Page 116 - Masonry and Concrete

Understanding Masonry
115
UNDERSTANDING MASONRY

improves workability and increases resistance to frost action and the
scaling caused by chemical removal of snow and ice. Air-entrained
mixes are not as strong as ordinary portland cement mixes, and exces-
sive air is detrimental in mortar and grout because it reduces bond to
masonry units and reinforcing steel.
Air-entrained cements are used primarily in horizontal applica-
tions where exposure to ponded water, ice, and snow is greatest.
Entrained air produces tiny voids in concrete or mortar into which
freezing water can expand without causing damage. Masonry paving
with mortared joints may enjoy some of the benefits of air-entrainment
in resisting the expansion of freezing water. Although masonry indus-
try standards limit the air content of masonry mortar, the benefits of
higher air contents in resisting freeze-thaw damage to paving may out-
weigh the decrease in bond strength. Since mortared masonry paving
systems are generally supported on concrete slabs, the bond strength
of the masonry is less important than its resistance to weathering. In
paving applications, lower bond strength might be tolerated in return
for increased durability.
In the United States, portland cement is packaged in bags contain-
ing exactly one cubic foot of material and weighing exactly 94 lbs. This
standardized packaging allows consistency in proportioning and mix-
ing mortar and grout by either weight or volume measurement.

Lime: The mortar used in most historic buildings was made only
with lime and sand and did not contain any cement. Lime mortars
were strong and durable but cured very slowly by a process called car-
bonation. Construction was also slow because the mortar had to gain
strength before it could support very much weight. The invention of
portland cement in the early 1800s changed the way mortar was made
by substituting cement in the mix for a portion of the lime. Contempo-
rary cement and lime mortars are now made with a higher proportion
of cement than lime. Although this has reduced curing time and
speeded up construction, the trade-off is that the higher portland
cement content makes fresh mixes stiff and hardened mortar brittle. A
cement mortar made without any lime is harsh and unworkable, high
in compressive strength, but weak in bond and other required charac-
teristics. The continued use of lime, although reduced in proportion,
has many beneficial effects in masonry mortar and grout.

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