Kevin R. Pruski, Ronald D. Medlock, and Mary Lou Ralls, Texas Department of Transportation
The goal of the AASHTO Technology Implementation Group (TIG) is to facilitate rapid acceptance and implementation of high-payoff and innovative technologies. In 2001, the TIG selected prefabricated bridge elements and systems as one area for implementation.
Prefabrication and HPC
Prefabrication provides more control over the construction environment, minimizes work-zone risks, and reduces inconvenience to the public. It also alleviates time pressures during construction by removing concrete strength gain from a project’s critical path. Combining prefabrication and high performance concrete (HPC) improves concrete quality as a result of the controlled environment in which the components are constructed. The improved concrete matrix provides a more durable concrete.
Texas Experience
The Texas Department of Transportation (TxDOT) has long used prefabricated bridge members to improve constructibility. In the mid-1950s, Texas began using precast, prestressed concrete beams with standard cross sections. Now, they are the “work horse” of the Texas highway bridge construction program. Beginning in 1963, Texas developed a composite concrete deck system consisting of precast, prestressed concrete deck panels with a cast-in-place concrete topping. Acceptance of this partially prefabricated deck system was slow, but today, the use of panels is the contractor-preferred system for constructing bridge decks. Texas now routinely considers prefabrication to address traffic disruption and constructibility issues on specific projects.
The following three examples demonstrate the growing popularity of prefabrication in Texas:
- The Louetta Road Overpass project in Houston was the first bridge in the country to use HPC for both the superstructure and the substructure. This project combined prefabricated highstrength concrete beams and partial-depth deck panels with high-strength concrete precast, hollow core, post-tensioned piers.
- While planning to replace 113 spans of an elevated structure in Houston’s central business district, designers realized that a conventional bridge substructure with cast-in-place bent caps would require 18 months to complete. User delay costs were estimated at $100,000 per day. TxDOT instead opted to use precast bent caps and completed the work in just 95 days.
- The Lake Ray Hubbard bridge project near Dallas is a 102-span bridge over water. Precast bent caps were used to speed construction and simplify concrete delivery for the substructure. This project used HPC with 35 percent of the cementitious materials consisting of ground granulated blast-furnace slag (GGBFS) to reduce permeability and, thereby, improve durability. GGBFS concrete has an initial slower strength gain than conventional concrete. However, the use of precast caps allowed this slower strength gain to occur off-site while foundations and columns were constructed on-site.
TxDOT promotes the use of HPC in its bridges. The longer curing period and strengthgain time required for some HPC mixtures can be better managed using prefabricated elements and systems. In addition, finishing and curing requirements for HPC are achieved more efficiently by using prefabricated elements. While highstrength HPC is used when needed for beam design, TxDOT is concentrating on the use of conventional strength HPC for its improved durability performance.
Summary
Prefabrication offers many advantages to the owner as well as to the general public.