Prestressed concrete could create stronger bridges with longer spans

Construction worker gets ready to pour concrete

Image: College of Engineering

The state of Texas has more bridges than any other state in the nation, with more than 50,000 total. Maintaining structurally sound, shorter- and longer-span bridges was the driving force in two collaborative projects recently completed by Texas A&M University professor Mary Beth Hueste and the Texas Department of Transportation (TxDOT).

Hueste, a Zachry Department of Civil Engineering professor at Texas A&M and Texas A&M Transportation Institute (TTI) research engineer, is dedicated to furthering the understanding of the use of new materials and designs for bridge structures, with a focus on prestressed concrete bridge systems. Prestressed members are put into a state of compression using high-strength steel tendons before external loads are applied.

“The use of precast, prestressed concrete bridge girders in Texas and other parts of the U.S. has proven to provide economical bridge systems that have a number of benefits,” Hueste said. “Producing the girders at the precast plant leads to enhanced quality because there is more control of the materials and the manufacturing process at the plant during fabrication. By investigating new bridge systems that utilize precast girders, TxDOT and other bridge owners have additional options for bridge designs that can be selected when the site conditions or other factors make precast concrete the optimal alternative.”

The first project centered on continuous prestressed concrete girder bridges. Most Texas bridge structures are constructed with precast concrete girders with a cast-in-place concrete bridge deck. The bridge girders are fabricated at a precast plant where they are prestressed to avoid cracking of the concrete and to achieve longer span lengths compared to conventional reinforced concrete bridges. However, the precast girder units are limited to 160 feet due to weight and length restrictions on transporting them from the plant to the bridge site. This project’s primary focus was to develop innovative and economical alternatives for longer-span bridges, with main spans up to 300 feet.

Hueste and her team of researchers found that with in-span spliced girder technology and continuous prestressing installed at the bridge site, the span length of precast concrete girder bridges can be nearly doubled.

The central focus of this second project was to investigate a new bridge system for short-span bridges with spans up to about 50 feet.

Conventional slab beam bridges have precast concrete slab beams placed immediately adjacent to one another with a cast-in-place topping slab. While this bridge system is commonly used for short-span bridges due to its shallow profile, it is more expensive than typical prestressed I-beam bridges.

This project examined the use of slab beams that are spread apart with less expensive precast concrete panels between beams and a cast-in-place concrete deck. The goal was to investigate the design, constructability and performance of this new bridge system, and to provide guidance for future designs.

To better understand the spread slab beam bridge system, the team built a full-scale prototype with widely spaced beams at the Texas A&M RELLIS Campus. There they were able to assess constructability and in-service performance.

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