroads as their engineering is refined.

Three new FRP technologies were in the spotlight in 2011, with rigidified FRP tube arches, hybrid composite beams and reinforced thermoplastics technology this year being named as focus areas by the AASHTO Subcommittee on Bridges and Structures’ Technical Committee T-6: Fiber Reinforced Polymer Composites.

Concept of the superstructure placement for the I-93 ‘Fast 14’ Rapid Bridge Replacement Project in Medford, Mass. completed in the summer of 2011.

Maine DOT has volunteered to be the lead state, taking on the next step in the implementation process, which will include conducting a market analysis and developing a marketing plan for implementation. Other state DOTs represented on the team include Massachusetts, Michigan, Missouri and New York, along with the Maine Composites Alliance and the University of Maine.

“For nearly 30 years, FHWA has supported research and development technology transfer, deployment and standardization of FRPs as a promising solution for bridge construction and rehabilitation,” Louis N. Triandafilou, P.E., FHWA Office of Infrastructure team leader, Bridge & Foundation Engineering Team, said this summer.

“After a long history of worldwide research, use of FRP composites in seismic retrofits and bonded repairs has become almost commonplace,” Triandafilou said. “Also, highway agencies are applying this technology to a growing number of projects involving bridge deck panels and reinforcing bar and prestressing applications. However, despite widespread government and industry support, there has been little self-sustaining, competitive deployment of this technology.”

Nonetheless, several emerging FRP composite technologies could play an important role in future rehabilitation and replacement, Triandafilou said. “Some promising emerging approaches are focused field applications of rigidified FRP tube arches, hybrid composite beams and reinforced thermoplastics.”

FRP is a general term for polymer-matrix composites reinforced with cloth, matting, strands or other fibers, Triandafilou said. FRP composites consist of thermoset resins, which, once cured, cannot be returned to an uncured state. Reinforced thermoplastic resin composites, on the other hand, can be softened repeatedly by heating or hardened by cooling. In the softened state, workers can reshape these composites by means of molding or extrusion. “FRP and reinforced thermoplastic composites have the potential to create cost-effective, durable and long-lasting bridge structures,” Triandafilou said.

* Rigidified FRP tube arches are derived from a kit consisting of three main components: carbon- and glass-FRP composite tube arches, a self-consolidating concrete (SCC) mix design, and corrugated fiberglass panels, Triandafilou reports. “Once on site, workers inflate the 12- to 15-inch- diameter diam tubes and bend them around arch forms,” he said. “The crew then uses a vacuum-assisted transfer molding process to infuse the tubes with resin. The tubes, which cure in a matter of hours, function as stay-in-place forms for the SCC, eliminating the need for temporary formwork, and provide structural reinforcement for the concrete in the longitudinal direction, in shear, and as confinement, eliminating the need to install rebar.”

* Hybrid composite beams combine the properties of concrete, steel and FRP composites in beam fabrication, Triandafilou says. “This combination results in stronger and lighter-weight bridge members,” he said. Hybrid composite beams offer the possibility of cost-effective spans and corrosion resistance, he added.

* Reinforced thermoplastics consist of 65-percent high-density polyethylenes blended with 35-percent polystyrene or polypropylene glass fibers, Triandafilou says. “The resulting materials have a high resistance to corrosion, rotting and insect infestation, making them excellent candidates for replacing deteriorated railroad ties,” he added. “Reinforced thermoplastics also possess favorable durability and toughness characteristics without chemical additives. Favorable engineering properties such as flexural, compressive and shear stress make these materials a viable alternative for highway bridge applications.”

SCC Boosts

Concrete Bridges

Self-consolidating concrete (SCC), also known as self-compacting concrete, is a highly flowable, non-segregating concrete that spreads into place, fills formwork, and encapsulates even the most congested reinforcement, all without any mechanical vibration, reports the National Ready Mixed Concrete Association (NRMCA). Its use is simplifying bridge construction both in the field and precast bridge component fabrication in the plant.

SCC is defined as a concrete mix that can be placed purely by means of its own weight, with little or no vibration. Adjustments to traditional mix designs and the use of superplasticizers creates flowing concrete that meets tough performance requirements, NRMCA says. If needed, low dosages of viscosity modifier can eliminate unwanted bleeding and segregation.

The flowability of SCC is measured in terms of spread when using a modified version of the slump test (ASTM C 143), according to NRMCA. The spread (slump flow) of SCC typically ranges from 18 to 32 inches depending on the requirements for the project. The viscosity, as visually observed by the rate at which concrete spreads, is an important characteristic of plastic SCC and can be controlled when designing the mix to suit the type of application being constructed.

In precast concrete components, SCC has the ability to eliminate inadequate consolidation in thin sections or areas of congested reinforcement, which leads to a large volume of entrapped air voids and compromises the strength and durability of the concrete. Because SCC is designed to consolidate under its own mass, it has the potential to eliminate this problem.

However, with SCC, when the flow rate is high, the potential for segregation and loss of entrained air voids increases. This can be fixed by designing a concrete with a high fine-to-coarse-aggregate ratio, a low water-cementitious material ratio (w/cm), good aggregate grading and a high-range water-reducing (HRWR) admixture. Viscosity modifying admixtures (VMAs) also are used to reduce the tendency for segregation and enhance the stability of the air-void system.

For the rigidified FRP tube arches described above, the SCC mix incorporates HRWRs to achieve enhanced flowability, and VMAs to achieve stability, eliminating aggregate segregation. The mix also includes set retarders (for stabilizing hydration), shrinkage reducing admixtures, and 0.375-inch pea gravel aggregate.

In a September, 2011 technical paper from the Illinois Center for Transportation, University of Illinois at Urbana-Champaign, Transfer and Development Links in Prestressed Self-Consolidating Concrete Bridge Box and I-Girders, authors Bassem Andrawes and Andrew Pozolo said the American precast industry has taken significant strides to adopt SCC in commercial projects, though concern about early-age bond behavior has limited the material’s application in prestressed members.

Placement of fiber-reinforced polymer (FRP) deck panels on steel girders of a 125-foot through-truss bridge at Maryland S.R. 24, north of Baltimore, near Rock Creek State Park.

To explore the application of SCC in Illinois bridge construction, Illinois DOT and the Illinois Center for Transportation sponsored a three-phase study investigating the bond behavior of steel strands in pretensioned bridge box and I-girders. In the first phase, 56 pullout tests were conducted to compare the performance of seven-wire strands embedded in SCC to that of strands in conventionally consolidated concrete blocks.

In the second phase, transfer lengths of prestressing strands in two 28-foot SCC hollow box girders and two 48-foot SCC I-girders were determined experimentally. In the third phase, development lengths of strands in the four girders were determined through a series of iterative flexural tests.

They found that pullout test results at various ages showed strand performance in SCC to be comparable with strand performance in the conventionally consolidated concrete.

I-girders were found to perform adequately in both shear and flexure even when the embedment lengths were lower than the predicted development length values, which ranged from 73.9 to 81 inches. “With satisfactory pullout behavior and adequate transfer and development lengths, it is reasonable to conclude that the SCC mixture in this study had sufficient bond to prestressing strands,” the authors conclude.

Defending Use of Fly Ash

The Environmental Protection Agency has taken aim at coal combustion fly ash used in precast and cast-in-place concrete, a move that seriously concerns the people who design and build bridges.

Fly ash is the residue of the burning of pulverized coal in thermal power plants. The ash particles are collected mechanically or by electrostatic precipitators. Fly ash is a pozzolan, meaning it is a siliceous and aluminous material that, in the presence of water, will combine with an activator (lime, Portland cement or kiln dust) to produce a cementitious material, according to Fly Ash Facts for Highway Engineers, a publication of the FHWA and authored by the American Coal Ash Association (ACAA).

Fly ash use on federal-aid highway projects was encouraged by its classification as a “recovered” product under the federal Resource Conservation and Recovery Act (RCRA), which generally mandates use of fly ash in cement or concrete in construction projects using $10,000 or more of federal funds.

The pending EPA classification of fly ash as hazardous waste has the potential to disrupt this accepted use of fly ash in the production of high-performance concrete. But legislation protecting fly ash was approved this October by the U.S. House of Representatives, and at press time awaited action by the U.S. Senate Environment and Public Works Committee.

Five Democrat and five Republican senators have filed the bipartisan Coal Residuals Reuse and Management Act (S.1751), creating national disposal standards for coal ash while protecting the material from a hazardous waste designation.

S.1751 is patterned after the bill of the same name that passed the House of Representatives in mid-October, with 37 Democrats voting yes.

Sen. John Hoeven (R-N.D.) observed that states can manage the disposal of coal byproducts with good environmental stewardship while permitting beneficial uses like building bridges, roads and buildings that are stronger and less expensive.

“Years of research have shown that coal ash should not be regulated as a hazardous waste,” said Sen. Kent Conrad (D-N.D.), a cosponsor of the legislation. “Doing so would only force unworkable requirements on our state’s utilities, resulting in serious economic consequences and the loss of good-paying jobs.”

Pavement preservation expert Larry Galehouse talks about tools and strategies that make sense for surviving the Great Recession

By Kirk Landers

“In times like these, the pavement manager’s most basic strategy is to keep sound pavements sound and keep bad pavements from becoming unsafe or unusable.”

So spoke Larry Galehouse, director of the National Center for Pavement Preservation, in an interview with Better Roads.

Indeed, Galehouse, one of the nation’s foremost experts on pavement preservation, sees the diminished road budgets of the Great Recession as a litmus test for pavement management strategies.

“The agencies that have pursued the traditional ‘worst first’ strategy, giving priority to rebuilding bad pavements, are feeling the budget shortfalls most acutely,” he notes. “Agencies that have given priority to prevention — to keep good pavements in good condition — are in much better shape.”

And that homily is Galehouse’s advice to pavement managers dealing with severely constrained budgets. “It costs a lot less to extend pavement life while the pavement is healthy than it does to rehabilitate or rebuild a pavement that has deteriorated too far,” he observes.

The key to executing a pavement preservation strategy is to bring the right prevention tool to the right pavement at the right time, he says. The challenge is to select the treatment with the greatest benefit for that particular pavement, and Galehouse notes that it takes a lot of up-front work to make that diagnosis.

“For example,” says Galehouse, “you need to identify subsurface failure spots, dig them out and repair them before applying a surface treatment.”

And Galehouse stresses that pavement preservation priorities apply to concrete and asphalt pavements alike.

Asphalt Interventions

One of the least-expensive asphalt pavement treatments that Galehouse often recommends today is the use of a rejuvenator shortly after the surface course is laid. A true rejuvenation of an asphalt surface requires the introduction of maltene fractions. Thus, rejuvenators containing maltenes – the oily, resinous component of asphalt – increase the asphalt binders’ resistance to oxidation by improving the chemistry and prolonging its flexibility.

Rejuvenator treatments can be repeated every few years to keep the surface pavement supple and weather-resistant, typically prolonging its life by two to three years. The maltene-based rejuvenator is clear and doesn’t affect paint lines.

“It’s important to get the true, maltene-based rejuvenator if a change in binder chemistry is desired,” says Galehouse, adding that there are many other products on the market. “I suggest getting references from other agencies about how the product worked on past projects,” he says.

Rejuvenators are often applied after a road pavement or airport landing strip has been retexturized, says Galehouse. “Today’s retexturizing technology is fast and inexpensive, and it improves pavement friction.”

The roughened surface accepts the rejuvenator treatment more efficiently and the process improves the tractive qualities of the aggregate. Galehouse warns that pavements with poor-quality aggregate will polish again relatively quickly, while good-quality aggregate will keep its texture for a long time.

Crack Treatments

As highway agencies have migrated to a prevention-first philosophy of road management, emphasis on crack treatment has grown. The process is inexpensive and has been shown to extend pavement life by two years and often more.

Galehouse considers crack treatment an important tool in the pavement manager’s toolbox.

“There are two approaches,” he says. “Crack sealing is a series of steps that first machines a reservoir in the crack, cleans the reservoir with compressed air, and then fills it with sealant. This approach can be very effective when used on the right pavement at the right time.” Galehouse estimates that crack sealing typically extends pavement life at least two to four years.

Crack filling, a process in which debris is blown out of the crack and the crack is filled with sealant, is used for nonworking cracks and wider cracks. Galehouse says crack filling typically adds about two years to the life of a pavement.

“Both of these processes are pothole preventers,” he says. “And the longer you can prevent potholes, the longer you can avoid more-expensive interventions like milling and overlays.”

Surface Treatments

Chip sealing has evolved as rapidly as any preservation technology over the past decade, says Galehouse, as contractors and suppliers have stepped up the quality of materials and application techniques. “It’s more of a science now,” says Galehouse, “though there are still people who don’t recognize it as such.”

Perhaps the most dramatic leap forward in chip sealing will be offered through the SHRP2 (Strategic Highway Research Project) Project R-26 in which chip seals with carefully selected aggregate and emulsion applied with precise construction technique and finished with a fog seal can be placed on high-volume roads. This will finally demonstrate that chip seals can perform excellently on high-volume roads when care is taken in design and construction, says Galehouse.

“A high-quality chip seal applied to a sound pavement prevents sunlight and water from destroying the pavement,” says Galehouse. “It also adds macro-texture to the road surface to enhance traction, and with a fog seal it provides more visibility to paint markings by improving contrast.”

Other surface treatment interventions include slurry sealing and microsurfacing. Slurry seals are a mixture of fine aggregate, emulsified asphalt, water and additives placed by special machines in a thin coat, one stone thick. The slurry seal fills hairline cracks and delays pavement oxidation, and is appropriate for urban/suburban neighborhood roads in good condition.

Microsurfacing is a slightly thicker intervention than the slurry seal, combining polymer-modified asphalt emulsion, crushed aggregate, water and other additives in a carefully specified mix design, and placed by specialized equipment. “Microsurfacing adds thickness to the pavement structure, so it can correct rutting and minor raveling, and improve friction,” says Galehouse. “It is also designed to stand up to high-traffic volumes and heavy loads.”

Overlays

The next stop on the prevention continuum for asphalt pavements is the ultra-thin overlay — typically, 3/4-inch thick or less. “This is an intervention for a sound pavement,” says Galehouse. “With the advances in mix design and placement practices, it has become a very effective tool. It protects the surface of the original asphalt, fills minor imperfections, and improves ride quality.”

Ultra-thin overlays can also be designed to deliver other benefits. Use of a rubberized asphalt binder, for example, can mitigate traffic noise with great effectiveness. An open-graded friction course design can reduce spray during rain and enhance the quality of runoff water.

Thin overlays — up to 1.5 inches in thickness — cost more, but by virtue of their thickness can smooth out deeper imperfections, and achieve greater smoothness that improves the ride quality of the pavement.

Milling is the tool when the surface pavement has deteriorated beyond the point where lesser interventions can restore its condition. Milling is also employed in metropolitan areas where a simple overlay will not align properly with gutters.

“This is the Cadillac fix in the prevention tool box,” says Galehouse. “You mill off a thickness of deteriorated pavement and overlay with new asphalt to restore the ride quality. It’s a lot cheaper than waiting until you have to do a total structural rehabilitation, but for stretching budget dollars, you want to work as far up the deterioration curve as possible.”

In-place recycling technologies can also be very effective in treating aging pavements, says Galehouse. “It’s important to make sure the project you have in mind is a good fit for the technology, whether it’s hot-in-place or cold-in-place,” says Galehouse. “The best procedure is to get a good, reputable contractor to evaluate the project in terms of its appropriateness for (either).”

When in-place recycling technologies are viable, they bring a cost advantage to the project as well as environmental advantages, such as less energy consumption, lower CO2 emissions, and a 100-percent recycling of the existing resource.

Concrete Pavement Interventions

Contractors and pavement managers have developed an elaborate toolbox for concrete pavement prevention, notes Galehouse.

Joint resealing lies at the low end of the cost spectrum, followed by diamond grinding and partial and full-depth repairs.

“Cost analysis is key, especially with the more-expensive interventions” says Galehouse. “You have to weigh the cost of repair against the cost of replacement. So, for example, if you had to replace every other panel on a stretch of road, it would probably make much more sense to just replace that stretch of road.”

In many cases, says Galehouse, concrete pavements just need diamond grinding to remove surface imperfections and improve smoothness. Over a period of time, concrete slabs can settle due to movement of the road’s subbase. Most of the imperfections occur where the panels abut.

“If you have vertical displacement – called faulting – of the slabs, you might consider retrofitting dowel bars to stabilize the joint and improve the transfer of loading between slabs,” says Galehouse. “If they are tied together, just diamond grind it and seal the joint.”

While some pavement managers aren’t convinced that joint sealing improves concrete pavement performance, Galehouse does advocate the practice.

“The important thing is to keep the incompressibles out of the joint,” he says. Incompressibles include foreign objects that can clog joints and prevent the slabs from flexing as temperatures change and cause the panels to expand and contract. “By keeping joints sealed, you keep out the material that can cause blow-ups,” says Galehouse. “Seals also help protect the pavement from water seeping into the base and creating a ‘pumping’ action that forms voids in the subbase and cause cracks and even breaks in the panel.”

Joint seals typically last 10 to 12 years before leaks appear, says Galehouse.

Does prevention pay off with concrete roads? “If we take good care of our concrete roads with the tools we have today,” says Galehouse, “they will last far beyond what we have come to expect — over 50 years for good concrete.”

Coping with Our Times

There are still pavement managers in America who give their worst pavements first priority in budgeting, Galehouse notes, and their systems are suffering the most from the diminished budgets of the Great Recession.

“In good times or bad, the strategy that makes the most sense is to first keep your good pavements good — your dollars go further and your system stays stronger,” says Galehouse. “Then you keep your marginal pavements from deteriorating any further — to minimize safety concerns and the cost of the ultimate repair. And then you rehabilitate bad pavements as dollars allow, starting with safety concerns.”

Galehouse concedes that today’s tight budgets constrict everyone, but those pursuing sound management strategies that stress prevention will outperform the others, he says.

“Agencies that follow an asset management approach will come out of this cycle in good shape,” he concludes.

Larry Galehouse

Larry Galehouse is a licensed professional engineer and a licensed professional surveyor. His experience includes tenure with an engineering consulting firm and with a large state DOT. In 2003, he helped found the National Center for Pavement Preservation located at Michigan State University in Lansing, Mich. Galehouse has been a leader in pavement preservation initiatives within AASHTO, NACE, FHWA and TRB. Contact and learn more about the National Center for Pavement Preservation at pavementpreservation.org.

By John Latta

Britain’s premier war hero, Admiral Horatio Lord Nelson, victor of the Battle of Trafalgar, lost an eye in battle early in his career. Years later, he put a telescope to his blind eye, mid-battle, and reported that he saw no signal to disengage the enemy. Such willful willingness to fight is part of Nelson’s legend. In Washington, we see what the Brits might call a “Reverse Nelson.” Our politicians, rather than not seeing what is there, are seeing what isn’t there. They see various user-fee alternatives to higher fuel taxes as saviors of highway funding. Maybe. But not in this decade.

by John Latta, Editor-in-Chief, jlatta@rrpub.com

We also have what we might call a “Half Nelson.” Private investment money for transportation infrastructure is out there, waiting, in very large sums. Our politicians look but don’t see it clearly. They keep adjusting their telescope’s eyepiece trying to focus on exactly where it is, who has it, how it might be invested, and on and on. But nothing is happening except talk and eyepiece fiddling.

We need to light a fire under them. A new report from U.S. DOT (CR-2011-147) Office of Inspector General, entitled Financial Analysis of Transportation-Related Public Private Partnerships, might be a dandy fire-starter. It set out “to (1) identify financial disadvantages to the public sector of PPP transactions compared to more traditional public financing methods; (2) identify factors that allow the public sector to derive financial value from PPP transactions; and (3) assess the extent to which PPPs can close the infrastructure gap.”

The report is not benign. The Reason Foundation’s Bob Poole in his Surface Transportation Innovations newsletter Number 94 has called it, among other things, biased against PPPs, “rather bizarre,” and “a very disappointing piece of work.” The report’s modeling, analysis and assumptions are subject to Poole’s sharp criticisms, and Poole says anti-PPP forces are already using it to support their case. FHWA Administration Victor Mendez, rather bravely I thought, reacted to the report by writing a memo to OIG to put his position strongly, and it is included in the report as an Appendix.

I want to be a debate crasher, barging in and stirring things up. Waving this document over my head, figuratively anyway, may just cause enough fuss, enough debate and arguing, points and counterpoints, to light some fires. It may be controversial; I certainly hope it is because that might get people out of talking mode and into doing mode.

The OIG report it is worth reading, including, especially, Mendez’s Appendix. (oig.dot.gov/library-item/5599). And so are Poole’s well-reasoned comments at http://www.reason.org/newsletters/stinnovations/. Maybe we’ll have a three-cornered dialectic, adding a “Full Nelson” of volatile debate to the mix.

The workshop is being held before contracts are advertised for the $1.9 billion Martin Luther King/Midtown Tunnel project to help area business owners participate in the bidding process.

“Small businesses make up the backbone of the American economy,” said U.S. Transportation Secretary Ray LaHood. “Helping small business owners compete for road and highway contracts creates jobs and is an investment in the nation’s economic recovery.”

The MLK/Midtown Tunnel project will ease local congestion and improve Hampton Boulevard between Portsmouth and Norfolk. The project, under discussion for decades, will include an additional two-lane tunnel parallel to the existing Midtown Tunnel under the Elizabeth River. It will also feature a new one-mile, limited-access four-lane roadway to extend SR 58 south to I-264.

“By helping to level the playing field for small businesses, we improve the bidding process while keeping the cost of highway projects low and putting people back to work,” said Federal Highway Administrator Victor Mendez.

Since last year, FHWA has hosted similar meetings around the country to focus on opportunities for minority- and women-owned small and disadvantaged businesses. The meetings helped prepare small businesses to compete for federal projects worth more than $21 billion.

Technologies taking form under the watchful eyes of the National Asphalt Pavement Association and the Federal Highway Administration

By Mike Anderson

There is little doubt that even the most staid and traditional of road agencies and contractors are at least warming to the idea of a cooled-down approach to asphalt production and application only unveiled to the United States nine years ago. The ability to cut fuel consumption, greenhouse emissions and employee risk has that effect.

Some industry stakeholders are even convinced warm-mix asphalt (WMA), regardless of how the ever-evolving category here is ultimately defined, will turn out to be the equivalent to what hot-mix asphalt (HMA) has long been. In other words, there won’t be any more HMA applications per se; they’ll all be what we are now loosely terming as WMA.

With warm-mix asphalt, “it’s important that we add the technology as part of the mix design,” says FHWA’s Mike Corrigan, “That way, we can identify where there potentially could be issues, especially where you have a high-traffic application such as an Interstate.”

The Federal Highway Administration (FHWA) and National Asphalt Pavement Association (NAPA) may not be ready for such a proclamation quite yet; the two organizations, however, continue to team up to try to get the asphalt industry’s collective head around WMA. Their Second Annual International Warm-Mix Conference is scheduled for Oct. 11-13 in St. Louis, Mo. A dedicated website maintained for FHWA by NAPA, www.warmmixasphalt.com acts as an ongoing source of education, news, research and products on the market.

More than 30 warm-mix asphalt products or technologies are being marketed and sold in the U.S., “which has been a challenge for us,” says Mike Corrigan, FHWA program manager, “although a good challenge, because it’s made us focus really on warm mix as a category. Instead of focusing on individual technologies, we’ve had to take a step back and say, ‘Let’s focus on warm mix as a material and what the performance of that warm mix is.’”

Will warm-mix asphalt be the new norm? Time will soon tell.

Speaking at NAPA’s Asphalt in Depth Conference held in June in Nashville, Corrigan did issue a disclaimer about the products listed on the website, ranging from emulsions to mixing systems, sourced from long-established suppliers and market newcomers alike. “The one thing I do like to point out is that just because it’s on warmmixasphalt.com does not mean it’s been through any kind of large scrutiny or evaluation whether it’s appropriate technology. A lot of them have had a lot of successes, but the only criterion for us to really list them on the site is the fact that they have their own website that we can link to, to provide people information. We’re not trying to say that we’re endorsing that technology or that it’s a proven technology, just that there is information available.”

Ongoing research, including at the National Center for Asphalt Technology (NCAT) test track at Auburn University in Alabama, is addressing a wide range of questions, reports NAPA and FHWA:

Can warm-mix pavements be opened to traffic quickly after construction?

What are the performance characteristics of these pavements?

In the case of technologies developed in other countries, can they be adapted to the U.S. where climate conditions are often more extreme?

If the production temperature is lower, does that mean that the binder does not age as much?

Will the potential for thermal cracking be reduced?

Will the potential for rutting be different?

Will the contractor have to use a different grade of asphalt binder?

What changes for the mix design procedure will be required?

And will the performance-graded binder in a warm mix perform differently from pavements produced at a higher temperature?

“A lot of questions get asked about higher-volume traffic – ‘Are we building warm mix on Interstates?’ – and the answer is, ‘Yes,’” says Corrigan, pointing to established usage in such traffic-heavy states as Texas, Florida and now New York.

Speaking at the Nashville conference, contractor Mike Law outlined conclusions from a Kentucky evaluation of WMA application, including:

Average densities were equal to if not superior to HMA counterparts.

Temperatures were on average 60 degrees F cooler than HMA control sections.

Workability still seems to be a concern for some contractors.

Actual savings will depend on plant efficiency, fuel prices, additive costs and aggregate moisture.

Average permeability readings were similar in HMA and WMA sections.

And, on average, WMA holds its temperature between the paver and roller better than HMA.

“Quality of warm mix must be as good as or better than HMA, or it is not acceptable,” says Law, of Scotty’s Contracting & Stone, Bowling Green, Ky.

Asphalt production expert Bob Frank of New Jersey-based Compliance Monitoring Service puts ongoing evaluations another way: What are the concerns with HMA? “Quite generally,” he says, “it’s things being where you don’t want them, whether it’s residual moisture in the aggregate where the dryer didn’t adequately dry the mix at the lower production temperatures, or moisture showing up in your baghouse fines due to the lower exhaust temperatures.”

Of the 358 million tons of asphalt produced nationwide last year, about 10 percent was warm mix, says Corrigan. “That was a tremendous increase in percentage from the year before,” he says, “and I think we’re going to continue to see that steady climb.” When he introduced Corrigan to the Nashville gathering, NAPA President Mike Acott called WMA “the future of flexible pavement in the United States.”

A heads-up, says Kentucky contractor Law: “If you’re not doing warm mix today, your competitor is.”

Federal Highway Administration (FHWA)

Evonik Cyro LLC announces the application of the PARAGLAS SOUNDSTOP TL4 System as part of highway projects in Florida, California, British Columbia, Ohio, Tennessee, and New Jersey. The lightweight, crash safe noise barrier system has been successfully tested under NCHRP 350 Level 4 conditions which include two crash tests involving an 8,000-kg single unit truck impacting at 80 km/hr and 15 degrees and a 200 KG pickup truck impacting at 100 km/hr.

Evonik Cyro’s TL4 system incorporates transparent PARAGLAS SOUNDSTOP GS CC noise barrier sheet, which features its unique fragment retention feature, meets the international EN1794 standard regarding noise barriers. If subjected to an impact at high enough energy where breakage occurs, large fragments are retained thus preventing any potential secondary damage from falling debris. In many countries around the world GS CC is the only product approved for use on bridges.

The SOUNDSTOP TL4 System weighs as little as 25 pounds per square foot compared to other TL4 compliant noise barrier walls, typically made from reinforced concrete, weighing 1200 pounds per linear foot.  “This light weight could dramatically reduce the cost of bridge construction by eliminating one or more load bearing girders needed to support the heavier concrete wall,” according to Charles Boyd, structures engineer with the Florida Department of Transportation.

“The  SOUNDSTOP TL4 System offers the impact resistance and safety necessary for elevated roadways, while providing unobstructed views to maintain natural aesthetics and light transmission,” comments Nate Binette, market manager, Noise Protection Products, Evonik Cyro LLC.

Dr. Ron Faller of the Midwest Roadside Safety Facility provided engineering services for the development and design of the TL 4 System. The Midwest Roadside Safety Facility is the FHWA certified laboratory that conducted the crash test.  “In addition to the prevention of falling debris, this system acts to prevent vehicle snagging and occupant compartment intrusions which are also requirements of the TL-4 impact standards for attachments to existing roadside safety features.  We actually observed that the TL4 System contributed to vehicle stability during the test” stated Faller.

Used throughout Europe for more than 30 years, SOUNDSTOP sheet was introduced in North America in 2002 and has since garnered significant success with scores of installations in more than 18 states.  SOUNDSTOP GS CC sheet maintains visibility for landscapes while still providing noise reduction for the surrounding area.

The TL4 system, as well as other affordable lightweight systems offered by Evonik Cyro, provides an attractive solution for bridges requiring noise abatement while also minimizing dead load.

PARAGLAS SOUNDSTOP panels offer very low life cycle costs. Due in part to the hard, smooth surface the panels have little to no maintenance costs.  he panels remain clean through exposure to rain and facilitate graffiti removal using a simple spray-on solution and pressure wash. The sheet is extremely resistant to the elements, with excellent UV light stability and light transmission, and will not yellow over time like polycarbonate and other materials.

Panels are available in transparent or translucent clear, and various color shades including Midnight Blue, Steel Blue, Sky Blue, Forest Green, Sea Green, Spring Green, and Smoky Brown.

By Kirk Landers

America was just emerging from the oil-embargo recessions of the ‘70s when Ford ended a long, miserable period of automobile engineering by introducing several stunning new models, including an ‘80s reprise of the Boss 302 Mustang. It combined the head-snapping performance of the first pony cars with a level of fuel efficiency once thought impossible in a V8 engine.

kirk.landers@att.net

An act of providence gave me almost exclusive access to the cars and to Ford’s proving ground facilities for a morning. I used the opportunity to drive the vehicles maniacally in several different venues. It was while driving the Mustang on the handling track — and spinning out repeatedly — that I realized America was once again fielding cars that could go fast enough to kill yourself in.

It was an odd sensation and it has never left me.

On one hand, as a young automotive editor, it was a moment of ecstasy in a day filled with unending joy. We had been driving ugly, underpowered econoboxes for nearly a decade. Unless you built your own car, driving had become tedium. I used to dream of getting out of my wretched company car and shooting it with a pistol, like a cowboy putting down a horse with a broken leg. It was the only moment of pleasure I got from the vehicle.

Enter the re-engineered 302-cubic-inch V8 Mustang. It seemed like — and was — the beginning of a new era for car enthusiasts in America.

But there was this other rush, too, a more sober one: You really could kill yourself in a vehicle like this, one that begged to be driven hard right up to the time you wrapped it around a tree or rolled it a few times in to a ditch.

That moment of rational insight didn’t change my appetite for hot cars, though my choices have been an eclectic spectrum defined on one end by a Corvette and on the other by a Mini Cooper S. But it did plant a thought that changed how I drive, and changed my perspective on highway safety.

In the ‘90s, when I got involved with road issues as a construction writer, I learned that rural roads produce a much higher rate of fatalities than other driving venues in America. I understood. Quiet, bucolic country roads can produce the same kind of narcotic effect that hot cars do, encouraging people to focus on the aesthetics of the driving experience rather than the weighty responsibilities that come with it.

My Mustang moment also changed my attitude toward safety advocates, who had been painted as overbearing, car-hating bureaucrats in the automotive press of the ‘70s and ‘80s.

That evolution came full circle last year, when the Federal Highway Administration issued a report showing that the rate of driving fatalities per vehicle miles driven had fallen to the lowest number in decades. FHWA attributed the breathtaking improvement to a combination of proactive measures, including programs to improve the safety of rural roads, to increase seat belt usage, and to crack down on impaired and/or distracted drivers.

As one who considered much of that effort to be political propaganda, I have to say, I’m impressed. And I was wrong. I thought 50,000 traffic fatalities annually, give or take a few thousand, was a reasonable number and one that we would never really improve upon. Fortunately, others were not encumbered with my beliefs and made real progress in an important area.

Finally, one of the most striking things about the programs that produced this improvement is that they cost very little. In a political culture where opportunistic politicians and media personalities rant endlessly about government inefficiency, this example of remarkable efficiency has gone largely unreported.

I can’t make up for that, but I can say thanks to all who persevered and overcame. Your work has made our lives better.

A strong foundation is the key to a strong pavement structure

By Tom Kuennen, Contributing Editor

A pavement is only as strong as its foundation. Without an adequate base or foundation, a road simply cannot stand up to long-term traffic volumes, increasing vehicle weights and speeds, and the assault of the elements.

Strong subbases bolster the base and pavement layers above.

While the concept of a subbase is simple, the reality is that in today’s world of advanced technology, a subbase design and construction can be complex and demanding. Subbase design may require analysis of existing, virgin and reclaimed materials, application and mixing of stabilization chemicals, installation of stabilization fabrics, and measurement of compaction using “smart” technology built into dirt rollers. Subbases must also be drained and protected from frost.

For new alignments, stabilize the subgrade prior to work on subbase, base and pavement layers.

And the new philosophy of mechanistic-empirical design, as articulated by the American Association of State Highway & Transportation Officials (AASHTO) with the National Cooperative Highway Research Program (NCHRP), is bringing a new rigor to the design and construction of subbases.

The result will be better-performing pavement structures.

Damage from Inadequate Subbases

Subbases inadequate for the traffic loads they carry will manifest their shortcomings in a variety of ways.

A new Cat 160M2 motor grader preps the subbase prior to base placement.

The most common clue to base failure-related pavement woes in is fatigue cracking. Fatigue, or bottom-up, cracking results when traffic load stresses propagated to asphalt pavement foundations cause foundation cracks to work their way upward through the pavement.

In asphalt pavements, it’s manifested as a series of interconnected cracks resembling an alligator hide, hence its popular name alligator cracking. It develops into many-sided, sharp-angled pieces, usually less than 12 inches on the longest side.

If not stabilized, expansive subbase and base layers will heave and destroy pavement.

Low-severity fatigue cracking characterizes an area of cracks with no or only a few connecting cracks. The cracks are not spalled nor sealed, and pumping of base materials out the cracks is not evident. In moderate fatigue cracking, the interconnected cracks form a complete pattern, cracks may be slightly spalled and may be sealed, and pumping is not evident. High-severity fatigue cracking is an area of moderately or severely spalled interconnected cracks forming a complete pattern, the pieces of which may move when subjected to traffic loads. Cracks may be sealed, and pumping may be evident.

In portland cement concrete (PCC) pavements, longitudinal cracking describes cracks that are mostly parallel to the pavement centerline, and are attributed to subgrade heaving that pushes upward against the rigid slab and cracks it.

“Daylighted” permeable base – exposed here at the shoulder – is a lower-cost PCC pavement drainable design that doesn’t require separation fabric or drainage systems.

Base and subbase layers that are composed of expansive soils with an abundance of clay must be stabilized, frequently done with cement. This is particularly true of soils in Louisiana, Texas and the American Southwest. Expansion of these base and subbase layers will cause heaving in the pavement, forcing it upward, causing it to fissure and break. The pavement likely will have to be completely reconstructed.

Layers of Pavement Structure

The pavement structure is composed of layers beginning with the subgrade, topped by the subbase, the base course, and lastly one or more surface courses. On roads with lighter traffic loads, the surface course(s) may rest directly on the subbase.

The surface courses can be a single course of PCC, although simultaneous twin lifts of PCC are being studied (see “Research that Can Change the Way We Work,” April 2011, pp. 26-39); or one, two or even three courses of hot-mix asphalt or its warm- and cold-mix permutations. These will rest on base and subbase layers that can be unbound, bound, or stabilized by a variety of methods, including cement- or lime-slurry, dry cement or lime, asphalt emulsion, or foamed asphalt.

The subgrade is the graded, prepared ground beneath the subbase layer. It’s been described as the point at which excavation ceases and construction starts, and supports the entire pavement structure and traffic loads.

In practice, the subbase becomes the main load-bearing layer of the pavement, evenly spreading the traffic loads across the subgrade. The materials used may be soil-aggregates, unbound granular material, or bound granular material.

Soil-aggregate subbases consist of soil from the subgrade, combined with mineral aggregate present on the road surface, with or without additional aggregate. ASTM D1241-07, Standard Specification for Materials for Soil-Aggregate Subbase, Base and Surface Courses describes soil-aggregate as: sand-clay mixtures; gravel; stone or slag screenings; sand; crusher-run coarse aggregate consisting of gravel, crushed stone, or slag combined with soil mortar; or any combination of these materials. These subbase materials are spread, shaped and compacted in accordance with Department of Transportation (DOT) contract documents.

They differ from granular subbases, which are composed of granular material that may be present on the roadbed, plus a specified quantity of virgin aggregates – with or without recycled materials – that meet strength, abrasion and gradation specs. The granular mixture is placed on a subgrade, uniformly moistened, shaped and compacted to spec.

Aggregates used in granular base and subbase applications generally consist of sand and gravel, crushed stone or quarry rock, slag, or other hard, durable material of mineral origin, according to the Federal Highway Administration (FHWA). The gradation requirements vary with type base or subbase.

“Granular base materials typically contain a crushed stone content in excess of 50 percent of the coarse aggregate particles,” according to the FHWA. “Cubical particles are desirable, with a limited amount of flat or thin and elongated particles. The granular base is typically dense-graded, with the amount of fines limited to promote drainage.”

Granular subbase is also dense-graded, but tends to be somewhat coarser than granular base, FHWA says. The requirement for crushed content for granular subbase is not required by many agencies, FHWA says, although provision of 100-percent crushed aggregates for base and subbase use is increasing in premium pavement structures to promote rutting resistance.

“A granular subbase course is that part of the pavement structure constructed to provide a foundation for the base course, to distribute the superimposed loading to the subgrade and to provide drainage beneath the base and surface courses,” states the Wisconsin DOT in its Construction and Materials Manual. “It usually consists of natural sand or a mixture of sand with gravel, excavated and constructed with grading equipment as an item under a grading contract.”

Before placing the subbase material, the subgrade or foundation must be properly prepared, the Wisconsin DOT says. “It should be smooth, shaped to conform to required crown and grade, and be compacted to the required density.”

Where travel of the placing equipment ruts or disturbs the foundation, means must be employed to correct these conditions ahead of placing the subbase material, the Wisconsin DOT warns. “If the subbase is constructed on a rutted foundation, the roadbed will not drain properly and areas of weakness may develop in the pavement structure. Placing, shaping and compacting the subbase material to conform for its full width to the required grade, section and density is necessary for satisfactory construction of the proposed base course. The inspector should frequently check the subbase course for correct depth and spread.”

Draining the Subbase

Wisconsin DOT warns of the danger of water in pavement structures. It’s commonly said that “water is the enemy of pavements.” Therefore, whatever can be done to keep water out of the pavement structure is effort well spent.

As noted below, saturated pavement structures will actually pump water and base fines out of HMA fatigue cracks or along the sides of PCC slabs, indicating subbase and base layers in dire straits.

Pavement structures will contain water in its free state, as capillary water between the granular material, bound moisture, or water vapor. Free water is the form of most concern, engineers say, because it can do the most harm and is the only form of water that can be significantly removed by gravity drainage.

The subgrade, granular subbase and other pavement layers always are constructed with cross slope to facilitate drainage. Rain or melt water will enter pavement through cracks and joints in the driving surface. A properly designed pavement will use gravity to encourage water to find its way through voids in the granular base and subbase following the slope, to either exit the structure into side ditches, or into a built-in pavement drain that will take it to ditches and ultimately to a creek, wetland or bioswale.

Permeable road bases are made of an open-graded granular material that allows free flow of water through the subbase or base layer, and then out to a drainage appurtenance. The permeable base may be unbound or bound, as in the case of the cement-treated permeable base – which adds structural strength – and may be separated from the subgrade by an impermeable drainage fabric that keeps fines from migrating from the subgrade into the subbase.

‘Daylighted’ Permeable Bases

Optimal use of fabric requires a drainage system, but a lower-cost design for PCC pavements — the “daylighted permeable base” — allows free draining of water to roadside.

“Daylighted permeable bases are well-suited for roadways with flat grades (1 percent or less) and shallow ditches, where it is difficult to outlet drainage pipes at an adequate height above the ditch,” says the FHWA in its 2009 Tech Brief publication, Daylighted Permeable Bases.

“Daylighted permeable bases have been used for more than 20 years in the United States to remove infiltrated water from pavement structures,” FHWA writes. “[W]hen appropriately used, designed, constructed and maintained, daylighted permeable bases have the potential to perform just as well as edge-drained permeable bases, for about the same or even lower cost.”

Two types of materials have been used for daylighted permeable bases, FHWA says. The first is an unstabilized large-sized stone, also called a rock base, typically constructed about 18 to 24 inches thick. The second type of material is a permeable base gradation such as would be used for an edge-drain system, either untreated or treated with asphalt or portland cement, and typically constructed about 4 to 6 inches thick. The permeability requirements and asphalt or cement content required to maintain long-term stability are the same for daylighted permeable bases as for edge-drained permeable bases, FHWA says.

A permeable daylighted base needs a suitable separator layer beneath it to prevent subgrade fines from migrating up into and clogging the base, but not necessarily a fabric, FHWA reports. “This may be an appropriately graded untreated aggregate subbase, an appropriate geotextile fabric, or a layer of subgrade soil treated with sufficient lime or cement to achieve good long-term stability and resist erosion,” the agency says.

Download the complete report at fhwa.dot.gov/pavement/concrete/pubs/hif09009/hif09009.pdf

Impact of New Design Guide

Part of the new complexity of subbase design, and ultimately construction, derives from the ongoing adoption of new highway pavement design procedures set forth in the Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures, Final Report (NCHRP, 2004), now referred to as the Mechanistic-Empirical Pavement Design Guide (MEPDG), and in the process of adoption by DOTs from coast-to-coast.

Mechanistic-empirical are big words that describe a very simple concept. Mechanistic refers to the interaction between the materials and structure of a pavement, and how it stresses and strains under load deflection. The MEPDG paradigm relates these pavement mechanics to empirical or experimental performance data obtained in field or lab.

The guide uses mathematical models to describe this relationship, and the primary basis for all mechanistic-based pavement performance predictions methods is cumulative axle load applications.

“The benefit of a mechanistic-empirical approach is its ability to accurately characterize in situ material (including subgrade and existing pavement structures),” says the Washington State DOT in its online tutorial. “This is typically done by using a portable device to make actual field deflection measurements on a pavement structure to be overlaid. These measurements can then be input into equations to determine existing pavement structural support (often called backcalculation) and the approximate remaining pavement life. This allows for a more realistic design for the given conditions.”

The existing 1993 edition of the AASHTO Guide for Design of Pavement Structures is based on empirical equations derived from the well-known, but outdated, AASHTO Road Test. This program conducted performance testing between 1958 and 1960 of a limited number of structural sections at one location, Ottawa, Ill., and based on much-reduced traffic levels compared those of the 21st century.

Under the new design guide, a designer of any pavement must first consider site conditions such as traffic, climate, subgrade, existing pavement condition for rehabilitation and construction conditions in proposing a trial design for a new pavement or rehab. Then, using the software, the trial design will be evaluated through prediction of key distresses and smoothness. If the trial does not meet the demanded performance criteria, the pavement design must be revised until it does.

The new guide also incorporates procedures for performing traffic analyses, includes options for calibrating to local conditions, and incorporates measures for design reliability. Engineers can use the guide to analyze common causes of pavement distress, including fatigue, rutting and thermal cracking in asphalt pavements, and cracking and faulting in concrete pavements.

Reclaimed Materials in Bases

There is no question that recycled concrete aggregate (RCA) also may be used in road subbases and bases, so long as it is treated as an engineered material — that is, crushed, screened, processed and tested as though it were a virgin aggregate. (See Better Roads, Two for the Price of One, April 2010, pp. 16-29.)

In that Road Science Tutorial, we reported that TxDOT has researched and used RCA with good success for about 17 years. In the years 2006-2008, TxDOT saved approximately 1.8 million tons of virgin aggregates by incorporating RCA in cement treated base, flexible base, continuously reinforced concrete pavement (CRCP), filter dams, gabion walls, concrete traffic barriers, flowable fill and select backfill for mechanically-stabilized earth walls. “This equates to an estimated savings of $12.6 million from reduced or eliminated landfill and virgin aggregate associated costs,” TxDOT reports. “Savings from using RCA has the potential to increase tenfold based on current availability of RCA.”

But recycled aggregate from structures may perform just as well as RCA from demolished highways, say Dana V. Martin and Gregory W. Halsey, undergraduate research assistants, and Jeffrey S. Melton, research assistant professor, Department of Civil Engineering, University of New Hampshire-Durham, in their 2011 Transportation Research Board paper, Comparison of Building Derived Aggregate in Comparison to Crushed Stone.

Use of recycled concrete aggregate for road construction has become a widely accepted practice throughout the United States, and has proven to be an excellent substitute for crushed stone in road base applications. More than 45 states allow its use in highway construction, they write, adding the most common source of RCA is from the demolition of highway infrastructure. “While the use of RCA has become commonplace,” they say, “the use of building-derived aggregate (BDA) in roadway construction has not.”

BDA derives from the construction and demolition industry, which generates millions of tons per year throughout the United States, the researchers say. An inherent trait of BDA is that there are a variety of other materials present, such as brick, porcelain, cement-based masonry units and other inorganic materials, they write.

“The presence of these other materials has created a barrier in the use of BDA for roadway construction,” say Martin, Halsey and Melton. “The AASHTO standard for the use of crushed concrete in road base applications, M 319, allows only 5-percent brick by mass to be used.” But it’s common for BDA to have up to 10-percent brick by mass present, they write, which has precluded its use. The presence of nonconcrete materials in BDA has created a perception that it does not perform as well as RCA or crushed stone.

Their research, performed at the University of New Hampshire and funded by FHWA through the Recycled Materials Research Center there, has shown that BDA is a usable substitute for crushed stone. As it is also important to understand the long-term effects of using BDA, their study quantifies the longer-term performance and associated effects of using BDA in road construction.

On the other hand, the stiffness increase was almost 50-percent more than that of the crushed rock, and did not decrease with time, they say. “If this trend continues,” they report, “it would suggest that the presence of so-called deleterious materials like brick and tile is not significant, and that the BDA can be used as base course aggregate.”

Reclaimed asphalt pavement (RAP) is useful as an additive to crushed angular aggregate or pit-run granular soils for road subbases and bases in Montana, according to research from Montana State University.

In research prepared for the Montana DOT, Evaluation of the Engineering Characteristics of RAP/Aggregate Blends, by Robert L. Mokwa and Cole S. Peebles, Department of Civil Engineering, Montana State University-Bozeman, research and tests were conducted to evaluate the suitability of such RAP blends.

The study examined changes that occur in the engineering properties of aggregate materials when mixed with RAP. In addition to a thorough evaluation of published literature on the subject, an extensive suite of laboratory tests were conducted using four different aggregates blended with asphalt millings over a broad range of mix percentages.

Laboratory investigations suggest that the engineering properties of RAP-blended soils are comparable with those of virgin aggregates, they say.

“Gradation analyses indicate that the addition of RAP to virgin materials does not significantly change the particle size distribution,” Mokwa and Peebles say. “The outlook for the continued implementation of RAP as an additive to granular base and subbase materials for use in highway construction looks promising. Results from the extensive suite of laboratory tests indicate that blending asphalt millings with granular cohesionless material, like crushed aggregate or pit-run cohesionless soil, results in only minor changes to the engineering properties of the virgin material.”

Also, pit-run steel slag is extensively used for subbase construction in some areas, especially where weak subgrade conditions exist.

Steel slag is a crushed product having hard, dense, angular and roughly cubical particles. “Steel slag meets the requirements of ASTM D 694 and D 1241, of national agencies, and of local highway departments for macadam and crushed aggregate bases,” reports supplier Phoenix Services, Uniontown, Pa. “Local highway department standards or the producer’s recommendations are applicable for both base and subbase courses.”

Steel slag for use in bases and structural fills — where very high stabilities are required — may require proper selection, processing and aging (weathering) before use, Phoenix says. “Steel slag may contain free lime (CaO or MgO) that may cause the slag to be expansive or cause differential movement when used as a base,” Phoenix reports. “Steel slag is not recommended for use in rigid, confined applications such as concrete aggregate, base or fill under structures or floor slabs, or backfill against structures or bridge abutments.”

Both systems will deliver real-time information on parking availability through Intelligent Transportation Systems. The Federal Highway Administration (FHWA) is providing the grants under the Truck Parking Facilities Discretionary Grants Program.

The program helps improve safety on the nation’s interstates by promoting projects that allow trucks to park safely and securely in areas away from moving traffic, instead of alongside the road itself, according to the FHWA.

The increase in traffic volume comes as the U.S. in 2009 posted its lowest number of traffic fatalities and injuries since 1950.

“More driving means more wear and tear on our nation’s roads and bridges,” said LaHood. “This new data further demonstrates why we need to repair the roads and bridges that are the lifeblood of our economy.”

LaHood noted that Americans drove 0.7 percent more, or 20.5 billion additional vehicle miles traveled (VMT), in 2010 than the previous year.

Travel increased by 0.6 percent, or 1.4 billion VMT, in December 2010 compared to the previous December. It is the tenth consecutive month of increased driving.

The new data, from the Federal Highway Administration’s (FHWA) monthly Traffic Volume Trends report, show the South Gulf area, a bloc of eight states ranging from Texas to Kentucky, experienced the greatest regional increase in December 2010 at 46.6 billion VMT, an increase of 624 million miles traveled compared to the previous December.

With an increase of 11.1 percent, or 156 million additional miles traveled, Nebraska led the nation with the largest single-state increase that month, and rural driving outpaced urban driving across the country.

“These data are critical to identifying and evaluating patterns of use on America’s road system, which help us to make decisions about investments in critical infrastructure,” said Federal Highway Administrator Victor Mendez. “Repairing our nation’s roads, bridges and tunnels will help us ensure safety, strengthen the economy and build for the future.”

To review the VMT data in FHWA’s Traffic Volume Trends reports, including that of December 2010, go to www.fhwa.dot.gov/ohim/tvtw/tvtpage.cfm.