Giant precast endwalls support a road above a stream in a small Maryland town.
By Joy LePree
Officials in Taneytown, Md., went to extremes in the summer of 2002 during the construction of a road that connected two housing developments. Since the road – dubbed the Crossing at Baumgardner Street – had to pass over a sizeable stream, engineers knew tremendous endwalls would be required to support the avenue and the three 72-inch culvert pipes needed for the project. Weighing in at 80,000 pounds apiece and measuring more than 28 feet long and more than 10 feet high, the large precast endwalls used in the Roberts Mill Run housing development project are thought to be the biggest of their kind in the state of Maryland.
While the town may be small, it is growing rapidly thanks to the addition of new housing developments. However, in the case of the Roberts Mill Run housing project, there was a problem. “In order for traffic to get from the new subdivision into the existing one and out onto the main road, a new road had to be built – but it had to go over a large stream,” says Gary Hardman, city manager for Taneytown. “Crossing that stream was absolutely essential if we were going to connect the two developments.”
In order to do that, the stream had to be temporarily diverted so that the two giant endwalls could be installed and three 72-inch culvert pipes could be pushed through pre-engineered holes in the endwalls. The endwalls, which had to be identical, were to be 101/2 feet high and 281/2 feet long and would be placed directly opposite each other to stabilize the new road that would be built above them.
The real trick to this sizeable project was doing it in a timely fashion. “They needed the roads built quickly so that traffic could pass over them almost immediately, and adding to the criticality was the fact that you don’t want to divert a stream for too long,” says Hardman.
The contractor saw this as his biggest challenge. “Working with three 72-inch pipes was tricky enough because they were the biggest we’ve ever handled, but disturbing the stream was the real challenge because it’s something we didn’t want to do for any longer than absolutely necessary,” says John Wiley, superintendent of utilities with project contractors Gray & Son in Butler, Md. “The longer you disturb a stream, the greater the chance for disaster.”
For this reason, Wiley and his team specified that the endwalls be made of precast concrete. “Not only does precast save money when compared to cast-in-place, but it’s also a huge time savings,” he says. “Because we used precast for this project, we only had to divert the stream for four days. Using cast-in-place would have taken at least two weeks, and that’s too long when it comes to diverting a stream.” Wiley explains that over a two-week period, a storm could have caused the diverted stream to flood or the weather could have turned nasty while the forms were in place waiting for the concrete to cure.
“There was no question in my mind that precast had to be the chosen material for this application,” Wiley continues. “It was just a matter of finding someone who could cast such large endwalls in one piece, because the town engineer insisted that each one must be cast as one giant endwall instead of making two smaller pieces for each.”
Casting the big ones
Specifying precast for the endwalls was the easy part of this project, but actually casting them was the hard part, according to Greg Ouimette, president of Frederick Precast Concrete in Frederick, Md.
“The process of casting these huge endwalls was pretty intense,” recalls Ouimette. “We made the hole formers for the pipes in our fab shop and put them together with the forms in our shop. Then we had to perform several inspections before we could even begin to pour. It was one of the most detailed processes I’ve ever seen,” he says.
And pouring had to be done very carefully using a total of four buckets. “One bucket had to go on the end of each endwall, and pouring had to occur simultaneously so the concrete wouldn’t set up on one end before the other end was finished,” explains Ouimette. “It took us about four hours to pour, and during that time we were monitoring the quality of the concrete to make sure it wasn’t setting too quickly.”
The precaster stripped the forms the following day, and the endwalls were allowed to cure for another two weeks before they would be touched again. “We wanted to be sure they would reach full strength,” says Ouimette. “The mix was a 4,500-psi mix at 28 days, but we reached 5,800 psi in two weeks. So we were confident when the contractor was ready for us.”
An event to be seen
The giant endwalls were loaded onto lowboy trailers because they were too tall to be shipped on a regular trailer. The pieces were carefully removed from the trailers using pre-engineered lift points and installed on the site in less than one hour.
“This was remarkable to watch,” says Hardman. “These things were each about the size of a bus, and they were off the truck and in place so quickly I couldn’t believe it.”
Even Ouimette and Wiley were impressed with the job. “These guys set 160,000 pounds of solid concrete in less than 60 minutes,” says Ouimette. “It was truly amazing.”
“I believe these are the biggest endwalls in the state of Maryland, so it was really pretty cool that we were able to make and install them without any problems,” says Wiley.
Once the endwalls were in place, the three culvert pipes were pushed through the pre-engineered holes, cut to size, grouted and tamped in place. Baumgardener Street was built in less than a week following this process.
“Precast really worked out great for this project,” says Hardman. “It’s really a pretty nice setup there, and we were really pleased that it was able to be done so quickly and with such great results.”
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 Name: Roberts Mill Run
Owner: City of Taneytown, Md.
Contractor: Gray & Son, Butler, Md.
Precast Manufacturer/Engineer: Frederick Precast Concrete Inc., Frederick, Md.