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CONCRETE CURING & POUR PLANNING · CONCRETE COMPRESSIVE STRENGTH CALCULATOR

Compressive Strength of Concrete at 28 Days

At 28 days, M25 concrete should average about 33.0 MPa on 150 mm cubes, which is the characteristic strength plus 8 MPa (EN 1992-1-1). That is a crushing load of about 743 kN. Individual cubes vary, so acceptance is judged on groups of results.

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kN
Concrete
MPa
Compression test33.0 MPa at 28 daysCube between platensmyconstructioncalculator.comNot to scale
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Results

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Compressive strength
33.0MPa
In psi4,789 psi
Expected mean at 28 days33.0 MPa
Result ÷ expected100 %
Result ÷ specified strength132 %

Cube strength is higher than cylinder strength: compare with a cube grade (IS 456 M-grades, or the second number of EN C25/30).

Acceptance uses groups of results, not one specimen: see IS 456 Table 11 or ACI 318 §26.12.3.

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Working
  1. Loaded area of a 150 mm cube = 22,500 mm² (34.88 in²)
  2. Strength = 743.0 kN ÷ area = 33.0 MPa
  3. Expected mean = (specified + 8 MPa) × exp{0.25 × [1 − √(28 ÷ 28)]} = 33.0 × 1.000 = 33.0 MPa (EN 1992-1-1 Eq. 3.2, standard 20 °C curing)
Results are estimates for planning and material ordering only. They are not a substitute for structural design or approval by a qualified engineer. Read the full disclaimer.
Formula


Expected mean at 28 days = (fck + 8 MPa) × exp{s × [1 − √(28 ÷ 28)]}
s = 0.20 rapid, 0.25 normal, 0.38 slow cement
Cube load = strength × 22,500 mm² (150 mm cube)


Worked Example


M25 concrete, normal cement, 28 days:


    

  1. Mean 28-day strength = 25 + 8 = 33 MPa
    2. 

  2. Ratio = exp{0.25 × (1 − √(1))} = 1.000
    3. 

  3. Expected mean = 33 × 1.000 = 33.0 MPa, a load of about 743 kN on a 150 mm cube
    4. 



Expected Strength at 28 Days by Grade


Mean cube strength, standard curing at 20 °C. Load is for a 150 mm cube with normal cement.






Grade
Rapid cement
Normal cement
Slow cement
Cube load (normal)






M20
28.0 MPa
28.0 MPa
28.0 MPa
630 kN




M25
33.0 MPa
33.0 MPa
33.0 MPa
743 kN




M30
38.0 MPa
38.0 MPa
38.0 MPa
855 kN




M35
43.0 MPa
43.0 MPa
43.0 MPa
968 kN




M40
48.0 MPa
48.0 MPa
48.0 MPa
1,080 kN






More Concrete Strength Tests
Frequently Asked Questions
What is the compressive strength of M20 concrete at 28 days?
About 28.0 MPa on average with normal cement (mean strength 28 MPa × 1.000). With rapid cement expect about 28.0 MPa, and with slow cement about 28.0 MPa.
Why is 28 days used for concrete strength?
By 28 days most cements have gained most of their strength, so codes specify strength at that age. Concrete keeps gaining slowly after 28 days, especially with fly ash or slag cements.
Is a 28-day cube below the grade a failure?
Not on its own. Acceptance uses groups of results: under ACI 318 the average of three consecutive tests must reach f'c and no test may be more than 500 psi below it; IS 456 Table 11 sets the Indian rules. A low result calls for investigation.
References
  • EN 1992-1-1 Eq. 3.2 (strength development) and Table 3.1 (fcm = fck + 8 MPa)
  • IS 516 (cube tests), ASTM C39 (cylinder tests); acceptance: IS 456:2000 Table 11, ACI 318-19 §26.12.3