New Approaches

Project challenges and lessons learned in building precast concrete bridge systems create new approaches for Michigan’s DOT.

By Corey Rogers, P.E., Photos courtesy of MDOT (michigan.gov/mdot)

The Michigan Department of Transportation (MDOT) uses a Prefabricated Bridge Element Systems (PBES) approach under its Accelerated Bridge Construction (ABC) policy. Rapid bridge construction means less inconvenience and delay for the public, businesses and transportation users. Construction details and challenges encountered on two projects with multiple precast concrete bridge elements led to important improvements for the PBES approach going forward.

PBES Bridge Project 1: Parkview Avenue over
U.S. 131 substructure – smooth sailing in six-week schedule

In 2008, the Michigan Department of Transportation (MDOT) began PBES construction on Parkview Avenue Bridge over U.S. 131 in Kalamazoo County. The challenge was to build a 249-ft, four-span bridge on a 23-degree skew in 12 weeks. The solution was precast concrete elements. Time was of the essence, and an ambitious schedule was proposed to mitigate the impacts to traffic in this populated area of Kalamazoo.

The first six weeks of construction involved demolition of the existing structure and construction of the substructure. The substructure design was semi-integral, comprised of a single row of HP 12 x 53 H-piles supporting the precast abutment stems. The piers were supported by cast-in-place (CIP) spread footings. The CIP footings, concrete diaphragms, back wall and bridge railing were the only elements of the bridge cast in the field.

After demolition of the existing structure, the pier footings were cast. In order to ensure proper fit-up of the columns and the footings, a block-out template was created detailing the location of the threaded rod inserts in the bottoms of the columns and used to lay out the voids in the footings for the footing-to-column connections. After the footing concrete reached a compressive strength of 2,500 psi, the threaded rods were inserted into the bottoms of the columns, and the columns were set over the respective voids in the footings. The voids were grouted and the connections completed. The tops of the columns did not utilize the threaded inserts but had the connecting resteel precast with the columns to match the corrugated ducts in the pier caps. The corrugated ducts were then grouted after fit-up to complete the column-to-pier cap connections.

The abutment stems were precast, incorporating pile sleeves. In this regard, the pile driving operation and location tolerances were critical, and the contractor ensured all the pile sleeves in the abutments lined up appropriately. The sleeves were then grouted and allowed to reach a compressive strength of 3,000 psi before beginning the erection of the superstructure. The PBES allowed MDOT to complete the substructure within the six-week schedule by eliminating forming, large pours, sequencing and lengthy cure times – essentially reducing it to a grouting operation.

PBES Bridge Project 2: M-25 over the White River-two weeks ahead of schedule

M-25 over the White River is located along Lake Huron on the east coast of Michigan’s thumb in Huron County. Due to scour concerns and existing conditions, the bridge was programmed for replacement in 2011. This route is frequently traveled by tourists seeking recreational activities such as boating, fishing and camping. In order to reduce impacts to traffic and mitigate negative impacts to the local businesses, ABC techniques were a primary objective for MDOT.

A prefabricated bridge element system utilizing precast concrete abutments and precast decked box beams was chosen to expedite construction of a 48-ft-long, single-span bridge. MDOT’s first experience with decked box beams presented some unique challenges but resulted in a very successful project. Conventional CIP construction would have resulted in a 20-week closure of M-25. The goal of the White River Bridge replacement was a construction schedule of 12 weeks, which was achieved through the use of precast elements.

Conventional construction requires forming, placing and curing concrete in the field. Structural concrete must reach an acceptable strength prior to loading and moving on to the next stage in construction. MDOT specifications for bridge deck pours require a seven-day wet cure prior to loading and subsequent concrete pours. The use of precast decked beams eliminates the need for a seven-day wet cure, significantly reducing time on projects where every day counts.
The reinforced precast concrete segments consisted of two abutment stems, two wing wall segments, two slope walls and eight beam-deck segments. All were fabricated in an off-site precast plant and trucked to the job site.

The existing White River Bridge was removed and the footings were cast. After reaching a 2,500 psi compressive strength, the precast abutment segments were lifted into place with a 500-ton crane and positioned on the footing. The abutment stem segments and wing walls were connected to the footing with non-shrink, grout-filled mechanical splices. The vertical key between the two abutment segments was grouted, and the closure pour between the wing walls and the abutments was completed. Each abutment was assembled in one day.

The modular decked box beams were comprised of 21-in. prestressed box beams pre-topped with 10½ in. of deck. Typical bridge decks are approximately 9 in. thick; however, additional deck thickness was included to allow for deck surface corrections and ride quality of the structure. The beam-deck segments were spaced 6 ft on center and rested on the abutments via elastomeric bearings. Between each modular section, top and bottom longitudinal reinforcement was threaded through the overlapped D-hoops within the closure pour block-out.

The beams were preloaded to reduce variations in camber among the beams to ensure a smooth transition at the closure pour joint. High Performance Superstructure Concrete (HPSC), capable of attaining 7,000 psi compressive strength, was used for the closure pours between beam-deck segments. Transverse post-tensioning tendons comprised of four ½-in.-diameter strands were installed in 5-in. ducts cast into the six diaphragms of the modular sections, stressed to 182 kips and grouted with HPSC.

Upon removal of the preloading, irregularities were still present in the deck due to slight camber variations and imperfections from lifting-loop locations. The deck was diamond-ground and coated with a thin epoxy overlay to improve aesthetics and ride quality, and provide a waterproof barrier. The project was completed in 10 weeks – two weeks ahead of schedule, and proved to be another successful endeavor utilizing precast concrete elements.

Post-construction lessons learned

The collaboration between MDOT and the contractor was remarkable. Innovations and efficiencies made in the field during construction and recommendations to improve similar types of construction in the future were discussed. The MDOT Cass City Transportation Service Center managed the project and detailed all facets of the project, including a post-construction meeting that focused on recommendations for future improvements.

Recommendations brought forward
after construction:

1. Verify the availability of HPSC near the project location, especially in rural and remote project locations.
2. Use recessed lifting loops on the precast segments. Recessed lifting loops eliminate the need for cutoffs and allow for easy mortar patching and increased longevity of the patch.
3. Eliminate the preloading guesswork. A table estimating the required preloading, based on a given camber deflection, would assist contractors in obtaining materials and expedite the preloading process.
4. Create a strength-testing procedure for expansive grouts.
5. Use a Qualified Products List or testing to verify required strength throughout the construction process. Contractors may not be familiar with the process to create and test the cubes necessary for grout testing.
6. Use a thin epoxy overlay. Although the deck was precast 1½ in. thicker to accommodate the need for grinding imperfections, grinding often results in an unattractive deck appearance, micro-cracking and loss of the densified top layer of protective concrete. A thin epoxy overlay seals the deck and provides an aesthetically pleasing wearing surface.

PBES going forward

On the Parkview Bridge Project, MDOT implemented the PBES philosophy using multiple precast bridge elements, thus laying the foundation for future precast projects throughout Michigan. Ultimately, a bridge replacement that typically would have taken about seven months to complete was finished in about five months, even though the deck panel error resulted in a two-month delay. But more importantly, MDOT gained invaluable experience in PBES construction and techniques related to proper prefabricated element fit-up.

The efforts put forth by MDOT project staff and the contractor to document the positives and negatives of these projects are a huge asset to the department. The successes and the lessons learned validated the effectiveness of precast concrete bridge elements and have allowed MDOT to develop a PBES implementation policy with guidance on future project selection. By sharing lessons learned statewide, future decision making is improved and the effectiveness of precast elements can be utilized to the greatest benefit for both MDOT (in terms of quality construction) and the public (in terms of reduced impacts and delays).

Precast concrete elements have earned their place in bridge construction, and as engineers become more comfortable with their use, precast systems will surely become a standard building block for MDOT.

Corey Rogers, P.E., bridge construction engineer, bridge field services division, MDOT, is responsible for creating alignment in bridge construction practices, troubleshooting construction related issues and implementing new innovations in bridge construction on a statewide basis. Contact him at
[email protected].