Which type of mortar is most appropriate for load-bearing masonry walls below grade?
Correct Answer
A) Type M mortar
Type M mortar has the highest compressive strength (2500 psi minimum) and is specifically designed for below-grade applications and high-load bearing situations. It provides excellent durability and resistance to freeze-thaw cycles.
Why This Is the Correct Answer
Type M mortar is the correct choice for load-bearing masonry walls below grade because it has the highest compressive strength at 2500 psi minimum, making it ideal for high-stress applications. Its superior strength and durability make it well-suited for below-grade conditions where walls must resist soil pressure and moisture. Type M also provides excellent resistance to freeze-thaw cycles and harsh environmental conditions commonly found in below-grade applications.
Why the Other Options Are Wrong
Option B: Type O mortar
Type O mortar has the lowest compressive strength (350 psi) and is only suitable for non-load bearing applications such as interior partitions and repointing, making it completely inadequate for load-bearing below-grade walls.
Option C: Type N mortar
Type S mortar has medium-high compressive strength (1800 psi) and is used for above-grade load-bearing walls and areas requiring good lateral strength, but it lacks the maximum strength needed for below-grade load-bearing applications.
Option D: Type S mortar
Type N mortar has medium compressive strength (750 psi) and is primarily used for above-grade applications like general construction and interior walls, making it insufficient for the high-stress demands of below-grade load-bearing walls.
Memory Technique
Use 'MOST' for Type M: Maximum strength, Outdoor harsh conditions, Structural load-bearing, Tough below-grade applications
Reference Hint
Florida Building Code Chapter 21 - Masonry, or ACI 530 Building Code Requirements for Masonry Structures - Section on mortar types and applications
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