Concrete Products

APR 2018

Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.

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48 • April 2018 BRIDGE DECK SYSTEM: COMPOSITE WITH GIRDERS A new-design deck in Kearney, Neb., behaves as fully composite with the girders, note Raed Tawadrous, M.S., P.E. and George Morcous, Ph.D., P.E., University of Nebraska at Lincoln, and Marc Maguire, Ph.D., Utah State University at Logan, in their 2018 TRB paper, Per- formance Evaluation of a New Precast Concrete Bridge Deck System. "Precast concrete (PC) deck systems have several advantages over cast-in-place (CIP) concrete decks commonly used in highway bridge construction, such as improved construction quality, reduced con- struction duration, and minimized maintenance cost," Tawadrous, Morcous and Maguire say. Composite PC deck systems, they add, are commonly used due to their structural efficiency and reduced over- all depth and cost of the bridge superstructure. "Existing systems use either continuous open channels (troughs) or discrete openings (pockets) over girder lines to accommodate the shear connectors (i.e. studs, rods, bars) of the supporting girders," they write. "These troughs or pockets are then field-grouted using flowable concrete/grout to connect the precast components," Tawadrous, Morcous and Maguire add. "Deck surface is usually covered by an overlay, similar to CIP concrete decks, for protection, which increases construction duration and cost. In addition, transverse joints between adjacent deck panels are either conventionally reinforced or longi- tudinally post-tensioned using embedded ducts, 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." In response, a new precast concrete deck system was developed jointly by the Nebraska Department of Transportation and University of Nebraska-Lincoln to address the shortfalls of existing systems. The system has these features and performance characteristics: •  Shear connectors are spaced at the largest spacing allowed by AASHTO LRFD to simplify panel and girder fabrication and erection; • Full-width full-depth long PC deck panels are designed to reduce the number of panels to be produced, transported, and erected, as well as the number of transverse joints; •  No reinforcing bars and post-tensioning ducts are used at the transverse joints to simplify panel forming and avoid conflicts/mis- alignments between adjacent panels during erection; • Covered shear pockets are used to minimize penetrations to panel surface, which eliminate the need for deck overlay; • Panels are transversely pre-tensioned and longitudinally post-ten- sioned to control panel cracking during construction and in service; • Post-tensioning strands are located 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; and, •  Live load continuity reinforcement is located at the haunch area below the deck panels and over each girder line to enhance deck durability. The new precast concrete deck system was first implemented in the construction of the Kearney East Bypass in summer 2015. This project has twin bridges: southbound bridge with CIP concrete deck; and northbound bridge with the new PC deck system. "Live load testing was conducted to evaluate the performance of the new system as well as a twin bridge constructed using the conven- tional CIP concrete deck," say Tawadrous, Morcous and Maguire. "Also, finite element analysis (FEA) was conducted to model the structural behavior of the new system." Based on the results of the live load testing of that bridge and comparison with CIP concrete deck bridge and FEA, the authors conclude: •  The new precast concrete deck system was fully-composite with the supporting prestressed concrete girder. This was proven by comparing the measured strains and deflections with those obtained from the CIP concrete deck bridge. Therefore, the use of threaded rods as discrete shear connectors and HSS formed shear pockets at 4 ft. spacing was adequate; •  Placing negative moment reinforcement in the haunch area over each girder line was sufficient for achieving live load continuity. This was proven by comparing the measured strains and deflections with those obtained from the CIP concrete deck bridge; •  Placing the deck post-tensioning strands in the haunch area over each girder line was not only simpler in construction, but also struc- turally efficient as it results in pre-compression of the PC deck panels and transverse joints, which was evident in linear variable differential transformers (LVDT) measurements; • The unreinforced post-tensioned transverse joint between adjacent precast concrete deck panels performed very well under live load with no signs of relative movements across the 24 joints in either horizontal or vertical directions, which also indicated the adequacy of the new post-tensioning system; and, • Moment distribution factors of the new system were accurately pre- dicted by the FEA, which were significantly lower than those predicted by AASHTO LRFD provisions for type "k" bridges. TECHNICAL TALK BY TOM KUENNEN Panel lifting showing shear pockets on precast bridge deck section in Kearney, Neb. On precast bridge deck in Kearney, Neb., end panel showing post-tensioning anchorage block. PHOTOS: Tawadrous, Morcous and Maguire

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