Residential Building Inspector B1Floor ConstructionMedium
A builder is installing a concrete slab-on-ground for a garage. The plans specify a 4-inch thick slab. What is the minimum compressive strength required for this concrete at 28 days, according to the IRC?
- A3,500 psi
- B2,500 psi
- C2,000 psi
- D3,000 psi
Show answer & explanationAnswer & explanation
Correct answer: B. 2,500 psi
IRC Table R402.2 (Footings, Foundation Walls and Slabs) specifies a minimum compressive strength of 2,500 psi at 28 days for concrete slabs on ground, for applications such as garages, porches, and carports, when not exposed to severe weathering.
Why the other options are wrong
- A. This strength is typically for more demanding structural concrete or severe exposure conditions.
- C. This strength is generally considered too low for structural slabs.
- D. This strength is often used for foundations or concrete exposed to weathering, but not the minimum for a standard garage slab.
Concrete Compressive Strength (Slab-on-Ground)
The minimum resistance of concrete to axial compression, measured in pounds per square inch (psi) at 28 days, as required by code for various applications.
- Minimums vary by application and exposure conditions.
- 2,500 psi is common minimum for garage slabs.
- Higher strengths may be needed for severe weathering or structural elements.
Memory trick: Strength, Thickness, Support, and Protection are Key.