CTQP Concrete Field Inspector 4 — Questions and Answers
Question 1: What is the primary purpose of using a vibrator during concrete placement?
- To accelerate cement hydration and reduce set time
- To eliminate entrapped air voids and achieve consolidation (Correct answer)
- To distribute supplementary cementitious materials evenly
- To increase the water-to-cement ratio for better workability
Correct answer: To eliminate entrapped air voids and achieve consolidation
Internal vibration consolidates fresh concrete by liquefying it momentarily so air voids escape and the mix fills the form completely.
Question 2: According to ASTM C94, how many total drum revolutions are permitted at mixing speed before discharge?
- 100 revolutions
- 200 revolutions
- 300 revolutions (Correct answer)
- 400 revolutions
Correct answer: 300 revolutions
ASTM C94 limits the total drum revolutions at mixing speed to 300 or the time limit of 90 minutes, whichever is reached first.
Question 3: A field inspector observes the vibrator being inserted more than 18 inches apart. What is the likely consequence?
- Over-consolidation between insertion points
- Under-consolidation leaving unvibrated zones (Correct answer)
- Increased air content between insertion points
- Faster set time due to heat from multiple insertions
Correct answer: Under-consolidation leaving unvibrated zones
Vibrator insertion spacing exceeding the radius of action (typically 1.5× vibrator diameter) leaves poorly consolidated zones between insertions.
Question 4: What does the water-to-cementitious materials ratio (w/cm) primarily control?
- The workability of fresh concrete only
- The color and surface texture of hardened concrete
- The strength and durability of hardened concrete (Correct answer)
- The setting time of the concrete mix
Correct answer: The strength and durability of hardened concrete
The w/cm ratio is the single most important factor governing concrete strength and durability — lower ratios yield stronger, more durable concrete.
Question 5: In cold weather concreting, what minimum temperature must fresh concrete be maintained at during placement and protection per ACI 306?
- 32°F (0°C)
- 40°F (4°C)
- 50°F (10°C) (Correct answer)
- 55°F (13°C)
Correct answer: 50°F (10°C)
ACI 306 requires fresh concrete to be maintained at a minimum of 50°F (10°C) during placement and for the required protection period.
Question 6: Which of the following is a correct procedure when the slump test cone is lifted?
- Lift slowly and tilt the cone to the side in one motion
- Lift straight upward in 5 ± 2 seconds without twisting (Correct answer)
- Lift rapidly in under 1 second to avoid dragging concrete
- Tilt 45° first, then lift vertically over 10 seconds
Correct answer: Lift straight upward in 5 ± 2 seconds without twisting
ASTM C143 requires the cone to be raised smoothly and vertically in 5 ± 2 seconds without lateral or twisting motion.
Question 7: What does segregation in concrete refer to?
- Separation of cement paste from supplementary cementitious materials
- Separation of coarse aggregate from the mortar fraction (Correct answer)
- Separation of air bubbles from the paste during vibration
- Separation of admixtures from the mix water
Correct answer: Separation of coarse aggregate from the mortar fraction
Segregation is the undesirable separation of the coarse aggregate from the mortar, often caused by improper handling, over-vibration, or excessive w/c ratio.
CTQP Concrete Field Inspector 4
What is the primary purpose of using a vibrator during concrete placement?