Precast/Prestressed Elements Drive Accelerated Bridge Construction
- Published: Thursday, 23 February 2012 17:43
- Written by CP Staff
The growing support for prefabricated bridges, bridge elements and pavements at the federal and state levels is driving research in precast/prestressed concrete products, and that was evident at the 91st Transportation Research Board (TRB) meeting last month in Washington, D.C.
Research in prefab bridges (accelerated bridge construction, or ABC) and pavement slabs is being supported by the Federal Highway Administration through its Highways for Life program. There, Prefabricated Bridge Elements and Systems and Precast Concrete Pavement Systems are among the “vanguard technologies” receiving strong research and promotional support for implementation.
“Highways for Life offers incentives to highway agencies to adopt innovations and customer-focused performance goals in building better highways and bridges,” said FHWA’s team leader Byron Lord. “The program also helps private industry move promising prototypes from late-stage development into the marketplace. The approach employs communication tools, training, technical assistance, and highway community stakeholder involvement.” (More information on Highways for Life may be found at http://www.fhwa.dot.gov/hfl/technology/trnews.cfm.)
For the first time, registration reached 12,000 at TRB, where delegates heard or studied over 4,000 peer-reviewed technical papers or poster presentations on transportation design, planning, construction, materials and operations. Concrete Products was there. Here is a roundup of some of the new research of significance to the readers. For more information about TRB, visit www.trb.org.
Precast Bridge for Seismic Zones in Washington State
Washington State has plunged into ABC using precast concrete bridge bent connections that are suitable for high seismic zones, said Bijan Khaleghi, Ph.D., P.E., S.E., state bridge design engineer, Washington State DOT, in his paper, Highways for Life Projects and Accelerated Bridge Construction in Washington State.
FHWA is actively promoting accelerated bridge construction in an effort to reduce construction time while improving work-zone safety, and minimizing environmental impacts, Khaleghi said. “The Every Day Counts initiative promotes Highways For Life projects allowing states to implement the new and innovative technologies for better performance of prefabricated bridge elements in seismic zones,” he said.
“Prefabricated bridge components are in increasing demand for accelerated bridge construction,” he observed. “Precasting eliminates the need for forming, casting and curing of concrete in the work zones, making bridge construction safer while improving quality and durability. Precast concrete bridge systems provide effective and economical design solutions for new bridge construction as well as for the rehabilitation of existing bridges.”
But the proper seismic design entails a detailed evaluation of the connections between precast components, as well as the connection between superstructure and the supporting substructure system, Khaleghi said, adding “In seismic regions, provisions must be made to transfer greater forces through connections and to ensure ductile behavior in both longitudinal and transverse directions.”
Precast connections are typically made at the beam-column and column-foundation interfaces to facilitate fabrication and transportation, he said. “However, for structures in seismic regions, those interfaces represent the locations of high moments and large inelastic cyclic strain reversals,” Khaleghi said. “Devising connections that are not only sufficiently robust to accommodate those cyclic loads, but are also readily constructible, are the primary challenges for ABC in seismic regions.”
In response, Khaleghi described a precast concrete bridge bent system suitable for ABC in seismic regions: “A precast bent system with grouted duct moment resisting connection was used for the conceptual design. Using precast elements, with a small number of bars and ducts, it is possible to assemble a bridge bent quickly. The connection between column and cap beam is made with large bars that project from the top of the column and are grouted into ducts in the cap beam.
“The advantage of a small number of large bars is the reduction in the number of alignments needed. The proposed system uses large diameter ducts to maximize assembly tolerances.”
This grouted-duct concept was applied to a three-span prestressed precast concrete bridge in high seismic zone of urbanized western Washington State. This project was the first application by Washington State DOT that used precast concrete for bridge girder support precast bent caps. “Based on the project success, the owner anticipates incorporating this method as an available practice for future designs,” Khaleghi said.
The bridge uses wide flange WF74G girders to span a wetland, a railroad right-of-way and an urban arterial. “Precast concrete girders were the best choice for the superstructure,” he said. “They are durable and have low maintenance and life cycle costs. Precasting the girders increases the public’s safety and convenience during construction by minimizing road closures and eliminating falsework over traveled lanes. The substructure cross beam was precast in order to save construction time. The use of precast concrete made duplicating the cast-in-place design feasible.”
The author found:
- The use of precast bent caps results in cost savings by eliminating the need for elevated falsework and its foundation. It also improves worker safety as rebar and concrete can be placed at the ground level.
- The column-to-cap beam connection is made with a small number of large bars column grouted into ducts in the cap beam. Their small number, and the correspondingly large ducts sizes that are possible, lead to a connection that can be assembled easily on-site.
- Precast/prestressed concrete bridge systems are an economical and effective for rapid bridge construction. Precasting eliminates traffic disruptions during bridge construction while maintaining quality and long-term performance.
- The development length of a reinforcing bar grouted into a corrugated steel pipe is much shorter than suggested by current code equations. A simple equation based on this research results in development lengths of typical grouted bar-duct sleeve connections.
Michigan DOT Places First Fully Prefab Bridge
There are lessons to be learned from construction of Michigan DOT’s first full prefabricated bridge that followed ABC principles, said Upul Attanayake, Ph.D., P.E., Osama Abudayyeh, Ph.D., P.E., Haluk Aktan, Ph.D., P.E., Western Michigan University Department of Civil and Construction Engineering, and Janine Cooper, P.E., Michigan DOT, in their paper, “The First Fully Prefabricated Bridge System in Michigan: Observations and Recommendations.”
“Implementation of accelerated bridge construction technologies and structural systems is gaining momentum as the bridge community is educated through workshops and demonstration projects,” the authors said, adding that component tolerances are critical when using prefabricated components.
After many years of service, the Parkview Bridge over U.S. 131 in southwest Michigan needed a major repair or a complete replacement. A decision was made to replace the existing bridge using rapid bridge construction techniques.
The replacement was designed to have a 23-degree skew with four spans to carry three lanes of traffic, the authors wrote. All the major bridge elements including piers, abutments, I-girders, and full-depth deck panels were prefabricated off-site. The superstructure is composed of PC-I Type III AASHTO girders, the deck 48 9-in. thick, partial width, precast reinforced concrete panels.
Precast girders were pre-tensioned and flared coil inserts were embedded in their top flanges, used to insert shear studs that facilitate girder-deck connection through shear pockets. Once the north and south panels were installed on-site, they were connected transversely using a reinforced cast-in-place closure.
“The north and south panels were cast to different widths; thus, the closure was about 5-ft. offset from the bridge centerline,” Attanayake, Abudayyeh, Aktan and Cooper said. “Transverse connection between full-depth deck panels was established using grouted shear keys and longitudinal post-tension. The special provisions required the use of non-shrink grout.“
In the substructure, each abutment consists of two precast stems, they wrote. The stem splice was formed using cast-in-place concrete. Each abutment is supported on 16 H-piles. The plans specified placing of precast abutment on the grade level, maintaining pile embedment of 2.5 ft. and filling the pile sleeve with grout. Each pier consists of four precast concrete columns that are mounted on a cast-in-place footing. The design specified connecting the pier columns to the footing using square pockets casted in the footing. The pier columns support a precast pier cap.
Special provisions to implement this new technology were prepared by Michigan DOT and were supplied to the contractor, the authors note, adding: “Precast full-depth deck panels were placed in four days. After placing the panels on the girders, longitudinal post-tensioning duct misalignment was noticed. This was due to a calculation error by the contractor during the prefabrication process. Bridge owner, design engineers, and the contractor explored all potential solutions to salvage some or all of the deck panels that were already placed on the girders. Several options were considered, however, the contractor chose to re-cast the deck panels after accurately incorporating the skew in the calculations. The ‘cast-match’ technique was used to ensure the correct alignment of post-tensioning ducts.”
Promoting the prefabrication at the job site or at a nearby location owned by the department may reduce construction costs and can minimize the impact of load restrictions, said Attanayake, Abudayyeh, Aktan and Cooper. However, setting up a certified plant at or near the site for a small bridge can be costly.
“Casting of large components such as abutment stems, pier columns, and pier caps can be an option due to their weight and less complicated details,” they added. “Fabrication of circular columns requires vertical formwork while pouring concrete and curing. Columns with rectangular sections can make the fabrication process easier, hence allowing on-site production. Language should be considered in the special provisions to complete prefabrication of components before demolition of the existing structure.”
Interstate Placement of Precast, Prestressed and CIP Slabs
Both standard precast and precast/prestressed pavement slabs are performing in a high-level Interstate highway installation, said M. Shabbir Hossain, Ph.D., P.E., and Celik Ozyildirim, Ph.D., P.E., Virginia Center for Transportation Innovation & Research, Charlottesville, Va., in their paper, Precast Concrete Pavement on I-66 in Virginia.
“To expedite construction and reduce the associated traffic delay and provide longevity, precast concrete slabs have been used for more than 10 years with successive improvements in processes and systems,” the authors wrote. “The Virginia DOT has recently tried two precast systems along with conventional cast-in-place repairs on I-66 near Washington, D.C.”
There, the existing pavement was jointed reinforced concrete pavement. One precast system used reinforced slabs with doweled joints and was called Precast Concrete Pavement (PCP). The other system used transversely prestressed slabs, post-tensioned in the longitudinal direction, and was called Prestressed Precast Concrete Pavement (PPCP).
Three different mixtures were used for the CIP patches, PCP panels, and PPCP panels, with varying cement contents of 846 lbs./yd., 518 lbs./yd. and 602 lbs./yd., respectively, and water/cement ratios of 0.32, 0.34 and 0.36, respectively, they reported. PCP panel mixtures used 172 lbs./yd. of slag, and PPCP panel mixtures had 150 lbs./yd. of Class F fly ash. Cast-in-place cylinders were cured using a standard wet cure, but the cylinders for both precast systems were cured using radiant heat.
Concrete placed in both precast systems exhibited satisfactory workability, air content and strength, Hossain and Ozyildirim said. All 15 batches of cast-in-place concrete achieved required 2,000 psi compressive strength in four hours, except one batch that reached 1,990 psi.
The cementitious materials for the PCP system contained 25 percent slag cement and had a low w/cm, leading to low permeability values that averaged 1,493 coulombs, they wrote. The compressive strengths were higher than the 4,000 psi design strength at both seven and 28 days. Compressive strength requirements for PPCP systems were 3,500 psi for detensioning and 5,000 psi at 28 days. Strengths were higher than the specified values. One set of two cylinders was tested for permeability and exhibited a very low average value of 601 coulombs.
CIP patches were placed 9-in. deep, while both PPCP and PCP slabs were 8 3/4-in. deep. A total of four lanes on I-66, including shoulder, were replaced using PPCP panels, each 10 ft. long. The inside two 12-ft. lanes were installed first, then the outer lane along with the shoulder was repaired using one 27-ft. wide slab. The whole system consisted of several types of panels: joint panels, anchor panels, base panels and closure pours. Several PPCP panels were post-tensioned together, creating 100-ft. to 160-ft. sections. Each section was tied to another with a doweled expansion joint. At both ends of each section there were joint slabs containing five block-out slots for longitudinal post-tensioning.
“Both the PCP and PPCP systems are performing satisfactorily after one year of traffic and the contractor was also satisfied with the constructability,” Hossain and Ozyildirim said. “One of the major concerns is the potential for wide expansion joints in the PPCP and eventual loss of support there. There were very few cracks in the PPCP section, mainly originating from grouting holes, cracks in the block-out patches, cracks and loss of epoxy at lifting hook holes, and corner breaks. PCP slabs showed some mid-slab cracks immediately after opening to traffic but they are still tight and stable after 1.5 years of traffic.”
New Design Bridge Approach Slabs Can Slash Costs
New-design cast-in-place and precast/prestressed bridge approach slabs can save money in the Show Me State, but the precast alternate would provide even more savings in construction time and money than CIP, said Ganesh Thiagarajan, Ph.D., P.E., Sheetal V. Ajgaonkar, Ceki Halmen, Ph.D., P.E., Department of Civil Engineering, University of Missouri-Kansas City, and Michael G. Eilers, P.E., Coreslab Structures (Missouri), in their paper, “Cost Efficient and Innovative Bridge Approach Slab Design.”
“Concrete bridge approach slabs (BAS) are used at the interface between bridge abutments and pavements that rest on compacted embankment,” the authors wrote. “The objective of this research was to develop optimal-cost BAS designs for new and replacement slabs.”
The authors considered two solutions: a cast-in-place (CIP) design for new construction, and novel precast prestressed slab (PCPS) designs for new construction and replacement of bridge approach slabs. Practices of different U.S. state DOTs were evaluated and compared to Missouri (MoDOT) practice.
Based on those practices, a 20-ft. span was chosen, and finite element models were analyzed considering different slab lengths, thicknesses, and loss of support conditions to calculate maximum moments, deflections and end slopes, Thiagarajan, Ajgaonkar, Eilers and Halmen said.
“Recommended alternatives [were] estimated to have a lower cost, and an equal or better performance, compared to the current MoDOT designs,” they wrote. The final design recommendation for new bridge approach slabs was a cast-in-place 20-ft. span, 12-in. thick slab with a sleeper slab for major roads. The expected deflection and slope for the considered 25 percent void formation were within their allowable limits.
But a precast prestressed slab with transverse ties also was proposed as a solution. “From the cost observations it is evident that these slabs could be cost effective in new construction as well,” they write. “Hence, designs for both a 20-ft span (new construction) and 25-ft. span (old/replacement construction) [are] proposed. Sleeper slabs are recommended for both designs. It has been shown by a cost analysis that the proposed precast solution ($46,839-$48,514) compares equally with the proposed cast-in-place solution ($47,893) and can be adopted for new construction as well.”
The bridge approach slab recommended by this research cuts down almost 22 percent of the cost of construction compared with the current MoDOT bridge approach slab cost of construction, they concluded.