There are times when bridge designers are concerned with overhead clearances for highways beneath bridges, the need for increased span lengths without increasing girder depth, or may desire to reduce the number of girders for a bridge, all of which could require HPC. For states in the colder regions of the United States, de-icing chemicals are used for removing ice from driving surfaces. These chemicals pose problems on pavements and bridge decks. Due to the concrete's permeability as well as any cracking, chemicals can penetrate the concrete causing rebar corrosion and concrete deterioration over time in bridge decks shortening deck life.
Understanding the possible benefits of HPC, SDDOT's bridge designers have a desire to construct and evaluate four bridge decks as well as prestressed girders for two bridges. While HPC may appear to have higher initial costs, these higher costs may be offset through reduced girder depths, reduced number of girders, reduced number of substructure units, or reduced concrete deck thickness. HPC in concrete deck surfaces can result in a higher resistance to chloride intrusion, less surface cracking and greater resistance to freeze-thaw deterioration. Therefore, where HPC is utilized, there is a potential to reduce repair and replacement costs.
SDDOT has the opportunity to incorporate HPC into two bridge decks on I-229 in Sioux Falls as well as the decks and prestressed concrete girders for two additional bridges on I-29 north of Sioux Falls. However, trial HPC batches should be prepared and the necessary fresh and hardened concrete property testing must be conducted to ensure that the desired concrete properties are achieved. Some of the fresh and hardened concrete properties for each HPC mix are unknown and need to be determined prior to the construction of the girders and decks. Also, instrumentation may be needed to monitor the performance of the decks and girders. Some properties in question for HPC's are: 28 day compressive strength, obtaining strand release strength in a specified time, thermal properties, creep, shrinkage, and elastic shortening for high strength concrete with limestone and quartzite aggregates; permeability and crack potential of concrete utilizing fly ash, silica fume, or a combination of fly ash and silica fume with limestone and quartzite aggregates; etc.
Findings: The South Dakota Department of Transportation constructed two three span high performance concrete (HPC) bridges in the summers of 1999 and 2000. The twin prestressed girder bridges are located along Interstate 29 near Sioux Falls, South Dakota. In each bridge instrumentation was installed in two end span girders and in the deck of an end span of these structures. This report presents the results of the laboratory trial batches and testing to optimize HPC mix designs for the girders and the decks. Detailed strain histories in the girders and in the decks, and deflections of the girders prior to installation in the bridge and after they were installed in the bridge over the two year period are also reported.
For the high performance bridge deck concrete two different coarse aggregates were used (quartzite and limestone) and ten mixes were cast with each aggregate. In each mix the percentage replacement of cement by weight with silica fume and fly ash was varied, keeping the w/c ratio constant. For the high-strength bridge girder concrete, twelve mixes were cast varying both the percentage replacement of cement with silica fume and the w/c ratios. The percentage replacements of silica fume investigated were 7%, 10% and 12% and the w/c ratios investigated were 0.28, 0.30, and 0.32. All concretes were tested for compressive strength, static modulus, modulus of rupture and chloride permeability. The addition of fly ash and silica fume reduced the chloride permeability of concrete significantly while increasing the compressive strength. Based on the analysis of results obtained, one mix was chosen, as the best mix having all the properties required for a high performance bridge deck. Another high strength HPC mix was selected for the girders to satisfy the strength requirements for the early release of prestress strands and at 28 days. Detailed compressive strength time histories out to ages of one year were developed for the concrete used in the structures. Tests to determine the modulus of elasticity for both the girder and deck concrete were conducted at selected ages out to one year. Shrinkage blocks were cast and instrumented in order to monitor the development of shrinkage strains in the girders and the bridge decks. The total cost of the HPC bridges and the standard SDDOT present design bridges is almost the same. However the life-cycle cost may be cheaper because of the anticipated longer life and reduced maintenance costs for the HPC bridges. Conclusions and recommendations are included in the report.
Reduced permeability and low crack potential are the properties we desire to improve for the bridge decks. The bridge deck permeability should be reduced by a significant amount (50%) as compared to SDDOT's standard bridge deck mix. Trial mixes should include the use of fly-ash, silica-fume, a combination of fly-ash and silica-fume, limestone aggregates, and quartzite aggregates. Concrete properties should also be determined for mixes where the aggregate densities are optimized. Submit the selected mix design to the technical panel by April 1, 1998.
Increased 28 day and strand release compressive strengths are the properties we desire to improve for the prestressed girders. The compressive strength and strand release strengths will be determined by the Office of Bridge Design. Strand release strengths should be achieved in 18 hours. Trial mix designs should be conducted using both limestone and quartzite aggregates. Submit the selected mix design to the technical panel by October 1, 1998.