ASTM C39 Standard Explained: Concrete Compressive Strength Testing Guide
ASTM C39/C39M is one of the most widely used standards for determining the **compressive strength of cylindrical concrete specimens**. The test is fundamental to concrete quality control, structural evaluation, construction projects, and material testing laboratories.
The method determines compressive strength by applying a controlled axial compressive load to a prepared concrete cylinder until failure. The maximum load carried by the specimen is then used together with its cross-sectional area to calculate compressive strength.
Accurate ASTM C39 testing depends not only on the concrete specimen itself but also on correct specimen preparation, alignment, loading rate, testing machine performance, and calibration.
This guide explains the ASTM C39 test procedure, specimen requirements, compressive strength calculation, testing equipment, common sources of error, and the main differences between ASTM C39 and EN concrete compression testing methods
What Is ASTM C39/C39M?
ASTM C39/C39M is the **Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens**.
It is used to determine the compressive strength of prepared concrete cylinders and concrete cores by applying an axial compressive force until the specimen fails.
ASTM C39 test results are widely used for:
- Concrete quality control
- Evaluation of concrete mixture performance
- Verification of specified compressive strength
- Construction quality assurance
- Research and development
- Comparison of concrete materials and mixtures
ASTM C39 should be used together with the applicable standards governing specimen preparation, curing, capping or end preparation, and testing equipment.
What Does the ASTM C39 Test Measure?
The ASTM C39 compression test measures the **compressive strength of a cylindrical concrete specimen**.
Compressive strength represents the maximum compressive stress sustained by the specimen during the test.
The basic relationship is:
- Compressive Strength = Maximum Load / Cross-Sectional Area
or:
**fc = P / A**
Where:
- **fc** = compressive strength
- **P** = maximum load carried by the specimen
- **A** = calculated cross-sectional area of the specimen
Results are commonly reported in **MPa** or **psi**, depending on the measurement system used.
Why Is ASTM C39 Important in Concrete Testing?
Compressive strength is one of the most important properties used to evaluate hardened concrete.
ASTM C39 provides a standardized testing approach so laboratories and engineers can obtain consistent and comparable compressive strength data when the required procedures are followed correctly.
ASTM C39 results can help engineers:
- Verify specified concrete strength
- Monitor concrete production quality
- Evaluate changes in mixture performance
- Support quality-control decisions
- Identify potentially abnormal test results
- Evaluate concrete used in construction and infrastructure projects
However, compressive strength results can be affected by specimen preparation, curing, geometry, end conditions, alignment, loading procedure, and testing machine performance.
ASTM C39 Specimen Size and Requirements
ASTM C39 is primarily associated with cylindrical concrete specimens.
Common cylinder sizes include:
- **150 mm × 300 mm (6 × 12 in)**
- **100 mm × 200 mm (4 × 8 in)**
The cylinder generally has a height-to-diameter ratio of approximately **2:1** for standard molded specimens.
Specimen dimensions must be measured appropriately because the calculated cross-sectional area directly affects the reported compressive strength.
Before testing, specimens should also be inspected for visible damage, dimensional irregularities, and unsuitable bearing surfaces.
Equipment Required for ASTM C39 Testing
Reliable ASTM C39 testing requires a suitable compression testing system and correctly prepared specimens.
The concrete compression testing machine applies an axial compressive load to the specimen until failure.
The system should provide suitable capacity, accurate force measurement, appropriate bearing surfaces, proper alignment, and controlled loading suitable for the required test procedure.
For laboratories performing frequent concrete testing, automatic load control and digital data acquisition can improve repeatability and reduce operator-dependent variation.
The specimen is positioned between the bearing surfaces of the compression testing machine.
Proper platen geometry, surface condition, alignment, and spherical seating are important because uneven load transfer can influence specimen failure and the measured result.
Suitable measurement devices are required to determine specimen dimensions used in calculating the cross-sectional area.
Concrete cylinder moulds and controlled curing equipment are used during specimen preparation before the compression test.
The applicable preparation and curing procedures should be followed because differences in curing conditions can significantly affect measured concrete strength.
ASTM C39 Test Procedure: Step by Step
The exact test must always be performed according to the applicable current standard and laboratory procedures. At a high level, an ASTM C39 compression test follows these stages.
Before testing, inspect the specimen for visible damage or abnormalities that could influence the result.
Measure the required specimen dimensions and determine the cross-sectional area used for the compressive strength calculation.
Ensure that the specimen ends and machine bearing surfaces meet the applicable requirements.
Improper end conditions can create non-uniform stress distribution.
Place the cylinder centrally on the lower bearing block.
Correct centering is critical because eccentric loading can produce uneven stress distribution and unreliable results.
Apply the load continuously and without shock according to the loading requirements of the applicable ASTM C39 procedure.
A testing machine with controlled loading can help maintain a stable test process.
Continue the test until the specimen reaches its maximum load and the required failure condition is obtained.
Record the maximum load carried by the concrete cylinder.
Calculate compressive strength from the maximum load and specimen cross-sectional area.
The specimen's fracture behavior should also be observed and recorded as required.
Unusual failure patterns can indicate issues such as specimen defects, alignment problems, or unsuitable end conditions.
ASTM C39 Compressive Strength Calculation
For a cylindrical specimen:
- fc = P / A
For a circular cross section:
- A = πD² / 4
Therefore:
- fc = 4P / πD²
Where:
- fc = compressive strength
- P = maximum applied load
- D = measured specimen diameter
- A = cross-sectional area
Example ASTM C39 Calculation
Assume a concrete cylinder has:
- Diameter = 150 mm
- Maximum load = 800 kN
Cross-sectional area:
- A = π × 150² / 4 ≈ 17,671 mm²
Therefore:
- fc = 800,000 / 17,671 ≈ 45.3 MPa
The calculated compressive strength is approximately
- 45.3 MPa.
The actual test result and reporting process must follow the requirements of the applicable standard.
What Can Affect ASTM C39 Test Results?
Several factors can influence the measured compressive strength.
An incorrectly centered cylinder can experience eccentric loading and non-uniform stress.
Uneven or unsuitable specimen ends can prevent uniform load transfer through the cylinder.
Applying load incorrectly or inconsistently can influence test results and repeatability.
A compression testing machine that is not properly verified or calibrated can produce unreliable force measurements.
Temperature, moisture conditions, curing duration, and specimen handling can influence concrete strength development.
Cracks, chips, or other damage before testing may affect the measured strength and failure behavior.
Common ASTM C39 Testing Errors and How to Avoid Them
Common Causes of Testing Errors and Recommended Actions
Many testing errors can be minimized by following proper laboratory procedures and maintaining consistent testing conditions.
| Potential Problem | Possible Effect | Recommended Action |
|---|---|---|
| Specimen not properly centered | Eccentric loading and uneven stress distribution | Carefully center the specimen before testing |
| Unsuitable bearing surfaces | Non-uniform stress distribution | Inspect specimen ends and bearing platens before testing |
| Incorrect loading control | Inconsistent or unreliable results | Apply the load using controlled loading procedures |
| Machine calibration issues | Inaccurate force measurements | Perform scheduled verification and calibration |
| Improper specimen curing | Altered or inconsistent strength results | Follow the specified curing procedures |
| Damaged specimen | Abnormal failure pattern or inaccurate results | Inspect the specimen for visible damage before testing |
Consistent specimen preparation, proper handling, and well-maintained testing equipment are essential for obtaining accurate, reliable, and repeatable concrete compression test results.
ASTM C39 vs EN 12390-3: What Is the Difference?
ASTM C39/C39M and EN 12390-3 are both used for determining concrete compressive strength, but they should not be treated as identical test methods.
ASTM C39/C39M focuses on the compressive strength testing of cylindrical concrete specimens and cores.
Standard molded cylinders are commonly associated with ASTM-based concrete testing.
EN 12390-3 specifies a method for determining the compressive strength of hardened concrete test specimens and is widely used with European concrete testing practices.
Cube specimens are commonly encountered in EN-based testing, although the complete requirements should always be determined from the applicable standard and project specification.
Not automatically.
Specimen geometry, dimensions, preparation, curing, loading conditions, and applicable testing procedures can influence the reported compressive strength.
Therefore, a cylinder result should not simply be assumed to equal a cube result.
For a detailed explanation, see our guide to Concrete Cube vs Cylinder Strength.
How to Choose a Compression Testing Machine for ASTM C39
When selecting an ASTM C39 concrete compression testing machine, laboratories should consider more than maximum capacity.
Important factors include:
- Required testing capacity
- Force measurement accuracy
- Frame rigidity and stability
- Specimen dimensions
- Bearing platen configuration
- Loading control
- Level of automation
- Data acquisition and reporting
- Calibration and verification requirements
- Applicable ASTM and other international standards
A properly selected machine helps laboratories perform controlled and repeatable concrete compression tests while improving workflow efficiency.
ASTM C39 Concrete Compression Testing Solutions from BESMAK
BESMAK manufactures concrete compression testing systems for laboratories, universities, research institutions, construction companies, and quality-control facilities.
Depending on laboratory requirements, available configurations include wall-type and four-column hydraulic compression testing systems as well as integrated compression and bending testing solutions.
BESMAK concrete compression systems can be configured for ASTM C39 and related international concrete testing requirements, with options including automatic loading control, digital data acquisition, and advanced control systems.
Frequently Asked Questions About ASTM C39
ASTM C39/C39M is a standard test method used to determine the compressive strength of cylindrical concrete specimens.
It measures the compressive strength of a concrete cylinder by determining the maximum compressive load the specimen can sustain and relating that load to its cross-sectional area.
Common molded concrete cylinder sizes include **150 × 300 mm (6 × 12 in)** and **100 × 200 mm (4 × 8 in)**.
The basic equation is **fc = P/A**, where P is the maximum load and A is the specimen's cross-sectional area.
A concrete compression testing machine with suitable capacity, force measurement, bearing surfaces, alignment, and loading control is used.
Both are used for concrete compressive strength testing, but they follow different standard frameworks and specimen/testing requirements. ASTM C39 is specifically associated with cylindrical concrete specimens, while EN 12390-3 is widely used within European concrete testing practice.
Potential causes include improper specimen preparation or curing, poor end conditions, incorrect centering, unsuitable loading, specimen damage, and testing machine measurement or calibration problems.
Conclusion
ASTM C39/C39M provides a standardized method for determining the **compressive strength of cylindrical concrete specimens**.
Reliable results depend on the entire testing process: correct specimen preparation and curing, accurate dimensional measurement, proper centering, suitable bearing surfaces, controlled loading, and a properly maintained compression testing machine.
For laboratories performing ASTM C39 concrete testing, selecting equipment with appropriate capacity, accuracy, control, and data acquisition capabilities can improve testing consistency and laboratory efficiency.