THE CONCRETE BRIDGE MAGAZINE

SUMMER 2017

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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to end-of-slab) and was on a tangent alignment with a 0% gradient. The superstructure was 27 ft 4 in. wide (out-to-out), which included a roadway width of 24 ft from curb to curb and a 1-ft 8-in. combination curb/concrete rail. The superstructure consisted of four haunched cast-in- place reinforced concrete T-beams with a reinforced concrete deck, which was placed integrally with the T-beams. The reinforced concrete deck had an asphalt overlay of approximately 2½ in. The existing superstructure did not have bearing pads, per the original plans. The bridge substructure units are parallel to each other and perpendicular to the centerline. The abutments are concrete gravity-style with minimal reinforcement supported on timber piles approximately 35 ft in length. Connecting the two abutments are three reinforced concrete struts below the waterline. The wingwalls are oriented 45 degrees to the backwall. Need for Rehabilitation In 2011 the bridge structure was identified as requiring maintenance. The bridge safety report revealed the general condition of the existing concrete deck to be structurally deficient due to delaminations up to 2½ in. deep throughout the concrete deck, with the bottom of the deck also having delaminations and cracks. In addition, the concrete slab overhangs and railings had delaminated and spalled, exposing corroded reinforcing steel. It was also noted that the exterior beams had cracks, delaminations, and spalls along the sides and bottoms. After preliminary discussions with VDOT, a full bridge replacement was determined not to be a viable option due to its location within existing wetlands and the necessary permits required for such an extensive project. VDOT concluded that the most appropriate solution to rehabilitate the 70-year-old bridge structure was a superstructure replacement. An in-depth field investigation of the existing substructure was conducted to determine its condition and suitability to support the new superstructure. All of the visible concrete on the abutments was hammer sounded to record areas of delaminated and spalled concrete. A probing rod was used to determine the extent of features that were under water, such as the concrete struts. Design Aspects Various superstructure replacement options were evaluated, including prestressed hollow-core slabs with a reinforced concrete deck, VDOT p re c a s t c o n c re t e b u l b - t e e b e a m s with a reinforced concrete deck, and galvanized structural steel girders with a reinforced concrete deck. To determine the most appropriate solution, several factors were evaluated, including geometry, final conditions, maintenance of traffic, environmental issues, and structural design. Ultimately, the final decision centered on which option would not increase the dead load applied to the existing abutments while providing the best long-term, low- maintenance solution. The VDOT concrete bulb-tee beams met the geometric requirements and provided a much more durable option than a structural steel superstructure. While the structural steel option d i d o f f e r t h e m o s t l i g h t w e i g h t superstructure, it was determined not to be an appropriate long-term, low- maintenance solution for this location due to its proximity to the brackish water. The use of the hollow-core slabs per VDOT design guidelines would have required a reinforced concrete deck for this roadway classification. While this solution was efficient, the geometry and high dead load for this option did not meet the requirements for using the existing substructure. The VDOT concrete bulb-tee beams met the geometric requirements and provided a much more durable option than a structural steel superstructure in this tidal environment, but the use of normalweight concrete increased the dead load on the existing substructure. T h e re f o re , t o m i n i m i z e t h e d e a d load, the designers evaluated the use of various densities of lightweight concretes for both the bulb tees and the concrete deck. After discussions with VDOT and industry professionals, the bulb tees were designed using VDOT Class A5 lightweight concrete with a maximum density of 115 lb/ft 3 and a minimum compressive strength at 28 days of 5 ksi. The beams were designed for a minimum compressive strength VIRGINIA DEPARTMENT OF TRANSPORTATION, FREDERICKSBURG DISTRICT, OWNER BRIDGE DESCRIPTION: A 45-ft 2-in.-long, lightweight concrete prestressed bulb-tee beam bridge OTHER MATERIAL SUPPLIERS: Lightweight aggregate supplier: Carolina Stalite Co., Gold Hill, N.C.; Stainless-steel reinforcement supplier: SteelCON Supply Company, Jacksonville, Fla. STRUCTURAL COMPONENTS: Four lightweight concrete prestressed 29-in.-deep bulb-tee beams with an 8½-in.-thick cast-in-place lightweight concrete deck and a lightweight concrete semi-integral backwall and substructure modifications BRIDGE CONSTRUCTION COST: $526,880 ($360.78/ft 2 ) Two 29-in.-deep lightweight concrete bulb tees prior to deck placement. Photo: Virginia Department of Transportation. ASPIRE Summer 2017 | 13

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