# Concrete Strength Acceptance Calculator

Switches between statistical and non-statistical criteria automatically.

- Page: https://jianzhujisuan.com/en/concrete-test-strength
- Category: Code Lookup (https://jianzhujisuan.com/en/c/code-lookup)
- Chinese version: https://jianzhujisuan.com/zh/concrete-test-strength.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Strength grade C30, Number of specimens 10 sets, Mean strength mfcu 35 MPa, Standard deviation Sfcu 1.5 MPa, Minimum fcu,min 33 MPa): **Acceptance result Accept**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Strength grade | select | C30 | — |  |
| Number of specimens | number | 10 | sets |  |
| Mean strength mfcu | number | 35 | MPa |  |
| Standard deviation Sfcu | number | 1.5 | MPa |  |
| Minimum fcu,min | number | 33 | MPa |  |

## Formula

- **Statistical · mean**: mfcu ≥ fcu,k + λ1·Sfcu
- **Statistical · minimum**: fcu,min ≥ λ2·fcu,k
- **λ1 / λ2**: 10-14: 1.15 / 0.90; 15-24: 1.05 / 0.85; ≥25: 0.95 / 0.85
- **Non-statistical**: mfcu ≥ 1.15·fcu,k (1.10 for ≥C60), fcu,min ≥ 0.95·fcu,k

## Worked example

C30, 10 specimens, mean 35.0, standard deviation 1.5, minimum 33.0: statistical method with λ1 = 1.15 and λ2 = 0.90. Mean must be at least 30 + 1.15×1.5 = 31.73 and is 35.0; minimum must be at least 27.0 and is 33.0 - accepted.

## Questions

### Why switch to a stricter method below 10 specimens?

A standard deviation from a handful of results is unreliable and usually understates scatter, which would let bad batches pass. The code therefore drops the deviation and compensates with a higher mean requirement (1.15×). It is also why important structures require at least one set per 100 m³ or per shift - sample size is itself evidence.

### Can a batch pass if the mean is fine but the minimum is not?

No - both conditions must hold. A low minimum points to a weak spot, perhaps poor curing or a faulty specimen. Test the actual structure (rebound hammer or cores) to find out which, then follow the GB 50204 procedure. Making new specimens is not an acceptable answer.

### Which data should the standard deviation come from?

All specimens in the same batch - never a selected subset. Where a grade has several batches, the code allows pooling three or more consecutive batches, but they must share grade, mix, production process and age. Cherry-picking is the most common form of manipulation and the first thing auditors check.

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