CCIL Concrete 5 — Questions and Answers
Question 1: What is the primary function of a plasticizer (water reducer) admixture in concrete?
- Accelerate the setting time of the mix
- Reduce the water requirement while maintaining workability (Correct answer)
- Increase the air content for freeze-thaw resistance
- Delay hydration in hot weather concreting
Correct answer: Reduce the water requirement while maintaining workability
Water-reducing admixtures (plasticizers) allow a reduction in mixing water while maintaining equivalent workability, thereby improving strength and reducing w/cm.
Question 2: Concrete affected by delayed ettringite formation (DEF) typically shows damage that:
- Appears immediately after placement and finishing
- Manifests years after placement as internal expansion and cracking (Correct answer)
- Reduces strength within the first 7 days
- Only affects the surface paste, not aggregate bond zones
Correct answer: Manifests years after placement as internal expansion and cracking
DEF is a long-term durability issue where ettringite forms within hardened concrete years after curing, causing internal expansion and cracking.
Question 3: The initial set of concrete is defined as the point at which penetration resistance reaches:
- 500 psi (3.5 MPa) (Correct answer)
- 3500 psi (24.1 MPa)
- 1000 psi (6.9 MPa)
- 4000 psi (27.6 MPa)
Correct answer: 500 psi (3.5 MPa)
Per ASTM C403, initial set corresponds to a penetration resistance of 500 psi (3.5 MPa), while final set corresponds to 4000 psi (27.6 MPa).
Question 4: What does the term 'carbonation depth' in concrete indicate in a durability assessment?
- Depth of freeze-thaw damage from the surface
- Depth to which atmospheric CO₂ has reduced the concrete's pH (Correct answer)
- Thickness of the carbonated aggregate layer
- Depth of chloride penetration from deicing salts
Correct answer: Depth to which atmospheric CO₂ has reduced the concrete's pH
Carbonation occurs when CO₂ reacts with calcium hydroxide in cement paste, reducing pH from ~13 to ~9 and potentially depassivating reinforcing steel.
Question 5: In accelerated curing of concrete specimens using warm water or steam, which property may be adversely affected?
- Early-age strength gain
- Long-term strength compared to moist-cured specimens (Correct answer)
- Setting time accuracy
- Air content stability
Correct answer: Long-term strength compared to moist-cured specimens
Accelerated curing boosts early strength but can reduce long-term strength compared to standard moist-cured specimens due to less uniform microstructure development.
Question 6: When testing concrete for chloride ion penetration per ASTM C1202 (RCPT), results are classified as 'Very Low' when the charge passed is:
- Less than 4000 coulombs
- Less than 1000 coulombs (Correct answer)
- Between 1000 and 2000 coulombs
- Between 2000 and 4000 coulombs
Correct answer: Less than 1000 coulombs
ASTM C1202 classifies concrete as 'Very Low' chloride ion penetrability when the total charge passed is less than 1000 coulombs over the 6-hour test period.
Question 7: Which test method is used to determine the air void system characteristics (spacing factor, specific surface) in hardened concrete?
- ASTM C231 pressure meter test
- ASTM C457 microscopical analysis of hardened concrete (Correct answer)
- ASTM C173 volumetric method
- ASTM C138 unit weight method
Correct answer: ASTM C457 microscopical analysis of hardened concrete
ASTM C457 involves microscopical examination of a polished concrete section to characterize the hardened air void system, including spacing factor and specific surface.
CCIL Concrete 5
What is the primary function of a plasticizer (water reducer) admixture in concrete?