Characteristic 28-day compressive strength of concrete grade C30: definition of the characteristic cubic compressive strength fcu,k = 30 MPa (150 mm cube, 28 d, 95% guarantee rate) and its distinction from the design value fck

Subject Positioning

This card answers the frequent engineering question "what is the characteristic 28-day compressive strength of C30 (or any grade) concrete", clarifying the statutory definition basis of concrete strength grades — the characteristic cubic compressive strength fcu,k — and distinguishing it from the characteristic axial compressive strength fck adopted in structural design, so that the two kinds of "characteristic strength" are not mixed up.

Core Conclusions

1. By strength-grade definition, the characteristic 28-day compressive strength of C30 concrete is the characteristic cubic compressive strength fcu,k = 30 MPa (30 N/mm²).
2. Strength grades are classified by characteristic cubic compressive strength (MPa) into C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75, C80, C85, C90, C95, C100 (GB 50164-2011 Clause 3.2.2).
3. Definition of the characteristic cubic compressive strength (original wording of GB/T 50107-2010 Clause 3.0.2): "the characteristic cubic compressive strength shall be a value in the overall distribution of concrete compressive strength, measured at 28 d age by standard test methods on cubic specimens of 150 mm edge length produced and cured by standard methods, with the probability of the strength falling below this value being 5%"; GB 50164-2011 Clause 3.2.2 expresses it as "the compressive strength value with a 95% guarantee rate (in MPa)". Three key conditions: 150 mm standard cube specimen, 28 d age, 95% guarantee rate.
4. Basis distinction (easily confused point): fcu,k = 30 MPa is the "grade defining value / cube-specimen basis"; the characteristic axial compressive strength fck used in structural design is on a prism basis with lower values — for C30, fck = 20.1 MPa and the characteristic axial tensile strength ftk = 2.01 MPa (GB 50010-2010 Section 4.1, Table 4.1.3-1; converted from fcu,k by 0.88×αc1×αc2). In design, 30 MPa must not be used directly as fck.
5. Professional code citation chain: material indicators (fck, design axial compressive strength fcd, ftk, design axial tensile strength ftd, modulus of elasticity Ec) in various specialized codes are taken directly from GB 50010-2010 Section 4.1, e.g. Table 3.2.2 of GB 50923-2013 Technical Code for Concrete-Filled Steel Tube Arch Bridges; that code further specifies the concrete shear modulus Gc as 40% of Ec and Poisson's ratio μc as 0.2.

Applicability Boundaries

• The 30 MPa value is the "grade defining value / 95% guarantee-rate quantile of the overall distribution", not the measured value of any specific specimen; acceptance evaluation of measured strength (statistical / non-statistical methods) follows the evaluation rules of GB/T 50107-2010.
• The defining conditions are limited to standard production and curing, 150 mm cube specimens, 28 d age, and standard test methods; when specimen size, age, or curing conditions deviate from the standard, the grade defining value cannot be applied directly.
• Values of fck, ftk and other design indicators are governed by GB 50010-2010 Table 4.1.3-1 as the authoritative source; the citation basis in specialized codes follows their own tables (e.g. GB 50923-2013 Table 3.2.2).

Further Reading

• GB/T 50107-2010 Standard for Evaluation of Concrete Compressive Strength, Clause 3.0.2 (definition of characteristic cubic compressive strength) and the subsequent evaluation chapters
• GB 50164-2011 Standard for Quality Control of Concrete, Clause 3.2.2 (strength grade classification, definition of the 95% guarantee rate)
• GB 50010-2010 Code for Design of Concrete Structures, Section 4.1 (characteristic and design strength values, modulus of elasticity; Table 4.1.3-1)
• GB 50923-2013 Technical Code for Concrete-Filled Steel Tube Arch Bridges, Clause 3.2.2 and Table 3.2.2 (values of fck/fcd/ftk/ftd/Ec; Gc = 0.4Ec, μc = 0.2)