Bridge building, Texas style: precast bridge construction rules the road - casting trends - Advertisement

Concrete Producer, The, Nov, 2003 by Lloyd M. Wolf

The Texas Department of Transportation (TxDOT) is using innovative pretest and prefabricated design and construction techniques for bridges to reduce traffic disruption and construction project duration, while enhancing public safety and ease of construction for contractors. Texas needs more roads and bridges to meet the growing population's needs. But larger projects can hinder drivers. Construction congestion delays motorists and compromises driver and worker safety. Goods and services may not reach their destinations on time. TxDOT has responded with precast and prefabricated bridge construction, best implemented in the early phases of the design process, to increase worker safety and decrease impact on drivers,

TxDOT began using precast concrete beams in 1958 and deck panels in 1963. Precast interior bent caps were first used in the early 1990s, with the first large-scale use of precast caps supported on precast concrete piling to speed construction across the Redfish Bay and Morris & Cummings Cut in 1994. Work was completed 6 months ahead of schedule because the 74 precast bent caps eliminated much of the need to cast concrete over water.

Precast interior bent caps minimized traffic disruption On U.S. 290 in Austin in 1994 and in Houston 2 years later. In downtown Houston, a 113-span section of Interstate 45, known as the Pierce Elevated Freeway, was replaced in just 190 days; with the first concrete deck poured just 5 days after the demolition started. Precast elements also are being used on the Texas State Historic Railroad and Union Pacific Railroad underpass at state Route 46 in New Braunfels.

Lake Ray Hubbard

The plan used at the Lake Ray Hubbard Bridge shows the benefits that can be achieved when TxDOT, the construction community, and academia work together. Completed in January 2001, TxDOT bridge engineers and precast and construction representatives provided input. The study developed four categories of connection details: grout pockets, grouted vertical ducts, bolted connections, and grout-sleeve couplers.

Criteria used were the ability to resist large unbalanced moments, avoiding conflicts between cap longitudinal reinforcement and precast cap connectors, confinement of the connection region to develop its full capacity, providing durability of the connection, and protecting connectors from corrosion. The ease of setting the cap in final position, adjusting grade of the cap and bearing seats, and completing the connection also were very important.

The design was first used on the state Route 66 bridge over Lake Ray Hubbard, on the east fork of the Trinity River, in Rockwall. The contractor requested using precast caps on the eastbound structure to overcome some of the difficulties of cast-in-place construction on a long water crossing and to eliminate the need for a barge crane near a 345-kV power line on the south side of the structure. Construction time for each of the 43 sire-precast bent caps was decreased by almost a week on tasks associated with formwork, reinforcement. casting, curing, and inspection. Also, casting the caps on the ground instead of over the water enhanced worker safety.

Double-line grouted vertical duct connections were used to connect the precast caps to the cast-in-place columns. The grouted duct connection minimized the amount of connection grouting needed. Plastic ducts were user for durability because the duct went all the way up to the top surface of the precast caps. The cast-in-place columns were held in exact position with respect to each other with the column formwork work platform and uncoated reinforcing bars cast into the tops of the columns with the aid of a template.

The precast caps were held in position with adjustable friction collars on the columns, and a plastic collar formed the bedding layer at the tops of the columns. A pre-packaged non-shrink grout completed the connection by pressure grounting through the plastic collar at the bedding layer up to the top of the ducts.

75-ton precast caps

The same design procedure was extended for a hammerhead bent design with higher moments for the state Road 36 bridge over Lake Belton, on the Leon River near Belton. This 3800-footlong bridge is over the lake that is used for Waco's drinking water supply. The difficulties of cast-in-place concrete 40 feet above the water and environmental concerns dictated using precast bent cap construction, At 75 tons each, these were the largest precast caps TxDOT has used.

Because of the hammerhead bent design, there were three layers of main reinforcing bars in the top of the cap to provide sufficient moment capacity for the superimposed loads. This required using a four-line grouted duct connection that did not extend the full height of the cap, except for four ducts used to monitor connection grouting. Grout was pumped into the bottom of the galvanized metal ducts through a 3/4-inch inlet tube just above the bedding layer and vented out of the top of each sealed duct by a 1/2-inch diameter tube. The grout must be pumped in from the bottom and up to the top to drive air out of the connection. The limits of the bedding layer are oval because of aesthetic considerations in the design, so friction collars could not be used. This complicated sealing the bedding layer before pressure grounting and made using dry-pack grouting necessary to seal the bedding layer. Steel shims set bedding layer thickness and adjust cap elevation.