Page 108 - Design of Reinforced Masonry Structures

3.2 CHAPTER THREE

In masonry construction, mortar constitutes only a small proportion (approximately
7 percent) of the total wall area, but its influence on the performance of the wall is signifi-
cant. At a first glance, mortar gives the appearance of simply being a jointing material for
masonry units. Although the primary purpose of mortar in masonry is to bond masonry
units into an assemblage, which acts as an integral element having desired functional char-
acteristics, mortar also serves other functions [3.1]:

1. Bonds masonry units together into an integral structural assembly
2. Seals joints against penetration by air and moisture
3. Accommodates small movements within a wall
4. Bonds to joint reinforcement to assist in resisting shrinkage and tension
5. Bonds to ties and anchors so that all elements perform as an integral unit

3.2.2 Mortar Materials

Mortar consists of cementitious materials to which are added sufficient water and approved
additives so as to achieve workable plastic consistency. The cementitious materials may be
lime, masonry cement, mortar cement, and portland cement, and should not contain epoxy
resins and derivatives, phenols, asbestos, fiber, or fireclays.
The American Society for Testing and Materials (ASTM) maintains national standards—
ASTM C 270-05a [3.7]—for mortars, and materials commonly used as their constituents,
under the jurisdiction of ASTM Committee C-12 on Mortars for Unit Masonry. It contains
sections on scope, requirements, materials, test methods, construction practices, specifica-
tion limitations, references, and an appendix. According to this standard, mortar is classi-
fied into three categories:
1. Cement-lime
2. Mortar cement
3. Masonry cement

All three mortars have been used successfully, with each offering unique benefits. A brief
discussion of these types of mortar can be found in Ref. 3.9, which is summarized here.
Cement-lime mortars are mixed to meet minimum physical requirements. They are pro-
duced by blending lime-sand mortar with portland cement. A lime-sand mortar possesses excel-
lent workability and high water retention, while portland cement increases setting time and
provides additional strength. These mortars are further subclassified in various types. During the
initial stages of mortar specification development, mortars were subclassified as Types A, B, C,
and D. In 1954, the following designation for mortar types was adopted [3.9, 3.10]:

1. M (instead of A-1)
2. S (instead of A-2)
3. N (instead of A-B)
4. O (instead of A-C)
5. K (instead of A-D)
The new classification was apparently developed by dropping the alternate letters A, O, W,
and R from the phrase MASON WORK. Cement-lime is subclassified into four types: M, S,
N, and O. Both mortar cement and masonry cement are subclassified into six different types:
M, M, S, S, N, and O. Proportioning requirements for mortar, by volume, are listed in ASTM
C270-05a [3.7] (see Table A.1), and discussed in other references [3.8, 3.9].