Pitted Against the Potomac

Feb. 1, 2006

The Woodrow Wilson Bridge project, located approximately eight miles south and within sight of downtown Washington, D.C., involves replacement of the existing structure over the Potomac River and reconstruction of four major interchanges on the approach roadways in Maryland and Virginia.

The Woodrow Wilson Bridge project, located approximately eight miles south and within sight of downtown Washington, D.C., involves replacement of the existing structure over the Potomac River and reconstruction of four major interchanges on the approach roadways in Maryland and Virginia.

The new bridge, which is part of the I-95/I-495 Capital Beltway, comprises of two adjacent and independent bridges; each has 35 spans (including a 270-ft double-leaf bascule span) and is approximately 6,075 ft long. The Inner Loop bridge has a maximum deck width of 124 ft, while the Outer Loop bridge has a maximum deck width of 110 ft. Each bridge has the capacity to carry six lanes of highway traffic with the capability of adding a commuter rail line to each bridge in the future.

The steel plate girder bridge is supported on a substructure composed of precast and cast-in-place piers supported by concrete pile caps on steel pipe piles. The main objective for the precast and cast-in-place substructure concrete design mixes was to provide an economical design mix with a low permeability. During the design, it was decided that high-performance concrete (HPC) would be beneficial in meeting these criteria, and the engineer and sponsoring agencies decided to incorporate HPC in all concrete elements on the bridge.

Splash security

The water in the Potomac River at the project site is brackish, resulting in an exposure rating of moderate to aggressive. Particular attention was given to the underwater pile caps and areas of the piers within the splash zone. All reinforcing steel in this area is epoxy coated. The concrete placements for the pier foundations ranged from 700 to 6,000 cu yd. For the bascule foundation placement, the contractor cast 130 cu yd/hour continuously for 45 hours.

Strict mass concrete requirements were imposed to minimize thermal cracking in the large foundation concrete placements. The differential temperature between the center and surface of the placement was limited to 35ºF, and a maximum concrete temperature of 160ºF was specified. The contractor was required by specification to demonstrate his ability to meet this criterion by performing a thermal analysis. The substitution of 75% of cement with slag resulted in a lower heat of hydration and helped reduce the concrete temperatures. However, the contractor took additional precautions to meet these temperature requirements, and chose to use chilled water and ice in the mix for the mass placements during the summer months, when the maximum concrete temperature was a concern. At several locations, cooling pipes were installed in the concrete mass to minimize the core concrete temperatures. In the winter months, surface insulation and heaters were used to help meet the temperature-gradient requirements.

The precast segments using high-strength concrete for the upper portion of the V-pier ribs were cast on site on the Maryland and Virginia shores. Since the pier ribs are outside the splash zone and protected overhead by the bridge deck, plain reinforcing steel was specified since the material cost savings outweighed the limited benefits of using epoxy bars at these locations. Up to 75% of the cement could be replaced with slag per the specification. However, since slag slows the concrete strength gain, the contractor chose a final mix using 50% slag to allow for early stripping of the forms in the casting yard.

The two precast V-pier ribs are tied together at the top with two horizontal and parallel concrete tie beams that are hidden by the haunched steel girders. These pier tie beams are solid precast members, which are cast off site. These critical elements are post-tensioned to provide compression in the tie beam under all loading conditions. The tie beams have a compressive strength of 8,000 psi, with all other mix design criteria the same as the fixed pier ribs. The high-strength concrete mix is required in order to minimize the number of post-tensioning stressing stages for the tie beams during the construction of the pier.

For the cast-in-place bascule piers, the criteria for chloride permeability, strength, percent pozzolans or slag, water/cementitious material ratio and mass concrete temperature limits are the same as the fixed pier precast pier ribs. 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 roadway joint. Since the bascule pier design is sensitive to creep and shrinkage, additional tests were required to determine concrete modulus of elasticity and creep and shrinkage coefficients for the final concrete mix. Modulus of elasticity tests were specified and performed in accordance with the requirements of ASTM C469. In addition, creep and shrinkage tests were specified and performed in accordance with the requirements of ASTM C512. The results of these tests were considered when computing final camber values for the post-tensioned elements of the bascule pier.

The design goal for the cast-in-place bridge deck was to specify requirements for the concrete mix in order to obtain an economical mix that resulted in a durable slab that exhibited minimal cracking and had 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 75-year projected service life of the deck and bridge required by the FHWA, various deck concrete mixes and corrosion protection systems were evaluated.

In order to limit the chloride ingress to the concrete over the top layer of reinforcing steel, the project special provisions require the chloride permeability of the concrete to be less than 2,000 coulombs. To meet this requirement, the contractor was permitted by the specification to use up to 75% ground-granulated iron blast furnace slag. The slag also will provide improved flexural and compressive strength, durability and workability (especially in warm weather) and better consolidation of the concrete. Epoxy-coated reinforcing steel will be used for all reinforcing bars in the 10-in.-thick-fixed span deck. Calcium nitrate, a corrosion inhibitor, was used at a dosage rate of 2 gal per cu yd to protect the reinforcing steel and effectively increase the bridge service life.

Curing of the deck concrete requires the contractor to place two layers of wet burlap over the slab within 30 minutes of concrete placement. Mist spraying is required within 15 minutes of placement, and the burlap must remain saturated for the seven-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 will be applied to the finished deck. The sealer and calcium nitrate also will improve the corrosion protection at concrete cracks.

Refusing to retire

Based on the above requirements and using Fick’s Second Law of Diffusion applied to the uncracked concrete, it was estimated that it will require 60 years for chloride ingress to initiate corrosion activity at the level of the reinforcing steel, at a chloride concentration of 2 lb/cu yd. An additional 20 years for the corrosion propagation time (i.e., the time between chloride ions accumulating at the top reinforcing steel to corrosion damage) will provide a service life in excess of the required 75 years.

For the bascule-pier deck, the same requirements as for the fixed deck were applied with several modifications for the movable section. In order to minimize the size of machinery and the wear-and-tear on the machinery parts, a lightweight concrete (120 pcf/2 Mg/m3) was used for the 71?2-in.-thick deck. In addition, stainless-steel reinforcing bars (ASTM A955, Type 2205 Duplex or Type 316LN), without calcium nitrate, were specified for the bascule deck. Although more expensive than plain and 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 cost.

Currently, the contractors working on the Outer Loop Bridge are on schedule to meet the final completion date of May 2006. All traffic will then be diverted from the existing bridge to the new Outer Loop bridge to allow demolition of the existing bridge. The demolition will allow for construction and completion of the Inner Loop bridge with a planned opening date in mid-2008.

About The Author: Kite, Jarrett and Holt are with Parsons, Baltimore.

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