An interior nonloadbearing masonry repair is specified with Type O mortar. Which 28-day compressive strength value matches Type O in the common mortar property table?
Correct Answer
C) 350 psi
Type O mortar is associated with 350 psi minimum average compressive strength at 28 days. Other choices: 750 psi is Type N, 1,800 psi is Type S, and 75 psi is a historic-restoration value sometimes associated with Type K rather than standard Type O.
Why This Is the Correct Answer
Type O mortar is associated with 350 psi minimum average compressive strength at 28 days.
Why the Other Options Are Wrong
Option A: 75 psi
In masonry work, 75 psi is the wrong selection for Mortar type strength: 75 psi is a different published number than the cited Mortar type strength value; ASTM C270 mortar property specification values supports 350 psi.
Option B: 750 psi
The trap in B is 750 psi. That selection is wrong because 750 psi is a different published number than the cited Mortar type strength value; ASTM C270 mortar property specification values supports 350 psi; Type O mortar minimum average compressive strength is 350 psi.
Option D: 1,800 psi
Choice D (1,800 psi) is rejected because 1,800 psi is a different published number than the cited Mortar type strength value; ASTM C270 mortar property specification values supports 350 psi. The cited answer is 350 psi.
Memory Technique
File mortar type strength under a cover note: condition first, number or duty second, answer 350 psi.
Reference Hint
Open Modern Masonry - Brick, Block, Stone, 9th Ed., 2020; use the index entry or section for "ASTM C270 mortar property specification values" to confirm that Type O mortar is associated with 350 psi minimum average compressive strength at 28 days.
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