IS 456:2000 · Table 2, Table 5, Clause 6.2.2 — Grades, exposure, and elastic properties of concrete
Concrete grades used in India with their characteristic compressive strength (fck on 150 mm cubes at 28 days), modulus of elasticity (Ec = 5000√fck), flexural strength (fcr = 0.7√fck), maximum water-cement ratio, and typical applications. Grades M10–M20 are ordinary concrete, M25–M55 are standard concrete for design-mix RCC, and M60–M80 are high-strength concrete requiring special mix design and quality control per IS 456:2000.
• fck is characteristic compressive strength at 28 days on 150 mm cubes — multiply by 0.8 to get equivalent cylinder strength (f'c) used in ACI/Eurocode
• Modulus of elasticity Ec = 5000√fck MPa (IS 456 Clause 6.2.3.1) — this is the short-term static modulus; actual Ec can vary ±20% depending on aggregate type
• Flexural strength (modulus of rupture) fcr = 0.7√fck MPa — used for cracking moment calculations and pavement design
• Target mean strength for mix design = fck + 1.65 × standard deviation (IS 456 Clause 9.2.4.2) — for M25, target is typically 31.6 MPa
• Concrete gains ~67% of 28-day strength at 7 days and ~99% at 90 days; IS 456 allows age factor of 1.2 at 3 months for design (non-seismic)
• For water-retaining structures (IS 3370), minimum grade is M30 with max w/c ratio 0.45
• Never specify M15 or M10 for RCC members — IS 456 mandates M20 as the minimum for reinforced concrete
• High-strength grades (M60+) exhibit brittle failure — always design with confinement reinforcement per IS 13920 for ductility
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