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Seattle Cancer Care Alliance

Desperate times often call for expensive measures. When a cancer patient's affliction reaches a critical stage, radiation therapy may not be a truly desperate measure, but it's certainly an expensive one. It requires 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.

The clinic, built for an alliance among the Fred Hutchinson Cancer Research Center, the University of Washington, and Children's Hospital and Regional Medical Center, is designed to be one of the country's premier oncology treatment facilities. The $30 million first phase of construction features a 7-story ambulatory facility with 159,000 square feet of treatment space.

A key feature of the building, the first phase of which opened in January 2001, 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.

The need for expensive medical equipment was a foregone conclusion, but savings could be found in the building's construction. 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. This use of concrete was entered for a 2002 Washington Aggregates and Concrete Association Award of Technical Merit.

The concrete, which had an average density of more than 350 pounds per cubic foot (pcf), allowed the structural/civil engineer, KPFF Consulting Engineers, to avoid the use of steel plates or lead bricks in the vault walls and slabs. This concrete allowed the engineer to design the vault's ceiling slab with roughly 4-foot-thick concrete instead of another alternative: 8- to 9-foot-thick concrete that otherwise would have been necessary if it were of normal weight (roughly 150 pcf). A nuclear physicist confirmed that the heavyweight concrete would safely contain the radiation at the reduced wall thickness. (CONCRETE CONSTRUCTION magazine published an article in its May 1996 issue about a similar project in which a hospital in Pontiac, Mich., used 300-pcf concrete for a room containing its linear accelerator.)

"The ceiling was the critical piece," notes Greg McKinnon, quality assurance and marketing manager for Stoneway. He adds that every 5 pcf of additional density would allow a reduction in wall thickness of 1 inch. "The ceiling was designed for a 44-inch thickness at about 330 pcf, and we ran weights on every load; if it would have been 325 pounds, they would have had to go 45 inches," McKinnon says. "That's why they specified the unit weight, which was so critical."

Although ACI does not maintain records of the unit weights of concrete used in construction, it confirms that the heavyweight concrete used for this project is the heaviest ever: Interestingly, Appendix 4 in ACI 211.1, "Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete," categorizes heavyweight concrete as having a maximum density of 350 pcf.

Steel aggregate, SCC admixture used

Stoneway Concrete had established a reputation among the local structural engineering community as a custom solution provider, as evidenced by its innovating a process of truck-batching flowable fill (see the note at the end of page 25). Turner Construction, the general contractor, had arranged for one ready-mix supplier to provide the normal-weight concrete to the project, but the supplier was unable to design a heavyweight mix that would meet the 330-pcf specification.

Luckily, Stoneway had experimented with a mix that had been planned for use as a counterweight for a movable bridge project for the Washington State DOT. Although WsDOT eventually scrapped the idea of using the heavyweight concrete as a counterweight, Stoneway had achieved 305-310 pcf with trial mixes. Slight adjustments in the mix 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. "They were steel punchings that they punch out of steel plates," notes McKinnon. "They were about 3/8-inch flat, round disks."

Did the aggregate shape cause a problem? "It did," says McKinnon. "One of the things we found out was that we had a high entrapped-air content. I think that our entrapped-air content was around 9%, or 27 pounds per yard. So we had to use an air detrainer." To further maximize "functional weight" in the mix, 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. Besides, the concrete was so heavy that it would be difficult to work with once it was out of the truck. "One of the concerns was that once you'd get the stuff on the ground, you couldn't move it," McKinnon says. "We were concerned that vibrating was actually going to entrap air that we didn't want."

Craig Holt, the project superintendent for general contractor Turner Construction, adds that minimizing water was desirable because he was afraid that water and the other materials might segregate upon placement of such dense material.

Weighty Issues

Producing the concrete created some operational challenges that, fortunately, did not cause any equipment damage. "The biggest thing was that we had to make sure that we used our newer trucks that had stronger pumps," McKinnon says. "On one of our first test batches, we actually stopped the drum on the track and then we actually had to rock it a little bit before we could get it to turn because number one, the mix was very fluid and number two, it was very, very heavy.

"The other issue was getting the material into the batch plant. Our loader operator got an education. The first time he went into the pile of steel punchings to load the plant, he lifted up the back end of the loader. Then he was putting only about a yard in the bucket, and of course he had to fill the belt very slowly. We just had to remind everyone to trickle it onto the belt; they were virtually feeding as we were batching."

Another concern was payload capacity. "We were running 3, 3 1/2-yard loads that weighed maybe 9600 pounds per yard, so we had almost as much weight as a conventional 7-yard load, and it was all in the front of the drum," McKinnon says. "It was hard on frames and pumps."

Fortunately, the project required only about 80 yards of the heavyweight concrete. A 6-inch-thick layer of normal-weight concrete had already been poured for portions of the 2000-square-foot vault roof slab located at street level, so Stoneway drivers were able to deploy their trucks on top of it. Then they chuted the concrete into 42-inch-deep forms located directly above the accelerator and above parts of two rooms.

"It wasn't a particularly long haul," adds McKinnon. "We went very early in the morning on a Saturday because we were delivering 3 yards at a time, so we had to put a lot of trucks on it, and you can pour 3 yards pretty quickly. Also, we wanted to avoid any traffic issues."

Heavyweight Concrete Mix Design

The heavyweight concrete used for the ceiling of a linear accelerator
vault at the Fred Hutchinson Cancer Research Center clinic used steel
aggregate and a self-compacting concrete (SCC) admixture.

Material                                                  Amount/cy

Type I-II ASTM C 150 cement (lbs.)                             1290
Coarse aggregate (recycled steel punchings, lbs.)              8100
Water (gal.)                                                    325
Entrapped air (%)                                               1.0
W.R. Grace Adva Flow ASTM C 494 Type F HRWR (oz.)               103
Master Builders 100xr ASTM C 494 Type B (oz.)                   103
Air-detraining agent (oz.)                                       85

Water-cement ratio                                             0.25
Slump (in.)                                                       8
28-day compressive strength                          8000 (approx.)

For the story about Stoneway Concrete's process of truck-batching flowable fill, visit www.worldofconcrete.com, click on "Article Archive," enter keywords flowable fill, and find the article titled "Breaking the Rules for Lower Breaks."

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