CSLB General Building (B)Site Work, Foundations and ConcreteMedium
A contractor is placing a 4-inch-thick interior concrete slab-on-grade and must determine control joint spacing to minimize random shrinkage cracking. Using the standard industry rule of thumb, what is the maximum recommended joint spacing?
- A16 to 20 feet
- B24 to 30 feet
- C8 to 12 feet
- D4 to 6 feet
Show answer & explanationAnswer & explanation
Correct answer: C. 8 to 12 feet
The common industry rule of thumb is that control joint spacing in feet should be about 2 to 3 times the slab thickness in inches. For a 4-inch slab, this gives a spacing of 8 to 12 feet (4 x 2 = 8, 4 x 3 = 12).
Why the other options are wrong
- A. This spacing is too wide and increases risk of uncontrolled cracking.
- B. This far exceeds recommended spacing for a 4-inch slab.
- D. This spacing is too tight and would be unnecessarily costly.
Control Joint Spacing
Joints cut or formed into a concrete slab at regular intervals to control the location of shrinkage cracks.
- Rule of thumb: spacing (ft) = 2 to 3 times slab thickness (in)
- Joints should be cut within 6-12 hours of finishing for best crack control
- Panel aspect ratio should not exceed about 1.5:1
Memory trick: Thickness times two or three tells you where the joint should be.