Why we provide steel in concrete? (Interview Question)

Why we provide steel in concrete?

       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 fibers.
        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 Picture).
AIM:
        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 utilizing 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.

WHAT ARE THE ADVANTAGES OF PRESTRESSED CONCRETE OVER R.C.C? (INTERVIEW QUESTION)

WHAT ARE THE ADVANTAGES OF PRESTRESSED CONCRETE OVER R.C.C?
       Concrete weak in tension and strong in compression. Therefore, Reinforcement concrete system has been created to overcome the weakness of concrete with rapidly development in construction, the concrete technology has to walk parallel with this development. Therefore, the Prestressed concrete was created to overcome the limit of reinforcement concrete span.
Prestressed concrete is a concrete construction material which is placed under compression prior to it supporting any applied loads or defined as Structural concrete in which internal stresses have been introduced to reduce potential tensile stresses in the concrete resulting from loads.

The prestressing of concrete has several advantages as compared to traditional reinforced concrete without prestressing. A fully prestressed concrete member is usually subjected to compression during service life.
This rectifies several deficiencies of concrete.

Serviceability and Strength:
1-Reduces occurrence of cracks .
2-Freezing & thawing durability is higher than non prestressed concrete
3-Section remains uncracked under service loads
4-Reduction of steel corrosion
5-Increase in durability.
6-Full section is utilized
7-Higher moment of inertia (higher stiffness)
Less deformations (improved serviceability).
8-Increase in shear capacity.
9-Improved performance (resilience) under dynamic and fatigue loading.
10-In areas where there are expansive clays or soils with low bearing capacity, post-tensioned slabs-on-ground and mat foundations reduce problems with cracking and differential settlement.
11-Reduces self weight of building thereby reducing the lateral load resisting system.
12-Suitable for use in pressure vessels, liquid retaining structures.

APPLICATIONS:
1-High span-to-depth ratios
2-They do not crack under working loads, and whatever cracks may be developed under overloads will be closed as soon as the load is removed, owing to the cambering effect of pre-stress.
3-This becomes an important consideration for such structures as long cantilevers. Under live loads the def section is also smaller because of the effectiveness of the entire un-cracked concrete section.
4-Larger spans possible with prestressing (bridges, buildings with large column-free spaces)
5-Post-tensioning allows bridges to be built to very demanding geometry requirements, including complex  curves, and significant grade changes.
6-Another advantage of post-tensioning is that beams and slabs can be continuous, i.e. a single beam can run continuously from one end of the building to the other.
7-Crack control helps in constructing high performance water tanks
8-More aesthetic appeal due to slender sections
9-Applications of various prestressed techniques enable quick assembly of standard units such as bridge members,building frames, bridge decks providing cost-time savings

1-Rapid construction
2-Better quality control
3-Reduced maintenance
4-Suitable for repetitive construction
5-Multiple use of formwork
6-There is also a definite savings stirrups, since shear in post-tensioned concrete is reduced in the inclination of the tendons, and the diagonal tension is further minimized bathe presence of pre-stress.
7-A lower building height can also translate to considerable savings in mechanical systems and façade costs.
8-Thinner slabs mean less concrete is required. It means a lower overall building height for the same floor-to-floor height.
9-Pre-tensioning is suitable for precast members produced in bulk.
10-The high tensile strength & precision of placement gives maximum efficiency in size & weight of structural members.