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A concrete compressive strength of 10,000 psi (70 MPa) was specified for
the precast superstructure and column segments.
High Strength Concrete for the Roslyn Viaduct
Patricia Barnes, Larry McAllen, and Wayne Moore, Bayshore Concrete ProductsLong Island, NY, famous for heavy and congested traffic patterns, needed to replace a key bridge over Hempstead Harbor in Roslyn. The original steel superstructure of the Roslyn Viaduct opened in 1949 with two lanes of traffic in each direction. Today, it carries approximately 38,000 cars and trucks each day. This aging bridge had deteriorated and needed to be replaced with a modern, sustainable bridge.
A precast, prestressed, variable depth, concrete segmental structure was the chosen technology for the new viaduct. This is Long Island’s first precast concrete segmental bridge. Several factors went into the decision making process, including a streamlined construction timeline by using pieces that are all cast offsite and the service life of a precast concrete bridge, which is targeted to be 60 to 100 years. Bayshore Concrete Products Corporation (BCP) located in Cape Charles, VA, was contracted to produce all 348 superstructure segments and 64 pier box column segments.
The twin structures of the new bridge will each have nine spans and eight pier columns with each structure carrying the traffic in one direction. The span lengths vary from 121 to 292 ft (36.9 to 89.0 m) and thus, the radius of curvature of the bottom of each span varies. As a result, each segment is unique. The 64 pier columns segments have a ship-lap architectural profile achieved by using formliners.
BCP faced multiple challenges with this project. Production of the segments was accomplished with the use of four casting machines. Two casting machines produced the girder segments with depths ranging from 10 to 4 ft (3.05 to 1.21 m). Another casting machine produced the girder segments with depths ranging from 17 to 10 ft (5.18 to 3.05 m), and the fourth casting machine was used for all pier segments. The shallower girder segments were the most abundant and the most critical to the schedule, hence the reason for two casting machines.
No two adjacent segments were exactly the same size or shape. Except for the starter segment, all segments were match cast against each other. The varying depth of the segments required formwork modifications every time. Within the form crew, smaller crews prepared the form sections in advance. Each casting machine had three soffit tables to allow advance setup. The erector’s anticipated schedule was aggressive so the time for formwork changes between castings had to be kept short.
Geometry control was another challenge. Geometry control was achieved by using the short-line match casting method. A typical casting run would start with the starter segment and end at the first wet joint. Each span generally consists of 20 to 23 segments with wet joints at midspan and at five or six segments away from midspan. The first cantilever girder segments were match-cast to each side of the girder pier segment. This required the double handling of 85 to 95 ton (77 to 86 Mg) girder pier segments; once to cast the upstation cantilever and a second time to cast the downstation cantilever.
In addition, this project required the consistent production of quality segments with sensitivity to the aesthetics of the precast units. The project required that BCP develop a concrete mix design that had the flow and consolidation characteristics of self-consolidating concrete as well as stringent high performance concrete requirements. The project specifications required that the concrete achieve a strength of 10,000 psi (70 MPa) at 56 days for both superstructure and columns. This strength was typically achieved in 14 days using the following concrete mix proportions:
In addition to the typical field tests such as spread or slump, entrained air content, and temperature, each delivered load of concrete was tested for compliance to water content in accordance with AASHTO TP 23 (now T 318) Water Content of Freshly Mixed Concrete using Microwave Oven Drying. A water-cementitious materials ratio between 0.29 and 0.33 was required for an acceptable batch. To achieve these concrete qualities, it was necessary to incorporate a high-range water-reducing admixture, a water reducer-retarder, and a corrosion inhibitor.
The contract specifications also required that the concrete meet specific values for freeze-thaw durability, scaling resistance, chloride permeability, air content, modulus of elasticity, creep, and shrinkage. The specified values and actual results are shown in the following table:
Over 4.5 million lb (2 million kg) of reinforcement was required for this project with uncoated bars used for all reinforcement enclosed in the segment concrete and stainless steel bars used for the projecting reinforcement. The reinforcement was very congested making placement difficult and typical segments very heavy. Some segments had so much reinforcement that no light was visible through the casting cell.
Segments were cured using steam. Except during the winter, very little steam was needed since the concrete mix generated most of its own heat. The application of steam was mostly to maintain a moist environment within the curing enclosure. The shear size of some of the segments made curing a challenge. Thermocouples in the enclosure and embedded in the concrete were used to monitor temperatures during curing to ensure that enclosure temperature was uniform over the entire piece.
Finally, each segment, superstructure, and column had to be hand rubbed for a smooth finish and a silane coating had to be applied, then approved and accepted by the NYSDOT inspector. Tully Construction, the project contractor, also had a representative at BCP to inspect each segment. All of the pieces were barged close to the job site and then transported by road using special trailers.
For further information about the segment production, please contact the first author at firstname.lastname@example.org.
HPC Bridge Views, Issue 57, Sept/Oct 2009