A concrete mix design calls for a water-cement ratio of 0.45. If 600 pounds of cement are used per cubic yard, what is the required weight of water per cubic yard?
Correct Answer
A) 270 pounds
Water-cement ratio = weight of water ÷ weight of cement. 0.45 = W ÷ 600, therefore W = 0.45 × 600 = 270 pounds of water per cubic yard.
Why This Is the Correct Answer
Option A (270 pounds) is correct. The water-cement ratio (w/c) is defined as the weight of water divided by the weight of cement: w/c = W ÷ C. Rearranging: W = w/c × C = 0.45 × 600 = 270 pounds. This is the exact water quantity needed to achieve the specified w/c ratio.
Why the Other Options Are Wrong
Option B: 300 pounds
Option B (300 pounds) is incorrect. This result would correspond to a w/c ratio of 300 ÷ 600 = 0.50, which is higher than the specified 0.45. A higher w/c ratio reduces concrete strength and durability; using 300 lbs of water would not meet the mix design specification.
Option C: 330 pounds
Option C (330 pounds) is incorrect. This corresponds to a w/c ratio of 330 ÷ 600 = 0.55, significantly above the required 0.45. Using excess water weakens concrete through increased porosity and reduced cement hydration efficiency.
Option D: 360 pounds
Option D (360 pounds) is incorrect. This corresponds to a w/c ratio of 360 ÷ 600 = 0.60. This is a common value in older construction but would not satisfy a mix design specifying 0.45. Excess water is one of the leading causes of reduced concrete compressive strength.
Memory Technique
The w/c ratio formula is straightforward: Water = w/c × Cement. Think of w/c as the 'fraction of water per unit of cement.' With a 0.45 ratio and 600 lbs of cement, you get 45% of 600 = 270 lbs. Visualize: for every 100 lbs of cement, you add 45 lbs of water.
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