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What is Prestressed Concrete?

Standard concrete structures are vulnerable, brittle, experience cracking, and simply cannot hold up against tension. These weaknesses are why engineers in San Francisco patented prestressed concrete in 1886. This new process of prestressing concrete produces sturdier structures by reducing the deflection or momentum and increasing the concrete’s ability to withstand more intense external pressures and forces. However, as prestressed concrete was around for over half a century prior, in 1951, the first North American prestressed concrete project was completed in Philadelphia, Pennsylvania as the Walnut Lane Memorial Bridge. Here at Permacast, we are taking a deeper look at prestressed concrete, how it is created, its benefits, and what prestressed concrete is used for.

Workers creating prestressed concrete

How Is Prestressed Concrete Created?

Prestressed concrete is created by either a pre-tensioning or post-tensioning process where compressive stress is applied to the concrete. Steel tendons, such as high-strength wires or alloys are stretched to reinforce the stress into the concrete material to create a more flexible and durable product.

Pre-Tensioning

Before the concrete mix is poured into the mold to cure, strong steel tendons are pulled between 70% and 80% of their overall strength. Once stretched, concrete is then poured around these tendons to undergo the curing or precasting process. Once the material approaches its desired strength based upon the initial design, the forces are released so the steel can be reduced back to its original size. This process is a transfer of prestressed forces between the steel and the concrete as the concrete maintains this state of pre-stress and strength even after the steel shrinks back to size.

Post-Tensioning

Unlike the pre-tensioning process, the post-tensioning process allows for the concrete mixture to be poured and cured first. Then, as curved ducts are formed by molds made of plastic, aluminum, or steel, the concrete cures and hardens prior to the steel tendons being stretched into these ducts. Once inserted, the steel tendons are pulled, stretched, and anchored externally to obtain a pre-stressed state within the concrete.

Prestressed concrete with post tensioning

What Are the Benefits of Prestressed Concrete?

Prestressed concrete offers a slew of benefits, including additional length, affordability, flexibility for alterations, efficiency, attractive designs, and appealing acoustics. Prestressed concrete lowers the overall maintenance cost of a structure as it greatly reduces the chance of cracks and faults within the concrete and increases the structure’s long-term durability, fire resistance, and shock absorbency. While this material allows for longer and larger unsupported spans, prestressed concrete is an ideal building material to remove common design limitations. This slender and sleek design increases usable floorspace in parking garages, commercial buildings, shopping centers, auditoriums, and more, without sacrificing stability.

Coronado Bay Bridge in San Diego Prestressed Concrete

Types of Prestressed Concrete Structures

While almost any structure can be built with prestressed concrete, some of the most popular projects that are created from this material include bridges, parking garages, commercial buildings, foundations, highway overpasses, silos, water towers, and tanks. In addition, prestressed concrete is an efficient way to construct shopping centers, school gymnasiums, cafeterias, auditoriums, columns, walls, beams, and balcony elements.

Prestressed concrete is a significant contribution to the concrete, precast concrete, and cement industries. It has greatly improved the flexibility, strength, and efficiency of precast concrete making it more able to take on external power, forces, and comprehensive strength. To learn more, or inquire about your own concrete structure or project, contact the experts at Permacast.

 

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