Lightweight Concrete for a Segmental Bridge

Structural lightweight concrete can help increase the stiffness and decrease the mass of a bridge; both are qualities that make the structure more likely to survive an earthquake. Ordinarily, however, lightweight concrete, a mixture that uses aggregates like expanded shale and coal furnace slag, has been considered too expensive compared to the alternatives. But a design for the proposed 1.2 mi long Benecia-Martinez Bridge, which will carry Interstate 608 across San Francisco Bay shows that the material can be both affordable and offer engineers a number of advantages, including high strength, high thermal strain capacity and better protecton for reinforcing steel than normal concrete. Of the four concepts that were studied for the bridge: lightweight concrete segmental bridge, steel truss with a concrete deck, steel box girder and a cable stayed design, the lightweight concrete alternative was the cheapest. At $91.3 million, it is about $8 million less than the next least expensive plan. And by combining ductile strength and structure tuning, the solution should comply with the desired seismic performance for San Francisco Bay area bridges: surviving the maximum credible earthquake with unimpaired functioning.