CCIL Concrete Compressive Strength Testing 3 — Questions and Answers
Question 1: Which ASTM standard governs the practice of making and curing concrete test specimens in the field?
- ASTM C172
- ASTM C31 (Correct answer)
- ASTM C39
- ASTM C138
Correct answer: ASTM C31
ASTM C31 covers the standard practice for making and curing concrete test specimens in the field.
Question 2: What is the minimum inside diameter of a mold used to cast a cylindrical concrete specimen per ASTM C31?
- At least 1.5 times the maximum aggregate size
- At least 2.0 times the maximum aggregate size
- At least 3.0 times the maximum aggregate size (Correct answer)
- At least 2.5 times the maximum aggregate size
Correct answer: At least 3.0 times the maximum aggregate size
ASTM C31 requires mold diameter to be at least 3 times the nominal maximum size of coarse aggregate in the concrete.
Question 3: During compression testing, which specimen condition is most likely to cause a significant reduction in measured compressive strength?
- Specimen tested at exactly 28 days
- Specimen tested while saturated surface dry
- Specimen with a concave end surface (Correct answer)
- Specimen stored in lime-saturated water before testing
Correct answer: Specimen with a concave end surface
A concave end surface causes stress concentration at the edges rather than uniform load distribution, significantly reducing measured strength.
Question 4: What is the acceptable length-to-diameter (L/D) ratio range for a cylinder to be tested without applying a correction factor per ASTM C39?
- 1.0 to 1.5
- 1.5 to 2.0
- 1.75 to 2.25 (Correct answer)
- 2.0 to 2.5
Correct answer: 1.75 to 2.25
ASTM C39 states no correction factor is needed when the L/D ratio is between 1.75 and 2.25 for standard cylinders.
Question 5: What action should be taken when the L/D ratio of a concrete cylinder is less than 1.75?
- Discard the specimen and cast a new one
- Apply a correction factor from the ASTM C39 table to the measured strength (Correct answer)
- Test at double the standard loading rate to compensate
- Report the result without any modification
Correct answer: Apply a correction factor from the ASTM C39 table to the measured strength
ASTM C39 provides correction factors for cylinders with L/D ratios between 1.00 and 1.74 to normalize results to standard geometry.
Question 6: In a compression testing machine, what is the purpose of the spherically seated bearing block?
- To measure the applied load electronically
- To allow the platen to self-align and distribute load uniformly across uneven specimen ends (Correct answer)
- To prevent the specimen from sliding laterally during loading
- To absorb energy during sudden specimen fracture
Correct answer: To allow the platen to self-align and distribute load uniformly across uneven specimen ends
The spherically seated block allows slight angular adjustment so the platen contacts the full specimen face uniformly even if ends are not perfectly parallel.
Question 7: Which type of neoprene pad cap is appropriate for testing concrete with compressive strength expected between 10 MPa and 40 MPa (1,450 to 5,800 psi) per ASTM C1231?
- 50 durometer Shore A pads
- 60 durometer Shore A pads (Correct answer)
- 70 durometer Shore A pads
- 80 durometer Shore A pads
Correct answer: 60 durometer Shore A pads
ASTM C1231 specifies 60 durometer Shore A neoprene pads for concrete strengths in the 10 to 40 MPa range.
CCIL Concrete Compressive Strength Testing 3
Which ASTM standard governs the practice of making and curing concrete test specimens in the field?