A crew uses the common quality-concrete rule of thumb that contraction-joint spacing in feet should be about 24 to 30 times slab thickness in inches. For a 5 in. slab, what range results?
Correct Answer
C) 10.0 to 12.5 ft
For a 5 in. slab, 24 x 5 = 120 in. = 10.0 ft and 30 x 5 = 150 in. = 12.5 ft, so the range is 10.0 to 12.5 ft. Other choices use too small or too large a thickness multiplier.
Why This Is the Correct Answer
For a 5 in. slab, 24 x 5 = 120 in. = 10.0 ft and 30 x 5 = 150 in. = 12.5 ft, so the range is 10.0 to 12.5 ft. Other choices use too small or too large a thickness multiplier.
Why the Other Options Are Wrong
Option A: 6.0 to 7.5 ft
joint spacing, choice a: the number 6.0 to 7.5 ft comes from 30 / 5.
Option B: 8.0 to 10.0 ft
joint spacing, choice b: 8.0 to 10.0 ft omits a required add-on/subtraction; the numeric gap is 2.
Option D: 15.0 to 18.0 ft
joint spacing, choice d: 15.0 to 18.0 ft is off by 5 from the checked result 10.0 to 12.5 ft because it misuses the 5 item (thickness in inches).
Memory Technique
Keep a flashing lip beside joint spacing: if one number is skipped, the option drifts away from 10.0 to 12.5 ft.
Reference Hint
Open The Contractor's Guide to Quality Concrete Construction, 4th Ed., 2018; use the index entry or section for "Slab contraction joint spacing guidance" to confirm that For a 5 in.
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