Prestressed concrete: what is it? How Does It Operate?

In order to resist stress from external loads in the desired way during the service period, pre-stressed concrete is a type of concrete in which initial compression is applied before applying the external load. High-strength steel wire or alloys (referred to as "tendons") embedded in the concrete portion cause this initial compression.

What is the Need for Prestressed Concrete?

The question now becomes, what is the necessity for this complexity?

Well, let's review the fundamentals first before moving forward. Concrete, as we all know, is quite weak in tension but excellent in compression. Thus, we can observe that a tension zone developed in the bottom portion of the concrete following external force. Thus, it attempts to lengthen, and what happens? It breaks. To prevent concrete from cracking, we add steel bars to the bottom part so that it can withstand the majority of the tension. This is the conventional RC structure we use. What about those megastructures that have beam spans that are greater? Consider a flyover, the well-known Gateway Bridge in Australia, the Incheon Bridge in South Korea, or the nuclear reactor at Ringhals in Sweden, all of which have extremely high external loading.

For these greater beam spans, a standard RC structure calls for a deeper foundation, which is frequently too deep and leaves insufficient room underneath the bridge for ships to pass. Pre-stressed concrete is a novel concept that is now available. Prestressed concrete is not a very complicated concept. In actuality, prestressing is a far older practice in daily life. In the first chapter of his book "Design of Prestressed Concrete Structures," Tung-Yen Lin, a professor of civil engineering at the University of California, described a barrel as one composed of metal bands and wooden staves.


Perhaps millennia ago, the fundamental idea of prestressing was used in building when hardwood staves were looped around metal bands or ropes to create a barrel (see Figure 1). The staves were able to withstand hoop tension caused by internal liquid pressure because the bands were under tensile prestress when they were tightened. This, in turn, created compressive prestress between the staves. Stated differently, the staves and the bands underwent prestression prior to being exposed to any service loads.


In the same way, future loads that will produce tension balances the initial compression in prestressed concrete.

How Do You Work with Pre-Stressed Concrete?

I therefore already have our concept. We now understand the benefits of using prestressed concrete and when to do so. How then does it operate? In reality, steel wires with a high tensile strength are put into the beam section, stretched, secured, and then released. Tensile tensions in the concrete are now converted to compressive stresses as the steel tendon tries to regain its previous length. Following loading, the beam is subject to two different types of forces: 1. Internal prestressing force

2. External factors that need to balance each other out, such as dead load and live load. Looking at the moment diagram, we'll see something similar to this.

Materials for Pre-Stressed Concrete

Prestressed concrete should employ high-strength steel wire, high-strength seven-wire strands, or alloys of the grade and type (defined by the designer), according to AASHTO. Additionally, prestressed concrete needs to be stronger than regular RC. Concrete with a minimum 28-day cylinder strength of 5000 psi is often required. Why then is this concrete so strong? Tendon stress might cause the concrete to fracture or fail if it is not robust enough. For prestressed concrete, high compressive strength is also preferred since it provides greater resistance to shear and tension.


Furthermore, shrinkage cracks are less common in high-strength concrete. Its creep strain is smaller and its modulus of elasticity is higher. Consequently, there is not much prestressing loss. 

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