The festive grand opening of the award-winning Bagley Avenue Pedestrian Bridge, the centerpiece of the $230 million Gateway Project and vital Mexicantown link, was testimony to the rich heritage and deep connection between two neighbors physically separated since the late 1970s.
Neighbors once divided are now connected by this breathtaking structure that gracefully arcs across two of Michigan’s busiest freeways. To commemorate the historic, cultural and economic rejoining of the east and west sides of Detroit’s Mexicantown community, U.S. and Mexican government officials, along with visitors from across Michigan, joined local residents at a ribbon-cutting ceremony on May 5, 2010.
Although the high-performance pedestrian bridge will be used primarily by local residents, it also promises to bring significant economic benefits to area businesses. International visitors will frequent the site, because it is adjacent to the new Detroit Welcome Center (the first stop for travelers after the border crossing from Canada) and indoor Mexicantown Mercado. The pedestrian bridge is an important part of the $230 million Gateway Project, the largest project ever undertaken by the Michigan Department of Transportation (MDOT). The objective of this massive economic-development initiative is to ease traffic congestion at the Detroit-Windsor International Border Crossing by providing a direct connection to area freeways from the Ambassador Bridge, which handles about 25% of all surface trade between the U.S. and Canada. This vital international border crossing, the busiest in North America, is key to the success of thousands of businesses throughout Michigan and Ontario.
Concrete Gateway
For the largest and final phase of construction, more than 165,000 cu yd of concrete containing Lafarge Type I portland cement and NewCem slag cement was placed in reconstructing or replacing 2.5 miles of the I-75 and I-96 freeways, along with 23 ramps, 29 bridges and 3 miles of cast-in-place concrete retaining walls. Suitable for use in virtually all concrete applications, slag cement provides increased durability, reduced permeability and enhanced finishing characteristics. Its use also makes a significant contribution to sustainable construction. With the growing interest in sustainable, or “green,” paving mixes, NewCem cement brings the distinct benefit of being able to incorporate a supplementary cementitious material (SCM) into the concrete paving mix in an easy and efficient manner. Moreover, for the first time in Michigan, cement kiln dust (CKD) was used to stabilize the roadways’ high-clay soil subgrade.
“The CKD did a fantastic job of stabilizing the clays and unstable clay soils,” said John Staton, MDOT state materials engineer. “It is a way of utilizing a waste product for soil/subgrade stabilization in lieu of using lime or having to remove and replace poor-quality soil.”
Towering winner
The Bagley Avenue Pedestrian Bridge is considered one of the most exciting elements of the Gateway Project’s construction. Easily visible from the Ambassador Bridge and other points in the city of Detroit, this elegant $7 million cable-stayed structure spans 407 ft across I-96 and I-75 and rises 150 ft from the freeway floor. Its breathtaking design by VanTine Guthrie (now inFORM Studio)—winner of an award from the American Institute of Architects—was the result of a nationwide competition seeking a capstone for the project that would bring a cohesive element back to the community. Marked by a soaring 150-ft concrete pylon, the state-of-the-art bridge varies in width from 10 ft on the west approach to 31 ft on the east approach. It is supported by 15 tension cables radiating from the concrete tower. The 15 sinewy steel cables with five tuned-mass dampers are all located on the north side to allow for an open view toward the Detroit River: 10 support the west portion of the bridge and five provide lift on the east side.
There also are post-tensioning cables that are hidden inside the column and vary in length from 150 to 270 ft. The cable-stayed bridge uses two types of lighting—335 lights in all on the aprons and bridge—arrayed in a geometric pattern: Amber LED lights are set flush in grade, and a series of columnar lights on vertical poles extend the horizontal axis of the bridge. “The bridge is definitely a signature structure that involved complex engineering and erection methods throughout,” said Bob Jones, vice president at Walter Toebe Construction, which served as the prime contractor. “We’ve been in the business for nearly 90 years, and this project was just so unique and by far one of the most technically challenging we’ve done.” Building the 15-story concrete pylon was, of course, where the project started, and the pylon alone required 62 separate construction stages. Because the structure tapers on all four sides, leans in three different directions and twists as it rises up, accurate surveying at every stage was both challenging and critical.
“Tolerance for the entire length of the pylon was 2 in., so geometry was everything,” said Victor Judnic, MDOTs chief engineer for the project. “If we were getting near to being out of tolerance, the temporary cables holding the forms in place would be tensioned in any direction we needed to sway the pylon back into tolerance again so the next pour didn’t exaggerate the tolerance of the previous pour.”
“We were literally building out of plumb and had to make constant, meticulous adjustments to the temporary cable supports to get back into alignment,” added Jones. With each pour and every time forms were set, more stress would be put on the pylon, so the cables would have to be adjusted in a particular order and a particular sequence to particular values to account for that additional weight. In addition to this detailed and tedious alignment process, the pours were made slowly, several feet at a time, to ensure that the weight of the concrete did not compromise the forms.
Special cement blend
In recent years, the Federal Highway Administration has been focused on promoting the use of advanced material technologies, and the departments of transportation of many states have been increasingly specifying high-performance concrete (HPC) for bridge construction. These innovative mixtures incorporate SCMs, such as slag cement, fly ash and silica fume, to make concrete stronger, more durable and longer lasting. Ternary mixes involving a combination of SCMs with portland cement often offer the best performance.
“With the pedestrian bridge being such an integral part of the Gateway Project, it was very important to achieve the highest possible performance from the concrete used in the structure,” said Staton. The resulting 6,000-psi HPC mix specified for the cast-in-place pylon was produced with Lafarge’s Tercem 3000 cement. This precisely formulated blend of portland cement (71%), granulated blast furnace slag (25%) and silica fume (4%) works synergistically to meet MDOT requirements for high-strength, long-term durability and reduced permeability. It also offers excellent finishing qualities and improved resistance to alkali-silica reactions, as well as exceptional freeze-thaw resistance.
While enhanced strength, durability and other performance properties were the primary reasons for choosing this ternary cement blend, the substantial environmental benefits also were desirable. Its production saves virgin raw materials and makes use of a silica fume and slag cement, which are byproducts from silicon metals operations and steelmaking that might otherwise be disposed of in landfills. It also allows concrete producers the opportunity to optimize their use of portland cement, thereby reducing the carbon footprint and lowering the embodied energy of concrete.
To determine the optimum percentages of slag cement, silica fume and portland cement in the HPC for the pedestrian bridge, MDOT and Lafarge conducted laboratory and field, or “in-place,” pavement assessments over the course of several winters. Standard concrete tests were performed as well as tests for flexural beam strength and permeability analysis.
MDOTs first experience with the Tercem 3000 cement came with a visit to observe how the product was being used in the $226 million, three-year renovation and expansion of the University of Michigan football stadium. Based on that project, MDOT redesigned the Tercem 3000 percentages to their unique Gateway requirements. They then conducted performance evaluations over a three-year period. These performance evaluations took place on bridge overpasses in Monroe and Eaton counties. Pleased with the performance demonstrated in these overpasses evidenced by the reduced level of cracking and general improved durability, MDOT was satisfied that the improved results were attributed to their unique Tercem 3000 formulation. With the results seen in these projects, this ternary cement blend is now considered the mix of choice for bridge decks in Michigan.
All ingredients for the Tercem 3000 cement were blended at Lafarge’s Detroit Terminal and Blending Station.
“In addition to the product’s high-performance and sustainability benefits, a local supply of high-quality preblended cement was also a key consideration,” said Staton. “This highly automated cement terminal can create precisely proportioned and fully blended products, which provided a high degree of consistency, reliability and quality control to this project.”
Opened in April 2005, the $38 million Detroit Terminal and Blending Station includes the largest cement silo in North America, standing more than 180 ft high and 95 ft in diameter. The facility consists of one 17,000-ton-capacity main silo (for portland cement) surrounded by 11 peripheral bins (for a variety of specialized material), ranging in capacity from about 1,500 to 3,600 tons. Total capacity for the structure is approximately 44,540 tons. This capability allows fully blended, accurately proportioned cements to be delivered on demand, including high-performance binary, ternary or quaternary blended cements.
Keep it cool
Alignment was not the only challenge in constructing the signature concrete pylon. As with any mass concrete pour, especially during the summer, controlling heat buildup was a primary consideration, and strict temperature limits were established to prevent thermal cracking. The differential temperature between the center and surface of the pylon was limited to 35°F, and a maximum concrete temperature of 180°F was specified.
The forms were insulated to help keep the temperature differential within the required 35°F range. However, since summer temperatures would reach the mid-80s or higher, maintaining a maximum concrete temperature below 180°F would now prove to be especially challenging. An independent thermal-modeling analysis confirmed the mass concrete would exceed temperature limits unless it was cooled down and poured at 60°F.
Several methods, such as aggregate cooling pools and mechanical cooling, were evaluated, but the most consistent, reliable and effective solution was determined to be liquid nitrogen. For the first time in Michigan, freshly batched concrete was shocked on-site by injecting it with liquid nitrogen to drop the temperature.
The contractor and MDOT engineers independently monitored temperatures with their own gauges cast in various locations of the mass structure. This joint constant monitoring of the concrete as it cured ensured that the maximum and differential concrete temperatures never exceeded the specified limits. The Bagley Avenue Pedestrian Bridge has been a very successful project that has received much acclaim as it gracefully rejoins this vibrant community in southwest Detroit. The beauty of the plazas on both ends of the bridge (over 41,000 sq ft) will encourage tourism and have a positive economic impact on local businesses. The east apron, an open, public gathering space on the approach to the bridge, provides performance spaces for festivities and is the site for public art. This space opens to the courtyard in front of the new Mercado and to the Detroit Mexicantown International Welcome Center. Building upon the spirit of collaboration that has been the hallmark of the Gateway Project, MDOT sponsored a competition to select an artist to design public art for the bridge. The winning design, by Detroit artist Hubert Massey, used images of pride and heritage from the surrounding area. A truly inspiring community treasure, the public art was installed during the last phase of construction in Spring 2010.
