Page 151 - Masonry and Concrete

Masonry Construction Techniques
150 CHAPTER FIVE

using a one-cubic-foot measuring box made of plywood or lumber. The
person at the mixer can then determine the exact number of shovels of
sand which equal one cubic foot. Since the moisture content of the
sand will vary constantly because of temperature, humidity, and evap-
oration, it is good practice to check the volume measurement at least
twice a day and make adjustments as necessary to the number of shov-
els of sand being used. Some mechanical mortar mixers are equipped
with a measuring box which is convenient to use because it is hinged
to dump directly into the mixer.
Bond strength is an important physical property of masonry mortar,
which depends on many things, including workability and water con-
tent. Unlike concrete, which is mixed with as little water as possible to
produce acceptable workability, masonry mortar requires the maxi-
mum amount of water consistent with good workability. Mortar
requires more mixing water than concrete because excess water is
rapidly absorbed by the masonry units, immediately reducing the
water-cement ratio to normal levels and providing a moist environ-
ment for curing. Unlike concrete, masonry mortar is never specified by
water-cement ratio or slump limits. Optimum water content is best
determined by the mason’s feel of the mortar on the trowel. Dry mixes
do not spread easily, produce poor bond, and may suffer incomplete
cement hydration. Mixes that are too wet are also difficult to trowel
and allow units to settle after placement. A mortar with good worka-
bility is mixed with the proper amount of water. Mortar with good
workability should spread easily, cling to vertical unit surfaces,
extrude easily from joints without dropping or smearing, and permit
easy positioning of the unit to line, level and plumb. Thus, water con-
tent is essentially self-regulating—what is good for the mason on the
scaffold is also good for the mortar itself. Quality control, therefore,
should concentrate not on water content, but on assuring batch-to-
batch consistency in the proportioning of cementitious ingredients
and aggregate. Water should be added to the mortar mix by a consis-
tent measure of known volume such as a plastic bucket. With a water
hose, it is easy to get too much water. The water proportion will vary
for different conditions of temperature, humidity, unit moisture con-
tent, unit weight, and so on. The necessary water content for grout is
significantly higher than that for mortar because grout must flow read-
ily into unit cores and cavities and around reinforcement and acces-

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