Most concrete contractors have had plenty of experience placing normalweight (140- to 150-pcf) portland-cement concrete mixes. But what if a project calls for placing a mix that's almost twice that weight? What are the unique challenges of working with heavyweight concrete? In the spring of 1995, concrete contractor Colasanti Corp. had a chance to find out after negotiating a contract to place a 300-pcf concrete for the walls, ceiling, and foundation of a linear accelerator facility at North Oakland Medical Center, Pontiac, Mich. The 650-square-foot room, located in the basement level of the hospital, required the high-density concrete as a shield against the radiation generated by the linear accelerator. To achieve the concrete density required to absorb the radiation, the ready mix supplier used a combination of heavyweight aggregates--steel-shot fine aggregate and hematite iron-ore coarse aggregate--instead of stone.