Prestressed Concrete

An Introduction to Prestressed Concrete

Although concrete is highly versatile, one of its major drawbacks is its weakness in tension. Conventional iron-reinforced concrete faces significant limitations when used in beams, floors, or bridges. The advent of prestressed concrete in the post-WWII era helped overcome key issues with concrete construction. Today, prestressed concrete is used as a foundation slab for both residential and commercial construction.

The Emergence of Prestressed Concrete

The first known patent for prestressed concrete was actually filed in San Francisco in 1886, only a handful of years after reinforced concrete was first used. Although sound in principle, it did not emerge in practice for decades. As Europe faced a major shortage of steel after World War II, engineers needed ways to facilitate large-scale construction projects without using conventional reinforced concrete. Likewise, high-rise construction was running up against the functional limits of traditional concrete elements. A new innovation was necessary.

In the early post war years, European engineers and architects relied on prestressed concrete far more than their American counterparts: The first prestressed concrete building in the U.S. was the Walnut Lane Memorial Bridge of Philadelphia, which was dedicated in 1950. The bridge achieved a 30% savings against conventional concrete designs of the time, leading to a wave of interest in prestressed concrete that led to its wide adoption. By contrast, thousands of structures throughout continental Europe incorporated prestressed concrete during post war rebuilding.

While prestressed concrete slabs are important for a variety of uses, there other forms of prestressing used in construction projects. Concrete pipes can be prestressed, and depending on the design, a thinner concrete pipe wall may be allowed in the final project. Prestressing bridge elements is also very useful in adding strength to items such as beams and girders. With the history and styles of prestressed concrete covered, let’s take a look at the prestressing process.

How Prestressed Concrete Works

There are three major implementations of prestressed concrete:

·         Pre-Tensioned Concrete: In this form, concrete is cast around steel bars or cables under tension. The concrete naturally bonds to these “tendons” while it cures. Compression by static friction transfers the tension to the concrete once it is released. Subsequently, any tension on the concrete transfers readily to the tendons. Pre-tensioned concrete elements are common in beams, lintels, and floor slabs.

        Bonded Post-Tensioned Concrete: In this form, compression is applied in situ during curing. A duct of aluminium, plastic, or steel is used in casting and follows the area where tension would occur in the concrete. Tendons are pushed through the duct, then tensioned via hydraulic jack after hardening. Once tendons’ stretching meets design specifications, they are wedged in place and the duct is grouted.

·         Unboned Post-Tensioned Concrete: Here, the individual tendons retain freedom of movement relative to the concrete. Tendons are prepared with a coating of lithium-based grease, then given a plastic-based “shell” formed through extrusion. The steel cables are tensioned against anchors placed in the slab’s perimeter. This design provides the ability to de-stress the embedded tendons prior to repair.

Major Benefits of Prestressed Concrete

Today, prestressed concrete is widely used in the floors of skyscrapers and within containment structures of nuclear reactors. It can facilitate emergency repairs to a wide range of buildings due to its ability to be stressed and de-stressed, allowing it to prop up damaged walls until a solution is found. Its versatility allows it to be used in bridges and other projects.

The major benefit that popularized prestressed concrete is its relatively low cost. It improves the efficiency of vertical structures thanks to its thinner slabs, which translate into additional floors. Thanks to its span distance, it requires fewer joints when compared to traditional reinforced concrete construction. This also leads to greater usable floor space.

Prestressed concrete construction also has major benefits from the perspective of the end user. Joints are the focus of most repair work over the lifetime of a concrete building, so a reduced number of them naturally leads to lower maintenance costs over time. This improves safety for the tenants and maximizes the time that the building can be in use.

Commercial Adoption of Prestressed Concrete

Over the last several decades, prestressed concrete has seen widespread commercial adoption. It is popular in shopping centers, parking garages, and large office complexes. It is also commonly seen in public buildings including libraries, schools, and auditoriums.

Aside from cost, some of the factors driving its popularity include:

·         Versatility: High span length facilitates easier alteration of a structure’s interior and makes it somewhat simpler to plan additions. This ability is at a premium in dynamic, growing businesses across a range of industries.

·         Acoustics: Prestressed concrete provides acoustical benefits to a building interior that can make it suitable for musical performances and public address. Effective positioning of joints makes it possible to create longer open spaces.

Many of the most notable high-rise buildings of the post war period incorporated prestressed concrete. Although innovative approaches are becoming more practical than they once were, it is still important for engineers to understand the many applications of prestressed concrete. Moving forward into the 21st century and beyond, prestressed concrete is likely to remain an important tool in the design, development, and maintenance of a wide range of commercial structures.