ASTM C39 Compressive Strength Test of Cylindrical Concrete Specimens

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ASTM C39 / C39M : Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens

ASTM C39 determines the compressive strength of cylindrical concrete specimens such as molded cylinders and drilled cores. It is limited to concrete having a unit weight in excess of 50 lb/ft3 (800 kg/m3).

The test principle is straightforward:

a cylindrical concrete specimen is subjected to a continuously increasing axial compressive load until failure occurs. The maximum load sustained by the specimen is recorded and used to calculate the compressive strength in units of pressure (psi or MPa).

Compressive Strength = Maximum Load / Cross-sectional Area

The test aims to measure the inherent potential strength of the concrete mixture under ideal, controlled loading conditions. It is a quality control and acceptance test, not a direct measure of the in-place strength of a structural element.

Test Specimen Information of ASTM C39:

Parameter	Requirements
Typical Sizes	100 mm × 200 mm (4 in × 8 in), 150 mm × 300 mm (6 in × 12 in); aspect ratio (height/diameter) = 2.0; drilled cores must have diameter ≥75 mm and aspect ratio 1.75–2.5 (adjustments per ASTM C42/C42M)
Number of Test Specimens	3 but typically several specimens per set of cylinders.
End Preparation	Ends must be plane within 0.050 mm; options: sawing, grinding, or capping (sulfur capping per ASTM C167/C167M, neoprene pad capping per ASTM C1231/C1231M)
Sampling & Curing	Molded specimens per ASTM C31/C31M (field) or C192/C192M (lab); cured at 23.0 ± 2.0°C (73.4 ± 3.6°F) in saturated lime water or moist room until test age (7, 28, 56 days common)
Measurements	Diameter measured at 3 locations (top, middle, bottom); average for cross-sectional area calculation

ASTM C39 Compressive Strength Test of Cylindrical Concrete Specimens

Test Equipment associated with ASTM C39 concrete test:

Compression Testing Machine	A hydraulic or screw-type machine of sufficient capacity. It must comply with ASTM E4 (Practices for Force Verification of Testing Machines).
	Spherical Bearing Block (Upper Head): A freely moving, self-aligning head to ensure uniform load distribution without introducing bending stresses.
	Lower Bearing Plate: A rigid, non-tilting block.
	Load Indicator: A calibrated device (often digital) capable of indicating the total applied load.
Bearing Blocks (Platens)	Hardened steel blocks between the machine heads and the specimen. Supplementary bearing blocks like neoprene pad caps (per ASTM C1231) , sulfur mortar caps (per ASTM C617) are used for end preparation.
Verification Instruments	Calibrated load rings or proving rings for verifying the accuracy of the testing machine.

Why The Compressive Strength Is Important in Concrete Engineering?

Compressive strength is the most fundamental and widely used performance metric in concrete engineering, serving as the backbone for structural design, quality control, and material performance validation—this primacy stems from concrete’s inherent material behavior (it is ~10–20x stronger in compression than tension) and its dominant role as a load-bearing compressive material in nearly all concrete structures.

The Fundamental Structural Principle: Concrete Carries Compression;

Material Nature: Concrete is intrinsically strong in compression but weak in tension (about 1/10th of its compressive strength). Modern structural design leverages this by using steel reinforcement to handle tensile and shear forces, while the concrete is designed to carry the compressive loads. The entire philosophy of reinforced concrete is built upon the reliable compressive strength of the concrete matrix.

Primary Function: In structures, concrete is used in columns, walls, piers, damns, and the compression zones of beams and slabs. The load-bearing capacity of these elements is directly proportional to the concrete's compressive strength.

The Basis for Structural Design & Safety (The "f'c" Specification):

The Governing Number: The specified compressive strength (f'c) is the keystone parameter in all structural calculations.

Safety Factor: The entire structural safety margin is built upon a reliable, known f'c. Using concrete with a lower strength than specified compromises the built-in safety factors, potentially leading to excessive deflection, cracking, or even catastrophic failure.

Economic & Practical Optimization:

Material Efficiency: Specifying the correct f'c allows for efficient, cost-effective design. Over-specifying (e.g., using 6,000 psi where 4,000 psi suffices) unnecessarily raises material costs (more cement, admixtures). Under-specifying leads to oversized, clumsy members that waste space and materials.

Enables Innovation: High-strength concrete (e.g., 10,000+ psi) allows for the construction of slender columns in skyscrapers, longer bridges with fewer piers, and more architecturally daring structures.

Test Procedure (Step-by-Step details):

Conditioning: Remove the specimen from moist storage just prior to testing. Do not allow it to dry.

End Preparation: If necessary, cap the specimen using an approved method (e.g., neoprene pad system).

Measurement: Measure the diameter of the specimen to the nearest 0.01 in. (0.25 mm) at two locations, and the length if required. Calculate the average cross-sectional area.

Positioning: Center the specimen on the lower bearing block of the compression testing machine. Ensure the self-aligning spherical head is properly seated and free to rotate.

Loading: Apply the load continuously and without shock.

Loading Rate: The load must be applied at a rate within the range of 20 to 50 psi/sec (0.15 to 0.35 MPa/sec). Modern machines are programmed to control this rate.

For a 6x12 inch cylinder, this equates to a load rate of approximately 35 to 85 kips/second.

Failure: Continue applying load until the specimen fails (no increase in load can be sustained). Record the maximum load indicated by the machine.

Observations: Note the type of failure and any unusual features. A well-executed test typically results in a conical or shear fracture.

Calculation: Calculate the Compressive strength = Max Load / Cross-sectional Area. Report to the nearest 10 psi (0.1 MPa).

Related standard with the ASTM C39 for concrete compression testing:

BS EN 12390: Testing hardened concrete-Compressive strength of test specimens

AS 1012.9: Methods of testing concrete, Method 9: Compressive strength tests - Concrete, mortar and grout specimens

ASTM C513: Standard Test Method for Obtaining and Testing Specimens of Hardened Lightweight Insulating Concrete for Compressive Strength

UNE-EN 12390-6:Testing hardened concrete - Part 6: Tensile splitting strength of test specimens

ISO 4012: Concrete — Determination of compressive strength of test specimens

KS F 2405: Method of test for compressive strength of concrete

JIS A 1108: Method of test for compressive strength of concrete

JIS A 1113: Method of test for splitting tensile strength of concrete

IS 516: Methods of Tests for Strength of Concrete:

ACI 363.2R-11: Guide to Quality Control and Assurance of High-Strength Concrete

Concrete fracture reference:

Main Test Application:

Quality Control: Verifying the consistency of concrete production at a plant.

Acceptance Testing: Determining if concrete meets the specified strength requirements (fc') for a project. This is its primary legal/contractual use.

Estimating Strength in Structure: Testing cores drilled from a hardened structure (per ASTM C42/C42M) to investigate in-place strength.

Mixture Proportioning: Used in laboratory trials (per ASTM C192/C192M) to develop concrete mixtures.

UnitedTest Hydraulic Compression Strength Testing Machine to test cement, brick and concrete is offered by United Test. Use hydraulic power driving, intelligent measuring and control meter to collect and process data, consist of frame, oil source, control system, test fixture. Accuracy class 1, max. test load 2000KN, 3000KN.

Q1: Why is the compressive strength test (ASTM C39) so important in construction?

A: It is the primary quality control and acceptance test for structural concrete. The result (f'c) is the key number specified by structural engineers in design calculations. It verifies that the concrete placed in the structure has the necessary strength to carry the intended loads safely, ensuring the building's or bridge's structural integrity. It’s the concrete’s official "report card."

Q2: What is the significance of the "28-day strength"?

A: 28 days is a standardized, practical testing age chosen because normal concrete (with general-use cement) gains most of its potential strength by this time. It provides a common benchmark for all parties (engineers, contractors, suppliers) to judge compliance with the design specification (f'c). Concrete continues to gain strength slowly for years, but 28-day results are used for official acceptance.

Q3: How do I properly prepare the ends of a cylinder for testing?

A: According to ASTM C39, cylinder ends must be flat, parallel, and perpendicular to the axis. For cylinders that are not perfectly smooth (like molded cylinders), you must "cap" them. The two most common methods are:

Sulfur Mortar Capping (ASTM C617): A traditional method using molten sulfur compound.

Unbonded Neoprene Pad Caps (ASTM C1231): The modern, faster, and safer preferred method, which uses pre-tested rubber pads held in a steel retainer. This is now standard practice for most field testing.

Q4: What is a common mistake that leads to an incorrectly low strength reading?

A: The single most common field error is improper capping or bearing. If the ends are not parallel or the specimen is not centered correctly in the machine, it creates uneven stress (eccentric loading), causing premature failure and a falsely low strength reading. Other major errors are letting the specimen dry out before testing and applying the load at the wrong rate (too fast or too slow).

Q5: My cylinder failed at a strength below the specified f'c. Does this mean the structure is unsafe?

A: Not necessarily for a single cylinder. The building code (like ACI 318) has specific acceptance criteria based on the average strength of sets of cylinders, not a single result. A certain number of tests are allowed to be slightly below f'c as long as the rolling average is sufficiently above it. If strength is consistently low, engineers will investigate further, often by testing cores drilled from the structure (ASTM C42) to determine the actual in-place strength.

Q6: What is ASTM C39/C39M, and what is its core purpose?

A: ASTM C39/C39M is the global standard test method for determining the compressive strength of cylindrical concrete specimens (molded lab/field cylinders, drilled cores) with a density >800 kg/m³. Its core purpose is to measure the maximum axial compressive load a concrete specimen can withstand before failure, calculate compressive strength (fc′), and provide a validated metric for concrete quality control, structural design compliance, and project acceptance.

Q7: Can ASTM C39 be used for drilled concrete cores, or is a different standard required?

A: ASTM C39 is the test method for compressive strength testing of drilled cores, but core specimens require pre-test preparation and strength correction per ASTM C42/C42M (Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete).

Key ASTM C42 adjustments for cores tested via ASTM C39:

Core diameter must be ≥75 mm (3 in) and at least 3x the maximum aggregate size.

Aspect ratio must be 1.75–2.5 (correction factors apply for ratios outside 2:1).

Core ends must be prepared to ASTM C39 flatness/parallelism standards.

Strength corrections for damage during coring, voids, or non-vertical core alignment.

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