ASTM C39 Compressive Strength of Cylindrical Concrete Specimens

Scope:

The compressive strength of cylindrical concrete specimens is determined using the ASTM C39 test method. However, these results should be interpreted with caution because compressive strength is not an inherent attribute of concrete and is affected by a variety of factors such as specimen qualities, batching, mixing, sampling, molding, curing conditions, and fabrication procedures.

Read more: ASTM C170/C170M Test for Compressive Strength of Dimension Stone

Test Procedure: 

Apparatus:

Testing Machine

• The testing machine utilized for this test should have enough capacity and capability to provide the necessary loading rates.
• The regular validation of the testing machine’s correctness, including calibration checks, must be carried out.
• Verification is to be required within certain intervals, such as after repairs or revisions, or when there is reason to suspect inaccuracy.

Design of the Machine:

• The machine must be powered, and the load must be continually applied without shock.
• An additional mechanism for loading at a rate appropriate for verification should be in place.
• There should be enough room for test specimens and calibrating instruments to be accommodated.

Accuracy:

• A percentage error greater than 1.0% of the indicated load should not be allowed, and the accuracy of the testing machine is crucial.
• In verification, test loads are applied in an ascending sequence, and the suggested load is compared to the applied load using a calibration device.
• The standards should be specified for the loading range within which the machine meets during the verification procedure.
• Values within the acceptable allowable variation should not be corrected for the indicated load of the testing equipment.

Bearing Blocks

• Steel upper and lower bearing blocks with hardened bearing faces are a requirement.
• Bearing faces should be dimensioned larger than the specimen’s nominal diameter.
• The upper bearing block should be spherically seated, with the design of the spherical section intended to prevent persistent deformation under stress.

Lubrication and maintenance of the bearing blocks must be performed.

Spacers:

• If used, solid steel spacers should be employed, and they should fully support the lower bearing block and any spacers above.
• Direct contact with the specimen or unbonded caps must not be made by spacers.

Load Indication:

• A load indicator, either dial or digital, with specific reading and accuracy criteria should be featured by the testing machine.
• The graduated scale of the dial should be viewable to at least 0.1% of the full-scale load.
• If the load is digital, the numerical increment should not be greater than 0.1% of the full-scale load.

Documentation:

• It is considered critical that the calibration and maintenance of the testing machine be documented according to the criteria outlined in Practice C1077.

Procedure:

• As soon as practical after being removed from moist storage, compression tests on specimens that have been moist-cured should be performed. By doing this, it is ensured that the samples are evaluated appropriately. Unless otherwise specified by the test specifier, the test age for this method begins when casting specimens.
• Placing the Specimen: The lower bearing block should be placed on the table or platen of the testing apparatus with the hardened face facing up.
  The bearing faces on the specimen, spacers (if applicable), and the upper and lower bearing blocks should be wiped down.

The specimen should be cleaned and centered by unbonded caps that are being utilized.

The lower bearing block should have the specimen set on it and be positioned so that its axis is parallel to the center of thrust of the higher bearing block

• Block seating and zero verification: Ensure that the load indicator is set to zero before testing the specimen.

By hand, the bearing face of the test specimen should be made parallel to the top of the movable part of the spherically seated block by gently tilting it.

• Alignment must be confirmed when utilizing unbonded caps: If unbonded caps are employed, the specimen’s alignment should be checked after applying stress but before reaching 10% of the projected specimen strength.

It should be verified that the ends of the cylinder are centered within the retaining rings and that a deviation from vertical by more than 0.5° is not allowed.

If these criteria are not fulfilled by the alignment, the load should be released, the specimen should be recentered, and then reapplied.

• Rate of Loading: The load should be continuously applied without shock.

A stress rate of 0.25 ± 0.05 MPa/s [35 ± 7 psi/s] on the specimen must be achieved by supplying the load at a rate of motion.

During the first half of the projected loading phase, a higher rate of loading is allowed, but it must be administered carefully to prevent shock loading.

As the ultimate load approaches and the stress rate slows down as a result of specimen cracking, the rate of movement should not be changed.

• Recording Outcomes: Compressive stress should be applied until a clearly defined fracture pattern is exhibited by the specimen and a continuous reduction is shown on the load indicator.

The load should not be turned off on machines with a specimen break detector until it has fallen to less than 95% of the peak load.

Compression should be applied to the specimen until it is determined that the maximum capacity has been reached when testing with unbonded caps.

During the test, the maximum load carried by the specimen and the kind of fracture pattern that was observed should be noted.

• Fracture Pattern Analysis: If the fracture pattern differs from the usual patterns, a concise description of it should be provided

The broken concrete should be examined, and notes should be taken of elements such as the existence of sizable air voids, signs of segregation, and the direction of fractures through the coarse aggregate particles if the measured strength is lower than anticipated.

The end preparations should be checked to see if they were done in compliance with the applicable ASTM practices (C617/C617M or C1231/C1231M).

Calculation:

To calculate the specimen’s compressive strength:

fcm= 4000Pmax/D2

where:

fcm = compressive strength, MPa [psi],

Pmax= maximum load, kN [lbf], and

D = average measured diameter in millimeters (in.).

To calculate specimen density

s= (4109W)/(LD2)

where:

s= specimen density, kg/m3 [lb ⁄ft3],

W  = specimen’s mass in air, kg [lb],

 L = average measured length, in millimeters (in.), and

 D = average measured diameter, in millimeters (in.).

If the specimen density is dependent on submerged weighing, the specimen density should be calculated as follows.

s= (Ww)/(W-Ws)

where:

s= specimen density, kg/m3[lb ⁄ft3]

W= specimen’s mass in air, kg [lb],

Ws= apparent mass of the submerged specimen, kg [lb]

w= density of water at 23°C [73.5°F]

Video 01: Compressive Strength of Concrete Cylinder ASTM C39/AASHTO T22 Part-3

Preferred Test Sample Size: 

Number of test Specimens: 1 but typically several specimens per set of cylinders.

Test Specimen Size:  4″ x 8″ or 6″ x 12″ cylinders

Keywords: Compressive strength, Concrete

Conclusion:

The ASTM C39 compressive strength test determines the compressive strength of concrete specimens, assessing their suitability for structural application. It checks for diameter tolerance and perpendicularity. Loading samples to failure reveals the axial load resistance and structural appropriateness of the concrete.