Cancer treatment can require powerful, expensive equipment like the linear accelerator that the Seattle Cancer Care Alliance had installed in a recently built clinic on the Fred Hutchinson Cancer Research Center's Robert W. Day Campus in Seattle.

A key feature of the building is a 2-story concrete vault that houses the linear accelerator. To contain the radiation using conventional designs, the vault ceiling would require steel plates, lead bricks, or super-thick normal-weight concrete for density.

Local concrete supplier Stoneway Concrete assisted with the value engineering of the building by producing what is thought to be the heaviest concrete ever used. If only normal-weight concrete had been used, an 8- to 9-foot-thick ceiling would have been necessary, and the top of the vault would have been located 6 feet above the clinic's second floor. The heavyweight concrete kept the height of the vault ceiling even with the second floor, saving the owners 2000 square feet of floor space.

The concrete had an average density of more than 350 pounds per cubic foot (pcf). Slight adjustments in an existing mix design that minimized the amount of water and entrapped air in the mix would allow the supplier to actually exceed the specified density. The key to the heavy weight was steel aggregate that Stoneway obtained from a sheet-metal processing facility in California. Also, Stoneway used no fines or fly ash but designed for an extremely low water-cement ratio (0.25) and a high cement content.

The use of a self-compacting admixture also addressed the issue of air entrapment and reduced the amount of water, the lightest element in the mix next to air. Producing the concrete created some operational challenges that, fortunately, did not cause any equipment damage.

The article includes the mix design for the project's heavyweight concrete.