Concrete Strength Acceptance Calculator

How to work it out

1. Count the test groups n, which decides between the statistical and non-statistical method. Ten or more groups use the statistical method; fewer than ten use the non-statistical method. This is the first fork in the assessment.
2. The statistical method needs the mean and the standard deviation. The standard deviation reflects scatter, and a low value indicates a well-controlled plant.
3. Select coefficients by n: 1.15 and 0.90 for 10-14 groups; 1.05 and 0.85 for 15-24; 0.95 and 0.85 for 25 and above. More groups allow a lower mean requirement because the statistical confidence is higher.
4. The statistical method has two simultaneous conditions: mean ≥ fcu,k + λ1 × S, and minimum ≥ λ2 × fcu,k. Both must hold; failing either fails the batch.
5. The non-statistical method also has two: mean ≥ 1.15 × fcu,k (1.10 for C60 and above), and minimum ≥ 0.95 × fcu,k.
6. Record any failure properly: commission an accredited laboratory for cores or rebound corrected by cores, or have the designer check structural adequacy. Do not declare compliance yourself.

Where it goes wrong

• Using the statistical method with too few groups. Below ten, the standard deviation carries no statistical meaning. Conversely, using the non-statistical method with ten or more groups applies an unnecessarily strict criterion and fails batches that comply.
• Checking only one of the two conditions. An adequate mean with an inadequate minimum, or the reverse, still fails. This is the most common misjudgement.
• Computing the wrong standard deviation. The code requires the sample standard deviation (dividing by n − 1); the population form (dividing by n) gives a smaller value and a lenient verdict.
• Confusing 28-day with 7-day strength. Seven-day strength is typically 70-80% of 28-day and may only be used for early indication, never for assessment.
• Keeping 1.15 for C60 and above. High-strength concrete has its own reliability requirement, set at 1.10; using 1.15 misjudges the result.

Material and cost

Making, curing and testing one group of three cubes costs 30-80 CNY. Remedial action is expensive: core testing 300-800 CNY per core, and structural strengthening (bonded steel, carbon fibre, section enlargement) several hundred to over a thousand per square metre. Compliant sampling and curing discipline matters far more than saving on cubes.

On site

• Sample randomly at the point of placing, never select favourable cubes. Standard curing is 20±2°C at 95% relative humidity or above; cubes left lying on site do not represent the structure.
• Cubes cured alongside the structure indicate striking and stressing times; they are not valid for strength assessment. Different purposes entirely.
• A large standard deviation, typically over 4.5 MPa, signals poor plant control. Even with a compliant mean, the minimum is likely to fail.
• Batches are formed from the same grade, same mix and same production run. Never assess different grades together.

Basis

• GB/T 50107-2010 Standard for evaluation of concrete compressive strength
• GB 50204-2015 Code for acceptance of construction quality of concrete structures, concrete works
• GB/T 50081-2019 Standard for test methods of physical and mechanical properties of concrete