Alan Kite, Parsons
The Woodrow Wilson Memorial Bridge project, located approximately 8 miles (13 km) south and within sight of downtown Washington, DC, involves the structural replacement and capacity upgrades of the existing structure over the Potomac River. The new bridge is part of the I-95 and I-495 Capital Beltway. It is comprised of two adjacent and independent bridges known as the Inner and Outer Loops, which are 124 ft (37.8 m) and 110 ft (33.5 m) wide, respectively. Each bridge carries six traffic lanes with full-width shoulders. A sidewalk is provided on the north side of the Inner Loop bridge to connect the parks at each end of the bridge. The new bridge has 35 spans, including a double-leaf bascule span, and is approximately 6075 ft (1,852 m) long.
Deck Concrete
For the cast-in-place concrete bridge deck, the objectives were to obtain an economical concrete mix that will result in a durable deck with minimal cracking and a low permeability to minimize chloride intrusion. The bridge is located in a moderate to aggressive environment where deicing salts are used extensively in the winter. In order to meet these objectives and a required 75-year projected service life of the bridge, various deck concrete mixes and corrosion protection systems were evaluated.
In order to limit the ingress of chlorides to the concrete over the first layer of reinforcing steel, the project Special Provisions require the chloride permeability of the concrete to be less than 2,000 coulombs at 56 days. To meet these permeability requirements, the contractor is permitted to use ground granulated blast-furnace slag for as much as 75 percent of the cementitious materials content. The slag also provides improved flexural strength, compressive strength, durability, workability (especially in warm weather), and consolidation of the concrete. Epoxy-coated reinforcing steel is used in the 10-in. (254-mm) thick fixed span decks. Calcium nitrite, a corrosion inhibitor, is also used at a dosage rate of 2 gal/cu yd (9.9 L/cu m) to protect the reinforcing steel and effectively increase the bridge service life.
Curing of the deck concrete requires that the contractor begin fog spraying within 15 minutes of concrete placement and place two layers of wet burlap over the slab within 30 minutes of placement. The burlap must remain continuously saturated for the 7-day curing period. This curing method is expected to minimize the drying and plastic shrinkage cracks at the deck surface. After the deck has cured, two coats of linseed oil or a silane based sealer are applied to the finished deck.
Based on the above requirements and using Fick’s Second Law of Diffusion applied to the uncracked concrete, it will require an estimated 60 years for chloride ingress to initiate corrosion activity at the level of the reinforcing steel assuming that corrosion begins at a chloride concentration of 2.0 lb/cu yd (1.2 kg/cu m). An additional 20 years from the start of corrosion of the top reinforcing steel until corrosion damage occurs will provide a service life in excess of the required 75 years.*
For the bascule pier deck, the same requirements as those for the fixed decks are used with several modifications. To minimize the size of machinery and the wear-and-tear on the machinery parts, a lightweight concrete with a density of 120 lb/cu ft (1.92 Mg/cu m) is used for the 7.5-in. (190-mm) thick deck. In addition, solid stainless steel reinforcing bars (ASTM A955, Type 2205 Duplex or Type 316LN) are specified for the bascule deck. Although more expensive than uncoated or epoxy-coated reinforcing steel, the design team and sponsoring agencies decided that the advantages of better corrosion resistance, extended service life, and reduced maintenance cost offset the additional initial cost. The use of calcium nitrite is not specified for the bascule deck.
Substructure Concrete
For the precast and cast-in-place concrete V-piers, the main objective for the concrete mix design was to provide an economical design mix with a low permeability. The water in the Potomac River at the project site is brackish, providing a moderate to aggressive exposure. Particular attention was given to the underwater pile caps and areas of the pedestal within the splash zone. Concrete placements for the pier foundations range in size from 700 cu yd (535 cu m) to 6000 cu yd (4587 cu m). For the large bascule pier, the contractor cast 130 cu yd/hr (99.4 cu m/hr) continuously for 45 hours. All reinforcing steel in the pile caps and pedestals is epoxy coated.
Due to the large size of the foundation, strict mass concrete requirements were imposed to minimize thermal cracking. The differential temperature between the center and surface of the pour was limited to 35°F (19°C), and a maximum concrete temperature of 160°F (71°C) was specified. The contractor was required by specification to demonstrate his ability to meet this criterion by performing a thermal analysis. The use of slag for 75 percent of the cementitious materials resulted in lower heat of hydration and helped reduce the concrete temperatures. The contractor also chose to use chilled water and ice in the mix during the summer months when the maximum concrete temperature was a concern. In the winter months, the contractor used surface insulation to control the temperature gradient. At several locations, cooling pipes were installed to minimize internal concrete temperatures.
The precast segmental V-pier legs utilize two standard hollow box girder sections. The segments were match cast in long-line beds constructed within the project right-of-way. Up to 75 percent of the cementitious materials could be slag per the specification. However, since slag slows the strength gain, the contractor chose a final mix using 50 percent slag to allow earlier stripping of the forms. Also, since the pier legs are outside the splash zone and protected overhead by the bridge deck, uncoated reinforcing steel was specified since the economics outweighed the limited benefits of using epoxy-coated bars at this location.
The two precast pier legs at each foundation are tied together at the top with two parallel concrete tie beams as shown in the photograph. These tie beams are a solid precast member, which is cast off-site. These critical elements are post-tensioned to provide compression in the tie beams under all loading conditions. The tie beams have a specified compressive strength of 8000 psi (55 MPa), with all other mix design criteria being the same as for the pier legs. High strength concrete was required in order to minimize the number of stages of post-tensioning for the tie beams. For the cast-in-place bascule piers, the criteria for chloride permeability, compressive strength, percent pozzolans or slag, water-cementitious materials ratio, and mass concrete temperature limits are the same as for the fixed precast pier legs. Since the bascule pier design is sensitive to creep and shrinkage, additional tests were required to determine concrete modulus of elasticity, creep, and shrinkage for the final concrete mix. The results of these tests were considered when computing final camber values for the post-tensioned elements of the bascule pier. Epoxy-coated reinforcing steel is required throughout the bascule pier to provide a higher level of durability, especially since road salts can reach the pier from the bascule tail joint.
Schedule
The anticipated completion of the Outer Loop bridge is approximately mid-2006. All traffic will be diverted from the existing bridge to the Outer Loop bridge to allow demolition of the existing bridge and construction of the Inner Loop bridge. Completion is anticipated by mid-2008.
Further Information
For futher information about this project, go to www.wilsonbridge.com or contact the author at [email protected] or 410-223-2740.
*Grace Construction Products, 2000 “Durability Report: Corrosion Protection,” 20 pp.