Mark D. Whittemore, New Hampshire Department of Transportation
The success of New Hampshire’s first high performance concrete (HPC) bridge—Route 104 in Bristol(1)—made the decision to proceed with the next HPC bridge an easy choice. Actually, during New Hampshire’s early involvement in HPC, it was planned to make the second project serve as an experimental control to the Route 104 HPC bridge. However, soon after completion of the Route 104 bridge, the New Hampshire Department of Transportation (NHDOT) determined it would be better to look forward, rather than revert back to the conventional deck and girder concrete construction. The goal, therefore, was to build on the results of the Route 104 bridge, making adjustments where problems had occurred, and solidifying where successes had been achieved.
The second HPC bridge, also located in Bristol, carries NH Route 3A over the Newfound River and is about one mile from the Route 104 bridge. The new bridge is a 60-ft (18.3-m) long simplespan structure that is 30 ft (9.1 m) from curb to curb with one 5-ft (1.52-m) wide sidewalk. The superstructure consists of 3-1/2-in. (90-mm) thick precast concrete deck panels with a 5-1/2-in. (140-mm) thick cast-in-place (CIP) concrete deck, and four precast, prestressed concrete New England bulb-tee (NEBT) 1000 HPC girders.
The advantages of combining HPC with the NEBT girders became apparent early in the design process. Girder spacings were increased to 11 ft-6 in. (3.51 m) on center. This reduced the number of girders from five to four. The NEBT was also 5-1/2 in. (140 mm) shallower than the AASHTO/PCI Type III girder, providing additional vertical clearance over the design flood elevation. The same girder concrete compressive strength of 8000 psi (55 MPa) used on the Route 104 bridge was specified for use on Route 3A bridge. Attaining this strength consistently was an issue on the Route 104 bridge and, consequently, several modifications were made to the specifications on the Route 3A bridge.
First, the air entrainment requirement for the girders was reduced from a range of 5 to 8 percent to a target value of 5 percent with a lower limit of 3.5 percent. Justification for this was that the girders would not be subjected to wetting from melting snow and deicing salts, and research supported the freeze-thaw durability of concrete with air entrainment values as low as 3 percent.(2) Second, a more proactive approach was taken in pursuing the necessary trial batching for developing an acceptable concrete mix design. The precaster aggressively supported a cooperative effort and trial batches consistently achieved strengths required by the specification.
A big asset to the contractor in pursuing a condensed construction schedule was the use of partial depth precast, prestressed concrete deck panels. On the Route 104 bridge, the contractor expressed a need for a less expensive method of forming the deck. The use of deck panels on the Route 3A bridge helped to reduce the costs associated with special deck forms needed for the wider girder spacings.
Average 28-day compressive strengths of 9000 psi (62.1 MPa) were obtained in the field for the CIP deck concrete along with chloride permeabilities well below the specification goal of 1000 coulombs at 56 days. The proper finishing and curing of the deck was crucial in order to achieve an excellent and durable concrete surface. Using a work bridge behind the screed machine, cotton mats were spread out on the deck and most were wetted within 10 minutes after the screeding operations. The cotton mats were kept wet for seven days. Inspections to date have revealed an excellent surface with only four visible hairline cracks. The completed bridge was opened to traffic in June 1999. This bridge received the 2000 PCI Design Award for the best bridge with spans less than 65 ft (19.8 m).(3)
The NHDOT has been extremely pleased and satisfied with its second bridge using HPC. Several concerns from the first HPC project were addressed to our complete satisfaction. The girders, deck panels, and CIP deck have performed superbly. With another success for HPC, the Department is well on its way to making HPC the standard concrete practice for New Hampshire’s bridges.
Further Information
For further information about the Route 3A bridge, contact the author at 603-271-2731 or [email protected]. Information about the Route 104 and Route 3A bridges is available in the compilation described on Page 4 of this newsletter.
References
- Waszczuk, C. M., “Crack Free HPC Bridge Deck–New Hampshire’s Experience,”HPC Bridge Views, Issue No. 4, July/August 1999, p. 2.
- SHRP High Performance Concrete Showcase Notebook, Houston, TX, March 25-27, 1996, Ramon Carrasquillo’s presentation on Mix Proportioning, p. 4-18.
- Ascent Magazine, Precast/Prestressed Concrete Institute, Fall 2000, pp. 74-75.