Concrete Laboratory C-1231 January 2012

New Mexico Technician Training and Certification Program

Compressive Strength of Cylindrical Concrete Specimens Using Unbonded Caps

ASTM C-1231-00 (Modified)

This test method covers the apparatus, concrete test specimen, and the determination of compressive strength of cylindrical concrete specimens using unbonded caps of a defined hardness. Unbonded caps are not to be used for acceptance testing of concrete with compressive strength below 10 MPa (1,500 psi) or above 85 MPa (12,000 psi) (1.2).

Modifications: 1. Use talcum powder to dust pads prior to placing them into the retaining

rings. 2. The load shall be applied at a rate of 35 ± 7 psi/s.

Key Elements: 1. Obtain test specimen. Obtain specimen according to AASHTO T-23, Making and Curing

Concrete Test Specimens in the Field.

2. Inspect apparatus. Testing machine shall be of a type having sufficient capacity and capable of applying the load continuously and without shock. Verify that the upper bearing block moves freely. Calibrate when machine is originally installed or relocation of the machine. Recalibrate whenever there is reason to doubt the accuracy of the results (5.1.1.4). See Section 5 of AASHTO T-22, Compressive Strength of Cylindrical Concrete Specimens, for all requirements of testing machine. Neoprene pads shall be 12.5 mm ± 2 mm (0.5 in. ± 0.08 in.) thick and the diameter shall not be more than 2 mm (0.08 in.) smaller than the inside diameter of the retaining ring (5.2.1). Pads shall have a Shore A durometer hardness that meets Table 1. The tolerance on Shore A durometer hardness is ± 5 (5.2.2).

Pads shall be supplied with the following information: the manufacturer’s or supplier’s name; the Shore A hardness; the applicable range of concrete compressive strength from Table 1; the date pads were placed in service; and the number of uses to which they have been subjected (5.2.4 –5.2.5).

Retainer shall be made of metal that will prove durable in repeated use. The cavity in the metal retainers shall have a depth at least twice the thickness of the pad. The inside diameter of the retaining rings shall not be less than 102% or greater than 107% of the diameter of the cylinder. The surfaces of the metal retainer, which contact the bearing blocks of the testing machine, shall be plane to within 0.05 mm (0.002 in.). The bearing surfaces of the retainers shall not have gouges, grooves, or indentions greater than 0.25 mm (0.01 in.) deep or greater than 32 mm2 (0.05 in.2) in surface area (5.3).

Table 1 - Requirements for Use of Polychloroprene (Neoprene) Pads Shore A Cylinder Compressive Strength, Durometer Qualifications Maximum psi (MPa) Hardness Tests Required Reuses a 1,500 to 6,000 (10 to 40) 50 none 100 2,500 to 7,000 (17 to 50) 60 none 100 4,000 to 7,000 (28 to 50) 70 none 100 7,000 to 12,000 (50 to 80) 70 Required 50 Greater than 12,000 (80) not permitted a Maximum number of reuses. Will be less if pads wear, crack split.

Concrete Laboratory C-1231 January 2012

New Mexico Technician Training and Certification Program

3. Prepare the test cylinder. The specimens shall be either 150 x 300 mm (6 in. x 12 in.) or 100 x 200 mm (4 in. x 8 in.) cylinders. Neither end of test specimen when tested shall depart from perpendicularity to the axis by more than 0.5 degrees (approximately equivalent to a difference in height of 1.6 mm (1/16 in.) for a 300 mm (12 in.) long cylinder and approximately 1 mm (1/25 in.) for a 200 mm (8 in.) long cylinder) (6.1, Note 2). Measure the diameter of the cylinder. The diameter used for calculating the cross-sectional area of the test specimen shall be determined to the nearest 0.25 mm (0.01 in.) by averaging two diameters measured at right angles to each other at about mid-height of the cylinder. Do not test any cylinder for strength if any individual diameter of a cylinder differs from any other diameter of the same cylinder by more than 2.0 percent (6.1).

Note: Cross-sectional area = π(r²)

4. Check planeness of cylinder ends. Depressions under a straight edge measured with a round wire gage across any diameter shall not exceed 5 mm (0.20 in.). If the cylinder ends do not meet this tolerance then the cylinder shall not be tested unless irregularities are corrected by sawing or grinding (6.2).

5. Insert cylinder into testing machine. Unbonded caps are permitted to be used on one or both end of the cylinder in lieu of a sulfur cap (7.1). Examine pads for excessive wear or damage. Replace pads that have cracks or splits exceeding 10 mm (0.40 in.) in length regardless of the depth (Note 6). Do not flip the pads and do not exceed the maximum reuse according to Table 1, or the manufacturer’s recommendation. Dust and insert the pads in the retainers before they are placed on the cylinder (7.2) (Note 3). Center the unbonded cap or caps on the cylinder. Place the cylinder on the lower bearing block of the testing machine. Carefully align the axis of the cylinder with the center of the upper bearing block. Rotate the upper bearing block to ensure it moves freely. After application of load and before reaching 10% of the anticipated specimen strength, check to see that the axis of the cylinder is vertical within tolerance of 0.5 degrees (3 mm (1/8 in.) in 300 mm (12 in.), and 1.6 mm (1/16 in.) in 200 mm (8 in.)) and that the ends of the cylinder are centered in the retaining rings (7.3).

6. Apply load. Apply the load continuously and without shock. For hydraulically operated machines the load shall be applied at a rate of 35 ± 7 psi/s. The designed rate of movement shall be maintained at least during the latter half of the anticipated strength (7.5.1). During the application of the first half of the anticipated loading phase a higher rate of loading shall be permitted (7.5.2). Make no adjustment in the rate of

Concrete Laboratory C-1231 January 2012

New Mexico Technician Training and Certification Program

loading during the latter half of the loading. Continue to load specimen until complete failure is achieved.

7. Record results. Record the maximum load carried by the test specimen

8. Calculate the compressive strength. Divide the maximum load by the cross-sectional area of the test cylinder calculated in Key Element 2.

9. Report the following test specimen information. Test specimens identification number; diameter; cross-sectional area; maximum load; any defects in specimens or caps; age of specimen; and calculated compressive strength to the nearest 0.1 MPa or (10 psi) (9).

10. Use Safety Equipment. Use a protective cage around cylinder when testing cylinder compressive strength using unbonded cap or caps. Wear eye protection at all times.