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Author:
Talip BIYIK
How to Read and Evaluate a Concrete Test Report
A concrete test report shows the compressive strength results obtained when concrete specimens taken on site are crushed in the laboratory at a given age. Contractors, site supervisors and ready-mix suppliers use it to decide whether the concrete placed meets the strength class specified in the project. Reading it correctly requires knowing the specimen age and shape, what the strength class means and which conformity criterion applies.
This article explains what a concrete test report contains, how compressive strength is calculated, how classes such as C25/30 are read and how results are evaluated against EN 206, with a worked example.
What does a concrete test report contain?
| Information | Why it matters |
|---|---|
| Report number, laboratory and approver | Identity and validity of the document; verification on QR-coded reports. |
| Structure, element, placement location and strength class | Shows which element and class the result belongs to. |
| Sampling date and test date | Shows the age of the specimen when crushed. |
| Specimen shape and size | 150 mm cube, 100 mm cube or 150×300 mm cylinder; determines the value to compare with. |
| Curing condition | Shows whether the specimen was stored under standard conditions. |
| Mass and density | A hint about specimen quality; large deviations deserve attention. |
| Failure load (kN) | The maximum load read by the press. |
| Compressive strength (MPa = N/mm²) | The core of the evaluation. |
| Failure mode | Whether the fracture pattern is satisfactory; if not, the result is questioned. |
How is compressive strength calculated?
Compressive strength is the failure load divided by the loaded area: fc = F / A. A 150 mm cube has an area of 150 × 150 = 22,500 mm². If the press reads 675 kN, the strength is 675,000 N / 22,500 mm² = 30.0 MPa.
What does C25/30 mean?
A strength class has two numbers. The first is the characteristic compressive strength of a 150×300 mm cylinder, the second that of a 150 mm cube, in MPa. For C25/30, 25 MPa is expected for cylinders and 30 MPa for cubes. So the first step is to check whether the specimen is a cube or a cylinder and compare with the matching value.
| Class | Cylinder fck (MPa) | Cube fck (MPa) |
|---|---|---|
| C20/25 | 20 | 25 |
| C25/30 | 25 | 30 |
| C30/37 | 30 | 37 |
| C35/45 | 35 | 45 |
| C40/50 | 40 | 50 |
If other specimen sizes are used (for example 100 mm cubes), the specification and the relevant standard should define how results relate to the standard specimen size.
7, 28 and 56-day results
- 7-day result: an early indicator. Because the rate of strength gain depends on cement type and conditions, it is not used on its own for conformity, but a very low value is an early warning.
- 28-day result: conformity is usually assessed on 28-day strength.
- 56-day result: used for concretes that gain strength later, such as those with additions or blended cements, where the project provides for it.
We cover tracking test dates in concrete sample tracking.
How are results evaluated? EN 206 conformity criteria
Rather than a single cube, the mean of a group of results (fcm) and each individual result (fci) are assessed together. Which criterion applies depends on the purpose: the producer's production control or identity testing of concrete delivered to site. The basic criteria in EN 206 are:
| Case | Criterion for the mean | Criterion for each result |
|---|---|---|
| Producer: initial production (3 results) | fcm ≥ fck + 4 | fci ≥ fck − 4 |
| Producer: continuous production (at least 15 results) | fcm ≥ fck + 1.48σ | fci ≥ fck − 4 |
| Identity testing: 2–4 results | fcm ≥ fck + 1 | fci ≥ fck − 4 |
| Identity testing: 5–6 results | fcm ≥ fck + 2 | fci ≥ fck − 4 |
The criterion to use in your project must follow the specification, the current edition of EN 206 and its national complementary standard, and local regulations. The table is for general information.
Worked example
Three 150 mm cubes from a C30/37 pour give 28-day results of 38.2 / 36.5 / 39.4 MPa. For cubes, fck = 37 MPa.
- Mean: (38.2 + 36.5 + 39.4) / 3 = 38.0 MPa
- Lowest result: 36.5 MPa ≥ 37 − 4 = 33 MPa → the individual criterion is met.
- Identity testing (3 results): 38.0 ≥ 37 + 1 = 38 → met.
- Initial production criterion: 38.0 ≥ 37 + 4 = 41 → not met.
The same results are interpreted differently depending on the criterion, so always check which criterion a "conforming" decision on a report is based on.
What if the result is low?
- Check the report details first: specimen age, size, curing and failure mode.
- Review sampling and storage on site; badly stored specimens do not reflect the real strength of the concrete.
- If 56-day specimens exist and the project allows it, wait for their results.
- If needed, cores are taken from the structure to investigate in-place strength. The decision is made together with the designer and site supervision.
For testing laboratories: producing reliable reports
A reliable report starts with the laboratory's records. In our Testing Laboratory Software, test dates are calculated automatically from the sampling date, calculations use the laboratory's own Excel templates, the preparer and approver sign separately with e-signatures and reports can be verified by QR code. Average strength is compared with the lower value of the class, 100 mm cube results are converted to the 150 mm cube equivalent and a monthly conformity chart is produced per concrete class. Customers receive their reports through a portal.
Frequently Asked Questions
What does MPa mean on a concrete test report?
It is the unit of compressive strength; 1 MPa equals 1 N/mm². It is the failure load divided by the loaded area of the specimen.
What cube result should C25/30 concrete reach?
The characteristic cube strength is 30 MPa. Conformity is assessed on the mean of a group of results and on each result according to defined criteria, not on a single result.
Is concrete non-conforming if the 7-day result is low?
No. The 7-day result is an early indicator; conformity is usually assessed at 28 days. A very low 7-day result should still be treated as an early warning.
How is a concrete test report evaluated?
Check the specimen age and shape, compare the result with the characteristic strength for cubes or cylinders and apply the EN 206 conformity criterion stated in the specification.
Why do cube and cylinder results differ?
Specimen shape and slenderness affect strength; the same concrete gives a higher strength on cubes than on cylinders. That is why a strength class has two values.
How can a concrete test report be verified?
On QR-coded reports, scanning the code shows whether the report was issued by the laboratory and whether it has been revised. If in doubt, contact the laboratory directly.