Reinforced concrete is a material that combines concrete and certain types of reinforcement into a single composite whole. While steel bars, wire and mesh are the most widely used forms of reinforcement, other materials are used in specialized applications, such as carbon-filament reinforcement and steel fiber.

Concrete has a high compressive strength but low tensile strength. Steel, on the other hand, has a very high tensile strength (as well as a high compressive strength), but is much more expensive than concrete relative to its load-bearing capacity. By combining steel and concrete into one composite material, we are able to harness both the high tensile strength of steel and the compressive strength of concrete at a relatively low cost.

Combining steel and concrete in this way has some other advantages that are derived from the characteristics of the material. (These characteristics are summarized in Table 1,

Table-1 Characteristics of Steel and Concrete

Characteristics of concrete

Characteristics of steel

high compressive strength high compressive strength
low tensile strength high tensile strength
relatively high fire resistance relatively low fire resistance
Plastic and moldable when fresh Difficult to mold and shape except at high temperatures
relatively inexpensive relatively expensive

For example, the plasticity of concrete enables it to be easily molded into various shapes, while its relatively high fire resistance enables it to protect the steel reinforcement embedded in it.

Reinforced concrete designers aim to combine reinforcement with concrete in such a way that relatively expensive reinforcement can be incorporated to resist tensile and shear forces, while using a comparatively inexpensive concrete to resist compressive forces. may be used. ,

To achieve this goal, the designer needs to determine not only the amount of reinforcement to be used, but how it is to be distributed and where it is to be placed. These latter decisions are critical to the successful performance of reinforced concrete and require that, during construction, the reinforcement is exactly as specified by the designer.

Therefore, it is important that those who supervise the fixation of reinforcement at the workplace, and those who fix it, have a basic understanding of both the principles of reinforced concrete as well as the principles and practices of fixing reinforcement.

Like reinforced concrete, prestressed concrete is a composite material in which the concrete’s weakness under stress is compensated by the tensile strength of steel—in this case, steel wire, strand or bar.

The compressive strength of concrete is used to advantage by applying an external compressive force that either keeps it permanently in compression, regardless of the load applied (fully prestressed) during its service life, or any tensile Limits the value of stress that is produced under load (partial pressure).

Figure 1
Figure 1

Pre-compressing or prestressing concrete can be compared to lifting a row of books by pressing them together. Figure 1, The greater the number of books (longer span) the greater the force that has to be applied at either end of the row to prevent the row (beam) from collapsing under its own weight. The load applied to the top of the books would need to exert even more force to prevent the fall.

In reinforced concrete, steel reinforcement carries all tensile stresses and, in some cases, even some compressive stresses. In prestressed concrete, tendons are primarily used to hold the concrete in compression. The tendons are stretched (keeping them under tension) and then tied with hard concrete before they are released. The force in the tendons is transferred to the concrete by compressing it.

A fully prestressed concrete member designed to be permanently under compression, effectively eliminating most cracking. In this case, if the member is slightly overloaded, some stress cracks may form, but these will close and disappear once the overload is removed, provided the steel has not been stretched over its elastic limit. In partly prestressed members, some tensile stress, and therefore some cracking, is accepted at the final load of the design.

In reinforced concrete, steel is not designed to operate at high levels of stress, as the elongation of the steel will cause the concrete to crack. In prestressed concrete, steel carries a very high degree of tensile stress. While it is capable of doing so, there are some penalties attached. First, because of the forces involved, great care must be taken in stretching and securing the tendons. Stressful operations should always be performed, or at least supervised, by skilled personnel. Secondly, the structure must be able to compress, otherwise beneficial prestressing forces cannot act on the concrete. The designer must extend the structure so that the necessary movements can take place.

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.