What type of mortar is typically specified for below-grade masonry construction in contact with soil?
Correct Answer
A) Type S
Type S mortar is specified for below-grade masonry in contact with soil because of its high strength and resistance to lateral forces. It provides better durability and weather resistance than Types N or O in harsh conditions.
Why This Is the Correct Answer
Type S mortar is the correct choice for below-grade masonry in contact with soil because it has high compressive strength (minimum 1,800 psi) and excellent resistance to lateral forces from soil pressure. It contains a higher cement content than Types N or O, providing superior durability and weather resistance in harsh underground conditions. Type S mortar also offers better resistance to freeze-thaw cycles and moisture penetration, which are critical factors for below-grade applications.
Why the Other Options Are Wrong
Option B: Type M
Type O mortar is the lowest strength mortar (350 psi minimum) and is only suitable for non-load bearing interior applications and repointing historic masonry. It would fail quickly under the lateral soil pressures and harsh conditions found below grade.
Option C: Type O
Type M mortar, while having the highest compressive strength (2,500 psi minimum), is actually too rigid and has poor workability. Its high cement content makes it prone to shrinkage cracking, which would compromise waterproofing in below-grade applications where flexibility is needed.
Memory Technique
Think 'S for Soil' - Type S mortar goes with soil contact. Also remember 'Strong but not Stiff' - S is strong enough but won't crack like M.
Reference Hint
Florida Building Code, Chapter 21 - Masonry, or ACI 530 Building Code Requirements for Masonry Structures, Section on mortar specifications
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