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
B) 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 A: 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: 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.
Option D: 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.
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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