Ganapathy Murugesh, California Department of Transportation
The new Benicia-Martinez Bridge across the Carquinez Strait on I-680 is an engineering marvel with the incorporation of several unique design and construction features. This 1.4-mile (2.3-km) long crossing is an important addition that meets the California Department of Transportation’s (Caltrans) mission—“Caltrans Improves Mobility across California.”
Unlike most of the San Francisco Bay area crossings, which are seismic upgrade projects, the new Benicia-Martinez Bridge is a congestion relief project. Despite corridor expansions, the existing bridge created a severe bottleneck for traffic, which the new bridge relieves. The new bridge carries five lanes of northbound I-680 traffic across the Carquinez Strait between the cities of Martinez and Benicia, California. In addition, the new bridge is designed to carry one future lane of light rail, mass transit traffic.
Choice of Lightweight Concrete
Feasibility studies conducted during the late 1980s evaluated the use of four types of bridges: a steel-truss bridge; a steel box-girder bridge; a concrete cable-stayed bridge; and a lightweight concrete bridge built using the cast-in-place balanced cantilever technique. Cost was an important factor in these studies. Even excluding the 150-year life-cycle aspect, the initial cost of the lightweight concrete, cast-in-place segmental structure was lowest. It should be noted that a span of 525 ft (160 m) was used for these preliminary evaluations.
Significant Changes during Design
Interstate Route 680 was included on the California’s Lifeline route, which means, “the bridge needs to remain open to traffic after a major seismic event.” This resulted in the use of stringent design criteria. The proximity to the Green Valley Fault and the San Andreas Fault, together with the unique topography, created additional challenges to the design team. The U.S. Coast Guard required an increase to the original span lengths across the navigational channel from 525 to 663 ft (160 to 201 m). This change, combined with the seismic challenges, resulted in pushing the limits for this type of construction in a high seismic risk zone.
Design Summary
Span lengths on the cantilever portion of the new Benicia-Martinez Bridge range from 418 to 659 ft (127 to 201 m), which pushes the limit for this type of construction. Including the Caltrans-designed northern approach spans, the bridge comprises 22 spans, with 16 over water. The segment cross section consists of a single cell box girder with a total depth that ranges from 37.4 ft (11.4 m) over the piers to 14.9 ft (4.54 m) at midspan. The top flange excluding the barriers has a width of 78.7 ft (24.0 m), while the bottom-flange thickness varies from 5.9 ft (1.80 m) at the pier segment to 9.8 in. (250 mm) at midspan.
The superstructure was constructed using 15.8-ft (4.8-m) long segments with as many as 20 segments cantilevered on each side of a pier. Considering the fact that these long spans were designed for a high seismic risk zone, the bridge becomes the first of its kind—and a world-class structure.
Lightweight Concrete Adds Length
The key to achieving the span lengths was the choice of lightweight high performance concrete. The lighter the structure, the less massive is the structure that pushes the piers in a seismic event. But finding the right mix design required countless tests and evaluations of more than 30 concrete mix designs. Caltrans, the designers, and the contractor researched a variety of aggregates, admixtures, cement contents, and water-cementitious materials ratios, to achieve a concrete mix that met the engineering properties and the construction needs.
Sand-lightweight concrete is used for the entire superstructure except for the pier table segments. The sand-lightweight concrete uses normal weight sand and lightweight coarse aggregate to produce concrete that is lower in density. The anticipated higher creep and shrinkage and lower modulus of elasticity characteristics expected with the lightweight concrete, resulted in stringent material properties being specified for construction.
As finally designed, the segmental concrete specified has a density of 125 lb/cu ft (2000 kg/cu m), or about 16 percent less than conventional structural concrete. the designers considered using lightweight sand, which could have produced a concrete density of 110 lb/cu ft (1760 kg/cu m), but it would not meet all the necessary material property requirements. Lightweight concrete with normal weight sand and a density in the range of 120 to 125 lb/cu ft (1920 to 2000 kg/cu m) was specified to achieve higher compressive strength, higher modulus of elasticity, and less creep and shrinkage.
Segmental Concrete Properties
*From production concrete.The above table lists the specified values and average measured values from production concrete for density, compressive strength, modulus of elasticity, shrinkage, creep, and splitting tensile strength.
Concrete Mix Proportions
The above table lists the concrete mix proportions as 833 lb of cement, 49 lb of fly ash, 98 lb of metakaolin, 1233 lb of sand, 858 lb of lightweight aggregate, and 304 lb of water for a total water-cementitious materials ratio of 0.31.
In addition, a shrinkage-reducing admixture, hydration-stabilizing admixture, and high-range water-reducing admixture were included.
Challenges Faced during Construction
In all of the elaborate testing efforts, the design and construction team discovered that the lightweight concrete needed a high cementitious materials content to meet the modulus of elasticity requirements. The design mixes resulted in compressive strengths between 10,000 and 11,000 psi (69 and 76 MPa), while only 6500 psi (45 MPa) was needed by design. Special aggregates were used to achieve the desired properties. The fine sand was imported from Canada and the lightweight coarse aggregate came from North Carolina. Because of the high cementitious materials content, the lightweight concrete had a high heat of hydration. The specifications limited the maximum concrete temperature during curing to 160°F (71°C). To achieve this, the contractor used ice in the concrete instead of water and cooled the concrete with liquid nitrogen. A long wand with a nozzle was used to inject liquid nitrogen for a few minutes into the concrete in the trucks. The combination of ice and nitrogen lowered the concrete temperature to 40° to 50°F (4 to 10°C).
A system of PVC tubes in the segments carried water to cool the concrete during early hydration. Radiator-like tubes ran through the bottom slab, webs, top slab, and the flared connection of the web to the top slab. Thermocouples to measure internal concrete temperatures were spaced where the maximum heat would be generated. These methods controlled the heat of hydration well, even during hot summer months. Further discussion of the heat of hydration is provided in HPC Bridge Views, Issue No. 47.
Marine placement of lightweight concrete posed additional challenges. The contractor provided an on-site batching plant on the south shore of Carquinez Strait. Mixing trucks traveled from the batch plant to a barge and drove on board. The barge, which could carry four loaded mixing trucks at one time, transported the concrete to the desired pier cantilever location, where each segment was cast in one lift.
For some placements, the barges were secured to the side of a barge with a concrete pump that could pump the material vertically to a height of more than 181.5 ft (55 m). On board the pump barge, the concrete was remixed before being pumped vertically to the placement location. For other placements, the barges were secured to the side of a footing, where a tower crane lifted buckets of concrete to a remixer on the bridge deck from which the concrete was then pumped horizontally up to 330 ft (100 m) to the intended segment.
Through elaborate quality control measures by the contractor, quality and consistency of the lightweight concrete was achieved for over 60,000 cu yd (46,000 cu m) of placed concrete and over 2 years of production.
Conclusion
In spite of all the challenges encountered during the design and construction of this bridge, the choice of lightweight concrete was the right one that helped Caltrans build this important Bay crossing. A great partnering effort between the dedicated Caltrans staff, design team of TYLin/CH2M Hill Joint Venture, and the contractor, Kiewit Pacific, helped successfully complete this project.