The compressive strength test is one of the most important quality-control tests for concrete. ASTM C39/C39M covers the determination of compressive strength of molded concrete cylinders and drilled cores having a density greater than 800 kg/m³. The test involves applying a continuously increasing axial compressive load until failure and calculating the strength from the maximum load carried by the specimen divided by its cross-sectional area.
1. Testing Machine Requirements
The compression testing machine must have sufficient capacity and be capable of applying the specified loading rate continuously and without shock. The machine should be verified at least annually, with the interval not exceeding 13 months. Verification is also required after installation, relocation, significant repair or adjustment, or whenever load accuracy is in doubt. Within its verified loading range, the indicated load error must not exceed ±1.0%.
The machine uses hardened steel bearing blocks, including a spherically seated upper bearing block to ensure uniform load transfer to the specimen. Proper alignment and clean bearing surfaces are essential to avoid eccentric loading.
2. Specimen Checks Before Testing
Before testing, verify the following:
- No individual diameter should differ from another diameter of the same cylinder by more than 2%.
- The ends should not deviate from perpendicularity to the cylinder axis by more than 0.5°.
- The bearing surfaces should be plane within 0.050 mm. If not, the ends must be sawed, ground, or properly capped.
- Measure two diameters at right angles at approximately mid-height and use their average to calculate the cross-sectional area.
- The diameter should be determined to the nearest 0.25 mm.
3. Condition and Age of Specimen
Moist-cured specimens should be tested as soon as practicable after removal from moist storage and must be kept moist until testing.
The permissible testing-time tolerances include:
| Test Age | Permissible Tolerance |
| 24 hours | ±0.5 hour |
| 3 days | ±2 hours |
| 7 days | ±6 hours |
| 28 days | ±20 hours |
| 90 days | ±2 days |
4. Placing the Cylinder in the Testing Machine
Clean the bearing faces of both blocks and the specimen ends. Place the cylinder centrally on the lower bearing block and carefully align its axis with the center of thrust of the upper spherical bearing block.
Before loading, ensure that the load indicator reads zero. When the upper spherical block contacts the specimen, gently rotate its movable portion to obtain uniform seating.
5. Rate of Loading
This is a critical requirement.
The load must be applied continuously and without shock at a rate corresponding to a stress rate of:
0.25 ± 0.05 MPa/s
or
35 ± 7 psi/s
The specified loading rate should be maintained at least during the latter half of the anticipated loading phase. During the first half, a higher controlled loading rate is permitted, provided the specimen is not subjected to shock loading.
The loading rate should not be adjusted as the specimen approaches ultimate load and begins cracking.
6. Failure and Recording of Maximum Load
Continue loading until the specimen reaches its ultimate capacity and the load begins to decrease steadily with a well-defined fracture pattern.
Record:
- Maximum load carried by the specimen
- Type of fracture
- Any unusual fracture pattern
- Visible defects such as large air voids or segregation, particularly when the measured strength is lower than expected
Typical fracture patterns are illustrated in the figure shown below.
7. Calculation of Compressive Strength
The compressive strength is calculated as:
Compressive Strength = Maximum Load / Average Cross-Sectional Area
The result is reported to the nearest 0.1 MPa.
For specimens having a length-to-diameter ratio of 1.75 or less, a correction factor must be applied.
| L/D Ratio | Correction Factor |
| 1.75 | 0.98 |
| 1.50 | 0.96 |
| 1.25 | 0.93 |
| 1.00 | 0.87 |
For intermediate L/D ratios, interpolation is used.
Use the following tool to interpolate correction factor for L/D ratio other than the value stated in the above table.
8. What Should Be Included in the Test Report?
A complete test report should include:
- Specimen identification number
- Diameter and, where applicable, length
- Cross-sectional area
- Maximum load
- Compressive strength
- Type of fracture, if unusual
- Defects in the specimen or caps
- Age of the specimen
- Density, when determined
Conclusion
Specimen geometry, end preparation, moisture condition, alignment, machine calibration, and especially the loading rate can significantly influence the reported compressive strength. For reliable and comparable results, every stage of the test must follow the specified procedure.
