A community’s input and a design challenge lead MoDOT to use precast over cast-in-place concrete on a replacement bridge.
By Bridget McCrea
Time is money on any job site, where schedules must be met and all delays translate into lost revenue. While rebuilding a St. Louis bridge in 2003, the Missouri Department of Transportation (MoDOT) learned firsthand how well precast concrete can shave weeks off a production schedule while at the same time helping to overcome significant design challenges.
The project dates back to 2001 and involves the replacement of the Chambers Road Bridge and a local community that wanted to make sure the new bridge fit well with the area’s surroundings. Built in the 1940s, the bridge had outlived its life expectancy and was the first of three area bridges to be replaced by MoDOT. Because it was the first of the three, the design process was both lengthy and involved.
It also required the surrounding community’s approval. That meant all bridge elements – from the Mechanically Stabilized Earth retaining walls to the individual precast panels – had to have the look and feel of the local area.
Bill Schnell, MoDOT’s north area engineer, says the idea was to design a corridor that was both functional and aesthetically pleasing. “We consulted with a community advisory group and came up with a way to blend the bridge with the community and with the homes in the area – many of which have limestone foundations,” says Schnell. “Many of the structures are from the 1800s, and we wanted to maintain that look for the new bridge.”
To get there, MoDOT engineers initially specified 16 architectural panels to be cast in place against precast Mechanically Stabilized Earth (MSE) retaining walls that would be used to retain the bridge-approach pavements. The architectural panels varied in height and length, and had to extend approximately 3 to 5 feet above the bridge pavement parapet wall.
But the design posed several potential problems for the contractor. For starters, four of the 16 panels measured approximately 10 feet wide by 35 feet long, which led the contractor, Kozeny-Wagner, based in Arnold, Mo., to question the feasibility of a cast-in-place pour.
Also raising red flags was the fact that the panels were to be cast on top of cast-in-place spread footings and against the faces of precast MSE retaining walls. The face of the MSE panels had a 11/2 inch deep cut limestone formed texture, and the largest of the proposed architectural panels measured 10 feet wide, 12 inches thick and 35 feet in length. But there was no provision for the possible expansion and contraction of either the MSE wall or the architectural panels.
After reviewing an engineered precast alternate, the contractor, the MSE wall supplier and MoDOT all agreed that precast architectural panels would be the more viable option.
Schnell says the MSE panels, provided by Champion Precast Inc. of Troy, Mo., were necessary because of the bridge’s structure and because such elements can be surfaced with a variety of textured architectural finishes, allowing the walls to blend in naturally with the environment or to match existing structures. MSE retaining walls are used often in industrial, highway and bridge applications and in other settings.
“(Precast) MSE walls are a much faster and less expensive way to build a bridge, compared to cast-in-place concrete,” says Schnell. “They also allowed us to easily create the stone pattern to achieve the architectural look we wanted.”
The right solution
Brandy Broecklin, the resident engineer with MoDOT who oversaw the bridge’s construction, says the fact that the cast-in-place elements were to be placed flush against the MSE walls was a key design challenge. “The wall designer would not allow the architectural element to be flush against the MSE wall and its stone finish,” Broecklin recalls. “There would have been no bond-breaker between the two surfaces and it would have sheared the retaining wall, so we went with precast instead.”
And with that, McCann Concrete Products in Dorsey, Ill., got involved with the Chambers Road Bridge project. Mark Melvin, company vice president, says his firm worked closely with Kozeny-Wagner to design a precast alternative that would allow the MSE walls to move during freezing conditions or normal settlement – something cast-in-place would not have allowed. “The MSE wall producer was very much in favor of our product,” says Melvin. “From an engineering standpoint, they said they would rather have a precast panel floating out there on its own, rather than a cast-in-place option that would have sat flush up against the walls.”
Melvin says McCann Concrete Product’s design called for the precast panels to be set on the same cast-in-place footings that were planned for the field-poured panels. The precast architectural panels would be anchored to exposed reinforcing bars cast into the spread footings. Using a custom designed loop and pin system, they would be anchored into the back of the cast-in-place bridge parapet.
“This design not only allowed for the possible expansion and contraction of both the MSE wall and the architectural panels,” Melvin says, “but it also eliminated the difficult task of casting the large architectural panels against the MSE wall’s texture.” The contractor – who no longer had to worry about handling a 35-foot pour – the MSE wall supplier and the design engineer were all pleased that precast could be used in this application.
“It really wasn’t feasible to move the cast-in-place away from the wall,” concurs Forrest Bubolz, senior project manager with Kozeny-Wagner. “So while the project specified cast-in-place, precast was really the most efficient way to approach it.” Besides, says Bubolz, precast was also the most aesthetic choice. “With precast, we could better control the consistency in the appearance of the elements, and the patterns and coloring,” he explains. “The elements had to be stained after the fact, and we didn’t have to worry about the honeycombs or discolorations that occur in a cast-in-place pour. All of the streaking and honeycombing was taken out of the mix, thanks to the precast.”
Additionally, Melvin says the panels required two different architectural designs: a vertical ribbed liner that varied with the height of the panels, and a series of picture frames (shadow boxes) inset into one another, which varied with the height and length of the panels. “This factor was yet another reason the contractor chose precast panels over cast-in-place panels,” Melvin says. Once on the job site, he says setting and anchoring the 16 panels (which totaled 155 cubic yards of precast concrete) in place took a total of three days.
“The contractor originally estimated that the cast-in-place design would require 40 working days to complete,” says Melvin. Not having to worry about form blowouts at the base of a 35-foot pour or shifts in the liners during concrete placement also saved MoDOT and the contractor headaches. The precaster handled the PE-stamped shop drawings, form manufacture, architectural liner layout, anchoring design and delivery, while the contractor took care of unloading, setting and anchoring the panels in place.
Putting it together
Once on the job site, Bubolz says “consistency of every panel” was excellent, just as he expected. In other words, no honeycombs, no streaking and no unsightly blemishes to throw off the bridge’s architectural feel. “They showed up on the job site ready to go, and everyone commented on how they exceeded expectations,” says Bubolz, who estimates that using precast also saved about $16,000 in total project costs for MoDOT. “We passed that right along to the state.”
According to Melvin, the final product – once in place – was met with enthusiastic response from MoDOT and the contractor. “It took a lot of pressure off them, and let them concentrate on pouring the bridge instead of the panels,” says Melvin.
Perhaps most importantly, the use of precast meant a finished product was delivered to the job site, ready for installation. “The panels required no patching other than the lifting insert locations,” he says, adding that MoDOT saved a combination of time, money and hassle by making the switch to precast instead of cast-in-place. “In the end, they were completely satisfied with the aesthetics and quality of the precast panels.”
To find a manufacturer of this product in your area or for more information, visit NPCA’s website at www.precast.org or call toll free (800) 366-7731.
Project: Chambers Road Bridge
Owner: Missouri Department of Transportation
Engineers: Brandy Broecklin (MoDOT) and John Schnell (MoDOT)
Contractor/Installer: Kozeny-Wagner, Arnold, Mo.
Precast Manufacturers: McCann Concrete Products, Dorsey, Ill. (precast panels), and Champion Precast Inc., Troy, Mo. (MSE retaining wall panels)
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