A concrete mix design for coastal construction specifies a water-cement ratio of 0.40. If 650 lbs of cement is used per cubic yard, how many gallons of water should be added?
Correct Answer
A) 31.2 gallons
650 lbs cement × 0.40 w/c ratio = 260 lbs water. 260 lbs ÷ 8.34 lbs/gallon = 31.2 gallons.
Why This Is the Correct Answer
Option A is correct because the calculation properly applies the water-cement ratio formula. With 650 lbs of cement and a 0.40 w/c ratio, the water needed is 650 × 0.40 = 260 lbs. Converting to gallons using water's density of 8.34 lbs/gallon: 260 ÷ 8.34 = 31.2 gallons. This follows standard concrete mix design principles for coastal construction where precise water content is critical for durability.
Why the Other Options Are Wrong
Option B: 26.0 gallons
This answer of 26.0 gallons represents an incorrect calculation, likely from using the wrong conversion factor or mathematical error. It would result in a water-cement ratio lower than the specified 0.40, which could lead to inadequate workability and potential mixing problems in the concrete.
Option C: 20.8 gallons
This answer of 20.8 gallons is significantly too low and would create a water-cement ratio of approximately 0.32, well below the specified 0.40. This could result in a concrete mix that is too dry, difficult to work with, and may not achieve proper hydration.
Option D: 15.6 gallons
This answer of 15.6 gallons is far too low, resulting in a water-cement ratio of only 0.24. Such a low ratio would create an extremely stiff mix that would be nearly impossible to place and finish properly, leading to poor concrete quality and construction difficulties.
Memory Technique
Remember 'Water Weighs 8.34' - water density is 8.34 lbs/gallon. For w/c ratio: Water = Cement weight × ratio, then divide by 8.34 to get gallons.
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