Ric Maggenti, California Department of Transportation
Photo: California Department of Transportation
The environmental benefits aside, greener concrete has been chosen for many of the new east spans of the San Francisco-Oakland Bay Bridge as a practical construction material as well as for durability. In these cases, environmental benefits were not the motivation, but rather the need to meet requirements of design and construction.
There are four distinct construction projects completed or underway, some with multiple contracts, to build the 2.2-mile (3.6-km) long bridge across San Francisco Bay between Oakland and Yerba Buena Island. The new bridge will replace the seismically vulnerable east spans of the 1936 San Francisco-Oakland Bay Bridge.
The new bridge from east to west comprises the Oakland Touchdown, the Skyway, the Self Anchored Suspension (SAS) bridge, and the Yerba Buena Island Transition. The Oakland Touchdown uses low-level, post-tensioned, cast-in-place concrete box girder bridges. The west end of these twin parallel bridges connects to the Skyway. The Skyway, now completed, is a 1.5-mile (2.4-km) long precast segmental structure that used the balanced cantilever construction method. The SAS bridge is the signature structure and connects the Skyway to the Yerba Buena Island shore. The SAS bridge will be a single tower bridge with asymmetrical spans. The Yerba Buena Island Transition structures will be prestressed concrete box girder bridges. They will connect the west end of the SAS to the Yerba Buena Island Tunnel.
Construction began with the Skyway. For durable concrete, California Department of Transportation (Caltrans) has required that 25% of the cementitious material be fly ash in almost all of its structural concrete to mitigate Alkali Silica Reactivity (ASR) since 1997. Thus all the concrete on this massive project required at least 25% fly ash. Higher percentages of fly ash were utilized for the large footings and other mass concrete elements. For the pier concrete, the Contractor, instead of using fly ash, chose to use 50% ground granulated blast-furnace slag, which was the maximum percentage allowed by the 2001 specifications. Bid prices indicated that this can be an economical benefit to the contractor as there was no requirement or even encouragement for its use. Though by today’s rapidly changing standards, the amount of supplementary cementitious materials used was modest in the 450,000 yd3 (344,000 m3) of concrete in the Skyway, the Environmental Protection Agency in 2006 recognized Caltrans as a leader in the construction use of waste products.
The west end of the SAS terminates at a massive pier bent. Here the suspension cables will loop around on saddles and head back toward the east end. The span on the west side nearest San Francisco is shorter then the span east of the tower. This creates an uplift on the west side that is countered by massive concrete anchors as well as the weight of the 8200 yd3 (6300 m3) concrete bent cap. Four columns supporting each end of the bent rest on 63x63x33 ft (19x19x10 m) anchorage blocks. To satisfy the restrictive thermal and corrosion requirements, the concrete contained 674 lb/yd3 (400 kg/m3) of cementitious materials including 40% fly ash. In 2004, this was considered a high percentage for California bridge concrete. Compressive strengths were over 9000 psi (62 MPa) at 90 days.
The portion of the Oakland Touchdown now under construction is 1080 ft (330 m) long and has seven spans over six piers. Under the piers are mass concrete pedestals, which sit on mass concrete pile caps that make up the footing. The pile caps vary in size having a footprint from 46 ft (14 m) square to 52×72 ft (16×22 m). In a cost savings move, Caltrans proposed a passive thermal control plan using 50% fly ash mixes to replace the contractors active thermal control system, which used internal cooling pipes. These mixes had 337 lb/yd3 (200 kg/m3) of fly ash and 337 lb/yd3 (200 kg/m3) of portland cement. The water to cementitious materials ratio was 0.4, the maximum permitted by the specifications for corrosion control. The strength requirement was 5000 psi (35 MPa) at 90 days for the pedestal and 4350 psi (30 MPa) for the pile cap. The average measured strength for all the pedestals was 4620 psi (31.8 MPa) at 28 days and 5720 psi (39.4 MPa) at 56 days. The graph below shows the 28- and 56-day strengths for the pile caps. The average strengths were 4630 psi (31.9 MPa) and 5630 psi (38.8 MPa), respectively. The lowest strengths occurred on samples stored during a 2- to 3-month period when temperature control of the curing room was malfunctioning. A few samples tested at 7 days had average strengths of about 3000 psi (21 MPa). Concrete from four pedestals had an average 90-day strength of 6230 psi (43.0 MPa). One 180-day test result was 6830 psi (47.1 MPa).
As Caltrans strives toward complying with California’s Assembly Bill 32 to reduce greenhouse gases, the Bay Bridge project is an example showing that this need not be a tradeoff with efficient concrete mixes.
Further Information
For further information about the mass concrete used in the San Francisco-Oakland Bay Bridge, please contact the author at [email protected].