Ken Beede, California Department of Transportation
On October 17, 1989, the Loma Prieta earthquake caused the collapse of a 50-ft (15-m) long section of the upper deck of the East Span of the San Francisco-Oakland Bay Bridge. After completing emergency repair of the structure, the California Department of Transportation (Caltrans) undertook the design of a new East Span that would replace the 2.2-mile (3.5-km) long section of the bridge from Yerba Buena Island to the touchdown in Oakland.
Skyway
The new East Span includes the Skyway section—twin parallel 1.3-mile (2.1-km) long precast concrete segmental viaducts, each carrying a five-lane roadway and shoulders. The superstructure includes a total of 452 precast, post-tensioned, segmental units that are constructed with high performance concrete designed to provide a service life of at least 150 years.
The pier tables were cast-in-place and the cantilever segments were precast in Stockton, California and shipped by barge to the site. The inclined webs of the box consisted of lightweight concrete panels that were precast ahead of the main box section. The substructure involved driving large diameter cast-in-steel shell (CISS) piles up to 350 ft (107 m) deep, installing prefabricated steel footing boxes that are filled and encased in concrete, and placing variable height concrete columns, pier tables, and bridge fenders.
HPC Concrete
Prior to the start of construction, various concrete mix designs were developed and tested to determine compressive strength and modulus of elasticity at various ages. In conjunction with the initial mix evaluation, other tests were performed to determine set times, mix rheology, and thermal properties. The testing activities, material evaluations, and reviews described below were extensive; but the results demonstrated that high performance concrete could be obtained efficiently and on a continual basis, while providing a realistic 150-year service life.
Specific requirements for concrete used in the Skyway design included limits for compressive strength, durability, corrosion, and thermal control for the portions of the concrete designated as mass concrete. The specified properties of the superstructure concrete included compressive strength at ages up to 60 days, creep based on 365 days of loading, modulus of elasticity, and shrinkage. A summary of the specified properties is given at the end of this article.
Concrete Production
Multiple mixes were evaluated for use in the concrete placement of the CISS piles, footing and column structures, pier tables, and precast bridge segments. Special mixes were also evaluated to obtain suitable concrete for the structural lightweight panels and for the in-fill concrete for the box footing interior cells. Unique mixes were also designed for the concrete fenders, footing encasements, and closure placements.
In the preconstruction stage, it was determined that a dedicated on-site batch plant would be required to produce the degree of uniformity required for the critical cast-in-place concrete. Barges from the onshore batch plant in Oakland, California, transported the bulk of the concrete to the bridge site. Each barge was equipped with two 20 cu yd (15 cu m) mixers and a liquid nitrogen cooling system with fully operable admixture dispensers and supply tanks. Each barge operator was a licensed weighmaster, an ACI Grade 1 concrete field testing technician, and qualified mixer operator. A mixer on the concrete pump barge provided additional mixing and mix agitation of the fresh concrete between barge deliveries. The contractor was responsible for all aspects of the concrete production while Caltrans was responsible for the quality assurance testing.
An important requirement for the high performance concrete was the selection of the concrete materials including the cementitious materials, aggregates, and admixtures. The project cements included Type II/V and a modified high early strength cement.
The prime concrete aggregate was a Sechelt aggregate from British Colombia, Canada. This aggregate source is known for its uniformity, thermal transfer properties, and high quality. It has been used to produce concrete for more than 20 years with no known incidence of alkali reactivity or incompatibility with cement.
Both a Class F fly ash and a ground-granulated blast-furnace slag were used to supplement the cement and provide additional corrosion protection and thermal properties. Several concrete admixtures were used to help achieve specific mix requirements including reduced shrinkage, mix stabilization, increase in strength, and reduced bleeding capacity. Several of the concrete admixtures had been prequalified and were approved for use in the project concrete. The shrinkage-reducing admixture used in the superstructure concrete and the viscosity-modifying admixtures used in the self-consolidating concrete of the pile caps were specially approved for the use on this project.
Concrete for Skyway Superstructure
The above table lists the concrete constituent materials and their proportions for the precast and cast-in-place concrete mixes used in the superstructure segments. Typical measured properties for each concrete mix are provided.
Excerpted Special Provisions for the Lightweight Superstructure Concrete
Modulus of Elasticity
The modulus of elasticity of the portland cement concrete shall be at least 35,600 MPa (5160 ksi) at 28 days when tested in accordance with the requirements in California Test 522. The samples shall be moist-cured for 7 days, followed by air drying at 23°C (73°F) and 50 percent relative humidity until test age. The modulus shall also be reported at 3, 7, 56, and 90 days. Test specimen size shall be the same as used for compressive strengths. Test results shall be based on the average of three test specimens at each age.
Creep
The specific creep coefficient, as determined in accordance with the requirements in ASTM C 512, after 365 days of loading, shall not exceed 75 millionths/MPa (0.52 millionths/psi). Test specimens shall be 152×305 mm (6×12 in.) cylinders and shall be moist cured for 7 days, followed by air drying at 23°C (73°F) and 50 percent relative humidity. Test cylinders shall be loaded at 28 days to a stress of 20 to 40 percent of the 56-day design compressive strength shown on the plans but not less than 20 percent nor greater than 40 percent of the measured strength at 28 days. For submittal of the prequalification data, coefficients after 28, 56, and 90 days of loading shall be submitted and used to predict the coefficient at 365 days based on the procedures of CEB-FIP Model Code for Concrete Structure, by the Comité-Euro-International du Béton. Mix design approval shall be contingent upon the 365-day creep coefficient satisfying the stated requirement.
Shrinkage
The shrinkage strain of portland cement concrete shall not exceed 0.045 percent after 180 days of drying in accordance with the requirements in ASTM C 157. Sample size shall be 100x100x285 mm (4x4x11-1/4 in.). Samples shall be moist-cured for 7 days, followed by air drying at 23°C (73°F) and 50 percent relative humidity. Shrinkage strain shall be calculated as the change in strain from the beginning of drying at 7 days.
Source: Caltrans, Notice to Contractors, Skyway Project, Contract No. 04-012024, Special Provisions; Revised Field Edition, p. 154. Publication Date: April 2002. (U.S. customary units have been inserted by the Editor.)
Further Information
For further information, contact the author at [email protected] or see ASPIRE, Winter 2007 at http://www.aspirebridge.com/magazine/2007Winter/