Which spacing is the rule of thumb for a long, wide slab if thickness is known?

• A. 24-30 times the slab thickness in inches
• B. 8-10 ft apart
• C. 6-8 ft apart
• D. 40-50 ft apart

Answer: (A)

For long, wide slabs, cracking is managed by setting contraction joints at spacing that scales with how thick the slab is. The standard rule of thumb is to space joints at 24 to 30 times the slab thickness, with the thickness measured in inches (giving the spacing in inches). So a 6-inch slab would have joints about 144 to 180 inches apart (roughly 12–15 ft). Fixed spacings like 8–10 ft, 6–8 ft, or 40–50 ft don’t account for varying thicknesses and can either under-control or over-control cracks depending on how thick the slab is. This 24–30 times thickness rule provides a consistent, thickness-dependent guideline for effective crack control.