With higher-strength concrete and significantly more steel reinforcement than a typical prestressed element, the spliced girders essentially span longer. The girders are shipped in shorter sections and spliced on the bridge by post-tensioning to obtain a longer span. Bexar’s 82-inch-high, 8500 psi spliced girders allow the Sylvan Avenue Bridge to cross the 250-foot mid-span of the Trinity River with fewer bridge piers. Bexar has worked closely with TxDOT to overcome the challenges posed by the large amount of mild steel in the girders. “It’s easy to draw it on paper, but a different story when it’s for real,” says Hinojosa.
Controlling the curing temperature has also put the producer to the test. “The specification allows a maximum curing temperature of 170 degrees F, but to increase production you need to add as much Type III cement as possible without exceeding the allowable temperature,” he explains. “Fortunately, the weather has cooperated so far.”
The sheer size of the elements Bexar is producing is impressive. “The trend in precast infrastructure is longer and bigger concrete girders,” says Hinojosa. “This means the prestress forces are larger so the fabrication beds and trucking equipment need to be larger.” He describes the production process as very similar to casting standard precast/prestressed concrete girders, but with more pieces to the puzzle.
Because the modified TxDOT 82-inch girders are wider than standard TxDOT girders, the forms had to be designed with a wider soffit. Fort Worth-based Hamilton Form Company configured the special formwork so the soffit could be cut down after the Sylvan Avenue Bridge job and used to cast typical 82-inch girders.
The formwork package includes 7- and 10-foot-long end blocks for the 85-foot girders. Two 8 1/2-foot end blocks are used back-to-back at the center of each beam to create a blister, or the thickened section, that forms the interior diaphragm that bears on a pier.
Hamilton Form also produced bearing and embed plates for the $42 million project. The 35- by 60-inch embedded plates are 1 inch thick; each has forty 6- by 7/8-inch steel studs and weighs 620 pounds. The larger beams use 55- by 44-inch beveled, galvanized steel bearing plates that are 2 inches thick and weigh 1850 pounds a piece.
Hinojosa sees the experience as an investment in Bexar’s future. “The infrastructure market is competitive,” he says. “It requires a high degree of quality control.” By working through the challenges of producing more complex precast and prestressed elements, the producer is determined to keep concrete at the forefront of Texas infrastructure.
Northeast Tarrant County (NTE) 183 Extension
Before 2012, Fort Worth precast producer Speed Fab-Crete had never produced a sound wall panel. But when a subcontractor asked for help producing 170,000 square feet of precast sound walls for the $2.5 billion North Tarrant Express (NTE) 183 extension project, a new product line was born.
The contractor had never worked with precast, so Speed Fab-Crete worked with the engineer to develop the best system for the job. They decided on a series of panel sizes varying from 10 to 18 feet high, and up to 18 feet long.
The panels are produced with two 4-unit battery forms, designed by Hamilton Form, that can be adjusted to accommodate the different sizes of panels needed. Each wall panel has an integral column cast on one end.
One battery form is designed with the columns across the top of the form. The wall panels will be shipped to the jobsite in this orientation, and then rotated 90 degrees for installation. This allows for panels up to 18 feet tall to be produced.
The other form casts the column in its normal vertical position at the end of the panel, producing panels up to 18 feet long. Both battery molds are designed with internal headers to accommodate casting up to four panels at a time.
Panels are 6 inches thick. The integral column is 18 by 18 inches; a void centered in the outer edge allows for the adjacent wall panel to lock into place.
Interior sideforms are fixed in place on a sleeper system. The outer sideforms are split into two 21-foot sections that roll back on wheel carriages and tracks. A catwalk between the two interior sideforms allows access to the top of the mold. The mold’s top ties are welded into a single assembly for each panel cavity for quick installation and removal.
In addition to its flexibility for production, the mold design has an aesthetic benefit. “The system produces many wall height and width variations,” says co-owner Carl Hall. “It reduces the cookie-cutter look.” This system also has the benefit of building walls set to a radius. Speed Fab-Crete uses formliners to create a one-sided decorative finish per project specifications.
With the right mold design in place, the precasters went to work tackling some other logistical issues. “We were out of space in the main plant,” explains Hall. “So we relocated the inventory from a small storage area across the road. This allowed us to make space for casting the sound walls.”
The owners reassigned a product manager — a lifelong precaster — and hired extra labor to focus on the sound wall project. Even with the right personnel in place, “it was a pretty big learning curve,” says Hall. “Early on, because of the size of the molds, we had problems with vibration and trying to get a good finish.” The precasters worked out a sequence of external and internal vibration to eliminate bug holes and achieve surface finishes that met the strict standards of the North Texas Tollway Authority. In addition to maintaining a high level of quality, working on large infrastructure projects requires patience. “With roadway projects you are dealing with traffic, utilities, and very busy areas,” Hall says. “You have to be flexible to handle schedule changes or you’ll go crazy.”
For the NTE 183 extension, Speed Fab-Crete constantly coordinates its production schedule with the project plans to reduce the number of mold changes. The team tries to pour as many panels of each mold as possible before changing it out, while keeping up with the different panel sizes required for each phase of the project.
When the project is completed in the middle of 2015, the 13.5-mile Interstate Highway 820 and State Highway 121/183 (Airport Freeway) corridor in Northeast Tarrant County will be rebuilt, relieving traffic congestion along one of the busiest highway corridors in the Dallas-Fort Worth region.