You are estimating concrete for a foundation that measures 120' × 80' × 2' thick. Including 8% waste factor, how many cubic yards of concrete should you order?
Correct Answer
B) 768 cubic yards
Volume = 120' × 80' × 2' = 19,200 cubic feet. Convert to yards: 19,200 ÷ 27 = 711.11 cubic yards. With 8% waste: 711.11 × 1.08 = 768 cubic yards.
Why This Is the Correct Answer
Option B correctly follows the complete calculation process for concrete estimation. First, the volume is calculated in cubic feet (120' × 80' × 2' = 19,200 cf), then converted to cubic yards by dividing by 27 (19,200 ÷ 27 = 711.11 cy). Finally, the 8% waste factor is applied by multiplying by 1.08 (711.11 × 1.08 = 768 cubic yards). This accounts for both the unit conversion and the industry-standard waste allowance.
Why the Other Options Are Wrong
Option A: 711 cubic yards
Option A represents only the base concrete volume without the waste factor applied (711 cubic yards). This would result in insufficient concrete for the project since it doesn't account for the 8% waste that typically occurs during mixing, transport, and placement.
Option C: 855 cubic yards
Option C appears to use an incorrect waste factor calculation, possibly applying 20% waste (711 × 1.20 = 853) instead of the specified 8%. This would result in over-ordering concrete and unnecessary cost.
Option D: 924 cubic yards
Option D significantly overestimates the concrete needed, possibly from calculation errors in either the volume calculation or applying an excessive waste factor of approximately 30%. This would lead to substantial material waste and cost overruns.
Memory Technique
Remember '27 CUBES' - there are 27 cubic feet in 1 cubic yard (3×3×3). For waste factors, think 'W + 1' where W is the decimal waste (0.08 + 1 = 1.08 multiplier).
Reference Hint
Florida Building Code Chapter 19 (Concrete) or construction estimating references covering concrete volume calculations and waste factors
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