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Sand-lightweight concrete was used to reduce the shipping
and handling weight of the precast, prestressed concrete beams.
Jeff Danzer Bridge, Ohio
The Jeff Danzer Bridge, formerly known as the Dixon Mill Road bridge, is located over the Little Scioto River in Scioto County, Southern Ohio. The bridge is a 201-ft 6-in. (61.4-m) long, two-lane, single span, precast, prestressed concrete structure. It replaced a steel truss bridge that was functionally obsolete, posted for reduced loads, and in critical need of replacement. A simple span structure was chosen to span the River in order to apply for and receive a U.S. Corps of Engineers nationwide permit rather than endure the lengthy individual permit process. The profile grade was raised approximately 10 ft (3 m) above the existing grade to accommodate the deeper superstructure of a simple span. The owner was able to acquire property for right-of-way and relocate utilities prior to awarding a design-build contract. The specified options for the bridge type were limited to galvanized steel, cast-in-place concrete, and precast, prestressed concrete. Two bids were submitted. One utilized three lines of 96-in. (2.44-m) deep steel plate girders, while the winning bid utilized four lines of precast, prestressed concrete beams.
The bridge design load was HS20-44 with alternate military loading. The allowable tensile stress in the precompressed tensile zone was taken as 3 psi (0.25 MPa). The superstructure consists of four lines of 103-in. (2.62-m) deep precast, prestressed concrete spliced bulb-tee beams with a 61-in. (1.55-m) wide top flange and an 8-in. (203-mm) thick web. The beams are spaced at 8 ft 0 in. (2.44 m) on centers and support an 8-1/2-in. (215-mm) thick cast-in-place normal weight concrete deck. Section lengths for the beams were 75 ft and 125 ft 6 in. (22.9 and 38.3 m) with a 1-ft (305-mm) long closure pour. The individual sections were pretensioned for shipping and erection. The individual lengths were selected so that they could be transported and erected without difficulty. To reduce the beam weight for shipping, a sand-lightweight concrete was specified.
The sand-lightweight concrete was specified to have a unit weight of 125 pcf (2000 kg/m3) and compressive strengths of 6000 and 7000 psi (41 and 48 MPa) at strand release and 28 days, respectively. Actual strengths ranged from 6050 to 7790 psi (41.7 to 53.7 MPa) at release and 8820 to 10,320 psi (60.8 to 71.2 MPa) at shipping ages of 25 to 39 days. The concrete mix proportions for the sand-lightweight concrete are given in the following table:
The above table lists the concrete mix proportions as 750 lb of cement, 1132 lb of fine aggregate, 912 lb of coarse aggregate, 458 lb of lightweight aggregate, 270 lb of water, 5 fl oz of air entrainment, 6 fl oz of retarder, 52.5 fl oz of water reducer, and 256 fl oz of corrosion inhibitor.
A cast-in-place spread footing and abutment were used at one end of the bridge. At the other end, the abutment was supported on cast-in-place drilled shafts. A temporary concrete slab and steel bent were erected at the splice location. A semi-integral connection was provided between the beams and the abutments.
Post-tensioning of the beams was applied in two stages. The first stage was applied after the beams were erected and the splices cast. The second stage was applied after the deck was cast. The post-tensioning tendons consisted of two tendons with seventeen 0.6-in. (15-mm) diameter strands and two tendons with sixteen 0.6-in. (15-mm) diameter strands. The specified jacking forces were 750 and 710 kips (3.34 and 3.16 MN) for the larger and smaller tendons, respectively.
Construction of the bridge took only 120 days, which fulfilled the contract requirement for road closure. The bridge is named in honor of an outstanding Scioto County Engineer and serves as a monument to innovation. It is the longest single-span bridge in Ohio using precast, prestressed concrete girders. It is hoped that the bridge will serve as a prototype for single-span bridges in the future.
This article is extracted from an article in the PCI Journal Vol. 52, No. 5, September-October 2007 and additional information provided by Brian Slagle of Janssen & Spaans and Donald J. Bosse of Prestress Services Industries.
HPC Bridge Views, Issue 49, May/June 2008