CCIL Concrete 3 — Questions and Answers
Question 1: What does the water-to-cementing materials ratio (w/cm) primarily control in hardened concrete?
- Workability and color
- Strength and durability (Correct answer)
- Setting time and bleed rate only
- Air content and unit weight
Correct answer: Strength and durability
The w/cm ratio is the most influential factor controlling both compressive strength and durability of hardened concrete.
Question 2: According to ASTM C1064, the maximum allowable time between sampling fresh concrete and completing the temperature measurement is:
- 1 minute
- 5 minutes (Correct answer)
- 10 minutes
- 15 minutes
Correct answer: 5 minutes
ASTM C1064 requires the temperature of freshly mixed concrete to be measured within 5 minutes of obtaining the sample.
Question 3: Supplementary cementing materials (SCMs) such as fly ash are added to concrete primarily to:
- Increase bleeding and reduce set time
- Improve workability, reduce permeability, and lower heat of hydration (Correct answer)
- Replace all Portland cement content
- Accelerate early strength gain above 7 days
Correct answer: Improve workability, reduce permeability, and lower heat of hydration
Fly ash and other SCMs react pozzolanically to improve long-term strength, reduce permeability, and lower heat of hydration in concrete.
Question 4: Which ASTM standard covers the Standard Test Method for Density, Absorption, and Voids in Hardened Concrete?
- ASTM C138
- ASTM C642 (Correct answer)
- ASTM C231
- ASTM C805
Correct answer: ASTM C642
ASTM C642 covers the determination of density, absorption, and voids in hardened concrete specimens.
Question 5: In cold weather concreting per ACI 306, concrete temperature at the point of placement should be maintained at a minimum of:
- 2°C (35°F)
- 5°C (41°F) (Correct answer)
- 10°C (50°F)
- 13°C (55°F)
Correct answer: 5°C (41°F)
ACI 306 requires concrete temperature at delivery to be at least 10°C (50°F) for thin sections to support hydration and early strength gain.
Question 6: What is the primary purpose of entrained air in concrete used in cold climates?
- To reduce compressive strength to design values
- To provide microscopic voids relieving hydraulic pressure from freezing water (Correct answer)
- To increase workability without adding water
- To accelerate the hydration of cement
Correct answer: To provide microscopic voids relieving hydraulic pressure from freezing water
Entrained air bubbles act as pressure relief chambers for water expanding during freezing, protecting concrete from freeze-thaw damage.
Question 7: When a fresh concrete sample is obtained from a transit mixer for testing, which portion of the load should be discarded first?
- The last portion discharged
- The first and last portions discharged (Correct answer)
- Only the middle portion is used
- No portion should be discarded
Correct answer: The first and last portions discharged
Per ASTM C172, the first and last portions of the discharge are discarded; the representative sample is taken from the middle portion of the load.
CCIL Concrete 3
What does the water-to-cementing materials ratio (w/cm) primarily control in hardened concrete?