ACI Certification Strength Testing Technician 3 — Questions and Answers
Question 1: What is the specified rate of load application when testing concrete cylinders for compressive strength per ASTM C39?
- 20 ± 5 psi/s (0.14 ± 0.03 MPa/s)
- 35 ± 7 psi/s (0.25 ± 0.05 MPa/s) (Correct answer)
- 50 ± 10 psi/s (0.34 ± 0.07 MPa/s)
- 75 ± 15 psi/s (0.52 ± 0.10 MPa/s)
Correct answer: 35 ± 7 psi/s (0.25 ± 0.05 MPa/s)
ASTM C39 specifies a constant loading rate of 35 ± 7 psi/s (0.25 ± 0.05 MPa/s) applied continuously and without shock throughout the test.
Question 2: Per ASTM C39, the bearing surfaces of cylinders must not depart from a plane by more than:
- 0.001 inch (0.025 mm)
- 0.002 inch (0.050 mm) (Correct answer)
- 0.005 inch (0.13 mm)
- 0.010 inch (0.25 mm)
Correct answer: 0.002 inch (0.050 mm)
ASTM C39 requires bearing surfaces to be plane within 0.002 inch (0.050 mm), which is why capping or end grinding is typically required.
Question 3: Per ASTM C39, the diameter of a test cylinder used to calculate compressive strength must be the average of two measurements taken at:
- Both ends of the cylinder
- Right angles to each other at or near mid-height (Correct answer)
- The top and bottom third of the specimen
- Any two convenient locations on the surface
Correct answer: Right angles to each other at or near mid-height
ASTM C39 requires diameter to be determined from two measurements at right angles to each other taken at or near mid-height of the specimen.
Question 4: According to ASTM C39, what age tolerance is permitted when testing concrete cylinders specified for a 28-day strength result?
- ± 4 hours
- ± 8 hours
- ± 12 hours
- ± 20 hours (Correct answer)
Correct answer: ± 20 hours
ASTM C39 allows 28-day specimens to be tested within ± 20 hours of the specified test age.
Question 5: A cylinder measures 5.90 inches long and 3.00 inches in diameter, giving an L/D ratio of approximately 1.97. Which statement best describes how to handle this specimen per ASTM C39?
- Discard it — L/D below 2.0 is not acceptable
- Apply a correction factor because L/D falls below 1.94
- Test and report the result with no correction since L/D is between 1.75 and 2.10 (Correct answer)
- Re-cure the specimen until it reaches full length
Correct answer: Test and report the result with no correction since L/D is between 1.75 and 2.10
ASTM C39 correction factors apply only when L/D is below 1.75; an L/D of 1.97 falls within the acceptable range and needs no correction.
Question 6: Which fracture pattern described in ASTM C39 is characterized by a well-formed cone on one end with no significant cracking through the capping material?
- Columnar fracture
- Type 1 cone fracture (Correct answer)
- Diagonal shear fracture
- Side fracture at top
Correct answer: Type 1 cone fracture
A Type 1 fracture in ASTM C39 shows a well-formed cone on one or both ends with no cracks running through the caps, indicating good end conditions.
Question 7: When a test result is the average of two companion cylinders and one shows damage due to an improper cap (not a concrete deficiency), what should be done per ASTM C39?
- Average both cylinders as normal and report the result
- Discard the defective cylinder and report the companion result alone (Correct answer)
- Report the defective cylinder at zero strength
- Retest both cylinders after recapping
Correct answer: Discard the defective cylinder and report the companion result alone
If a cylinder fails due to an obvious fabrication or capping defect, its result is discarded and the strength is reported from the valid companion cylinder.
ACI Certification Strength Testing Technician 3
What is the specified rate of load application when testing concrete cylinders for compressive strength per ASTM C39?