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How Does Prestressed Concrete Work?

Satyajeet Vispute
Prestressing imparts strength to concrete, allowing one to build large structures by using it. We'll learn the details of concrete prestressing, and also explore the different methods used to achieve this important task.

Did You Know?

In 1928, Eugène Freyssinet, a French civil and structural engineer, became the first person to practically apply the concept of prestressing to overcome concrete's natural weakness against tension. He later went on to successfully build many bridges using prestressed concrete.
Researchers say that some amount of stress does you good, by pushing you to the optimal levels of your performance capabilities. Perhaps, they came up with this theory from the old saying, 'whatever doesn't kill you makes you stronger'.
Or maybe they were able to reach this conclusion by observing how much better prestressed concrete was able to perform as compared to its laid back counterpart - normal concrete!
Prestressing is a method employed in the field of civil engineering that helps get rid of the natural weaknesses of concrete. Prestressed concrete is widely used in the construction of long beams, wide floors, and large bridges.
In the next sections, we shall discover how prestressing of concrete makes it outperform regular concrete. But before that, let's first find out what prestressed concrete actually is.

What is Prestressed Concrete?

Modern-day human beings live in a jungle of concrete. Our homes, offices, skyscraper buildings, bridges, etc., are all made of concrete. Without a doubt, it is one of the most important materials, especially in the world of civil engineering.
However, this versatile building material has one inherent flaw. Though it has extremely high compressive strength, it has almost no tensile strength. To understand what this means, imagine a rectangular piece of cardboard supported at both ends.
If you push down lightly in the center, it will bend. The top or the concave portion of the bend is under compression, while the bottom or the convex portion is under tension.
When concrete is used in the construction of bridges, it is in a similar situation as the piece of cardboard in our example. Now, since normal concrete has enough compressive strength, the top of the bridge will remain intact under load.
However, the bottom of the bridge which is under tension is most likely to fail and crack due to concrete's lack of tensile strength. This could spell disaster.
However, civil engineers, aware of this problem, altered concrete in a way which increased its tensile strength greatly. The end result was what is known today as prestressed concrete. Thus, prestressed concrete is basically concrete which has been altered in a way to impart a high degree of tensile strength to it.

How Does Prestressed Concrete Work?

The process of prestressing concrete can be best explained through the following illustrations.

Fig. 1 shows a concrete beam which is supported on both ends, acting as a bridge. If a heavy load such as a truck were to move over it, the beam would bend.
This would cause the bottom part of it to crack, as the concrete there would be pulled apart by lateral tensile forces, as shown in Fig. 2. To remedy this, a steel cable is passed through the concrete, reinforcing it against the tensile force acting upon it. Fig. 3 shows this reinforced concrete at work.
Now, to increase the concrete's strength further, the steel cable is first stretched by applying external pulling forces to it at both the ends. Then, liquid concrete is poured over it and allowed to set, with the stretched steel cable in the middle of the structure.
Once the concrete hardens to a certain level, the external pulling forces on the steel rod are removed. The steel cable immediately tries to regain its original size and in the process, compresses the surrounding concrete, making it harder, resulting in the formation of prestressed concrete. This is shown in Fig. 4.
Prestressing makes concrete exceptionally strong as far as tensile strength is concerned, allowing it to bear large loads without bending or cracking.

Methods Used for Prestressing Concrete

There are basically two main methods used for making prestressed concrete.

❑ Pretensioned Concrete
This is the most basic method employed for prestressing concrete, and is similar to the one described in the given illustrative example. As the name itself indicates, in this method, steel is tensioned before the placement of the concrete.
High strength steel strands, also known as tendons, are first brought under tension by pulling them at both ends. Concrete is cast around them and allowed to cure (harden).
During the curing process, concrete bonds with the steel, and so when later the pulling forces are removed, as the steel tendons return to their original shape, the bonded concrete too gets pulled together, resulting in its compression and hardening.
Pretensioning requires strong anchor points on which the steel tendons can be tied and stretched. Such points are not always available on-site, and therefore, most pretensioned concrete parts are manufactured in factories and transported to construction sites.
This imposes a limit on the size and weight of the prestressed concrete parts. This technique is used for the construction of balconies, floor slabs, piles, and even some bridges.
❑ Post-tensioned Concrete
In this method, first, the concrete placement is done, and tension is applied to the steel cable later when the concrete hardens. This technique can be employed to tension concrete on-site. To understand this process, imagine a series of wooden blocks, each having a hole at their center. 
To understand this process, imagine a series of wooden blocks, each having a hole at their center. If one passes a rubber string through these holes, and holds its ends, the wooden blocks will sag down.
Now, if one fixes winged nuts to both ends of the rubber string, and starts winding it, the blocks will be drawn closer until they align themselves in a straight line. This is the basic principle of post-tensioning.
There are mainly two methods for post-tensioning concrete.

Bonded Post-tensioned Concrete
In bonded post-tensioning, concrete forms a bond with the steel cable before it is tensioned. High strength steel cables (tendons) are passed through ducts made in the slabs of concrete.
The concrete is first allowed to cure, and after it hardens sufficiently, a stressing jack is used to tension the steel tendons. As the tensioning process continues, the steel tendon gets elongated while the concrete is compressed.
When sufficient tension is attained, the prestressed steel is anchored firmly in place. The anchors are so designed that they are able to permanently maintain the steel cable in tension, which keeps the concrete in a state of compression.
◆ Unbonded Post-tensioned Concrete
In unbonded post-tensioning of concrete, the concrete is prevented from bonding with the steel cables. To achieve this, each individual steel tendon is coated with a layer of grease and wrapped in a plastic sheathing.
Once the concrete hardens, tension is transferred to it via the force created by the steel cables acting against the anchors embedded in the perimeter of the concrete slab. In unbonded post-tensioned concrete, each steel cable retains freedom of movement relative to the concrete itself. This allows for greater layout flexibility as compared to other forms.

Advantages of Prestressed Concrete

➤ Prestressed concrete significantly increases the strength of concrete. Therefore, the structures made using this process have a very low failure rate. They perform well under stress, and are much less prone to getting cracked.
➤ Being stronger than normal concrete, prestressed concrete structures can be made much more compact while still retaining their strength. Floor slabs and beams can be designed to be much thinner as compared to those made using reinforced concrete.
This proves to be very advantageous in multi-storey building construction, where the space saved in designing each level can add up to form extra usable floor space.
➤ Usually, in any construction, the joints between slabs are the most high maintenance area, since they are the weakest links in the structure. Using the prestressing technique, concrete slabs can be made larger, which results in lesser number of required joints.
➤ Large building and bridges can be built reliably using this technique.

Thus, the procedure of prestressing cures the concrete of its weakness against tensile forces, and allows it to be used in several high tension applications.