Reinforced concrete is a material that combines concrete and some form of reinforcement into a composite whole. Whilst steel bars, wires and mesh are by far the most widely used forms of reinforcement, other materials are used in special applications, e.g. carbon-filament reinforcement and steel fibres.
Concrete has a high compressive strength but a 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-carrying ability. By combining steel and concrete into a composite material, we are able to make use of both the high tensile strength of steel and the relatively low-cost compressive strength of concrete.
There are some other advantages to combining steel and concrete in this way which are derived from the characteristics of the materials. (These characteristics are summarised 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 mouldable when fresh
Difficult to mould and shape except at high temperatures
For example, the plasticity of concrete enables it to be moulded readily into different shapes, whilst its relatively high fire resistance enables it to protect the steel reinforcement embedded in it.
The aim of the reinforced concrete designer is to combine the reinforcement with the concrete in such a manner that sufficient of the relatively expensive reinforcement is incorporated to resist the tensile and shear forces which may occur, whilst utilising the comparatively inexpensive concrete to resist the compressive forces.
To achieve this aim, 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 positioned. These latter decisions are critical to the successful performance of reinforced concrete and it is imperative that, during construction, reinforcement be positioned exactly as specified by the designer.
It is important, therefore, that both those who supervise the fixing of reinforcement on the jobsite, and those who fix it, have a basic appreciation of 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 weakness of concrete in tension is compensated by the tensile strength of steel – in this case, steel wires, strands, or bars.
The compressive strength of the concrete is used to advantage by applying an external compressive force to it which either keeps it permanently in compression even when loads are applied to it during its service life (fully-prestressed) or limits the value of any tensile stress which arises under load (partial prestressing).
The pre-compressing or prestressing of concrete can be likened to picking up a row of books by pressing the books together Fig-1. The greater the number of books (the longer the span) the greater the force that has to be applied at either end of the row to prevent the row (the beam) collapsing under its own weight. A load applied to the top of the books would require an even greater force to be applied to prevent collapse.
In reinforced concrete, the steel reinforcement carries all of the tensile stresses and, in some cases, even some of the compressive stresses. In prestressed concrete, the tendons are used primarily to keep the concrete in compression. The tendons are stretched (placing them in tension) and then bonded to the hardened concrete before releasing them. The force in the tendons is transferred to the concrete, compressing it.
A fully prestressed concrete member is designed to be permanently under compression, effectively eliminating most cracking. In this case, if the member is slightly overloaded, some tension cracks may form but these should close up and disappear once the overload is removed, provided always that the steel has not been overstrained beyond its elastic limit. In partially prestressed members, some tensile stresses, and therefore some cracking, is accepted at the design ultimate load.
In reinforced concrete, the steel is not designed to operate at a high level of stress, as elongation of the steel will lead to cracking of the concrete. In prestressed concrete, the steel does carry very high levels of tensile stress. Whilst it is well able to do this, there are some penalties attached. Firstly, because of the forces involved, considerable care must be exercised in stretching the tendons and securing them. Stressing operations should always be carried out, or at least supervised, by skilled personnel. Secondly, the structure must be able to compress, otherwise the beneficial prestressing forces cannot act on the concrete. The designer must detail the structure so that the necessary movements can occur.