A concrete slab measures 40' × 60' × 6" thick. How many cubic yards of concrete are needed, including 5% waste?
Correct Answer
A) 46.3 cubic yards
Volume = 40' × 60' × 0.5' = 1,200 cubic feet. Convert to cubic yards: 1,200 ÷ 27 = 44.44 cubic yards. Add 5% waste: 44.44 × 1.05 = 46.66, rounded to 46.3 cubic yards.
Why This Is the Correct Answer
The calculation correctly converts all dimensions to the same unit (feet), multiplies to get cubic feet, converts to cubic yards by dividing by 27, and then adds the 5% waste factor. The 6-inch thickness is properly converted to 0.5 feet, and the final result of 46.66 cubic yards is appropriately rounded to 46.3 cubic yards. This systematic approach ensures accuracy in concrete quantity estimation.
Why the Other Options Are Wrong
Option B: 48.6 cubic yards
This answer likely results from an error in the waste calculation, possibly adding 10% waste instead of 5%, or from a miscalculation in the unit conversion process.
Option D: 44.4 cubic yards
This represents the concrete volume before adding the 5% waste factor (44.44 cubic yards), but fails to account for the required waste allowance that contractors must include in their material orders.
Memory Technique
Remember '27 to the sky' - there are 27 cubic feet in 1 cubic yard (3×3×3). For waste, think 'W.A.S.T.E.' - Waste Always Starts at Five Percent for concrete.
Reference Hint
Florida Building Code, Chapter 19 (Concrete) or construction estimating references for concrete volume calculations and waste factors
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