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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