ACI Certification Concrete Mix Design 1 — Questions and Answers
Question 1: According to ACI 211.1, which factor most directly influences the required water content for a given slump?
- Cement type
- Admixture brand
- Maximum size of aggregate (Correct answer)
- Color of aggregate
Correct answer: Maximum size of aggregate
ACI 211.1 tables show that water demand decreases as maximum aggregate size increases for a given slump, due to reduced total surface area.
Question 2: In the absolute volume method of mix design, the sum of volumes of all ingredients for one cubic yard of concrete must equal:
- 25 cubic feet
- 26 cubic feet
- 28 cubic feet
- 27 cubic feet (Correct answer)
Correct answer: 27 cubic feet
One cubic yard equals 27 cubic feet, so the absolute volumes of all ingredients—cement, water, aggregates, and air—must sum to 27 ft³.
Question 3: What is the approximate specific gravity of Type I portland cement used in absolute volume calculations?
- 2.45
- 2.65
- 3.15 (Correct answer)
- 3.50
Correct answer: 3.15
Portland cement has a specific gravity of approximately 3.15, which is used to calculate its absolute volume in mix design.
Question 4: A lower water-to-cement (w/c) ratio generally produces concrete with:
- Greater slump and lower strength
- Higher strength and improved durability (Correct answer)
- Higher water content and greater bleeding
- Reduced cement paste volume
Correct answer: Higher strength and improved durability
Reducing the w/c ratio decreases capillary porosity in the hardened paste, resulting in stronger and more durable concrete.
Question 5: According to ACI 318, the maximum w/cm ratio for concrete exposed to freezing and thawing in a moist condition is:
- 0.40
- 0.45 (Correct answer)
- 0.55
- 0.60
Correct answer: 0.45
ACI 318 limits the w/cm to 0.45 for concrete subjected to freezing and thawing in moist conditions to control permeability.
Question 6: The target mean compressive strength (f'cr) is set higher than the specified strength (f'c) to account for:
- Aggregate variability only
- Seasonal temperature changes
- Normal variability in concrete production and testing (Correct answer)
- Equipment calibration errors
Correct answer: Normal variability in concrete production and testing
f'cr is set above f'c to ensure that no more than a prescribed fraction of test results fall below f'c, given inherent production and testing variability.
Question 7: Supplementary cementitious materials (SCMs) such as fly ash are used in concrete mix design primarily to:
- Increase the water demand of the mix
- Eliminate the need for coarse aggregate
- Replace a portion of portland cement while maintaining or improving properties (Correct answer)
- Increase the unit weight of concrete
Correct answer: Replace a portion of portland cement while maintaining or improving properties
SCMs like fly ash partially replace portland cement, often improving workability and long-term durability while reducing heat of hydration and material cost.
ACI Certification Concrete Mix Design 1
According to ACI 211.1, which factor most directly influences the required water content for a given slump?