Precast Bridges

For bridges, as well as other commercial projects, precast and prestressed precast concrete have earned a place of distinction as a preferred building material. The reliability, affordability, and low maintenance alone add immeasurable value to this product.

History of Precast Bridges in the United States

Photo via Structurae (Walnut Lane Bridge (1950), Philadelphia, Pennsylvania (HAER, PA,51-PHILA,715-5))

The major precast bridge breakthrough occurred in the 1950’s and 1960’s as a result of the massive increase in road traffic and the construction of new interstates. Fast and affordable solutions were required to prevent the disruption of the moving traffic. The creation of bigger and more slender precast units was significantly aided by the entrance of long-line prestressing techniques into precasting factories which proved to be particularly useful for long spans and heavy weights for bridges.

First Prestressed Bridge

The Walnut Lane Bridge in Philadelphia, completed in late 1950, is regarded as the nation's first significant prestressed concrete bridge. The bridge was created by Belgian engineer Gustave Magnel and his pupil Charles Zollman. The post-tensioned concrete beams were all cast in a single piece at the bridge site. A cable was run end to end through a planned aperture after the concrete had dried and set. The cable was given tension by a jacking device, which anchored it in place. In 1989–1990, the initial beams and superstructure were changed.

Although pretensioned precast bridge beams were manufactured before 1948, that year saw the debut of the first producers who began advertising and creating precast bridge beam ranges. Tennessee is where the first precast bridge in the USA was built.

How Precast Bridges Are Built

All types of bridges use precast to build both the superstructure and the substructure including vehicular, pedestrian, railroad, pipeline, and dock bridges. Flat slabs, neighboring box beams, pretensioned beams, as well as spliced and curved girders are examples of superstructures. Precast end bents, piles, and pile bent caps, along with precast columns, are examples of substructures.

What is a prestressed concrete bridge?

Prestressing is a method of applying stresses of a preset magnitude through tendons (often steel cables) to generate compressive stress to increase tensile strength. Prestressing is frequently used in bridge components, as well as other precast products.

Pretensioning involves pouring concrete over already-tensioned wires and letting it set up to solidify and secure the cables in place. The ends of the tensioned cables are cut and the tension is transferred into the beam or slab once the concrete has hardened and cured.

Pretensioning, which is more difficult than post-tensioning, is the current standard method for creating prestressed bridge beams. In order to retain the steel cables, referred to as "strands," in a highly tensioned state while the concrete is being poured around them in molds, pretensioning calls for the creation of sizable "casting beds."

Concrete is post-tensioned, which involves adding compression after curing. The tendons are tensioned and wedged to maintain the tension before being supplied through ducts cast into the finished concrete product. The aperture is then grouted after this.

What type of concrete is commonly used for bridges?

Concrete beams, spandrels, columns, single and double tees, wall panels, segmental bridge units, bulb-tee girders, I-beam girders, flat slabs, and hollow-core slabs are typical examples of structures that use precast concrete.

Precast Bridge Components

Precast concrete is a superior option for bridges and other traffic related construction such as noise barriers, median barriers, and pavement systems due to its structural and aesthetic versatility, resilience, speed of construction, high level of quality and durability. 

Precast Piles

Precast piles are one of the main components of precast bridges commonly used in areas such as Mississippi, Alabama, and Louisiana. Prefabricated, highly durable prestressed concrete columns known as piles serve as the foundation or support for buildings and bridges. Concrete piles are reliable workhorses and outlast wood or steel piles in maritime and coastal conditions.

Biloxi Bay Bridge

After the original bridge was destroyed by Hurricane Katrina, the new Biloxi Bay Bridge was designed using various precast components including precast piles that were designed not only to withstand a Category 5 hurricane but also vessel collision loads.

Precast Pavement

Precast pavement and precast bridge decks add an additional benefit and the greatest option for a high-volume, continuously used roadway. PCI-certified precast concrete replacement slabs are a lasting investment for transportation clients faced with deciding how best to spend limited infrastructure funds.

Lake Pontchartrain Causeway

The Lake Pontchartrain Causeway is a prime example of how precast can be used on a highly traveled roadway. The bridge is the main mode of travel over Lake Pontchartrain and sees over 40,000 vehicles every day. When safety became a liability, a total precast solution was chosen to bring the bridge up to modern-day design standards.

Precast NEXT Beams

Precast bridge designers are finding new ways to bring innovation to the industry. Through the creation of the NEXT Beam, the demand for a more effective bridge beam design entered the transportation world and is gradually transforming how bridges are maintained or replaced with an emphasis on cost-effectiveness, longevity, and design flexibility.

Cribbs Mill Creek Drainage Bridge

Contech, formerly known as Forterra, was involved with providing NEXT beams for the Cribbs Mill Creek Drainage bridge in Alabama. It was the first project in Alabama to use NEXT Tee Beams. The NEXT beams provided a clean line profile that did not require joint lines and removed possible future joint maintenance.

Advantages of Precast Concrete Bridges

There are numerous benefits to using precast concrete for bridges.  Not only is precast produced in a quality-controlled environment, PCI certified producers adhere to a strict quality control program to ensure their precast components are of the highest quality.  

Precast concrete bridge components also have uniformity because they are manufactured using the same molds and techniques.This means when they arrive at the job site, they are ready to be erected without additional work or forming.

As an offsite construction option, precast is manufactured in plants away from the job site.  This means they are shipped precisely when they are needed and do not require laydown space on tight job sites. The offsite manufacturing proves to be the perfect solution for environmentally sensitive areas on major highways or rural bridge locations.

Why is precast concrete cheaper?

When it comes to bridges precast concrete is a more durable and economical solution than other construction materials.  Precast concrete lasts longer, requires less future maintenance, and is produced in quality-controlled environments which ensures the quality of the components.  DOTs around the US look to precast to build their infrastructure so it lasts for decades with little upkeep.

How long does precast concrete last?

Precast concrete can be designed to perform for 100 years or more with minimal maintenance. This means that bridges designed with high-quality precast components such as those from PCI-certified producers continue to give value long after the initial cost of installation.

Accelerated Bridge Construction

Bridge repair and replacement procedures continue to improve thanks to close cooperation between the Transportation Research Board, the FHWA, the American Association of State Highway and Transportation Officials, and academic/industry partners along with the development of strategies such as accelerated bridge construction.

Key Takewaways 

Precast, prestressed concrete has taken its place as the material of choice for bridge building and other transportation-related projects. Precast continues to be a “go-to” option for government agencies and their engineers across the nation today. This development is the result of precast producer dedication to creating and developing cutting-edge products and technologies with the goal of improving the efficiency of bridges and other transportation-related structures.