After five years of collaboration involving the Federal Highway Administration (FHWA), the Iowa Department of Transportation (IDOT), Iowa State University/Bridge
After five years of collaboration involving the Federal Highway Administration (FHWA), the Iowa Department of Transportation (IDOT), Iowa State University/Bridge Engineering Center, and Lafarge North America, the first highway bridge in North America built with Lafarge’s ultra-high-performance, fiber-reinforced Ductal concrete opened on May 5 in Wapello County, Iowa. The single-span Mars Hill Bridge consists of three 110-ft. Ductal girders with no rebar for shear stirrups. Erected under the Innovative Bridge Construction Program, it is part of the FHWA’s Bridge of the Future undertaking.
But the structure is only the initial step toward what Lafarge hopes will be the first all-Ductal bridge and deck with no steel reinforcing bars and specially designed, optimized forms.
POWERS OF PERSUASION
In 2003, Wapello County and IDOT were granted funding through the TEA-21 Innovative Bridge Construction Program for a project using ultra-high performance concrete (UHPC). Following many conversations over the previous two years with Lafarge about incorporating Ductal into a structure, it was decided that UHPC would be used in prestressed beams for a bridge replacement project in the southern part of the county.
The girders are prestressed using 0.6-in.-diameter low-relaxation strands, and no mild reinforcing steel is used except to provide composite action between the beams and a cast-in-place deck. To verify shear and flexural capacity of the bridge beams, 10-in. and 12-in. shear beams and a 71-ft.-long test beam were cast and tested by Iowa State University and the Center for Transportation Research and Education in Ames.
Casting of the 110-ft. production beams was completed in June and July 2005. Construction on the bridge began in August, with two of the three beams arriving at the job site in September and the third arriving in early October.
Two local precasters expressed an interest in casting the beams for the project and were certified by Lafarge to mix and cast the Ductal mix. Plant inspections were part of a certification process, and test batches were performed at each facility. However, the precasters expressed concerns about working with Ductal, including the high cost of the mix; longer batching time and additional cleaning time for mixers because of the steel fibers and fine aggregate; the chance of damaging mixing equipment due to the high mixing energy required; shrinkage values estimated to be twice the amount normally expected from standard mixes, because of the large amount of cement in the mix; long setting and curing times (about 40 hours; 48 hours of 195_F steam cure); and, lost production time in the casting beds. Ultimately, with experieince and optimization, precasters were able to resolve all of these concerns.
Although bids were received from local precasters for the 71-ft. test beam, three 110-ft. production beams, and additional smaller beams for shear testing, Lafarge’s Winnipeg, Canada, plant landed the contract, due in large part to its success with prior Ductal projects.
Vic Perry, Ductal/Lafarge vice president & general manager, explains that the design of the Mars Hill Bridge was actually a look at how UHPC would be used in the future. It goes without saying that steel and concrete bridges look different, but Ductal bridges look different than either, he says. So, we had to come up with a new solution, one that used optimal bridge shapes.
Lafarge worked with engineers from the Massachusetts Institute of Technology on beam design and construction plan preparation for an all-Ductal structure. The FHWA recognized that this was a radical new bridge shape. The density of Ductal is about 5 percent heavier than regular concrete if you use the same forms. Since the durability, strength and fabrication flexibility is higher with Ductal, we can make smaller elements, thus the entire bridge ends up weighing less than regular concrete, by as much as 50 percent in some cases, adds Perry, who was marketing director for Ductal in Paris for five years before heading up the North American Ductal business at Lafarge. Also with smaller pieces, we can use smaller cranes, or get a longer reach with a large crane, and you can transport more pieces on a single truck.
Despite Ductal’s aesthetic and performance properties, as well as compressive and flexural strengths up to 30,000 psi and 6,000 psi, respectively, Perry admits that there are some challenges to using Ductal on certain types of bridge projects like steel structures. For longer spans, the elements might be too light and you’d end up with vibration problems. This would have to be addrssed, particularly if the bridge featured a pedestrian walkway.
The FHWA recognized the advantages to an all-Ductal structure, according to Perry. With no rebar in the girders or deck, there’s no chance for corrosion. This is a major step toward longer bridge life, about two or three times longer by our estimates, he explains. But before we could take that step, we had to prove that we could build a thin-webbed, thin-girder bridge.
CAMBER AND GROWTH
Production beams for the bridge were successfully cast in Winnipeg, and fiber gauges were installed inside the final beam to allow researchers the ability to measure strains at strand release, during construction, and final dead and live loading.
Release camber for the 110-ft. beams was 1Ê in. After curing and thermal treatment, camber was measured at 4∫ in. before any dead load was placed upon the beams.
Two of the three 110-ft. beams arrived at the job on September 30, 2005, and were placed on the abutments by local contractor Bloomfield Bridge and Culvert; the thrid beam arrived and was placed on October 7. The 8-in., cast-in-place deck was poured in November, with concrete railing, approach guardrail, and final earthwork completed in early 2006.
FULLY OPTIMIZED FUTURE
Perry says that the next step for Ductal is a fully optimized bridge. We are testing Pi girders, which are basically double-Ts with bulbs on the bottom. We put strand on the bulbs and then we can create a 3-in.-thick, 8-ft.-wide deck with no rebar and a 2Ê-in. web, he says. We could place an 8-ft. section in less than an hour, actually more like 30 minutes. Then you just fill up the joint with grout, put on a nonskid surface, and you’re good to go.
The good news for Lafarge is that IDOT has plans on the board for just such a bridge to be completed in 2007, with FHWA currently carrying out full-scale load testing. We haven’t purchased a full line of steel forms for the optimized bridge just yet, but we’re close, Perry says. Lafarge recognized when it introduced Ductal to North America that it would take at least seven years to get the first optimized bridge built. Our focus was always on long-term objectives. Conventional beams are fine, but there is a need for new decks that last longer. So, Mars Hill is an important incremental step toward an optimized solution.