fck (Characteristic Strength)

fck is the characteristic compressive strength of concrete — the value of compressive strength below which not more than 5% of test results are expected to fall. For 'M25' concrete, fck = 25 N/mm² (MPa). It is the foundational design parameter in IS 456:2000 limit-state design. Determined by the 28-day cube test on 150 mm cubes per IS 516, with statistical acceptance per IS 456 Cl. 16: the mean of 4 consecutive samples must be ≥ fck + 4 MPa AND no individual sample below fck − 4 MPa.

For design, IS 456 Cl. 38.1 uses 0.67 fck divided by partial safety factor γc = 1.5, giving design compressive strength of 0.45 fck for concrete in flexure. So M25 concrete is designed at 0.45 × 25 = 11.25 MPa maximum compressive stress at ULS. The stress-strain curve idealised in IS 456 Annex B is parabolic up to strain 0.002 then constant up to ultimate strain 0.0035. Modulus of elasticity Ec = 5000√fck MPa (Cl. 6.2.3.1) — for M25, Ec = 25,000 MPa, used in deflection and axial-shortening calculations.

The IS 456 grade designation system runs from M5 (very low strength, blinding only) through M80 (high-strength concrete for tall buildings). Grade selection is a function of structural demand AND exposure: IS 456 Table 5 mandates minimum grade by exposure (M20 mild, M25 moderate, M30 severe, M35 very severe, M40 extreme). For pre-stressed concrete per IS 1343, minimum grades are M30 (post-tensioned) and M40 (pre-tensioned). The 'M' prefix stands for 'Mix' (not 'Maximum' or 'Mortar'); 'C' is the equivalent symbol in Eurocode 2. So M25 ≈ C20/25 in Eurocode notation (cylinder strength 20, cube strength 25).