DPR Cuts the Lead Using High-Density Concrete

At a renovation project involving a new linear accelerator at St. Mary’s Medical Center (SMMC), a member of Catholic Healthcare West, in San Francisco, DPR is using a material that puts a new spin on an old building staple: concrete.

Weighing twice as much as regular concrete—up to 300 pounds per cubic foot—high-density (HD) concrete offers comparable radiation shielding without the need for lead, steel or composite liners. By eliminating these liners, the total wall size can be reduced, as was the case at the SMMC cancer services renovation project.

“Typically, in these applications, you’ll find lead-lined walls that are four-to-eight-feet thick,” said Bill McCrea, an estimator with DPR’s Self Perform Concrete Group. “The HD concrete walls need to be only 24 to 42 inches thick. Additionally, at SMMC, we were able to reduce the use of lead to just 144 square feet.”

HD concrete saves on space and is more cost effective than lead shields, but it is not without challenges. For example, the product’s weight would disable the belting system used for mixing regular-weight concrete. Instead, a crane had to be used to load the HD materials into a truck for mixing. One of these trucks, which could transport approximately 10 cubic yards of normal-weight concrete, could only haul four cubic yards of HD concrete, requiring precise sequencing of mixing and moving while DPR completed the pour.

“The process took approximately 30 to 40 minutes per truck,” said DPR Concrete’s Roger Cook. “It was critical to stage arrivals of trucks so that a continuous pour was achieved and standby time was minimized.”

The weight also complicated the pouring process because it exceeded weight restrictions of the concrete pump’s boom. Because the facility was an existing structure attached to another building, the team couldn’t access it via a conveyor belt, as would be possible with new construction. Instead, they used a crane-and-bucket system to hoist the material up to the roof, then poured it a half yard at a time through PVC tremie and into forms.

On the carpenter side, this translated into DPR having to engineer forms that could support twice the pressure. Additionally, because HD concrete is not as flowable as regular concrete, both internal and external vibrators were fixed to the walls to ensure proper consolidation of the material.

DPR is now considering HD concrete for other hospital projects, including a linear accelerator facility where the budget was reduced approximately $800,000 over the original lead-lined walls approach, according to McCrea.

“There are a lot of reasons to look at HD concrete for projects where radiation shielding is an issue,” Cook said. “Not only do you end up creating the same shielding with thinner walls that take up less space in a facility, but it’s less caustic to the environment because it doesn’t include lead; it’s cleaner.”