A submittal lists standard 8 by 8 by 16 concrete masonry units. What actual unit size should the estimator use when allowing for a 3/8 in. mortar joint?
Correct Answer
A) 7-5/8 in. by 7-5/8 in. by 15-5/8 in.
A standard nominal 8 by 8 by 16 CMU has actual dimensions 7-5/8 in. by 7-5/8 in. by 15-5/8 in., leaving room for 3/8 in. mortar joints. Other choices: the nominal size includes joints, 7-1/2 in. dimensions do not match standard modular CMU, and mixing nominal length with actual height is inconsistent.
Why This Is the Correct Answer
A standard nominal 8 by 8 by 16 CMU has actual dimensions 7-5/8 in. by 7-5/8 in. by 15-5/8 in., leaving room for 3/8 in. mortar joints.
Why the Other Options Are Wrong
Option B: 8 in. by 8 in. by 16 in.
Choice B (8 in. by 8 in. by 16 in.) is rejected because 8 in. by 8 in. by 16 in. is a different published number than the cited CMU dimensions value; CMU modular sizes and actual dimensions supports 7-5/8 in.
Option C: 7-1/2 in. by 7-1/2 in. by 15-1/2 in.
In masonry work, 7-1/2 in. by 7-1/2 in. by 15-1/2 in. is the wrong selection for CMU dimensions: 7-1/2 in. by 7-1/2 in. by 15-1/2 in. is a different published number than the cited CMU dimensions value; CMU modular sizes and actual dimensions supports 7-5/8 in.
Option D: 8 in. by 7-5/8 in. by 16 in.
The trap in D is 8 in. by 7-5/8 in. by 16 in. That selection is wrong because 8 in. by 7-5/8 in. by 16 in. is a different published number than the cited CMU dimensions value; CMU modular sizes and actual dimensions supports 7-5/8 in.
Memory Technique
Treat CMU dimensions like a labeled shelf; only the matching condition belongs with 7-5/8 in. by 7-5/8 in. by 15-5/8 in.
Reference Hint
Open Modern Masonry - Brick, Block, Stone, 9th Ed., 2020; use the index entry or section for "CMU modular sizes and actual dimensions" to confirm that A standard nominal 8 by 8 by 16 CMU has actual dimensions 7-5/8 in.
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