The concrete, rendered lightweight by the Elemix additive, was used to fill the annular space between the excavated rock and the steel tunnel lining. “We used Elemix for several benefits,” said Frank Stevens, construction manager, Cowin & Co. “The lightweight concrete provided a strong backfill material at about one-third the density of normal concrete. The lower density allowed for rapid and continuous placement of the backfill without overloading the forms, saving substantial labor and overhead cost. We were impressed the Elemix concrete flowed long distances without segregation or bridging, ensuring complete backfill of the tunnel lining and consistent compressive strength.” Approximately 75,000 lb of Elemix was used as part of a total volume of 8,000 cubic yards of concrete. The slope has a 12% downgrade.