Why Does Concrete Need Reinforcement ?
Why we provide Reinforcement ? It is more important for all aspects. In this, we talk about why it is important and why we choose steel as reinforcement.
Read also Why Top Down construction Require.
The purpose of providing reinforcement in R.C.C. is :
- To take up all the tensile stresses develop in the structure.
- Increase the strength of concrete sections.
- To make the sections thinner as compare to plain concrete section.
- To prevent the propagation of cracks develop due to temperature and shrinkage stresses.
Although steel is the most suitable reinforcing material yet rusting of steel is the biggest disadvantage. If possible is porous or if cover to the reinforcement is not sufficient, steel gets rust and loses strength.
Characteristics of a Reinforcing Material
The reinforcing material should have the following characteristics :
- It should be cheap and easily available.
- Also, have more durable.
- It should be able to develop a good bond with concrete so that the stresses are easily transfer from one – material to another.
- Should have high tensile strength.
- It should have high modulus of elasticity.
- It should have co-efficient of linear expansion nearly same as that of concrete so the temperature stresses are not develop.
- Easy to cut bend or weld.
- It should be free from loose mill scales, loose rust and coats of paints, oil, mud or other coatings which may destroy or reduce bond.
SUITABILITY OF STEEL AS A REINFORCING MATERIAL
Steel fulfills almost all the characteristics require for a reinforcing material. Hence it is the most suitable reinforcing material. Steel is use as a reinforcing material because of the following reasons :
- It has high tensile strength.
- Also, have highly elastic.
- It can develop good bond with concrete as its co-efficient of expansion is nearly equal to that of concrete ( i.e., 11.7 * 10 /°C of steel and 9.9 x 10 /°C of concrete).
- It is easily available in India.
FORMS OF STEEL REINFORCEMENT
Steel can be provided in the form of:
- Mild steel plain bars conforming to IS: 432 (Part 1) – 1982
- (6) High strength deformed steel bars conforming to IS: 1786 – 1985
- (c) Hard-drawn steel wire fabric conforming to IS: 1566 – 1982
- (d) Structural steel conforming to Grade A of IS: 2062 – 1992 .
Mild Steel Plain Bars.
Mild steel is the most ductile amongst all kinds of steel. The mild steel bars are plain round and hot roll bars. These bars have plain surface and have sufficient bond with concrete. Also, These bars need hooks and bends at their ends for anchorages. These bars are bent easily and are weldable. Mild steel has a definite yield point. Plain mild steel bars (Fe 250 grade) are less commonly used in reinforced concrete, because they have less strength (250 N/mm2 yield strength) and cost approximately the same as high strength deformed bars. However, they are use in practice in situations where nominal reinforcement is to be provide.
The mild steel bars can yield on overloading. It results in an excessive cracking in concrete, thus giving sufficient warning before failure. Due to higher ductility, mild steel bars can be economically used in blast and earthquake resistant structures.
High Yield Strength Deformed Bars (HYSD Bars).
These bars are obtain by subjecting the mild steel plain bars to cold working by tensioning and twisting. The cold working process increases both the yield as well as the ultimate strength of steel. As the name indicates the high yield strength deformed bars have much high yield point, but the yield point is not define. The yield stress or characteristic stress is given by 0.2% proof stress.
These bars have some type of projections (or deformations or lugs or ribs) on the surface which act as a key for the concrete and check the slipping of bars. The lugs generally have an average spacing not exceeding 0.7 dia.(where dia. – Nominal diameter of bar). The height of lugs is at least 0.04 dia. to 0.05 dia.
The lugs must be present around at least 75% of nominal bar perimeter. The deformation are place so that the angle to the axis of the bar is not less than 45 degree. Generally, longitudinal ribs are also present on the surface of the bar.
HYSD Bars of Different Grades
The different grades of HYSD bars are :
- Fe 415 grade,
- And, Fe 500 grade
- Fe 550 grade
Fe refers to ferrous metal and the number following it refers to yield strength of steel in N/mm2.
Advantages of HYSD Bars.
- Due to high yield strength, the amount of steel require is considerably reduce.
- The twist or deform bars provide a better bond with concrete than mild steel bars. Due to increase bond strength. Deform bars do not need end hooks and require reduce length for overlaps etc. It results in further saving of steel.
- During the twisting process the defects, if any, in mild steel are notice and they are reject. Thus, each bar is severely test for defects.
Disadvantages of HYSD bars
- With increase in stress level the cracking tendency increase. Hence for Fe 500 or higher grade of steel it becomes necessary to use high grade concrete.
- Due to reduction in tension steel the design shear strength of concrete is reduce.