What is Prestressed Concrete?

Despite its excellent qualities as a structural material, concrete has some weaknesses, too. One is that it has almost no strength against tension. Concrete can withstand a tremendous amount of compressive stress, but when you try to pull it apart, it gives up quickly. Concrete’s other weakness is that it’s brittle. It doesn’t have any “give” or stretch or ductility. Combine these two weaknesses, and you get cracks. Concrete loves to crack. And if you’re designing or building something made of concrete, understanding how much and where it’s going to crack can be the difference between the success and failure of your structure.

To understand how an engineer’s design reinforced concrete structures, we first have to understand design criteria – or the goals of the structure. The apparent goal that we all understand is that it shouldn’t fall down. When a car drives over a bridge and the bridge doesn’t collapse, the structure is achieving its design criterion of ultimate strength. But, in many cases in structural engineering, avoiding collapse actually isn’t the limiting design criteria. The other important goal is to avoid deflection, or movement under load. Most structural members deflect quite a bit before they actually fail, and this can be bad news. The first reason why is perception. People don’t feel safe on a structure that flexes and bends. We want our bridges and buildings to feel sturdy and immovable. The other reason is that things attached to the structure like plaster or glass might break if it deflects too much.

In the case of reinforced concrete, deflection has another impact: cracks. The reinforcement within concrete is usually made from steel, and steel is much more elastic than concrete. So, to mobilise the strength of the steel, first, it has to stretch a little. But, unlike steel, concrete is brittle – it’s doesn’t stretch, it cracks. So that often means that concrete has to crack before the rebar can take up any of the tensile stress of the member. Those cracks not only look bad, but in an actual structure, they could allow water and contaminants into contact with the reinforcement, eventually causing it to corrode, weaken, and even fail.

One solution to this problem of deflection in concrete members is pre-stressing or putting compressive stress into the structural member before it’s put into service. This is usually accomplished by tensioning the reinforcement within the concrete. This gives the member compressive stress that will balance the tensile stresses imposed in the member once it is put into service. A conventionally reinforced concrete member doesn’t have any compression to start with, so it will deflect too much well before it’s in any danger of not being strong enough to hold the load. So with conventional reinforcement, you don’t even get to take full advantage of the structural strength of the member. When you prestress the reinforcement within concrete, you don’t necessarily increase its strength, but you do reduce its deflection. This balances out the maximum load allowed under each of the structural design criteria, enabling you to take fuller advantage of the strength of each material.

There are two main ways to prestress reinforcement within concrete:

Pre-tensioning: It’s pre-stressed because the steel is stressed before the member is put into service, but pre-tensioned because the steel is stressed before the concrete cures.
Post-tensioning: In post-tensioning, the steel is stressed after the concrete cures, but still before the member is put into service.
It’s important to point out that we didn’t necessarily make these beams stronger. Both the steel and concrete have the same strength as they would without prestressing the steel. But, we did increase the serviceability of the member by reducing the amount of deflection under load. Pre-stressed concrete is used in all kinds of structures from bridges to buildings to silos and tanks. It’s a great way to minimize cracking and take fuller advantage of the incredible strength of reinforced concrete. 

Source Credit: practical.engineering

The unparalleled growth in the construction sector has widened the scope for prestressed concrete. Keeping its growing importance in mind, PHI has brought out a textbook Prestressed Concrete by Muthu et al.

Interested to know more about this textbook: PRESTRESSED CONCRETE

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