REINFORCED CEMENT CONCRETE ( R.C.C ) AND STEEL REINFORCEMENT

R.C.C AND STEEL REINFORCEMENT

REINFORCED CEMENT CONCRETE ( R.C.C )

Concrete is strong under compression but weak under tension and shear. Whenever concrete members are likely to be subjected to tension. they are reinforced with steel.

 

 Steel used for reinforcement should have following qualities:

1) High tensile strength

2) Good bond with concrete

3) Nearly same coefficient of expansion as that of concrete

 

Advantages of R.C.C.

1. R.C.C. structures are durable if designed and laid properly. They can last up to 100 years.

2. R.C.C. sections can be given any shape easily by properly designing the form work. Thus, it is more suitable for architectural requirements.

3. The steel reinforcement imparts ductility to the R.C.C. structures.

4. R.C.C. is as compared to steel and prestressed concrete. There is an overall economy by using R.C.C. because its maintenance cost is low.

5. The raw materials which are required for R.C.C. i.e. cement, sand, aggregate, water and steel are easily available and can be transported easily.

6. R.C.C. is almost impermeable to moisture.

7. Properly designed R.C.C. structures are extremely resistant to earthquakes.

 

STEEL REINFORCEMENT

 

Many traditional materials such as bamboo and natural fibres have been tried as reinforcement in earlier times. 

                 

i. Steel is very strong in compression. tension. shear and torsion.

ii. It can develop bond with concrete.

iii. Steel is ductile in behaviour. This results in sufficient warning time before failure.

iv. The steel bars can be cut, bent, lifted and welded easily with commonly available tools and machines.

v. Steel has longer life.

vi. Steel is easily available.

vii. It coefficient Of expansion is nearly same as for concrete.

 

However, steel has a few disadvantages which are listed below.

a) The biggest disadvantage of steel reinforcement is rusting. If concrete is porous or if cover to the reinforcement is not sufficient, steel gets rusted and loses strength.

b) Steel loses its strength at high temperatures.

 

Classification of steel bars

 

In India, following types of steel bars are available:

1. Mild steel plain bar

a. Cold worked mild steel bars

b. Hot rolled mild steel bars

 

2. High Yield Strength Deformed steel

a) Cold worked deformed bars

i. GradeFe415

ii. Grade Fe500

c) Hot rolled deformed bars

 

3. Hard drawn steel wire fabric—Wire mesh

4. Rolled steel members—Angles, T-sections, Joists, Channels etc.

5. Thermo-mechanically treated bars (TMT) & Corrosion resistant steel (CRS) bars (Deformed  bars Latest in use).

Normally, Fe250, Fe415 and Fe500 are used in reinforced concrete work. Mild steel (Fe 250) is more ductile and is preferred for structures in earthquake zones or where there are possibilities of vibration, impact, blast, etc.

 

1. Mild steel bars are also known as Fe 250 because the yield strength of this steel is 250 N/mm2.

2. High Yield Strength Deformed Bars

These are also known as HYSD bars. They have higher percentage of carbon as compared to mild steel.

A twisted deformed bar has about 50% higher yield stress than plain bars. HYSD bars are preferred as reinforcement in R.C.C. over plain mild steel bars, due to following reasons.

a.  HYSD bars have yield strength, higher than that of plain mild steel bars.

b. The HYSD bars have better bond with concrete due to corrugations or ribs on the surface Of the bars.

 

3. Steel Wire Fabric

This mesh is used as reinforcement in Slabs, shells, pavements and roads, etc.

 

4. Structural Steel

Sometimes for very heavily loaded elements such as foundations and columns rolled sections like rolled Steel joists, channels Or angles are embedded in concrete and used as reinforcement.

 

5. CRS and TMT Rebars

The oxide layer formed on the surface of the rebar, especially due to phosphorus, is found to be relatively dense (with minimal pores) and acts as an inhibitor to corrosion.

 

The stage-wise heat treatment process results in a combination of a Strong outer layer (of tempered martensite) and a ductile inner core (of ferrite-pearlite), giving the bar a combination of strength and ductility. TMT bars are now being manufactured to yield very high of elongation in addition to high strength, use of Fe500 bars discouraged in earthquake-resistant design and detailing. However, in recognition Of the changes achieved IS 1786 has introduced recently new grades, called Fe 415D, Fe 5001), and Fe 550D, with percentage elongations of 18.0, 16.0, 14.5 respectively, and with minimum ultimate tensile strengths of 500, 565 and 600MPa respectively. These changes have to be now incorporated in the design code, IS 456.

 

Functions Of reinforcement in R.C.C

 

The following are the functions of reinforcement in R.C.C

i. TO resist the bending moment in flexural members like slabs, beams, etc.

ii. TO resist diagonal tension due to shear.

iii. To increase the load carrying capacity of compression members like columns.

iv. To resist the effects of secondary stresses like temperature, etc.

v. To reduce the shrinkage of concrete.

vi. To resist spiral cracking due to torsion.

vii. To prevent the development of wide cracks in concrete due to tensile strains.