Chloride damage spawns rehab of bridge decks.

Work on the first of two contracts rehabilitating the approach decks and related elements of New York’s Verrazano Narrows Bridge is underway for the Triborough Bridge & Tunnel Authority. The project’s design and materials specifications were influenced by the authority’s objective of achieving a 20-year life from the rehabilitated deck slabs.

The tests indicated that the deterioration was caused by chloride exposure over the 30 years the decks have been in service. The staged work is being done behind concrete barriers, closing one lane in each direction. Work must be shut down for the annual running of the New York City Marathon in the fall.

The Brooklyn approach ramps include 13 structures with a total deck area of 637,170 square feet. The Staten Island side has a combined deck area of 154,590 square feet. Some 150,000 square feet of the total deck area must be completely replaced.

Structural investigations and tests during the one-year design phase established a profile of the chloride contamination of the concrete, gained an indication of corrosion potential of the reinforcing steel and quantified the concrete’s compressive strength and resistance to freeze/thaw cycles. In all, 569 samples were collected at two-inch and six-inch depths at 286 locations across the concrete slab for chloride half-cell, compressive strength and freeze/thaw cycle tests. These tests showed that chloride had penetrated down to the lower mat of the reinforcing steel in many areas of the deck.

In a rather unusual application, relatively impermeable latex modified concrete (LMC) is being used structurally for the curb-to-curb replacement of the top three-inches of the nine-inch decks. This will not only remove the most contaminated portion of the concrete, but will also create a protective barrier for the underlying original concrete and reinforcing steel against future chloride intrusion. Where a total replacement of the nine-inch slab is required, the new slab will be seven-and-a-half inches of Class E concrete with a one-and-a-half-inch topping of LMC. The top surfaces will be grooved for skid resistance.

Approximately 10 percent to 50 percent of the deck surfaces had been patched over the years but never received an overlay. A study of repair options ruled out other alternatives such as sealers, traffic membranes, overlays and cathodic protection on the basis of cost, maintenance requirements and service life.

With a main span of 4,260 feet, the Verrazano remains the nation’s longest suspension bridge.

The scope of the project now underway encompasses the roadway decks, expansion joints, electric pull boxes and power conduits that were designed and built to accepted practices at the time of original construction in the 1960s.

The chloride infiltration attacked the bare decks relentlessly, and they eventually exhibited virtually every sign of damage, including efflorescence, honeycombing, hidden voids, spalls, cracks and delaminations.

Except for the Shore Parkway ramps on the Brooklyn side, all approach slabs will be replaced. Also, the existing pressure relief joints and about 20 feet of on-grade concrete pavement beyond the approach slabs are candidates for replacement.

In addition, some 675 manholes/pull boxes; 29,000 feet of conduits; 1.2 million feet of conductor; and 80 lighting fixtures were in poor enough condition to justify their repairs or replacement.

Greiner Inc., Rocky Hill, Conn., led the design team, which included NYC-based American Consulting Engineers and Metro Tech Consulting Services. The first contract for $25.8 million was awarded to Slattery Associates, New York City.