NASCLA Accredited Commercial General ContractorConcreteMedium
A concrete contractor is preparing to pour a 12-inch thick structural slab for a commercial building. The project specifications require a minimum concrete cover of 1.5 inches for all reinforcing steel exposed to weather. If the main reinforcement bars are #6 bars, what is the maximum recommended clear spacing between the horizontal main reinforcement bars to ensure proper concrete consolidation and bond?
- A12 inches
- B18 inches
- C9 inches
- D6 inches
Show answer & explanationAnswer & explanation
Correct answer: B. 18 inches
According to ACI 318, the clear spacing between parallel bars in a layer should not be less than the diameter of the bar, 1 inch, or 1.33 times the nominal maximum aggregate size. However, for slabs and walls, the maximum clear spacing should not exceed 18 inches or three times the slab thickness, whichever is smaller. In this case, three times the slab thickness (12 inches) is 36 inches, making 18 inches the governing maximum clear spacing.
Why the other options are wrong
- A. While 12 inches is a common spacing, the code allows for a greater maximum in this scenario.
- C. This spacing is also too restrictive and not the maximum allowed by code for this slab thickness.
- D. This spacing is too restrictive and not the maximum allowed by code for this slab thickness.
Reinforcement Bar Spacing (Slabs)
The clear distance between parallel reinforcing bars in concrete slabs, governed by ACI 318 to ensure proper concrete consolidation and bond.
- Minimum clear spacing: bar diameter, 1 inch, or 1.33x aggregate size.
- Maximum clear spacing (slabs): 18 inches or 3x slab thickness, whichever is smaller.
- Ensures concrete flow around bars and prevents segregation.
Memory trick: Space out your steel, or the concrete won't feel right, and the slab won't be tight.