IS Standards Series · Part 2 of 9
Material Properties — Concrete, Rebar & Structural Steel
Characteristic and design strengths for concrete grades M20–M55 per IS 456:2000, reinforcement grades Fe415–Fe600 per IS 1786:2008, and structural steel per IS 800:2007. Includes elastic moduli, durability limits, and exposure class requirements.
1. Concrete Grades — IS 456:2000
IS 456 designates concrete grades by the letter M followed by the characteristic cube compressive strength at 28 days in MPa. The characteristic strength fck is the value below which not more than 5% of test results are expected to fall.
| Grade | fck (MPa, cube) | fcd = 0.446·fck (MPa) | Min. Use |
|---|
| M15 | 15 | 6.7 | Plain concrete only (not RC) |
| M20 | 20 | 8.9 | Minimum for RC (IS 456 Cl.6.1.3) |
| M25 | 25 | 11.2 | Moderate exposure; seismic zones III–V |
| M30 | 30 | 13.4 | Columns, footings in aggressive soils |
| M35 | 35 | 15.6 | Prestressed concrete |
| M40 | 40 | 17.8 | High-rise columns, bridges |
| M45 | 45 | 20.1 | High-performance structures |
| M50 | 50 | 22.3 | Special applications |
| M55 | 55 | 24.5 | Maximum in IS 456 (higher grades per IS 1343) |
Minimum grades per IS 13920:2016: For seismic zones III, IV, and V, IS 13920 Cl.5.2 requires a minimum concrete grade of M20 for ordinary moment frames and M25 for ductile (SMRF) structures.
2. Concrete Properties — IS 456
Modulus of Elasticity
Tensile Strength
Stress-Strain for Design
IS 456 uses a parabolic-rectangular stress block for section design. The key parameters are:
- Peak design stress: 0.446·fck (= 0.67·fck/γc)
- Maximum strain at extreme compression fibre: εcu = 0.0035
- Strain at peak stress: 0.002 (onset of plateau)
Key Properties Summary
| Property | M20 | M25 | M30 | M35 | M40 |
|---|
| fck (MPa, cube) | 20 | 25 | 30 | 35 | 40 |
| fcd = 0.446·fck (MPa) | 8.92 | 11.15 | 13.38 | 15.61 | 17.84 |
| Ec (GPa) | 22.4 | 25.0 | 27.4 | 29.6 | 31.6 |
| fcr (MPa) | 3.13 | 3.50 | 3.83 | 4.14 | 4.43 |
| Unit weight (kN/m³) | 24 | 24 | 24 | 24 | 24 |
3. Reinforcement — IS 1786:2008
IS 1786 specifies high strength deformed (HSD) steel bars for reinforced concrete. The grade designation (Fe) is followed by the minimum yield stress in MPa.
| Grade | fy (MPa) | fu (MPa) | Elongation (%) | Note |
|---|
| Fe415 | 415 | 485 | ≥14.5 | Most common in older construction |
| Fe500 | 500 | 545 | ≥12.0 | Standard grade — current practice |
| Fe500D | 500 | 565 | ≥16.0 | "D" = improved ductility; required by IS 13920 for seismic zones III–V |
| Fe550 | 550 | 585 | ≥10.0 | High-strength; limited seismic use |
| Fe550D | 550 | 600 | ≥14.5 | Ductile variant of Fe550 |
| Fe600 | 600 | 660 | ≥10.0 | Special applications |
Design Strength and Properties
| Grade | fy (MPa) | fyd = 0.87·fy (MPa) | Es (GPa) |
|---|
| Fe415 | 415 | 361 | 200 |
| Fe500 | 500 | 435 | 200 |
| Fe500D | 500 | 435 | 200 |
| Fe550 | 550 | 479 | 200 |
| Fe600 | 600 | 522 | 200 |
IS 13920:2016 requirement: For seismic zones III, IV, V and all high-ductility structures, only Fe415D, Fe500D, Fe550D, or Fe600 is permitted — ordinary Fe500/Fe550 is not allowed as the enhanced elongation requirement is critical for ductile failure modes.
Standard Bar Diameters
IS 1786 bars are available in the following nominal diameters: 6, 8, 10, 12, 16, 20, 25, 28, 32, 36, and 40 mm. Cross-sectional areas:
| Dia. (mm) | Area (mm²) | Weight (kg/m) |
|---|
| 8 | 50.3 | 0.395 |
| 10 | 78.5 | 0.617 |
| 12 | 113.1 | 0.888 |
| 16 | 201.1 | 1.579 |
| 20 | 314.2 | 2.467 |
| 25 | 490.9 | 3.854 |
| 32 | 804.2 | 6.313 |
4. Structural Steel — IS 800:2007
IS 800:2007 covers structural steel design using the limit state method. Steel material is specified by IS 2062 (hot-rolled sections) and IS 8500 (special grades).
| Grade | fy (MPa) | fu (MPa) | Application |
|---|
| IS 2062 E250 | 250 | 410 | General structural steel (mild steel) |
| IS 2062 E300 | 300 | 440 | Bridges, higher-strength applications |
| IS 2062 E350 | 350 | 490 | Standard grade — current practice |
| IS 2062 E410 | 410 | 540 | High-strength; columns, trusses |
| IS 2062 E450 | 450 | 570 | Special high-strength applications |
| IS 2062 E550 | 550 | 670 | Heavy plate, high-rise construction |
Partial Safety Factors for Steel — IS 800
| Failure Mode | Symbol | Value |
|---|
| Yielding of gross section | γm0 | 1.10 |
| Rupture of net section | γm1 | 1.25 |
| Bolts bearing, friction | γmb | 1.25 |
| Welds | γmw | 1.25 |
Key Steel Properties
| Property | Value |
|---|
| Elastic modulus E | 2×10⁵ MPa (200 GPa) |
| Shear modulus G | 0.769×10⁵ MPa (76.9 GPa) |
| Poisson's ratio ν | 0.30 |
| Coefficient of thermal expansion | 12×10⁻⁶ /°C |
| Unit weight | 78.5 kN/m³ |
5. Durability & Cover Requirements — IS 456
IS 456:2000 Table 5 specifies five exposure conditions that determine minimum concrete grade and cover:
| Exposure Class | Environment | Min. Grade | Max. w/c | Min. Cover (mm) |
|---|
| Mild | Protected against weather, non-aggressive soil | M20 | 0.55 | 20 |
| Moderate | Sheltered from rain, buried concrete, RC in seawater splash | M25 | 0.50 | 30 |
| Severe | Alternate wetting/drying, marine above splash, de-icing salts | M30 | 0.45 | 45 |
| Very Severe | Seawater, abrasive conditions, sulfate soils | M35 | 0.45 | 50 |
| Extreme | Tidal/splash zones, aggressive chemicals | M40 | 0.40 | 75 |
Nominal cover: IS 456 Cl.26.4 specifies nominal cover (to outermost reinforcement including links). For fire resistance, additional cover may be required per IS 456 Cl.26.4.3. Cover tolerance is +10 mm / –0 mm.
Preliminary design only. This article is for educational purposes. Always verify material specifications with the project specification and a licensed structural engineer.