Page 123 - Masonry and Concrete

Understanding Masonry
122 CHAPTER FOUR

flexural bond strengths not only increase resistance to wind loads,
but to moisture penetration as well. Masonry cements are more
widely used than portland cement and lime for masonry mortars, and
the vast majority of projects which incorporate them perform very
well. On projects which have flexural bond failures or excessive
moisture penetration, the fault can seldom be attributed solely to the
use of masonry cement instead of portland cement and lime in the
mortar. Usually, there are other defects which contribute to the prob-
lems. Both masonry cement mortars and portland cement and lime
mortars are capable of providing what the industry considers ade-
quate flexural bond strength when they are properly designed, mixed,
and installed.
There are five common mortar types, designated as M, S, N, O, and
K. Each of the five types is based on standardized proportions of the
various ingredients and has certain applications to which it is particu-
larly suited. Type M, for instance, is a high-compressive-strength mix
recommended for masonry which may be subject to high-compressive
loads. Type S is a high-bond-strength mortar recommended for struc-
tures which require resistance to significant lateral loads from soil
pressures, winds, or earthquakes. Because of its excellent durability,
Type S mortar is also recommended for structures at or below grade
and in contact with the soil, such as foundations, retaining walls,
pavements, sewers, and manholes. Type N is a good general-purpose
mortar for use in above-grade masonry. It is recommended for exterior
masonry veneers and for interior and exterior loadbearing walls. This
medium-strength mortar represents the best compromise among com-
pressive and flexural strength, workability, and economy and is, in
fact, recommended for most masonry applications. Type O is a high-
lime, low-compressive-strength mortar. It is recommended for interior
and exterior non-loadbearing walls and veneers which will not be sub-
ject to freezing in the presence of moisture. Type O mortar is often
used in one- and two-story residential work and is a favorite of masons
because of its excellent workability and economical cost. Type K mor-
tar has a very low compressive strength and a correspondingly low
flexural bond strength. It is seldom used in new construction and is
recommended only for tuckpointing historic buildings constructed
originally with lime and sand mortar.

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