|Part 1--Selecting and Mixing Mortar Materials|
Formulas allow for variations of proportions within the types of mortar. The volume of sand should never exceed three times the combined volume of cement and lime. If you use too much sand, the mortar will be weaker and less workable. Most building codes and specifications call for ASTM mortar types M, S, N, or O (Table 1-1).
1-11. Type M. Type M mortar is composed of 1 part cement, ¼ part lime, and 3¾ parts sand. This mortar has a high cement-to-lime proportion that makes for a very strong mortar. This type of mortar is suitable for general use. It is recommended specifically for structures that are below grade and in contact with the earth, such as foundation, retaining walls, and walkways.
1-12. Type N. Type N mortar is composed of 1 part cement, 1¼ parts lime, and 6¾ parts sand. Type N has excellent workability because of its high lime content. It does not have the strength of Type M or Type S mortar. Type N-
1-13. Type O. Type O mortar is composed of 1 part cement, 2 parts lime, and 9 parts sand. It is extremely plastic and workable (notice the lime content) but has relatively low strength.
Type O should not be used if it will be subjected to freezing and thawing in the presence of excessive moisture. Many building codes do not allow Type O mortar for construction. However, some local building codes allow Type O to be used for load-bearing walls of solid blocks when the compressive stresses do not exceed 100 pounds per square inch. Before using this type mortar, check local building codes.
1-14. Type S. Type S mortar is composed of 1 part cement, ½ part lime, and 4½ parts sand. It is a very good general-purpose mortar used in above-grade exteriors that are exposed to severe weathering. Type S is also used in interiors and all load-bearing structures unless only Type M is specified. Type M and S are usually interchangeable.
|David L. Heiserman, Editor||
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Revised: June 06, 2015