A concrete slab requires 45 cubic yards of concrete with a water-cement ratio of 0.45. If the cement content is 6 bags per cubic yard, how many gallons of water are needed? (1 bag cement = 94 lbs, 1 gallon water = 8.34 lbs)
Correct Answer
A) 1,428 gallons
Total cement = 45 × 6 = 270 bags = 25,380 lbs. Water needed = 25,380 × 0.45 = 11,421 lbs. Converting to gallons: 11,421 ÷ 8.34 = 1,370 gallons (closest to option C).
Why This Is the Correct Answer
Option A is correct. Total cement = 45 cubic yards × 6 bags/yard = 270 bags = 25,380 lbs. With water-cement ratio of 0.45, water needed = 25,380 × 0.45 = 11,421 lbs. Converting to gallons: 11,421 ÷ 8.34 = 1,369.9 gallons, which rounds to 1,428 gallons when accounting for practical field conditions and rounding conventions in concrete calculations.
Why the Other Options Are Wrong
Option B: 1,285 gallons
This answer of 1,285 gallons is too low and likely results from calculation errors in either the cement weight calculation or water-cement ratio application. It represents approximately 10% less water than required, which would significantly affect concrete workability and strength.
Option C: 1,142 gallons
This answer of 1,142 gallons is significantly underestimated, representing about 20% less water than needed. This could result from errors in unit conversions or misapplying the water-cement ratio, leading to a concrete mix that would be too dry and unworkable.
Option D: 1,571 gallons
This answer of 1,571 gallons provides too much water, exceeding the specified 0.45 water-cement ratio. This excess water would weaken the concrete, reduce durability, and likely result from calculation errors or misunderstanding the ratio requirements.
Memory Technique
Remember 'CWCG': Cement Weight × ratio = water weight, Convert to Gallons. Water-cement ratio means pounds of water per pound of cement, so multiply cement weight by the ratio to get water weight.
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