Optimized Sections for High-Strength Concrete Bridge Girders--Effect of Deck Concrete Strength
CHAPTER 5. CONCLUSIONS AND RECOMMENDATIONS
Based on the analyses described in this report, the following conclusions are made.
- The use of high-strength concrete in bridge decks will not result in a reduction of deck thickness or in the amount of transverse reinforcement. Therefore, no corresponding savings will occur.
- The use of high-strength concrete in bridge decks allows for a slight increase in maximum span lengths of bulb-tee girders.
- An increase of 25 percent in the in-place cost of high-strength deck concrete will only increase the overall superstructure cost by 5 to 10 percent.
- The use of high-strength concrete in bridge decks will result in less live-load deflection.
Flexural Strength and Ductility
- The use of high-strength concrete in bridge decks did not affect flexural strengths of the shorter span girders. At the maximum span lengths for each girder concrete strength, the high-strength concrete in the decks had a slight effect in increasing the flexural strength and ductility of the section.
- A minimum specified deck concrete strength of 41 MPa (6,000 psi) should be used for span lengths in excess of 24.4 m (80 ft) when girder concrete compressive strength exceeds 41 MPa (6,000 psi). Until further analyses can be performed, the specified deck concrete strength should be at least 60 percent of the specified girder concrete strength at 28 days when the specified girder concrete strength exceeds 41 MPa (6,000 psi).
- The applicability of the AASHTO specifications for flexural strength design with high-strength concrete needs to be evaluated.
Prestress Losses and Long-Term Deflections
- The use of high-strength concrete in the decks did not affect the magnitude of the prestress losses or long-term deflections.
- Prestress losses in high-strength concrete girders will generally be less than the losses in lower strength concrete girders.
- The current AASHTO procedure for calculation of prestress losses needs to be modified to account for the properties of high-strength concrete.
- The use of high-strength concrete in girders in place of lower strength concrete will result in less initial camber and similar long-term deflections for the same span lengths.
- Deflection requirements may limit the span lengths for which high-strength concrete girders with high-strength concrete decks can be used.
The Federal Highway Administration should continue to pursue the use of high-performance concrete in bridge decks. The impact of the increased initial costs is likely to be small compared to the long-term benefits. In addition to specifying durability requirements for the deck concrete, a minimum compressive strength of 41 MPa (6,000 psi) should be specified when the girder concrete compressive strength at 28 days is specified to be in excess of 41 MPa (6,000 psi) and span length exceeds 24.4 m (80 ft).
The industry should continue to pursue the usage of concrete with compressive strengths up to 69 MPa (10,000 psi) for prestressed concrete girders. The present research has not identified any limitations that would prevent existing design procedures from being utilized for concrete compressive strengths up to 69 MPa (10,000 psi). Special attention should be given to the deflections of long-span girders.
Additional work should be undertaken to evaluate the applicability of current design procedures for bridges constructed with high-performance concrete. This is particularly important for the longer span lengths where the amount of prestressing will be large and the girders will be spaced close together so that the effective width of the top flange is limited. A rationale should be developed that addresses the effects of the difference in compressive strength between the deck and girder concretes. Additional work is needed to address long-term deflections of long-span girders.
In a previous report, it was concluded that the application of high-strength concrete in bridge girders is limited by the amount of prestressing force that can be applied to the cross section. (6) A reduction in the assumed prestress losses will allow a higher force to be used in design for the same amount of prestressing steel. There is, however, a lack of data about the creep and shrinkage of high-strength concrete as used in prestressed girders. As part of the ongoing showcase projects, FHWA should encourage the monitoring of prestress losses and measurement of creep and shrinkage properties of the concretes.
The authors would like to express their appreciation to the following individuals and organizations who provided information relative to this project:
- W. Vincent Campbell, Bayshore Concrete Products Corporation
- Reid W. Castrodale, Portland Cement Association
- Z. T. George, Texas Concrete Company
- Howard W. Knapp, Rocky Mountain Prestress, Inc.
- David Pellizzari, Alfred Benesch & Company
- Habib Tabatabai, Construction Technology Laboratories, Inc.
- Max J. Williams, Gulf Coast Pre-Stress, Inc.
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