Pre-stressed concrete is a form of concrete where initial compression is given in the concrete before applying the external load so that stress from external loads are counteracted in the desired way during the service period. This initial compression is introduced by high strength steel wire or alloys (called ‘tendon’) located in the concrete section.
Why is Pre-stressed Concrete Required?
Now, the question is, why we need this complexity?
Okay, before get started let’s go back to some basics first. We know, concrete is good at compression but very weak in tension. And so we see after external loading, in the bottom part of concrete, a tension zone occurred. So, it tries to be elongated and guess what? It cracks. This is why we add some steel bars at the bottom section so that it can resist most of the tension and save concrete from cracking. This is our traditional RC structure. But what about some megastructures with larger beam span? Think about a flyover or famous Gateway Bridge in Australia, Incheon Bridge in South Korea or Ringhals nuclear reactor in Sweden where external loading is very high.
In the way of traditional RC structure, for these larger beam spans, we should provide larger depth which is often too much that, for a bridge over a river, there will be no enough space under the bridge for ships to pass it. Here comes this new concept--- Pre-stressed concrete. The concept of prestressed concrete is not so difficult. In fact, the practice of prestressing things is very older in our day to day life. Imagine a barrel made of wooden staves and metal bands. Tung-Yen Lin, a civil engineering professor at the University of California, explained it in the introductory chapter of his book “Design of Prestressed Concrete Structures".
The basic principle of prestressing was applied to construction, perhaps centuries ago, when ropes or metal bands were wound around the wooden staves to form a barrel (see Figure 1). When the bands were tightened, they were under tensile prestress, which in turn created compressive prestress between the staves and enabled them to resist hoop tension produced by internal liquid pressure. In other words, the bands and the staves were prestressed before they were subjected to any service loads.
Like this, in prestressed concrete, initial compression is given to be balanced by future loading that will create tension.
How Pre-Stressed Concrete works?
So by now, I have had our concept. We have learned why and when we should use prestressed concrete. So, how does it work? In the real life, high tensile strength steel wires are inserted into the beam section and they are stretched and anchored, then released. Now the steel tendon wants to gain its original length and tensile stresses are transformed into a compressive stress in the concrete. Now after loading there are two kinds of forces on the beam,
- Internal prestressing force
- External forces (Dead load, Live load etc.)
Which must counteract each other. Observing the moment diagram, we will find something like this
Types of Pre-stressed Concrete
According to construction method, there are two kinds of prestressed concrete:
- Pre-tensioned concrete
- Bonded Post-tensioned concrete.
- Unbonded Post-tensioned concrete
In this method, wires or tendons are tensioned at first and concrete is poured later. It creates a good bondage between the tendon and concrete. As a result, the tendons are protected from corrosion and tensions are transferred directly. Tendons are anchored and stretched and the stress is transferred to the concrete when it is hard. Then the tendon tries to get back to the original length, but resisted by the bond between the concrete hence it induces compressive force in it.
Bonded Post-tensioned Concrete
Similar to pre-stressing but here concrete is poured first then tendons are tensioned. Tendons are placed at suitable places in the member and then casting is done. After the concrete becomes hard, the tendons are tensioned by hydraulic jacks against the concrete. When the tendons have tensioned sufficiently, according to design, they are fixed in position. After the jacks are removed, tension remains and it transfers pressure to the concrete. This method is widely used in building monolithic slabs for mega house construction projects where expansive soil sometimes creates problems. Moreover, post-tensioned concrete is also used in bridges.
Unbonded Post-tensioned Concrete
It is little different from bonded post-tensioned concrete. It allows freedom to move the cables. For this, each tendon is coated with grease and covered by plastic. Stress transfer to the concrete is achieved by the cables through anchors. There are some advantages of this like-
- The ability to distress tendons before trying to repair work.
- The ability to individually adjust cables
Also, there are some disadvantages like if damaged, one or more than cable can be distress itself and burst out of the slab.
Pre-Stressed Concrete Materials
According to AASHTO, high strength seven wire strand, high strength steel wire or alloys of grade and type (as specified by the designer) should be used in prestressed concrete. Also, stronger concrete is required in prestressed than normal RC. Generally, minimum 28-day cylinder strength of 5000 psi concrete must be used. So, why this high strength concrete? Well, if the concrete is not strong enough, it can be cracked or failed when it is stressed by tendons. As well as high compressive strength offers higher resistance to tension and shear and so it is desirable for prestressed concrete.
Moreover, high strength concrete is less subjected to shrinkage crack. It has a higher modulus of elasticity and smaller creep strain. As a result, loss of prestressing is small.