A pile cap bottom will bear on undisturbed soil with reinforcing steel placed before the concrete is cast. For concrete cast against and permanently exposed to earth, which minimum cover does ACI 318 require?
Correct Answer
A) 3 in.
ACI 318-19 Table 20.6.1.3.1 gives 3 in. as the minimum cover for reinforcement in concrete cast against and permanently exposed to earth. Other choices: 2 in. and 1-1/2 in. belong to formed concrete exposed to earth or weather for different bar sizes, while 3/4 in. is an interior slab, wall, or joist value.
Why This Is the Correct Answer
ACI 318-19 Table 20.6.1.3.1 gives 3 in. as the minimum cover for reinforcement in concrete cast against and permanently exposed to earth.
Why the Other Options Are Wrong
Option B: 2 in.
In concrete work, 2 in. is the wrong selection for Concrete cover: 2 in. is a concrete-cover value for a different exposure/member case; ACI 318-19 Table 20.6.1.3.1 keys this item to 3 in.
Option C: 1-1/2 in.
The trap in C is 1-1/2 in. That selection is wrong because 1-1/2 in. is a concrete-cover value for a different exposure/member case; ACI 318-19 Table 20.6.1.3.1 keys this item to 3 in.; Minimum cover for concrete cast against earth is 3 in.
Option D: 3/4 in.
Choice D gives 3/4 in. Its specific error: 3/4 in. is a concrete-cover value for a different exposure/member case; ACI 318-19 Table 20.6.1.3.1 keys this item to 3 in. The accepted answer is 3 in.
Memory Technique
Link concrete cover to its book condition; a different condition means a different answer than 3 in.
Reference Hint
Open ACI 318-19, Building Code Requirements for Structural Concrete; use ACI 318-19 Table 20.6.1.3.1 to confirm that ACI 318-19 Table 20.6.1.3.1 gives 3 in.
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