Ric Maggenti, California Department of Transportation
The new Carquinez Bridge is a 3000-ft (1-km) long suspension bridge spanning the Carquinez Strait at the north end of San Francisco Bay. The cables of the bridge are supported on concrete towers that rise 425 ft (130 m) above the water. The cables are anchored by concrete blocks with thicknesses up to 50 ft (15 m). Approximately 60,000 cu yd (45,000 cu m) of mass concrete are used in the footings under the two towers and in the four concrete anchors. Although a series of placements was used, each placement was large enough to be considered mass concrete.
Concrete Mix
The concrete mix was designed so that the heat of hydration would not be detrimental to the finished concrete. Too high a temperature or temperature gradient can compromise durability and strength due to thermal cracking, self desiccation, or chemical alteration of the paste resulting in possible delayed or secondary ettringite formation.
Measures taken to cope with the heat consisted of designing the mixes to minimize the heat generated while still achieving the specified strength, selecting concrete placement temperatures, and placing thermal blankets to control the temperature gradient. The specification included a maximum concrete temperature of 158°F (70°C) and a maximum temperature gradient of 36°F (20°C). The specified concrete compressive strength for the footings and anchors, except the temporary access chambers, was 3500 psi (24 MPa) at 56 days. The chambers, which were later filled with concrete, provided space for the spinning process of the wire strands to form the main cables. The 2600 cu yd (2000 cu m) of concrete used to fill these temporary access chambers had a specified concrete compressive strength of 1000 psi (6.9 MPa) at 28 days.
The required concrete properties were achieved by minimizing the cementitious materials content and replacing portland cement with fly ash. The concrete for the footing and anchor blocks contained 560 lb/cu yd (332 kg/cu m) of cementitious materials of which 35 percent was Class F fly ash. The concrete fill for the access chamber contained 376 lb/cu yd (223 kg/cu m) of cementitious materials of which 50 percent was Class F fly ash. The concrete fill temperature never rose above 91°F (33°C).
The actual field temperatures of the in-place concrete were predicted to a reasonable degree of accuracy by the contractor for over 40 placements. One comparison of measured peak temperature and temperature near the surface with predicted values was reported to be within 4°F (2°C). With this accuracy, it was shown that concrete temperature can be controlled to limit cracking. The mass concretes were, with a few exceptions, relatively crack free. Design and specifications engineers visiting the project saw that large concrete elements could be placed with a limited amount of cracking. Large areas of the concrete surface were visibly crack free.
One notable exception to crack-free concrete was the second lift of the south anchorage. The weather on the day of casting was dry and windy and the air temperature reached 104°F (40°C). Before initial set of the concrete, an extensive network of plastic shrinkage cracks developed on the surface because no curing was applied. Thermal blankets were placed five hours after the last concrete was placed. Many of the cracks were deeper than 14 in. (350 mm) and a few cracks extended into the entire depth of the placement. The cracks were treated with a high molecular weight methacrylate. Cores, some almost 10 ft (3 m) long, were taken through a random selection of cracks to ensure penetration and bond. Tensile splitting tests were conducted on several cores to verify an adequate repair. One area of the same placement was accidentally water cured because of a leaking cofferdam and remained crack free.
Use of Fly Ash
Prior to 1995, pozzolans, in the form of fly ash or natural pozzolans added at the batch plants, were limited by the California Department of Transportation (Caltrans) Standard Specifications to 15 percent of the total cementitious materials. This has changed and most concrete used by Caltrans now requires 25 percent fly ash. The use of fly ash to control heat in mass concrete has steadily increased. The foundation elements for structures built in Oakland in the early 1990s started out using 15 percent fly ash. Temperature monitoring showed heat to cause potential problems so up to 40 percent fly ash was substituted.
The use of fly ash at 35 and 50 percent of the total cementitious materials proved to be vital to control heat in the Carquinez Bridge project. The measured strengths for the structural concrete and the chamber fill exceeded the specified values. The concrete temperatures did not reach values that might result in delayed or secondary ettringite formation or gradients that would induce excess thermal stresses. The use of internal cooling pipes was not required. The mass concrete for this suspension bridge should serve its structural purposes as the mix designs accomplished their intended results.
Further Information
For further information, contact the author at [email protected] or 916-227-8755 or see the following reference: Maggenti, R. and Haylock, G., “California’s New Carquinez Bridge,” Concrete International, Vol. 25, No. 2, February 2003, pp. 56-60.