Speeding Up Time
The multiple values of accelerated bridge replacement
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.
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.
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.
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.”
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.
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.”