Optimized Sections for High-Strength Concrete Bridge Girders--Effect of Deck Concrete Strength
CHAPTER 5. CONCLUSIONS AND RECOMMENDATIONS
CONCLUSIONS
Based on the analyses described in this report, the following conclusions are made.
Cost Analyses
- 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.
RECOMMENDATIONS
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.
ACKNOWLEDGMENTS
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.
REFERENCES
- ACI Committee 363, State of the Art Report on High-Strength Concrete (ACI 363R-92), American Concrete Institute, Detroit, 1992.
- Carpenter, James E., "Applications of High-Strength Concrete for Highway Bridges," Public Roads, Vol. 44, No. 2, September 1980, pp. 76–83.
- Castrodale, R. W., Kreger, M. E., and Burns, N. E., A Study of Pretensioned High-strength Concrete Girders in Composite Highway Bridges–Design Considerations, University of Texas Center for Transportation Research, Research Report 381-4F, Austin, Texas, 1988.
- Zia, P., Schemmel, J. J., and Tallman, T. E., Structural Applications of High-Strength Concrete, North Carolina Center for Transportation Engineering Studies, Report No. FHWA-NC-89-006, Raleigh, North Carolina, 1989.
- Russell, B. W., "Impact of High Strength Concrete on the Design and Construction of Pretensioned Girder Bridges," Journal of the Precast/Prestressed Concrete Institute, Vol. 39, No. 4, July/August 1994, pp. 76–89.
- Russell, H. G., Volz, J. S., and Bruce, R. N., Optimized Sections for High-Strength Concrete Bridge Girders, FHWA, U.S. Department of Transportation, Report No. FHWA-RD-95-180, 1995, 165 pp.
- Rabbat, B. G., Takayanagi, T., and Russell, H. G., Optimized Sections for Major Prestressed Concrete Bridge Girders, U.S. Department of Transportation, Federal Highway Administration, Washington, DC, Report No. FHWA-RD-82-005, February 1982, 178 pp.
- Rabbat, B. G., and Russell, H. G., "Optimized Sections for Precast, Prestressed Bridge Girders," Journal of the Prestressed Concrete Institute, Vol. 27, No. 4, July/August 1982, pp. 88–104. Also reprinted as PCA Research and Development Bulletin RD080.01E, 1982, Portland Cement Association, 10 pp.
- Standard Prestressed Concrete Bulb-Tee Beams for Highway Bridge Spans to 150 ft, STD-115-87, Precast/Prestressed Concrete Institute, Chicago, Illinois, 1987.
- Garcia, A. M., "Florida's Long Span Bridges: New Forms, New Horizons," Journal of the Precast/Prestressed Concrete Institute, Vol. 38, No. 4, July/August 1993, pp. 34–49.
- Geren, K. L., and Tadros, M. K., "The NU Precast/Prestressed Concrete Bridge I–Girder Series," Journal of the Precast/Prestressed Concrete Institute, Vol. 39, No. 3, May/June 1994, pp. 26–39.
- Zia, P., High Performance Concrete in Severe Environments, SP140, American Concrete Institute, Detroit, 1993, pp. 3.
- Zia, P., Leming, M. L., and Ahmad, S. H., High Performance Concretes, A State-of-the-Art Report, Report No. SHRP-C/FR-91-103, Strategic Highway Research Program, National Research Council, Washington DC, 1991.
- Bruce, R. N., Martin, B. T., Russell, H. G., and Roller, J. J., Feasibility Evaluation of Utilizing High-strength Concrete in Design and Construction of Highway Bridge Structures, Final Report—Louisiana Transportation Research Center, Research Report FHWA-LA-92-282, Baton Rouge, Louisiana, 1994, 219 pp.
- Bridge Design Manual, Reinforced Concrete Superstructure, Vol. 2 Design Aids, State of Washington, Department of Transportation, September 1975, pp. 5–305.
- American Association of State Highway and Transportation Officials, Standard Specification for Highway Bridges, Fifteenth Edition, Washington, DC, 1992.
- Notes on ACI 318-89 Building Code Requirements for Reinforced Concrete with Design Applications, Portland Cement Association EB070D, 1990, 912 pp.
- Kaar, P. H., Hanson, N. W., and Capell, H. T., "Stress-Strain Characteristics of High Strength Concrete," Douglas McHenry International Symposium on Concrete and Concrete Structures, ACI SP55-07, American Concrete Institute, Detroit, 1978, pp. 161–185.
- Ahmad, S. H., and Shah, S. P., "Structural Properties of High Strength Concrete and its Implications for Precast Prestressed Concrete," Journal of the Prestressed Concrete Institute, Vol. 30, No. 6, November/December 1985, pp. 92–119.
- ACI Committee 318, Building Code Requirements for Reinforced Concrete, American Concrete Institute, Detroit, 1989.
- Martinez, S., Nilson, A. H., and Slate, F. O., "Spirally Reinforced High-Strength Concrete Columns," Research Report No. 82-10, Department of Structural Engineering, Cornell University, Ithaca, August 1982.
- Canadian Standards Association, CSA A23.3-94 Design of Concrete Structures, Rexdale, 1994.
- Pauw, A., "Static Modulus of Elasticity of Concrete as Affected by Density," ACI Journal, Vol. 32, No. 6, December 1960, Proceedings, Vol. 57, Paper No. 32, pp. 679–687.
- Collins, M. P., Mitchell, D., and Macgregor, J. G., "Structural Design Considerations for High-Strength Concrete," Concrete International, American Concrete Institute, Vol. 15, No. 5, May 1993, pp. 27–34.
- Popovics, S., "A Numerical Approach to the Complete Stress-Strain Curve of Concrete," Cement and Concrete Research, Vol. 3, No. 5, May 1973, pp. 583–599.
- Thorenfeldt, E., Tomaszewicz, A., and Jensen, J. J., "Mechanical Properties of High-Strength Concrete and Application in Design," Proceedings of the Symposium on Utilization of High-Strength Concrete, Tapir, Trondheim, 1987, pp. 149–159.
- Hognestad, E., A Study of Combined Bending and Axial Load in Reinforced Concrete Members, University of Illinois Engineering Experimental Station, Bulletin No. 399, June 1951, 128 pp.
- Hognestad, E., Hanson, N. W., and McHenry, D., "Concrete Stress Distribution in Ultimate Strength Design," ACI Journal, Vol. 52, No. 6, December 1955, pp. 455–479.
- Suttikan, C., A Generalized Solution for Time-Dependent Response and Strength of Noncomposite and Composite Prestressed Concrete Beams, Ph.D. Thesis, the University of Texas at Austin, 1978, 350 pp.
- Oesterle, R. G., Glikin, J. D., and Larson, S. C., Design of Simple-Span Precast Prestressed Girders Made Continuous, National Cooperative Highway Research Program, Report No. 322, Transportation Research Board, Washington, DC, 1989, 97 pp.
- ACI Committee 209, Prediction of Creep, Shrinkage, and Temperature Effects in Concrete Structures (ACI 209R-92), American Concrete Institute, Detroit, 1992.
- Hanson, J. A., Prestress Loss as Affected by Type of Curing, PCA Development Bulletin D075, Portland Cement Association, Skokie, IL, 1964.
- Burg, R. G., and Ost, B. W., Engineering Properties of Commercially Available High-Strength Concretes, PCA Research and Development Bulletin RD104, Portland Cement Association, Skokie, IL, 1994, 55 pp.
- Perenchio, W. F., and Klieger, P., Some Physical Properties of High-Strength Concrete, PCA Research Development Bulletin RD056, Portland Cement Association, Skokie, IL, 1978, 8 pp.
- Wolsiefer, J., "Ultra High-Strength Field Placeable Concrete with Silica Fume Admixture," Concrete International, American Concrete Institute, Vol. 6, No. 4, April 1984, pp. 25–31.
- Smadi, M. M., Slate, F. O., and Nilson, A. H., "Shrinkage and Creep of High-, Medium- and Low-Strength Concretes, Including Overloads," ACI Materials Journal, Vol. 84, No. 3, May–June 1987, pp. 224–234.
- Ghosh, R. S., and Timusk, J., "Creep of Fly Ash Concrete," ACI Journal, Vol. 78, No. 5, September–October 1981, pp. 351–357.
- Luther, M. D., and Hansen, W., "Comparison of Creep and Shrinkage of High–Strength Silica Fume Concretes with Fly Ash Concretes of Similar Strengths," Fly Ash, Silica Fume, Slug, and Natural Pozzolans in Concrete, SP Vol. 114, No. 5, American Concrete Institute, 1989, pp. 573–591.
- PCI Committee on Prestress Losses, "Recommendations for Estimating Prestress Losses," Journal of the Prestressed Concrete Institute, Vol. 20, No. 4, July–August 1975, pp. 44–75.
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