CCIL Concrete Compressive Strength Testing 2 — Questions and Answers
Question 1: According to ASTM C39, what is the required rate of loading for compressive strength testing of cylindrical concrete specimens?
- 0.05 to 0.15 MPa/s (7 to 21 psi/s)
- 0.15 to 0.35 MPa/s (20 to 50 psi/s) (Correct answer)
- 0.35 to 0.70 MPa/s (50 to 100 psi/s)
- 0.70 to 1.40 MPa/s (100 to 200 psi/s)
Correct answer: 0.15 to 0.35 MPa/s (20 to 50 psi/s)
ASTM C39 specifies a loading rate of 0.15 to 0.35 MPa/s (20 to 50 psi/s) during the final phase of loading.
Question 2: What fracture pattern in a cylindrical concrete specimen indicates a well-centered load with good end condition?
- Type 1 — cone fracture with no cracking through ends (Correct answer)
- Type 4 — diagonal fracture with no cracking at ends
- Type 5 — side fractures at top or bottom
- Type 6 — similar fractures at top and bottom
Correct answer: Type 1 — cone fracture with no cracking through ends
Type 1 (cone fracture) indicates uniform load distribution and satisfactory end conditions per ASTM C39 fracture pattern classifications.
Question 3: When using sulfur mortar capping for concrete cylinders, what is the minimum time the cap must cure before testing?
- 15 minutes
- 30 minutes (Correct answer)
- 2 hours
- 24 hours
Correct answer: 30 minutes
Sulfur mortar caps must cure for a minimum of 30 minutes before the specimen is tested per ASTM C617 requirements.
Question 4: A 4×8 inch concrete cylinder yields a compressive strength of 4,250 psi. What correction factor is applied relative to the standard 6×12 inch cylinder?
- Multiply by 0.87
- Multiply by 1.00 — no correction needed
- Multiply by 1.06 (Correct answer)
- Multiply by 0.94
Correct answer: Multiply by 1.06
Per ASTM C39, 4×8 inch cylinders may yield approximately 106% of the strength of 6×12 inch cylinders, requiring a correction factor of about 1.06.
Question 5: What is the maximum allowable departure from perpendicularity of the end surface of a concrete cylinder to its axis before testing?
- 0.25 degrees
- 0.50 degrees (Correct answer)
- 1.00 degree
- 2.00 degrees
Correct answer: 0.50 degrees
ASTM C39 allows a maximum departure from perpendicularity of 0.50 degrees (approximately 0.5% of diameter) for cylinder end surfaces.
Question 6: Which curing condition for standard-cured concrete cylinders is required per ASTM C31 prior to transportation to the lab?
- Store at ambient temperature with no moisture protection
- Maintain between 16°C and 27°C (60°F to 80°F) and protect from moisture loss (Correct answer)
- Immerse in water immediately after casting
- Store at 10°C (50°F) for the first 24 hours
Correct answer: Maintain between 16°C and 27°C (60°F to 80°F) and protect from moisture loss
ASTM C31 requires initial curing between 16°C and 27°C with protection from moisture loss until cylinders are transported to the lab.
Question 7: If two cylinders from the same sample set yield strengths of 3,820 psi and 3,540 psi, what is the percent difference and is it within ASTM C39 acceptable limits?
- 7.3% — within the 8% limit; average is reportable (Correct answer)
- 7.3% — exceeds the 6% limit; results must be investigated
- 7.3% — within the 10% limit; average is reportable
- 7.3% — exceeds the 5% limit; retest required
Correct answer: 7.3% — within the 8% limit; average is reportable
The difference is 280 psi / 3,820 psi ≈ 7.3%, which is within the ASTM C39 guideline of 8% between duplicate cylinders.
CCIL Concrete Compressive Strength Testing 2
According to ASTM C39, what is the required rate of loading for compressive strength testing of cylindrical concrete specimens?