Bijan Khaleghi, Washington State Department of Transportation
The Tieton River Bridge was the first use of self-consolidating concrete (SCC) for a precast, prestressed girder superstructure by the Washington State Department of Transportation (WSDOT). The project involved replacing two 77-year old bridges and widening lanes and shoulders along and over the Tieton River, about 14 miles (22.5 km) west of Naches, WA. Built in 1933, the original bridges were only 24 ft (7.3 m) wide and classified as structurally deficient. Design of the replacement bridge began in 2004, with construction completed in 2009.
The replacement Tieton River Bridge consists of a two-span continuous structure with span lengths of 80 and 167 ft (24.4 and 50.9 m) for a total length of 247 ft (75.3 m). The bridge is 32 ft (9.8 m) wide and carries two lanes of traffic. The bridge was designed per the 4th Edition of the AASHTO LRFD Bridge Design Specifications.(1) The bridge superstructure is composed of five WF74G wide-flange, precast, prestressed concrete girders spaced at 6 ft 9 in. (2.1 m) on center. The nominal depth of the precast girders is 74 in. (1.88 m) for both spans. The superstructure is composite with a 7.5-in. (190-mm) thick cast-in-place concrete deck.
SCC was used only for the prestressed concrete girders of the shorter span. The specified concrete compressive strengths for the SCC precast girders were 4500 psi (31 MPa) at release of the prestressing strands and 5700 psi (39 MPa) at 28 days. The specifications required a slump flow of 25 to 28 in. (635 to 710 mm) and an air content of 1.5%. The girders were lightly precompressed with twelve 0.6-in. (15.2-mm) diameter straight and three 0.6-in. (15.2-mm) diameter harped strands because WSDOT requires the use of the same type and number of girders in all spans of continuous bridges.
Self-Consolidating Concrete
WDSOT received Innovative Bridge Research and Deployment (IBRD) funding from the Federal Highway Administration for the Tieton River Bridge. The main objectives of the IBRD program are to promote, demonstrate, evaluate, and document the application of innovative designs, materials, and construction methods in the construction, repair, and rehabilitation of bridges and other highway structures. The use of SCC reduced production costs through faster placement and allowed for placement with fewer skilled workers. The SCC created a smooth surface to the girders without signs of bleeding or discoloration. On the other hand, the moisture content of the aggregate and variations in aggregate gradations have a greater impact on the properties of SCC than for conventional concrete.2
Concrete Mix Proportions
| Materials | Quantities (per yd3) | Quantities (per m3) |
| Cement, Type III | 658 lb | 390 kg |
| Fly Ash, Class C | 150 lb | 89 kg |
| Fine Aggregate | 1412 lb | 838 kg |
| Coarse Aggregate 1 AASHTO #67 | 781 lb | 463 kg |
| Coarse Aggregate 2 AASHTO #8 | 781 lb | 463 kg |
| Water | 275 lb | 163 kg |
| Entrained Air | 1.5% | 1.5% |
| High-Range Water- Reducing Admixture | 5.5 fl oz | 213 mL |
| Water-Cementitious Materials Ratio | 0.34 | 0.34 |
Structural Design
The requirement for high flowability of SCC dictates the use of higher cementitious materials content, a high-range water-reducing admixture, and less coarse aggregate content. These materials result in concrete properties that could be different from those of conventional concrete. This created a lack of confidence among WSDOT bridge designers in the use of SCC for structural applications. The structural design concerns related to the use of SCC for constructing precast, prestressed girders include the likely lower modulus of elasticity,3,4 greater shrinkage,3,4 possible larger prestress losses,3 and the reduced shear resistance5 resulting from the use of a smaller maximum aggregate size or a smaller volume of coarse aggregate. As a result, WSDOT requires the following design modification factors for use with SCC in precast, prestressed concrete girders:
| Property | Modification Factors |
| Modulus of elasticity | kSCC = 0.9 |
| Creep | kSCC = 1.5 |
| Shear (applies to Vc only) | φSCC = 0.7 |
| Shrinkage | kSCC = 1.10 |
Lessons Learned
This project did not tell us much about the structural properties of the SCC because the girders were deep, only 80-ft (24.4 m) long, and lightly prestressed with about 3/4 in. (19 mm) of camber. From the production perspective, WSDOT was concerned that SCC placed into a deep girder might segregate, but that did not happen. The placement went smoothly with fewer workers required than for a conventional concrete girder, and the finishing work was significantly reduced because of the high quality finish right out of the form. The concrete strengths at 28 days were comparable to those of the regular mixes. Although the material is more expensive, the reduction in labor more than compensates for the added cost. Our experience tells us that once we get a good handle on the design concrete properties, SCC is the way to go in the future. It will be more economical and the product will have a higher quality finish.
References
1. AASHTO LRFD Bridge Design Specifications, 4th Edition, American Association of State Highway and Transportation Officials, Washington, DC, 2007.
2. Carr, M. K. and Strickland, B., “Self-Consolidating Concrete for Beams Speeds Biloxi Bay Bridge Construction, HPC Bridge Views, Issue No. 50, July/August 2008.
3. Khayat, K. H. and Mitchell, D., “Self-Consolidating Concrete for Precast, Prestressed Concrete Bridge Elements,” Transportation Research Board, NCHRP Report 628, Washington, DC, 2009.
4. Zia, P., Nunez, R. A., and Mata, L. A., “Implementation of Self-Consolidating Concrete for Prestressed Concrete Girders,” Technical Report, North Carolina Department of Transportation, Research and Analysis Group, 2005.
5. Brewe, J. E. and Myers, J. J. “Shear Behavior of Reduced Modulus Prestressed High-Strength Self Consolidating Concrete Members Subjected to Elevated Concrete Fiber Stresses,” PCI National Bridge Conference, 2009.
Further Information
For further information about this article, contact the author at [email protected] or 360-705-7181.