A builder is installing a concrete foundation wall that will be 8 feet high and 10 inches thick. The wall will retain 6 feet of unbalanced fill. What is the minimum concrete compressive strength required by Michigan Building Code?
Correct Answer
D) 3,000 psi
Michigan Building Code requires minimum 3,000 psi compressive strength for concrete foundation walls retaining more than 4 feet of unbalanced fill.
Why This Is the Correct Answer
Option D is correct. The Michigan Residential Code (aligned with IRC R402.2) requires a minimum concrete compressive strength of 3,000 psi (f'c = 3,000 psi) for foundation walls retaining more than 4 feet of unbalanced fill. This ensures adequate structural capacity to resist lateral earth pressures.
Why the Other Options Are Wrong
Option A: 4,000 psi
4,000 psi is incorrect. While 4,000 psi concrete provides excellent strength, it exceeds the code minimum for this application. It may be required for garage slabs, driveways, or foundations in severe or very severe weathering regions, but not as the minimum for a standard residential foundation wall retaining 6 feet of fill.
Option B: 3,500 psi
3,500 psi is incorrect. This value is between the standard 3,000 psi minimum and the more demanding 4,000 psi requirement. It is not a standard code threshold for residential foundation walls retaining fill in Michigan.
Option C: 2,500 psi
2,500 psi is incorrect. This lower strength may be used for interior concrete slabs not subject to freezing or for certain light-duty applications, but it is insufficient for foundation walls retaining more than 4 feet of unbalanced fill. The increased lateral pressure requires at least 3,000 psi.
Memory Technique
Memorize the rule: '3,000 psi when fill exceeds 4 feet.' Think of it as '3K for 4+ feet of fill.' If the unbalanced fill is 4 feet or less, lower concrete strength may be acceptable; once it exceeds 4 feet (as in this case with 6 feet), you need 3,000 psi minimum.
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