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Concrete and Steel

It is essential that the designer has to acquire a fair knowledge of the materials to be used in the design of reinforced concrete structure.

Plain concrete is prepared by mixing cement, sand (also known as fine aggregate), gravel (also known as coarse aggregate) and water with specific proportions. Mineral admixtures may also be added to improve certain properties of concrete. Thus, the properties of concrete regarding its strength and deformations depend on the individual properties of cement, sand, gravel, water and admixtures. Clauses 5 and 6 of IS 456:2000 stipulate the standards and requirements of the individual material and concrete, respectively. Plain concrete after preparation and placement needs curing to attain strength. However, plain concrete is very good in compression but weak in tension. That is why steel is used as reinforcing material to make the composite sustainable in tension also. Plain concrete, thus when reinforced with steel bars in appropriate locations is known as reinforced concrete.

The strength and deformation characteristics of concrete thus depend on the grade and type of cement, aggregates, admixtures, environmental conditions and curing. The increase of strength with its age during curing is considered to be marginal after 28 days. Blended cements (like fly ash cement) have slower rate of strength gain than ordinary Portland cement as recognized by code.

Depending on several factors during its preparation, placement and curing, concrete has a wide range of compressive strength and the material is graded on the basis of its compressive strength on 28th day also known as "characteristic strength".

Steel is used as the reinforcing material in concrete to make it good in tension. Steel as such is good in tension as well as in compression. Unlike concrete, steel reinforcement rods are produced in steel plants. Moreover, the reinforcing bars or rods are commercially available in some specific diameters. Normally, steel bars up to 12 mm in diameter are designated as bars which can be coiled for transportation. Bars more than 12 mm in diameter are termed as rods and they are transported in standard lengths.

Steel has several types or grades. The four types of steel used in concrete structures as specified in cl. 5.6 of IS 456 are given below:

Cl.5.6 of IS 456: Reinforcement

  • Mild steel and medium tensile steel bars conforming to IS 432 (Part 1)
  • High yield strength deformed (HYSD) steel bars conforming to IS 1786
  • Hard-drawn steel wire fabric conforming to IS 1566
  • Structural steel conforming to Grade A of IS 2062.

Mild steel bars had been progressively replaced by HYSD bars and subsequently TMT bars are promoted in our country. TMT bars are widely used in all projects.

5.6.1 All reinforcement shall be free from loose mill scales, loose rust and coats of paints, oil, mud or any other substances which may destroy or reduce bond. Sand blasting or other treatment is recommended to clean reinforcement.

5.6.2 Special precautions like coating of reinforcement may be required for reinforced concrete elements in exceptional cases and for rehabilitation of structures. Specialist literature may be referred to in such cases.

5.6.3 The modulus of elasticity of steel shall be taken as 200 kN/mm2. The characteristic yield strength of different steel shall be assumed as the minimum yield stress/O.2 percent proof stress specified in the relevant Indian Standard.

Stress-strain curves for reinforcement:

Figures 1 and 2 shows the representative stress-strain curves for steel having definite yield point and not having definite yield point, respectively. The characteristic yield strength fy of steel is assumed as the minimum yield stress or 0.2 per cent of proof stress for steel having no definite yield point. The modulus of elasticity of steel is taken to be 200000 N/mm2.

For mild steel (Fig. 1), the stress is proportional to the strain up to the yield point. Thereafter, post yield strain increases faster while the stress is assumed to remain at constant value of fy.

For cold-worked bars (Fig. 2), the stress is proportional to the strain up to a stress of 0.8 fy. Thereafter, the inelastic curve is defined as given below:

Stress Inelastic strain
0.80 fy Nil
0.85 fy
0.90 fy
0.0001
0.0003
0.95 fy
0.975 fy
0.0007
0.0010
1.00 fy 0.0020

Linear interpolation is to be done for intermediate values. The two grades of cold-worked bars used as steel reinforcement are Fe 415 and Fe 500 with the values of fy as 415 N/mm2 and 500 N/mm2, respectively.

Considering the material safety factor ym of steel as 1.15, the design yield stress (fyd) of both mild steel and cold worked bars is computed from  fyd = fy /ym

Accordingly, the representative stress-strain curve for the design is obtained by substituting fyd for fy in Figs. 1 and 2 for the two types of steel with or without the definite yield point, respectively.

Acknowledgements: The content is sourced with due acknowledgement from the following references for knowledge dissemination.

  • Indian Standard Plain and Reinforced Concrete: Code of Practice (4th Revision), IS 456: 2000, BIS, New Delhi.
  • Lesson 2, Properties of Concrete and Steel, Version 2 CE IIT, Kharagpur