Page 111 - Design of Reinforced Masonry Structures

MATERIALS OF MASONRY CONSTRUCTION 3.5

in the laboratory are maintained in the field. Water added in the field as determined by the
mason for both methods of specifying mortar. Either proportions or properties specification,
but not both, should be specified. If the project specifications do not indicate which of the
two specifications is to be followed, the properties specifications govern, unless that data
qualifying the mortar under the property specification are presented to and accepted by the
specifier. Excessive amounts of pigments used to achieve mortar color may reduce both the
compressive and bond strength of the masonry. As pointed out earlier, admixtures containing
excessive amount of chloride ions are detrimental to items (reinforcing steel, joint reinforce-
ment, wall ties, metal connectors and inserts, etc.) placed in mortar or grout.

3.2.5 Selection of the Right Mortar Type
There is no single mortar mix that is uniquely suitable for all applications. No one mortar type
rates the highest in all areas of applications. No single mortar property defines mortar qual-
ity. Therefore, it is very important to understand the selection of the right type of mortar as it
influences both the construction process and the quality of finished product. ASTM Standard
specifications provide a means for specifications to identify acceptable materials and products
without limiting those items to specific brands of manufacturers. Project specifications should
reference ASTM C270-05a, the Standard Specification for Mortar of Unit Masonry.
The different mortar types are used for a variety of masonry applications. Type N mortar
is a general-purpose mortar that provides good workability and serviceability. It is com-
monly used for interior walls, above-grade exterior walls under normal conditions, and
for veneers. Type S mortar is used for structural load-bearing applications and for exterior
applications for at or below grade. In addition, it also provides increased resistance to
freeze-thaw deterioration. Type M is high-strength mortar, which may be considered for
load-bearing or severe free-thaw applications. Type O is a low-strength mortar that is some-
times used for interior masonry or pointing. Special attention should be given when severe
exposure conditions are expected. Type O mortar should not be used in saturated freezing
conditions. Table 3.1 (adapted from ASTM C270-05a) provided guidelines for selecting
mortar unreinforced (plain) masonry.

TABLE 3.1 Guide for Selection of Masonry Mortars
Mortar type
Location Building segment Recommended Alternative

Exterior, above grade Load-bearing walls, N S or M
Non-load-bearing walls O * N or S
Parapet walls N S
S † M or N†
Exterior, at or below Foundation walls, retaining walls, N S or M
grade manholes, sewers, pavements,
walks, and patios
Interior Non-load-bearing partitions O N
*Type O mortar is recommended for use where the masonry is unlikely to be frozen when saturated or unlikely to
be subjected to high winds or other significant lateral loads. Types N or S mortars should be used in other cases.
†Masonry exposed to whether in nominally horizontal surface is extremely vulnerable to weathering. Mortar for
such masonry should be selected with due caution.