U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
|Accelerating Infrastructure Innovations|
Publication Number: FHWA-RD-99-107
Date: September 1999
Prince George's County, Maryland, plans to build 11 or 12 new bridges within the next 3 years-a threefold increase over its typical construction schedule. If Edward Binseel, associate director of the Prince George's County Department of Public Works and Transportation, has his way, some of the bridges will be designed and built using high-performance concrete (HPC). All will be simple spans ranging from 7.3 to 18m (24 to 60 ft) feet in length, and all will be financed by the county, with no State or Federal funding.
"In a time when maintenance resources are shrinking and are expected to continue to shrink, something has to change," says Binseel. "We have to design the bridges in such a way that we increase their durability and longevity while reducing maintenance costs. We think HPC will give us that durability and longevity."
After reading an article on HPC in Focus in 1998, Binseel began calling around, seeking information on HPC specifications and the names of bridge designers who had "used HPC specs and gotten the bugs out."
One feature of HPC that especially attracts Binseel-and one that he has incorporated into projects for the last 4 years-is restricting the water-cement ratio. "By going to a water-cement ratio of 0.40, we can produce a denser, less porous concrete," says Binseel. "A low ratio, which implies an additional amount of cement, gives us the longevity and durability we need. If one consequence of the design is added strength, we'll accept that-although we're not seeking that directly, the way a more conventional design would."
Binseel says the industry's lack of familiarity with HPC poses a potential barrier to his agency's use of HPC. "It's one thing to specify, and it's another to bring the industry along with you," he says. He believes that any barriers can be overcome by respecting the education and resources of the consulting architect/engineers his agency uses for design, construction management, and inspection. "For this round of projects," says Binseel, "we're currently scoping the bridge requirements. When the consultant recommends a concrete structure, we'll specify that HPC be incorporated into the design. The consultant will then need to learn what's necessary to prepare the design and specifications accordingly. He knows what we're trying to achieve. There hasn't been a problem so far."
Binseel says he was prepared to have to pay higher initial costs, but he has not found that to be necessary. In recent projects, he has added an inch or so of concrete cover-bringing the total cover to between 5 or 6 cm (2 or 2.5 in), which requires an additional 15 m3 or so (20 cubic yd) of concrete for a typical bridge. At approximately $65/cubic yd, the additional concrete costs only $1,300-"negligible on a million-dollar structure," says Binseel.
"There seem to be fewer construction-related problems than were expected with a stiffer mix, although it's also possible we're not being charged for them. All of the design changes we have incorporated to date have been to prevent chloride ion entry. We're using a very conservative 2- to 2.5-inch concrete cover to protect the reinforcing steel, and that, coupled with the extra cement and the reduced water/cement ratio, has enabled us to achieve a denser concrete that allows less migration of chloride ions."
Binseel says that "HPC may offer economic advantages because it results in greater strength and requires fewer structural members, but most important to us, it yields a long-lasting, more durable bridge that should require less maintenance over time."
"We're on our own at the moment," says Binseel. "Other counties have been slower to move and to adopt these design changes. But I'm not willing to wait that long. I've read the research results, and there seems to be a good track record. I'm willing to try it."
For further information, contact Edward Binseel at 301-883-5624 (fax: 301-883-5131).