A concrete mix design specifies a water-cement ratio of 0.45. If 600 pounds of cement are used, how many gallons of water should be added?
Correct Answer
D) 32.4 gallons
Water needed = 600 lbs × 0.45 = 270 lbs. Since water weighs 8.34 lbs per gallon, 270 ÷ 8.34 = 32.4 gallons. The water-cement ratio is critical for concrete strength and workability.
Why This Is the Correct Answer
The water-cement ratio of 0.45 means 0.45 pounds of water per pound of cement. With 600 pounds of cement, we need 600 × 0.45 = 270 pounds of water. Converting to gallons requires dividing by water's weight of 8.34 pounds per gallon: 270 ÷ 8.34 = 32.4 gallons. This calculation properly accounts for the unit conversion from weight to volume.
Why the Other Options Are Wrong
Option A: 27 gallons
This answer (45 gallons) appears to come from dividing 270 pounds by 6 instead of 8.34, showing a fundamental error in water weight conversion.
Option C: 45 gallons
This answer (36 gallons) likely comes from using an incorrect water density (7.5 lbs/gallon) or rounding errors in the calculation process.
Memory Technique
Remember '8.34' as 'Ate Three Four' - the weight of water per gallon that you 'ate, then three, then four' to memorize this critical conversion factor.
Reference Hint
ACI 211 - Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete, or concrete technology sections in construction reference manuals
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