TM 5-809-3/NAVFAC DM-2.9/AFM 88-3, Chap. 3
of concrete units which can affect the structural
flexural strengths of the masonry assemblage. In
performance of installed masonry include: absorp-
addition, mortar compensates for dimensional and
tion; moisture content; shrinkage potential; tem-
surface variations of masonry units, resists water
perature expansion/contraction; compressive
and air penetration through masonry, and bonds to
strength; and flexural strength.
metal ties, anchors, and joint reinforcement so that
(1) Absorption. Absorption of a concrete ma-
they perform integrally with the masonry units.
sonry unit, determined in accordance with ASTM
a. Cementitious materials. Cementitious materi-
C 140 is the total amount of water, expressed in
als used are portland cement, ASTM C 150; or
pounds per cubic foot, that a dry unit will absorb
portland blast furnace cement, ASTM C 595; and
and is somewhat related to density.
lime, ASTM C 207; or masonry cement, ASTM C
(2) Moisture content. Moisture content is ex-
91. Masonry cement has limited applications. Mor-
pressed as a percent of the total water absorption
tar made with portland cement, lime, aggregate
possible for a given concrete masonry unit. Dimen-
(sand) and water is preferred since all constituents
sional changes of concrete masonry due to changes
are well defined. While both types of mortar have
in unit moisture content can have serious effects
similar attributes and requirements, the discussion
upon the structure depending upon the nature of
herein applies specifically to mortar made with
the boundary conditions and size of a given ma-
portland cement, lime, and aggregate. In general, it
sonry element. The most common effect is shrink-
may not be possible to specify a mortar, which will
age cracking due to a loss of moisture. Moisture
be optimal for both workability and strength. A
loss is affected by the humidity of the air sur-
mortar which is workable with the masonry units
rounding a particular masonry element. Moisture
being used under site environmental conditions will
conditions, and thus cracking potential, may be
usually result in a masonry assemblage with
significantly different for interior and exterior
acceptable strength and good quality joints.
elements.
b. Aggregate (sand). Well-graded sand, ASTM
(3) Shrinkage potential. Shrinkage potential
C 144, with a uniform distribution of particle sizes
characteristics of a given unit, determined accord-
is necessary to produce a workable mortar which is
ing to ASTM C 426, depend upon the method of
dense and strong in the hardened state. Sand on the
manufacture and the materials. The linear shrinkage
finer side of the permitted gradation range will
potential values given in the appropriate ASTM*s
produce a more workable mortar than a mortar
represent an attempt to equalize drying shrinkage
made with coarser sand. However, the mortar with
for units of different shrinkage potential
finer sand requires more water to be workable and
considering differences in humidity conditions.
is therefore weaker. The particles of manufactured
(4) Temperature expansion/contraction. As
sand are sharp and singular and tend to produce a
is the case for most materials, concrete masonry
less workable mortar than that made with natural
expands and contracts with temperature changes.
sand of rounded particles. More water may be
(5) Compressive strength. The compressive
required to obtain adequate workability of mortar
strength of concrete masonry units is established in
made with manufactured sand than that made with
accordance with ASTM C 140. This test is a
natural sand, resulting in a lower strength due to
measure of unit quality. The compressive strength
the higher water-cement ratio.
of the masonry units, along with the mortar
strength, provide the basis for assuming the com-
270, mortar may be specified either in terms of
pressive strength of the masonry assemblage. Fac-
proportions (by volume of portland cement, hy-
tors which affect compressive strength include:
drated lime, and aggregate) or in terms of proper-
water-cement ratio, degree of compaction, and
ties (required compressive strength). The propor-
cement content. Minimum compressive strength
tion method is the only method allowed by the
requirements are presented in the appropriate
guide specification. It should be noted that mortar
ASTM*s for the various kinds of units.
conforming to the proportion specifications of
(6) Flexural strength. Flexural strength,
ASTM C 270 may have compressive strength far in
modulus of rupture, is basically a measure of the
excess of the minimum values prescribed for the
tensile strength of a masonry unit and is somewhat
property method.
correlated to the unit compressive strength.
d. Mortar types. The four types of mortar given
in ASTM C 270 are; in order of descending
3-4. Mortar. Mortar, ASTM C 270, is a mixture of
strength; M, 5, N, and 0. Generally as strength
cementitious materials, aggregate and water.
increases, workability decreases. Since a good mor-
Mortar serves to bond masonry units together to
tar must have a combination of strength and
form a composite structural material. As such,
workability, the mortars on the extremes (M and 0)
mortar is a factor in the compressive, sheer, and
3-4
Previous Page
