TM 5-809-3/NAVFAC DM-2.9/AFM 88-3, Chap. 3
should not be used. Although S and N are allowed
values for sand and lime illustrates a typical Type S
in the guide specifications, Type S exhibits the best
mix of 1 part cement, part lime and 4 parts
overall qualities of strength and workability and
sand. The figure indicates that the expected cube
normally should be specified.
strength is approximately 3700 pounds per square
inch, using these proportions. The band between
e. Water retentivity. Mortar exposed to air tends
the two sloping straight lines reflects the range of
to lose water by evaporation. Mortar in contact
proportions as prescribed in ASTM C 270.
with masonry units tends to lose water to the units
h. Factors affecting mortar to unit bond.
because of the suction of the units. Retentivity is
the mortar property associated with resistance to
Because mortar not only seals masonry against
such water loss and resultant loss of workability.
wind and water penetration, but also binds masonry
Lime in mortar improves the water retentivity and
units together, strength and bond of mortar are
workability. Ideally, retentivity of a mortar should
essential to well-constructed masonry. Two forms
be compatible with the suction of the units used
of bond strength are important for structural pur-
and environmental conditions, such as, temperature
poses, tensile bond strength and shear bond
and humidity, so that adequate workability is
strength. Tensile bond is required to resist forces
maintained. Water content of mortar should be as
perpendicular to a mortar-unit joint while sheer
high as possible consistent with proper workability
bond is required to resist forces parallel to such
and suction of the masonry units to maximize bond
joints. The factors which affect bond are basically
of the mortar to units. Water retentivity is
common to both with the exception of the influence
measured by methods described in ASTM C 91.
of compression on shear bond. Factors affecting
Units with high suction require the use of mortar
bond strength include:
with high retentivity to prevent excess and rapid
(1) Mortar properties.
water loss and reduced workability. It is noted in
(a) Cement content. Other factors equal,
ASTM C 67, in the case of clay-unit masonry, that
greatest bond strength is associated with high
mortar which has stiffened due to water loss
cement content.
because of suction results in poor bond and water
(b) Retentivity. Bond strength is
permeable joints. It is suggested in ASTM C 67
enhanced if high retentivity mortar is used with
that clay masonry units with initial rates of
high-suction units and low absorption.
absorption in excess of 30 grams per minute per 30
(c) Flow. Bond is enhanced by using the
square inches be wetted prior to placing to reduce
maximum water content consistent with good
suction. If wetting is done, care should be taken to
workability considering unit properties and envi-
ensure uniformity.
ronmental conditions.
f. Flow. Flow determined by methods of ASTM
(d) Air content. Bond decreases with
C 109 is a rough measurement of workability, but
increasing air content.
is not a test amenable to construction sites. No
generally accepted procedure has been developed
(2) Masonry unit properties.
for field measurement of workability; the mason is
(a) Surface texture. Mortar flows into
the best judge.
voids, cracks, and fissures and forms a mechanical
g. Factors affecting mortar compressive
attachment to the surface of the unit.
strength. Mortar compressive strength, typically
(b) Suction. For a given mortar, bond
measured by uniaxial compression of 2 inch cubes
strength decreases as unit suction increases. This is
in accordance with ASTM C 109 is a measure of
perhaps due to the rapid loss of water to the unit
relative mortar quality. Because of several factors,
on which mortar is placed. The mortar becomes
such as, state of stress, water content, and
less workable and bond becomes less reliable.
dimensions, the compressive strength of a mortar
(3) Workmanship factors.
cube is not directly related to compressive strength
(a) Time. The time lapse between
of mortar in a masonry joint. The basic factors
spreading mortar on a unit and placing a unit upon
which affect uniaxial cube compressive strength,
that mortar should be minimized to reduce the
however, are essentially those which affect mortar
effects of water loss from mortar due to suction of
performance in masonry, such as, proportions of
the unit on which it is placed.
portland cement, hydrated lime and sand, water
content, admixtures, air content, mixing time, and
(b) Movement. Movement of units after
sand characteristics. The proportions are critical
placing can reduce, if not break, bond between
factors affecting cube compressive strength. The
mortar and unit.
variation in mortar cube strength due to mix
(c) Pressure and tapping. Units must be
proportions is illustrated in figure 3-3. The circled
placed on mortar with sufficient downward pres-
3-5
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