Advantages and Disadvantages of Prestressed Concrete

In karate demonstrations, the pile of concrete that they break is made out of plain concrete. When a tensile force such as a karate chop is applied to it, it cracks and crumbles. If, however, these were to be replaced by prestressed concrete blocks, the pile would be humanly impossible to break.

Concrete is used in various types of construction. The name 'prestressed' itself indicates that this particular type of concrete has been put under stress, before building a floor over it. However, instead of buckling under the pressure, it manages to become stronger, and is able to withstand much larger 'tension' than normal concrete ever would.

But how does concrete do this. What are the benefits and drawbacks of prestressing concrete? Let's find out the answers to these questions, which will give a better understanding of prestressed concrete.

Concrete in its normal state has an extremely high amount of compressive strength. This makes it possible to be used for building structures that have to bear compressive loads. For example, it is used to build columns and pillars to support the various slabs in large buildings.

However, compared to its compressive strength, concrete has almost no tensile strength. Therefore, if normal concrete is used to build the slabs, it would sag under the compressive pressure of the weight upon it, and eventually crack and crumble.

To remedy this weakness in concrete, the method of prestressing is applied. In its most basic form, prestressing is done as following.

A number of steel cables are brought under tension by applying pulling forces on their ends, and are arranged in the concrete block mold.

Then, liquid concrete is poured in the mold and is allowed to harden, which causes bonding to occur between it and the steel cables inside. After that, the pulling forces are released. As the cables try to regain their original form, they pull with them the bonded concrete, thus creating compression.

This induces stress in the internal particles of the concrete, strengthening it, and making it ready to be used for construction. Since stressing of concrete is done prior to putting it to use, it is called prestressed concrete.

Prestressed concrete has a large amount of, both, compressive as well as tensile strength. It is used to build long bridges, building slabs, etc.

1) High Tensile Strength and Crack Resistance

A normal concrete slab, if put under tension, sags down under the pressure of the weight. In this position, the top of the slab is under compression, while its bottom is under tension. Since concrete can withstand large amounts of compression, the top of the slab is able to bear the load.

However, concrete is weak against tensile forces. The bottom of the slab begins to crack, until the whole slab crumbles down.

Prestressed concrete has a high amount of tensile strength, and is thus able to bear large loads without cracking or failing.

2) Lower Depths

Owing to its high tensile strength, prestressed concrete can be used to build structures having considerably lower depths, as compared to reinforced concrete structures. This has two main advantages.

If used for building slabs, the greater slenderness of this concrete, as compared to reinforced concrete, saves on space requirements, which can result in additional usable space becoming available, especially in multi-storied buildings where the space saved in the construction of each level adds up cumulatively.

The second advantage of lower depths structures is that they have lower weights, and so supporting columns in buildings too can be made smaller, which saves on construction cost and effort.

3) Longer Spans

Prestressed concrete can be used to build structures having a longer span as compared to reinforced concrete.

In the construction of buildings, this means that fewer columns will be required to support the slabs, and also the spacing between them can be kept much larger. For bridges, using prestressed concrete can allow the engineers to build long spanning bridge sections, which won't fail under load.

4) Rapid and Reliable Construction

Prestressed concrete blocks are manufactured in industries in several standard shapes and sizes. They are known as precast blocks. Since they are professionally manufactured, they have extremely good build quality, while at the same time they provide all the strength benefits of precast concrete.

They can be directly trucked to the construction site and used to rapidly complete the construction work. The structures built using these blocks are known to have a better overall quality, and require less long-term maintenance.

1) Greater Building Complexity

Prestressing concrete on the construction site is a challenging and complex process. One must have thorough knowledge of each step which is involved, along with complete knowledge of the use of the various equipment that are required.

Precast concrete structures once made, are difficult to alter, and hence, the complexity of initial planning too is increased. Also, since the margin for error is very low, a lot of care needs to be taken while constructing with it.

2) Increased Construction Cost

Prestressing concrete requires expertise and special equipment, both of which can be expensive to obtain. Even the cost of precast blocks is significantly higher than reinforced blocks.

In the construction of residential buildings, the additional tensile strength offered by prestressed concrete may be unnecessary, as simple reinforced concrete, which is much cheaper, is strong enough to fulfill almost all load requirements.

3) Need for Quality Control and Inspection

The procedure used for prestressing needs to be checked and approved by quality control engineers. Each prestressed concrete structure has to inspected in order to ensure that it has been subjected to the appropriate amount of stress. Too much stress is also bad, and it can damage the concrete, making it weaker.

Prestressed concrete structures offer superior tensile strength as compared to normal and even reinforced concrete ones, but they are complicated to design and much costlier to build.

For low tension applications, such as slabs in buildings, using prestressed concrete is largely impractical. Hence, the decision to use prestressed concrete must be made only if the project specifications and requirements demand it.