What Is ASTM C39 Test for Compressive Strength of Concrete?
Concrete is strong in compression. That sounds obvious, but knowing how strong a particular batch of concrete actually is requires more than simply putting a specimen into a testing machine and recording the maximum load.
In construction projects, concrete compressive strength is one of the most important indicators used to evaluate whether concrete has achieved the required performance. It is used for quality control, mix verification, specification compliance, and the evaluation of concrete production and placement.
One of the most widely recognized methods for this purpose is ASTM C39/C39M, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens.
The current edition is ASTM C39/C39M-26. The standard covers the determination of compressive strength of cylindrical concrete specimens, including molded cylinders and drilled concrete cores. It applies to concrete with a density greater than 800 kg/m³ (50 lb/ft³).
1. What Is the ASTM C39 Test?
ASTM C39 is a standardized test method used to determine the compressive strength of cylindrical concrete specimens.
In simple terms, a concrete cylinder is placed between the compression platens of a calibrated testing machine. The machine applies an increasing axial load until the specimen fails. The maximum load reached before failure is then used to calculate the compressive strength.
Basic relationship:
Compressive Strength = Maximum Load / Loaded Area
fᶜ = P / A
Where:
• fᶜ = compressive strength of concrete
• P = maximum load applied to the specimen
• A = cross-sectional area of the specimen
For a circular cylinder:
A = πD² / 4
where D is the average measured diameter.
The calculation is straightforward. In practice, however, the quality of the result depends heavily on specimen preparation, curing, end condition, alignment, loading rate, machine accuracy, and operator technique.
2. What Does ASTM C39 Actually Measure?
ASTM C39 measures the compressive strength of cylindrical concrete specimens at the time of testing.
It does not directly measure every aspect of concrete performance. It does not, by itself, determine tensile strength, flexural strength, permeability, abrasion resistance, chloride resistance, freeze-thaw resistance, or long-term durability.
What it provides is a standardized measurement of the maximum compressive stress that the tested concrete cylinder or core can withstand under the specified test conditions.
The measured value can be affected by specimen size and shape, batching and mixing, sampling, molding, fabrication, age, temperature, and moisture conditions during curing.
Good compressive strength testing therefore starts long before the specimen reaches the compression machine.
3. Why Is ASTM C39 So Important in Concrete Testing?
For most concrete laboratories, the compressive strength test is one of the routine tests performed on hardened concrete.
Quality Control
Ready-mix concrete producers and construction laboratories use compressive strength results to monitor consistency between batches and identify changes in production.
Specification Compliance
Concrete is often supplied according to a specified compressive strength. Testing according to ASTM C39 provides a standardized method for comparing measured strength with project requirements.
Mix Design Verification
During concrete mix development, compressive strength results help engineers determine whether a proposed mixture can achieve the required performance.
Research and Development
Universities, research laboratories, concrete producers, and material manufacturers use C39 testing when developing new concrete mixtures.
4. What Kind of Specimens Are Used?
ASTM C39 is specifically intended for cylindrical concrete specimens. These may include molded concrete cylinders and drilled concrete cores.
Common cylinder dimensions used in concrete laboratories include:
• 100 × 200 mm
• 150 × 300 mm
• Other dimensions permitted by the applicable project and standard requirements
The laboratory needs to ensure that specimen dimensions, preparation, aspect ratio, end condition, and testing procedure are compatible with the applicable requirements.
5. How Is the ASTM C39 Compressive Strength Test Performed?
Step 1: Prepare the Concrete Specimen
The concrete cylinder is molded, compacted, identified, and cured according to the applicable specimen-making and curing procedure.
Step 2: Check the Specimen
Before testing, the operator checks the specimen condition and dimensions. The diameter is important because the cross-sectional area is used in the strength calculation.
Step 3: Prepare the Specimen Ends
The two ends of a concrete cylinder need to provide a suitable bearing surface. Depending on the applicable procedure, end preparation may involve grinding, bonded caps, or unbonded caps.
Step 4: Position the Cylinder in the Compression Testing Machine
The specimen is placed between the upper and lower bearing blocks. Correct alignment is essential to avoid eccentric loading.
Step 5: Apply the Load
The compression machine applies load continuously until the specimen fails. ASTM C39 specifies a stress rate of approximately 0.25 ± 0.05 MPa/s for the applicable portion of loading.
Step 6: Record the Maximum Load
The machine records the maximum load reached by the specimen. The result is then used to calculate compressive strength.
6. How Is Concrete Compressive Strength Calculated?
The calculation is based on maximum load divided by loaded cross-sectional area.
fᶜ = P / A
For a cylindrical specimen:
A = πD² / 4
Therefore:
fᶜ = 4P / πD²
Example:
Cylinder diameter = 100 mm
Maximum load = 800 kN
Cross-sectional area:
A = π × 100² / 4 ≈ 7,854 mm²
Maximum load:
P = 800,000 N
Compressive strength:
fᶜ = 800,000 / 7,854 ≈ 101.9 MPa
7. Why Do Concrete Compressive Strength Results Vary?
Some variation is normal. Important sources include:
• Specimen preparation and compaction
• Curing conditions
• End preparation
• Specimen alignment
• Loading rate
• Testing machine condition and verification
• Material variability
A low result does not automatically mean the concrete mix is defective. The laboratory should also review sampling, molding, curing, dimensions, end preparation, machine verification, loading rate, and failure characteristics.
8. What Equipment Is Required for ASTM C39 Testing?
Concrete Compression Testing Machine
This is the main piece of equipment. Depending on workload and concrete strength range, machines may be available in capacities such as 1000 kN, 2000 kN, 3000 kN, 5000 kN, or higher.
Bearing Blocks and Platens
These transfer the machine load to the specimen. Their condition affects load distribution and repeatability.
Load Measurement System
The load cell or equivalent load-indicating system provides the force measurement used in the final calculation.
Measuring Tools
A laboratory may also need vernier calipers, balance, specimen identification tools, curing records, and data recording software.
9. How to Choose an ASTM C39 Compression Testing Machine?
Check Maximum Expected Load
Estimate maximum concrete strength and specimen area before selecting machine capacity.
Consider Loading Control
A servo-hydraulic system with automatic loading control can help maintain a consistent loading rate.
Consider Machine Stiffness
High-strength concrete can fail suddenly. Machine rigidity and specimen protection should therefore be considered.
Check Calibration and Verification
Ask about calibration, verification, load measurement accuracy, certificates, and service intervals.
Think About Laboratory Workflow
High-volume laboratories may benefit from automatic control and data recording, while smaller laboratories may prefer a simpler digital system.
10. ASTM C39 vs Other Concrete Strength Tests
ASTM C39/C39M - Compressive strength of cylindrical concrete specimens
ASTM C78/C78M - Flexural strength of concrete
ASTM C496/C496M - Splitting tensile strength of cylindrical concrete
ASTM C617/C617M - Capping cylindrical concrete specimens
ASTM C1231/C1231M - Unbonded caps for concrete cylinders
ASTM C31/C31M - Making and curing concrete specimens in the field
ASTM C192/C192M - Making and curing concrete specimens in the laboratory
11. Common Mistakes in ASTM C39 Testing
Mistake 1: Testing an improperly prepared cylinder
Mistake 2: Ignoring end conditions
Mistake 3: Poor alignment
Mistake 4: Incorrect loading rate
Mistake 5: Using an unverified machine
Mistake 6: Choosing machine capacity by guesswork
Mistake 7: Treating every low result as a concrete mix problem
12. ASTM C39 Testing for High-Strength Concrete
As concrete strength increases, failure can become more sudden and energetic. For high-strength and ultra-high-strength concrete, laboratories should consider sufficient machine capacity, machine stiffness, reliable load measurement, stable hydraulic control, appropriate bearing assemblies, specimen protection, and operator safety.
ASTM is also maintaining and revising the C39 series, so laboratories should confirm the edition specified by the project rather than assuming that an older procedure is automatically acceptable.
13. ASTM C39 and Concrete Quality Control
ASTM C39 is normally part of a larger quality-control system.
Fresh concrete:
Sampling → Slump → Temperature → Air content → Density
Specimen preparation:
Molding → Compaction → Identification → Curing
Hardened concrete:
Compressive strength → Other mechanical or durability tests
Quality evaluation:
Test result → Specification comparison → Trend analysis → Corrective action
The objective is not simply to obtain a compressive strength number. The objective is to generate repeatable, traceable, technically defensible data.
14. Practical Checklist Before Performing an ASTM C39 Test
Specimen
• Correct identification
• Correct test age
• No unacceptable damage
• Dimensions acceptable
• Ends properly prepared
Machine
• Suitable capacity
• Load measurement system verified
• Platens and bearing blocks clean and undamaged
• Loading conditions controllable
Test Setup
• Specimen centered
• Loading axis aligned
• Safety protection in place
• Data acquisition working
Test
• Continuous loading
• Required loading rate maintained
• Maximum load recorded
• Failure mode documented
Report
• Dimensions recorded
• Strength calculated correctly
• Unusual observations noted
• Test traceable to equipment and operator
15. Final Thoughts
ASTM C39 looks like a straightforward compression test: put a concrete cylinder into a machine, apply load, record the peak value, and calculate the strength.
In a professional laboratory, it is more complicated than that.
The reliability of the final result depends on the entire testing chain-from sampling and specimen preparation to curing, end preparation, machine verification, loading control, measurement, and reporting.
That is why selecting an ASTM C39 compression testing machine should not be based on machine capacity alone.
A good system should provide:
adequate capacity + accurate load measurement + stable loading + proper specimen alignment + reliable data recording + calibration support.
When these elements are properly controlled, ASTM C39 becomes a reliable tool for understanding concrete quality and making engineering decisions based on measurable evidence.
For laboratories, ready-mix concrete producers, construction companies, universities, and material-testing organizations, a properly configured compression testing system is an important part of the concrete quality-control process.
The machine produces the load. The laboratory produces the result. The quality of the procedure determines whether that result can be trusted.
FAQ
Q1:What is ASTM C39 used for?
ASTM C39/C39M is used to determine the compressive strength of cylindrical concrete specimens, including molded cylinders and drilled cores.
Q2:What is the ASTM standard for concrete cylinder compression testing?
The relevant standard is ASTM C39/C39M – Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. The current ASTM listing identifies C39/C39M-26 as the active edition.
Q3:What machine is used for ASTM C39?
An appropriately sized concrete compression testing machine or universal testing machine with suitable compression fixtures can be used. The machine must have sufficient capacity, controlled loading capability, and verified accuracy.
Q4:What is the ASTM C39 loading rate?
The commonly specified stress rate is 0.25 ± 0.05 MPa/s (35 ± 7 psi/s) for the applicable loading portion of the test.
Q5:How is concrete compressive strength calculated?
The basic calculation is fᶜ = P/A, where P is maximum load and A is the cross-sectional area.
Q6:Why can two concrete cylinders from the same batch have different strengths?
Normal variation can occur because of sampling, molding, compaction, curing, specimen dimensions, end preparation, alignment, loading, and material variability.