University of New Mexico

Civil Engineering Department

Civil Engineering Materials Laboratory, CE 305L

Compressive Strength of Cylindrical Concrete Specimens

ASTM C39

Scope

This ASTM test method covers the determination of the unconfined compressive strength of cylindrical concrete specimens. The test method consists of applying a compressive axial load to molded cylinders (or cores) at a rate which is within a prescribed range until failure occurs. The compressive strength of the specimen is calculated by dividing the maximum load attained during the test by the cross-sectional area of the specimen.

References

ASTM C39 Compressive Strength of Cylindrical Concrete Specimens

ASTM C617 Capping Cylindrical Concrete Specimens

ASTM C192 Making and Curing Concrete Test Specimens in the Laboratory

Capping Cylindrical Concrete Specimens (ASTM C617)

Apparatus

Capping Plate (mold)

Alignment Device (guide bars)

Melting Pot

Fume Hood (exhaust fan)

Materials

Moist-cured concrete cylinders (with no moisture on the surface)

Sulfur mortar (5000 psi strength at 2 hours)

Procedure

Download Capping and Unconfined Compression Procedures

Download the Unconfined Compression Excel Form

1) Prepare sulfur mortar for use by heating to about 265°F (130°C). Fresh sulfur mortar must be dry at the time of placement in the melting pot (dampness will cause foaming). Note: The flash point of sulfur mortar is approximately 440°F (225°C).

2) Oil the capping plate lightly.

3) Stir the molten sulfur mortar immediately prior to pouring each cap.

4) Dry the ends of the moist-cured specimens to preclude the formation of steam and foam pockets in the caps.

5) Pour the molten sulfur mortar into the capping plate (mold). Lower the specimen, using the alignment device guide bars, ensuring that the axis of the specimen is perpendicular to the plate.

6) The molded end caps on the specimen should have a minimum thickness of 1/8 ” (3 mm) but less than 5/16” (8 mm).

7) After the sulfur mortar has set, remove the specimen from the mold plate using a slight twisting motion.

8) Repeat this process, capping both ends of the specimen.

9) Maintain the specimen in a moist condition between the completion of capping and the time of testing.

Compressive Strength of Cylindrical Concrete Specimens (ASTM C39)

Apparatus

Compression Test Machine

Materials

Capped cylindrical concrete specimens

Procedure

1) Maintain the specimen in a moist condition up to the time of compression testing. Compression tests are made as soon as practicable after removal from moist storage. The specimens are tested in this cured moist condition.

2) Wipe clean the bearing surfaces of the upper and lower platens of the compression testing machine. Also, wipe clean both end caps of the test specimen.

3) Center the specimen on the lower platen of the testing machine.

4) Carefully align the axis of the specimen with the center of thrust of the spherically seated upper platen.

5) Bring the upper platen to bear on the specimen, adjusting the load to obtain uniform seating of the specimen.

6) Apply the load at a loading rate of 20 to 50 psi/s (140 to 350 lb/s for 3” diameter cylinders, 250 to 630 lb/s for 4” diameter cylinders, 560 to 1400 lb/s for 6” diameter cylinders). The time to failure for 3000 psi concrete is 1 to 2.5 minutes.

7) Apply the load at the prescribed loading rate until the specimen fails. Record the maximum load (lb). Note the type of failure and the appearance of the concrete (see Figure 1).

Figure 1. Types of Concrete Fracture.

Click here to view a Quicktime movie of an unconfined compression test

Report

1) Report the size (diameter and length) and the age of the specimen.

2) Record the maximum load to the nearest 10 lb.

3) Report the type of failure and appearance of the concrete.

4) Calculate the unconfined compressive strength of the specimen by dividing the maximum load by the cross-sectional area of the specimen. Report this strength to the nearest 10 psi.

Assignment

1) Measure the unconfined compressive strength of previously prepared specimens at 7, 14, and 28 days. Perform these tests per the defined test schedule.

2) Plot the results for all data generated by the class (both sections if applicable), showing unconfined compression strength (y-axis) versus time in days (x-axis). Calculate average strength values from the above data and plot.

3. For purposes of design of concrete structures, the 28 day strength is typically used. What is f_cr' based on the acquired test results?

Liquid sulfer placed in mold

Capping the Cylinder

Capping Results

Capped cylinder placed in the UTM

Sample Data from CE 305 Fall 2002

Unconfined compression test of 3500 psi concrete.

Sample result of shear failure