Page 112 - Masonry and Concrete

Understanding Masonry
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UNDERSTANDING MASONRY

strength, but compressive strength is usually less important in
masonry mortar and grout than bond strength. Good bond between
the mortar and units provides physical stability as well as resistance
to wind loads and moisture penetration. A mortar or grout mix which
produces good bond will have a lower compressive strength than con-
crete, but only moderate compressive strengths are required for most
masonry construction.
Mortar makes up only a small part of masonry construction, but its
influence on performance and appearance are much greater than the
proportion implies. The ingredients used to make mortar and grout
directly affect the performance of the finished masonry. Cement pro-
vides strength and durability. Lime adds workability, water retention,
and elasticity. Sand serves as a strong and economical filler. High-
quality mortar and grout require high-quality ingredients.

4.6.1 Mortar and Grout Properties

The term fresh mortar refers to the wet mix of ingredients before
they begin to cure. When the material begins to set but is not fully
cured, it is called green mortar and after it has fully cured, it is
called hardened mortar. Fresh mortar and grout must be workable,
and hardened mortar and grout must have good bond strength and
durability. The quality of the ingredients, the proportions in which
they are mixed, and the way the mix is handled, placed, and cured
affect these properties.
Workability significantly influences most other mortar characteris-
tics. Workability is not easy to define, but a workable mortar has a
smooth consistency, is easily spread with a trowel, and readily
adheres to vertical surfaces. Well-graded, smooth sand enhances mor-
tar workability, as do lime, air entrainment, and proper amounts of
mixing water. The lime gives plasticity and increases the capacity of
the mix to retain water. Air entrainment introduces minute bubbles
which act as lubricants in improving workability, but air entrainment
must be limited in mortars because it reduces bond strength. Where
concrete mixes perform best when mixed with the minimum amount
of water necessary, masonry mortar is just the opposite. Mortar
requires the maximum amount of water consistent with workability.
Variations in units and in weather conditions affect optimum mortar
consistency and workability. For example, mortar for heavier units

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