Elmer E. Marx, Alaska Department of Transportation & Public Facilities
Since the early 1970s, the Alaska Department of Transportation & Public Facilities has been building adjacent, precast, prestressed concrete deck bulb-tee girder bridges using high strength concrete (HSC). Most of these bridges are jointless utilizing either integral or semi-integral abutments. Typical girder spans range from 85 ft (26 m) for a 42-in. (1.065-m) deep section to 145 ft (44.2 m) for a 66-in. (1.675-m) deep section. Girder spacing is usually between 6.6 ft (2 m) and 8.2 ft (2.5 m).
Initially, design concrete strengths were 5500 psi (38 MPa) at release and 6500 psi (45 MPa) at 28 days. As the years passed, the specified concrete strength increased. It was presumed that improved durability would be one benefit of the increased concrete strength and no performance requirements such as chloride permeability, abrasion resistance, or freeze-thaw resistance were specified. Time has thus far proven the assumption true.
By the late 1990s, concrete release strengths of 7500 psi (52 MPa) and 28-day strengths of 8000 psi (55 MPa) were specified. The need to consistently obtain the high release strength in a short period of time, such as 18 hours, resulted in actual 28-day concrete strengths of 10,000 psi (70 MPa) or higher. As the specified concrete release strength has increased, fabricators have maintained their daily production cycle and no significant cost increase has occurred.
HSC has permitted an increase in the girder prestressing force. The combination of high strength concrete and increased prestressing force has resulted in both longer girder spans and wider girder spacings. By increasing the girder span length, Alaska’s most cost-effective bridge system can be used more often. By increasing the girder spacing, the cost of a typical bridge has been reduced by eliminating girder lines.
With the deck bulb-tee girder, bridge construction time is significantly reduced compared to conventional cast-in-place (CIP) deck systems. A typical highway overpass is often built in less than three months from the mobilization of equipment to the installation of the bridge railing. This is particularly important in Alaska where the construction season is short and CIP concrete is not readily available outside the major population centers.
A disadvantage of the deck girder system is its heavier weight compared to the standard bulb-tee girder. Transportation and erection equipment requirements are, therefore, increased. However, the reduction in the total number of girder lines has reduced the number of girders that must be transported to the site.
Because the concrete deck and girder are cast concurrently, the bridge deck concrete strength and durability are of exceptional quality not otherwise available when using CIP concrete. The deck is an integral component of the flexural system and is designed to remain in compression in the longitudinal direction under all service load combinations. In addition, a full width waterproofing membrane and asphalt overlay have generally been used on bridges built since the early 1980s. Consequently, there has been almost no girder-related maintenance required on the 212 bridges of this style built since 1973. Although traffic volumes are low compared to other states, Alaska has more severe environmental conditions. Studded tire and chain usage is high and may occur for up to six months per year. Deicing chemicals are used in much of the state, particularly in the corrosive maritime regions where snowfall is heavy and the number of freeze-thaw cycles is high.
Both the long- and short-term costs of the deck bulb-tee girder bridge have been unbeatable for the Alaska market. As the usage and quality of HPC continue to improve, Alaska will continue to lengthen bridge spans and increase girder spacings. Alaska’s most cost-effective structure will become even more economical.
Further Information
For more information, contact the author at (907) 465-6941 or [email protected].
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