Craig Abernathy, Montana Department of Transportation
In the summer of 2002, the Montana Department of Transportation (MDT) initiated a research project on Secondary Road No. 243 near the town of Saco in northeast Montana. This research opportunity was afforded by the construction of three bridges with the same geometry on the same route within 1/4 mile (400 m) of each other. The variability in conditions between test sites typically encountered in large scale field investigations was minimized in this situation. Notably, the bridges would have a common quality of construction and would experience the same vehicular and environmental conditions. This situation offered the opportunity to evaluate the relative performance of three different bridge decks.
Each bridge consists of three spans with a total length of 146 ft (44.5 m) and a width of 27.6 ft (8.4 m). The superstructure consists of four lines of AASHTO Type I precast, prestressed concrete beams spaced at 7.9 ft (2.4 m) centers with a cast-in-place reinforced concrete deck approximately 8 in. (200 mm) thick. Epoxy-coated reinforcement is used in the deck. The bridge decks were cast in the second quarter of 2003.
The objective of the project was to investigate the performance of the following three types of concrete bridge decks:
- Conventionally reinforced deck made with standard concrete, designed and constructed following MDT’s standard practices.
- Deck with reinforcement designed according to the empirical design approach of the AASHTO LRFD Bridge Design Specifications, made with standard concrete, and constructed following MDT’s standard practices.
- Conventionally reinforced deck made with high performance concrete (HPC).
The specifications for the HPC deck required a minimum cementitious materials content of 615 lb/cu yd (365 kg/cu m), a silica fume content of 5 to 7 percent by weight of the cementitious materials, a maximum water content of 270 lb/cu yd (160 kg/ cu m), a slump of 1.6 to 3.1 in. (40 to 80 mm), an air content of 5 to 7 percent, and a minimum compressive strength of 4500 psi (31 MPa) at 28 days. Subsequently, the use of fly ash up to a maximum of 20 percent by weight of cementitious materials was permitted. Each deck was required to be water cured for 14 days using a burlap cover and fogging nozzles.
Each bridge deck was instrumented to monitor strains during live load tests, long-term deflections, and long-term strains. In addition, crack and corrosion monitoring will be performed on a regular basis.
Measured compressive strengths at 28 days for the HPC ranged from 7270 to 8340 psi (50.1 to 57.5 MPa) and for the standard concrete ranged from 3920 to 4840 psi (27.0 to 33.4 MPa).
Crack mapping of the decks approximately 5 weeks after their construction showed that hairline cracks had formed over the bents of all bridges except one bent with the HPC deck. An inspection of the decks approximately 9 months after casting indicated only one additional crack—a full depth diagonal crack near a corner at the abutment of one bridge. After 12 months, no additional cracking was observed and all cracks are still considered hairline.
In addition to monitoring the bridge instrumentation, MDT has a program underway to develop a cost effective HPC for use in bridge deck applications. The program includes testing for compressive strength, modulus of elasticity, rapid chloride permeability, chloride penetration resistance, freeze-thaw durability, scaling resistance, and shrinkage.
Further Information
For further information, contact the author at [email protected] or 406-444-6269. For project reports, go to www.mdt.state.mt.us/research/projects/mat/high_concrete.shtml.