A concrete mix design specifies a water-cement ratio of 0.45. If 600 pounds of cement are used per cubic yard, how many gallons of water should be added?
Correct Answer
B) 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.
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 600 pounds of cement, the water needed is 600 × 0.45 = 270 pounds. Converting to gallons using water's density of 8.34 pounds per gallon: 270 ÷ 8.34 = 32.4 gallons. This calculation properly applies the water-cement ratio and converts weight to volume using the correct density of water.
Why the Other Options Are Wrong
Option A: 27 gallons
27 gallons is incorrect because it appears to use an improper conversion factor or calculation method. This would result from either using the wrong water density (perhaps 10 lbs/gallon instead of 8.34) or miscalculating the total water weight needed from the water-cement ratio.
Option C: 36 gallons
36 gallons is incorrect because it likely results from using an incorrect water density conversion factor, possibly 7.5 pounds per gallon instead of the correct 8.34 pounds per gallon. This would give 270 ÷ 7.5 = 36 gallons, but uses the wrong density value.
Option D: 45 gallons
45 gallons is incorrect because it appears to ignore the proper conversion from pounds to gallons entirely. This might result from simply dividing 270 pounds by 6 instead of using water's actual density of 8.34 pounds per gallon, leading to a significantly overestimated water volume.
Memory Technique
Remember 'Water weighs 8.34' - sounds like 'Water weighs ate-thirty-four.' For water-cement ratios, always multiply cement weight by the ratio, then divide by 8.34 to get gallons.
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