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10,000 psi (69 MPa) concrete was used in the beams.
High Strength Concrete for the Hoosick River Crossing
Ben Cota, J.P. Carrara & Sons, Inc.The new four-span Hoosick River Crossing carries Route 7 across the Hoosick River in the town of Hoosick, Rensselaer County, in the northeast corner of New York State. The new bridge replaced an existing steel bridge that was originally built in 1932 and rehabilitated in 1976.
The new bridge consists of two end spans with a length of 115 ft (35.1 m) and two interior spans of 137 ft (41.8 m). The cross section utilizes five beam lines of New England bulb tees (NEBT 2000) with a depth of 78.7 in. (2000 mm) at a spacing of 9.0 ft (2.7 m) and a 9½-in. (240-mm) thick cast-in-place concrete deck.
Precast, Prestressed Concrete Beams
The longer precast, prestressed concrete beams contain a total of fifty-two 0.6-in. (15.2-mm) diameter strands including 10 draped strands and 6 top strands. The total prestressing force was 2.3 million lb (10 MN). The shorter beams contain a total of fifty 0.6-in. (15.2-mm) diameter strands and required a prestressing force of 2.2 million lb (9.8 MN). Bursting reinforcement at the ends of the beams consists of pairs of No. 6 stirrups at 2¾ in. (70 mm) centers. Shear reinforcement consists of pairs of No. 4 stirrups at a spacing varying from 4 in. (100 mm) at the ends of the beams to 24 in. (610 mm) at midspan. The longer and shorter beams weighed 73 and 62 tons (66 and 56 Mg), respectively.
High Strength Concrete
The specified concrete compressive strengths for the precast, prestressed bulb-tee beams were 7100 psi (49.0 MPa) at prestress transfer and 10,000 psi (69.0 MPa) at 56 days. An air content of 5 to 9% was specified. The concrete was placed at an 8 to 10 in. (200 to 250 mm) slump. The following concrete mix proportions were used:
2. ¾ in. (19 mm) maximum aggregate size
Prior to approval of the concrete mix proportions, the New York State Department of Transportation required the mix design to satisfy the state requirements for freeze-thaw durability, scaling resistance, modulus of elasticity, shrinkage, creep, and chloride penetration. In addition, the concrete was required to contain a minimum of 5% silica fume measured as a percentage of the total cementitious materials. The goal is to achieve a bridge with a 75-year service life. The concrete mix was qualified for use on this specific project.
Concrete for each longer beam was produced in four batches of 9.25 yd3 (7.1 m3) each for a total of 37 yd3 (28.3 m3). For the shorter beams, six batches of 10.8 yd3 (8.3 m3) each were used to produce two beams. Ambient temperatures during production ranged from 65 to 85°F (18 to 29°C) and concrete temperatures at time of placement ranged from 75 to 86°F (24 to 30°C), which were less than the specified maximum temperature of 90°F (32°C). The water content of the first batch for each placement was measured using AASHTO T 318, Water Content of Freshly Mixed Concrete Using Microwave Oven Drying. Slump and air content were measured for each batch. Cylinders representing each batch were cast and cured with the beams until the specified strength was achieved.
Although the concrete had a measured slump of 8½ to 10¼ in. (215 to 260 mm), both internal and external vibration were used to ensure adequate concrete consolidation in the areas of congested reinforcement. Measured air contents ranged from 5 to 8½% with an overall average of 6.3%. The average measured compressive strengths at transfer and 28 days were 7580 and 11,270 psi (52.3 and 77.7 MPa), respectively.
Two lessons were learned about the concrete mix design:
For more information about this project, contact the author at firstname.lastname@example.org or 802-388-6363.
Beams for the Hoosick River Bridge.
HPC Bridge Views, Issue 66, Mar/Apr 2011