on the effect of mortar moisture content on bond
At first glance, these results seem to support
were conducted together, and their results are
current guideline recommendations that mortar
presented jointly in Figure 15. The results in Fig-
be allowed to fall to 6% moisture before it is sub-
ure 15 represent the average of five specimens.
jected to freezing. However, it is clear that the
Appendix B shows the individual test results. The
10% moisture content shows the same bond
mortars were made with a type M masonry
strength as the 6% does. We conclude, from both
cement. The KC1 mortars were made with 16%
compression and bond strengths, that mortar can
water content, which was the same initial water
be allowed to freeze at an earlier age than now
content as the admixture-free mortars included in
allowed in current guidelines. The admixtures
Figure 15. The primary finding is that KC1 mortar
can lessen cold-weather constraints.
developed a stronger bond to the masonry units
than did normal mortar. This could be due to an
Freezethaw durability
increase in the plasticity of the mix, which could
Objective
have resulted in better mechanical anchoring into
Masonry has historically been viewed as a du-
the pores of the bricks.
rable construction material. Long-term perfor-
Effect of mortar moisture content at
mance of concrete masonry structures exposed to
time of freezing on bond strength
the weather in cold regions requires the use of
This experiment was designed to define the
units that can resist the potentially destructive
effect of mortar moisture content at the time of
forces imposed on the units by cyclical freezing
freezing (a function of absorption, age, and tem-
and thawing. The severity of the exposure and
perature) on the bond strength of a masonry
the properties of the masonry unit control the
assembly. The initial moisture content of all mor-
long-term performance of the units.
tars was 16%. The moisture content of the mortar
The objective was to evaluate several test
at time of freezing was regulated by allowing the
methods that could be used as indicators of the
masonry assemblies to stand at room temperature
freezethaw durability of concrete masonry
until curves like those in Figure 6 indicated that a
units. Two freezethaw test methods that subject
prescribed moisture content had been achieved.
water-saturated specimens to cyclical freezing
and thawing were considered: ASTM C 666, "Re-
At the prescribed moisture content, the specimens
were brought into a 10C (14F) room. Twenty-
sistance of Concrete to Rapid Freezing and Thaw-
four hours later, the specimens were returned to a
20C (68F) room for 28 days. The "bond wrench
the FreezeThaw Durability of Manufactured
Concrete Masonry Units and Related Units." The
test" followed. The results presented in Figure 15
first of the two methods was originally devel-
represent the average of five specimens (individ-
oped for structural concrete, and the second is a
ual test specimen results are included in Appen-
new method that was developed specifically to
dix B).
evaluate dry mix, no-slump concrete products
Of the sets tested using the control mortar (no
such as concrete masonry units.
admixture), the one with the highest tested bond
Knowledge about the air void structures of
strength had the highest moisture content: 16%.
other related materials has provided insight into
These prisms were placed into the cold room im-
freezethaw durability performance. Therefore,
mediately after construction. These results may be
two other test methods were also considered to
explained by the fact that the mortar in these
determine if they might provide similar insights
prisms was in a plastic state when water expan-
into concrete masonry unit durability: ASTM C
sion within the mortar occurred and thus was able
457, "Microscopic Determination of Air-Void
to accommodate volume changes. Bond strengths
Content and Parameters of the Air-Void System in
decreased with decreases in mortar moisture con-
Hardened Concrete," and ASTM D 4404, "Deter-
tent (achieved by longer delay periods between
mination of Pore Volume Distribution of Rock
prism construction and placement in the cold
and Soil by Mercury Intrusion Porosimetry."
room) until a rebound in strength is observed
To evaluate these methods, six different sets of
with the estimated 6% mortar moisture. This
concrete segmental retaining wall units were
rebound effect is consistent with mortar compres-
used. The units are manufactured using the same
sive strength development as discussed under
materials and methods as conventional concrete
Critical Maturity in the Freezing Strength section of
masonry units.
this report.
17
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