ACI Concrete Materials & Mix Proportioning 2 — Questions and Answers
Question 1: What does a low water-to-cement (w/c) ratio primarily produce in hardened concrete?
- Higher strength and lower permeability (Correct answer)
- Lower strength and higher workability
- Increased bleeding and segregation
- Faster setting time only
Correct answer: Higher strength and lower permeability
A lower w/c ratio reduces the capillary porosity in hardened concrete, resulting in higher strength and lower permeability.
Question 2: Which type of Portland cement is specifically designed for use in sulfate-exposed environments?
- Type I
- Type II
- Type III
- Type V (Correct answer)
Correct answer: Type V
Type V Portland cement has very low tricalcium aluminate (C3A) content, making it highly resistant to sulfate attack.
Question 3: Fly ash used as a supplementary cementitious material in concrete is a byproduct of:
- Steel manufacturing
- Coal-fired power plants (Correct answer)
- Cement kiln operations
- Oil refining processes
Correct answer: Coal-fired power plants
Fly ash is a pozzolanic byproduct collected from the flue gases of coal-fired electric power plants.
Question 4: According to ACI mix design principles, what is the purpose of the 'trial batch' step?
- To determine cement type required
- To verify calculated proportions produce desired fresh and hardened properties (Correct answer)
- To measure aggregate moisture content
- To establish the mixing time
Correct answer: To verify calculated proportions produce desired fresh and hardened properties
Trial batches confirm that calculated mix proportions achieve the target slump, air content, and compressive strength before full-scale production.
Question 5: What is 'bleeding' in fresh concrete?
- Loss of cement paste through formwork gaps
- Migration of mix water to the concrete surface (Correct answer)
- Excessive air entrainment escaping
- Aggregate sinking below the paste layer
Correct answer: Migration of mix water to the concrete surface
Bleeding occurs when water migrates upward through fresh concrete to the surface due to settlement of heavier solid particles.
Question 6: What is the approximate specific gravity of normal-weight Portland cement?
- 1.5
- 2.3
- 3.15 (Correct answer)
- 3.85
Correct answer: 3.15
Portland cement has a specific gravity of approximately 3.15, which is used in absolute volume mix design calculations.
Question 7: Silica fume added to concrete primarily improves which property?
- Workability and pumpability
- Air entrainment stability
- Strength and reduced permeability (Correct answer)
- Extended set time
Correct answer: Strength and reduced permeability
Silica fume is an extremely fine pozzolan that reacts with calcium hydroxide to form additional C-S-H gel, significantly improving strength and reducing permeability.
ACI Concrete Materials & Mix Proportioning 2
What does a low water-to-cement (w/c) ratio primarily produce in hardened concrete?