What is the minimum grade of concrete for RCC as per IS 456?

M20 (fck = 20 MPa). This is specified in Clause 6.1.1 and Table 5. For prestressed concrete, minimum is M30.

What does 'M' in M20 stand for?

'M' refers to Mix. The numeral (20) is the 28-day characteristic compressive strength in MPa on 150 mm cubes.

What is the modulus of elasticity for M25 concrete?

Ec = 5000√25 = 25,000 MPa (as per Clause 6.2.3.1). This is the short-term static modulus. For long-term, divide by (1 + creep coefficient).

IS 456:2000 — Plain and Reinforced Concrete — Code of Practice

IS 456:2000 — Clause 6.1

Grades of Concrete

Clause 6.1 classifies concrete by its 28-day characteristic compressive strength (fck) tested on 150 mm cubes. Grades range from M10 (ordinary) to M80 (high-strength). The grade designation 'M' refers to mix and the numeral to the specified characteristic compressive strength in MPa.

Key Requirements

•Minimum grade for reinforced concrete is M20
•Minimum grade for prestressed concrete is M30
•Minimum grade for post-tensioned concrete with tendon ducts ≥ 50 mm is M35
•For aggressive exposure (severe/very severe/extreme), minimum grade per Table 5 overrides
•Characteristic strength is the value below which not more than 5% of test results are expected to fall

Reference Tables

Table 2 — Grades of Concrete (Clause 6.1)

Group	Grade	fck (MPa)
Ordinary	M10	10
Ordinary	M15	15
Ordinary	M20	20
Standard	M25	25
Standard	M30	30
Standard	M35	35
Standard	M40	40
Standard	M45	45
Standard	M50	50
High Strength	M55	55
High Strength	M60	60
High Strength	M65	65
High Strength	M70	70
High Strength	M75	75
High Strength	M80	80

Concrete Properties by Grade (IS 456 Clause 6.2)

Grade	fck (MPa)	Ec (MPa)	fcr (MPa)
M20	20	22361	3.13
M25	25	25000	3.5
M30	30	27386	3.83
M35	35	29580	4.14
M40	40	31623	4.43
M45	45	33541	4.7
M50	50	35355	4.95
M55	55	37081	5.19
M60	60	38730	5.42

Ec = 5000√fck (short-term static modulus). fcr = 0.7√fck (flexural tensile strength).

Formulas

Ec = 5000 √fck

Short-term static modulus of elasticity

Ec = Modulus of elasticity in MPafck = Characteristic compressive strength in MPa

fcr = 0.7 √fck

Flexural tensile strength (modulus of rupture)

fcr = Flexural tensile strength in MPafck = Characteristic compressive strength in MPa

Practical Notes

✓M20 is the de facto standard for residential slabs and beams in India — almost all site RCC unless structural design demands otherwise.

✓M25 is increasingly preferred for multi-storey buildings (G+4 and above), especially in moderate/severe exposure zones.

✓The Ec formula (5000√fck) gives a theoretical value; actual site Ec can be 20-30% lower due to aggregate quality, curing, and age. IS 456 Amendment 4 (2013) acknowledges this.

Common Mistakes

⚠Using M15 for RCC work — IS 456 mandates minimum M20 for all reinforced concrete.

⚠Confusing cube strength (fck) with cylinder strength (fc') — IS 456 uses 150 mm cubes. Cylinder strength ≈ 0.8 × cube strength.

⚠Ignoring exposure-based minimum grade — Table 5 may require M30 or higher even if structural design only needs M20.

Frequently Asked Questions

Related Resources

Cl. 8.2 Cl. 38.1 Cl. 39.3 Concrete Grades & Properties Concrete Grade Selection Guide Mix Design Concrete Grades M20, M25, M30 Concrete Grade Conversion: M20/M25/M30 to ACI and Eurocode Equivalents

View all 15 clauses of IS 456:2000 →