Which mortar type is specified for underground masonry?

Which mortar type is specified for underground masonry?

• A. Type S
• B. Type O
• C. Type N
• D. Type M

Answer: (D)

Underground masonry must endure moisture, soil pressure, and potential freeze‑thaw cycling, so the mortar chosen needs very high strength and durability. Type M mortar provides the highest compressive strength among common masonry mortars (minimum about 2500 psi), making it well suited for below‑grade work such as foundations, basement walls, and other masonry that remains in contact with earth and moisture. That high strength helps minimize cracking and deterioration in challenging underground conditions, which is why it is the preferred choice for underground applications. The other types are more appropriate for above‑grade or less demanding scenarios: Type S is strong but not as high as Type M and is typically used for exterior above‑grade walls in seismic or weather-resistant situations; Type N is a general-purpose, mid-strength mortar for above‑grade load-bearing walls; Type O is a low-strength mortar used for non-structural interior work.

Underground masonry must endure moisture, soil pressure, and potential freeze‑thaw cycling, so the mortar chosen needs very high strength and durability. Type M mortar provides the highest compressive strength among common masonry mortars (minimum about 2500 psi), making it well suited for below‑grade work such as foundations, basement walls, and other masonry that remains in contact with earth and moisture. That high strength helps minimize cracking and deterioration in challenging underground conditions, which is why it is the preferred choice for underground applications. The other types are more appropriate for above‑grade or less demanding scenarios: Type S is strong but not as high as Type M and is typically used for exterior above‑grade walls in seismic or weather-resistant situations; Type N is a general-purpose, mid-strength mortar for above‑grade load-bearing walls; Type O is a low-strength mortar used for non-structural interior work.