A New Precast Concrete Deck System for Accelerated Bridge Construction
Full-depth precast concrete deck systems have several advantages over cast-in-place (CIP) concrete decks in bridge construction, such as improved construction quality, reduced construction time and impact on traveling public, possible weight reduction, and lower life-cycle cost. This paper presents the development of a new full-depth precast concrete deck system for accelerated bridge construction. The new system overcomes the drawbacks of existing systems, such as the large number of panels, joints, and openings; complexity of panel production and erection; need for an overlay; and difficulty of post-tensioning and grouting operations. The developed deck system consists of full-width precast concrete deck panels that are 12 ft (3.6 m) long to minimize the number of panels and joints. Panels have covered pockets at 4 ft (1.2 m) spacing in each girder line for shear connectors. These pockets were designed to minimize deck surface penetrations and eliminates the need for a deck overlay. Deck panels are prestressed in both directions: pre-tensioned in the transverse direction and post-tensioned in the longitudinal direction using a new approach that eliminates duct installation and grouting operations. These unique features enhance deck durability and improve its economic competitive with CIP concrete decks. The paper also presents the construction sequence and a case study for the implementation of the new system.
Precast concrete deck systems have several advantages over cast-in-place (CIP) concrete decks commonly used in highway bridge construction. Examples are improved quality and safety, reduced construction duration, and minimized maintenance cost. The quality of precast concrete decks is superior to CIP concrete decks as production occurs in a controlled environment eliminating the variability in environmental conditions, casting crews, and curing procedures. Moreover, shrinkage cracking in bridge decks is minimized in precast systems due to the relatively small panel size, two-way prestressing, and delayed composite action with supporting girders.
Composite full-depth precast concrete deck systems are commonly used because of their structural efficiency and reduced overall depth and cost of the superstructure. Existing full-depth full-width composite deck systems use either continuous open channel along each girder line (trough) or closely spaced discrete openings (pockets) to accommodate the shear connectors of the supporting girders. The troughs or pockets are then field grouted and the deck surface need to be protected by an overlay similar to CIP concrete decks, which increases construction duration and cost. In addition, transverse joints between adjacent deck panels are either conventionally reinforced or longitudinally post-tensioned using embedded ducts. These transverse joints and/or ducts have also to be field grouted, which complicate panel fabrication and erection processes and, consequently, reduce the attractiveness of precast concrete deck systems as an accelerated, economical, and durable alternative to CIP concrete decks.
Simplify the fabrication of the supporting girders by using discrete clustered shear connectors at 4 ft (1.2 m) spacing (the largest spacing allowed by AASHTO LRFD (2014)).
Fabricate the largest possible full-depth full-width precast concrete deck panel (12 ft (3.6 m) long) to minimize the number of panels and joints that need to be field cast. Also, no reinforcing bars and/or ducts are projecting out of the panel to void any potential corrosion during the service life.
Use covered shear pockets in the panel with minimal penetrations to eliminate the need for deck overlay. Lifting inserts can be located at the shear pockets.
Prestress the panel in both directions: transversely using pre-tensioning and longitudinally using post-tensioning to control cracking and increase the service life of the deck.
Position post-tensioning strands underneath the deck soffit and above the girder top flange (i.e. haunch area) to eliminate the need for placing post-tensioning ducts inside the panels, threading strands through embedded ducts and across the joints, and grouting the ducts after post-tensioning, which are tedious and laborious operations.
The new precast concrete deck system was recently developed jointly by Nebraska Department of Roads (NDOR) and University of Nebraska-Lincoln (UNL). Several analytical and experimental investigations were conducted at UNL to evaluate the structural performance and constructability of the new system. For more information about these investigations, refer to Morcous, et al. (2013) and Morcous, et al. (2015). Also, the new system was implemented to the construction of the Kearney East Bypass project located in Kearney, NE in the summer of 2015. The next section presents the construction sequence of the new deck system. The following sections present the implementation project.
2 Construction Sequence
Fabricate precast/prestressed concrete I-girders with shear connectors at 4 ft (1.2 m) spacing.
Fabricate full-depth full-width precast/prestressed concrete deck panels with covered shear pockets and 4 in. diameter (100 mm) grouting holes, which will also host the lifting inserts.
Erect all girders and check their alignment using a digital template to ensure that shear connectors in all girders match the shear pockets in deck panels within the specified tolerance.
Form, reinforce, and pour end and intermediate diaphragms up to the girder top flange.
Conduct shim shots on the edges and center of each girder line to determine the actual profile of the cambered girders prior to panel erection.
Attach the deck support system made of bent plates by welding them to the girder top flange to achieve the desired deck elevation and a minimum haunch thickness of 3 in. (75 mm).
Attach extruded polystyrene panels to the diaphragm concrete between girders to fill the gap between the deck panels and diaphragm concrete between girders.
Adjust the height of shear connectors (by cranking or cutting) to achieve the specified embedment in the deck shear pockets (usually 5 in. (125 mm)).
Attach compressible material to the top of the deck support system to fill any gaps between the deck soffit and the deck support system.
Lay down post-tensioning strands longitudinally on top of each girder and thread them through the end deviators embedded at the ends of each girder line.
Lay down negative reinforcement bars on the top flange of each girder over the intermediate support. Bars should be staggered and supported using chairs to avoid conflict with strands.
Erect each deck panel using the specified lifting points and place it on the deck support system.
Use backer rod to fill the gaps between adjacent deck panels and clean/moist the joint surfaces.
Pour flowable concrete/grout in the transverse joints between deck panels.
Raise post-tensioning strands at the ends of each girder line to the mid-height of the deck and thread them through the holes of the anchor plates.
Tension individual post-tensioning strands using mono-strand jack starting from the middle and moving symmetrically outward. Repeat this process for each girder line starting from the middle one and moving symmetrically outward.
Pour flowable concrete/grout to fill the gap between the deck soffit and girder top flange as well as shear pockets using the provided holes. Pouring should continue until the concrete overflow from every hole.
Form, reinforce, and pour approach slabs and the area around post-tensioning blocks.
Form, reinforce and pour the rails.
Grind the top surface of the finished deck up ½ in. (12 mm) to ensure a leveled ridding surface.
3 Implementation Case Study: Kearney East Bypass Project
The Kearney East Bypass project is located in Kearney, NE over the US-30 and Union Pacific rail road tracks. The project consists of twin two-span bridges: a south bound bridge with cast-in-place concrete deck; and north bound bridge with precast concrete deck. Construction of the superstructure (girders, deck, and rail) of the two bridges was done concurrently starting in May 2015 and was completed in September of 2015. Each span is 166 ft (50 m) long and consists of five NU1800 precast/prestressed concrete I-girders 71 in. (1.8 m) deep with 8 ft 6 in. (2.6 m) spacing. Bridge width is 41 ft 8 in. (12.6 m) and has a 14o skew. Both cast-in-place and precast concrete decks are 8 in. (200 mm) thick and have closed cast-in-place concrete rail. The following sections summarize the fabrication of precast concrete girders and deck panels of the north bound bridge conducted by Coreslab Structures Inc. at the Plattmouth, NE plant, and their erection conducted by Hawkins Construction, Inc.
3.1 Girder Production
3.2 Deck Production
3.3 Girder and Panel Erection
The research project was sponsored by Nebraska Department of Roads (NDOR). The contributions of NDOR engineers and staff under leadership of Mark Traynowicz are highly appreciated. Thanks to Todd Culp, Andy Flanagan, and Lance Overland from Coreslab Structures, Inc.; Jim Gregory and Jessie Ridder from Hawkins Construction, Inc.; and Afshin Hatami, Micheal Asaad and Raed Tawadrous from UNL.
- American Association of State Highway and Transportation Officials (AASHTO): AASHTO LRFD Bridge Design Specifications: Customary U.S. Units, Washington, D.C., 7th Edition (2014)Google Scholar
- Morcous, G., Tadros, M.K., Hatami, A.: Implementation of precast concrete deck systems NUDECK (2nd generation). Technical report M323, Nebraska Department of Roads (NDOR) (2013)Google Scholar
- Morcous, G., Hatami, A., Asaad, M.: Evaluating the constructability of NUDECK precast concrete deck panels for kearney bypass project. Technical report M336, Nebraska Department of Roads (NDOR) (2015)Google Scholar