A concrete mix design calls for a water-cement ratio of 0.45. If 500 pounds of cement are used, how many gallons of water should be added?
Correct Answer
D) 27 gallons
Water needed = 500 lbs × 0.45 = 225 lbs of water. Since water weighs 8.33 lbs per gallon: 225 ÷ 8.33 = 27 gallons. The water-cement ratio is critical for achieving proper strength and workability.
Why This Is the Correct Answer
Option B is correct because the water-cement ratio of 0.45 means 0.45 pounds of water per pound of cement. With 500 pounds of cement, we need 500 × 0.45 = 225 pounds of water. Since water weighs 8.33 pounds per gallon, we divide 225 ÷ 8.33 = 27 gallons. This calculation properly applies the water-cement ratio and converts weight to volume using water's density.
Why the Other Options Are Wrong
Option A: 22.5 gallons
This answer uses an incorrect water-cement ratio calculation or wrong water density, possibly calculating 250 pounds of water instead of 225 pounds.
Option B: 30 gallons
This answer incorrectly assumes water weighs 10 pounds per gallon instead of 8.33 pounds per gallon, resulting in 225 ÷ 10 = 22.5 gallons.
Memory Technique
Remember '8.33' as 'Ate Thirty-Three' - water weighs 8.33 pounds per gallon, which is essential for all concrete mix calculations
Reference Hint
ACI 318 Building Code Requirements for Structural Concrete, Chapter 19 (Concrete Quality) or concrete mix design tables in construction materials references
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