A concrete mix design calls for a water-cement ratio of 0.45. If 600 pounds of cement are used, how many gallons of water are needed?
Correct Answer
A) 32.4 gallons
Water needed = 600 lbs × 0.45 = 270 lbs of water. Since water weighs 8.34 lbs per gallon: 270 ÷ 8.34 = 32.4 gallons. The water-cement ratio is critical for concrete strength and durability.
Why This Is the Correct Answer
Option B is correct because it follows the proper two-step calculation process for water-cement ratio problems. First, multiply the cement weight by the water-cement ratio to get water weight in pounds (600 × 0.45 = 270 lbs). Then convert pounds to gallons using water's density of 8.34 lbs/gallon (270 ÷ 8.34 = 32.4 gallons). This systematic approach ensures accurate concrete mix proportioning.
Why the Other Options Are Wrong
Option B: 27 gallons
This answer likely results from using an incorrect water density conversion factor, possibly 10 lbs/gallon instead of the correct 8.34 lbs/gallon, leading to an underestimation of required water volume.
Option C: 40.5 gallons
This answer appears to use an even lower conversion factor (around 6.67 lbs/gallon), significantly overestimating the water volume and demonstrating confusion about water's actual weight per gallon.
Option D: 36 gallons
This answer suggests dividing 270 lbs by 7.5 instead of 8.34, which is an incorrect water density value that would overestimate the water volume needed.
Memory Technique
Remember '8.34' as 'Ate Three Four' - water weighs 8.34 pounds per gallon, a fundamental constant in concrete calculations.
Reference Hint
ACI 211 - Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete, or concrete materials chapter in construction reference manual
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