Page 113 - Masonry and Concrete

Understanding Masonry
112 CHAPTER FOUR

must be more dense to prevent uneven settling and to keep excessive
mortar from being squeezed out of the joints. Hot summer tempera-
tures require a soft, wet mix to compensate for evaporation.
Mortar is subject to water loss by evaporation, particularly on hot,
dry days. Retempering (the addition of mixing water to compensate for
evaporation) is good practice in masonry construction. A partially
dried and stiffened mortar will have lower bond strength if the evapo-
rated water is not replaced. Mortar which has begun to harden as a
result of cement hydration, however, should be discarded. Since it is
difficult to tell whether mortar stiffening is due to evaporation or
hydration, the suitability of mortar is judged on the time elapsed after
initial mixing. When air temperatures are above 80°F, mortar may be
1
safely retempered if needed during the first 1- 2 to 2 hours after mix-
ing. When temperatures are below 80°F, mortar may be retempered for
1
2- 2 hours after mixing. Industry standards recommend that all mortar
1
be used within 2- 2 hours, and permits retempering as frequently as
needed within this time period. Tests have shown that the decrease in
compressive strength is minimal if retempering occurs within recom-
mended limits, and that it is much more beneficial to the performance
of the masonry to maximize workability and bond by replacing evapo-
rated moisture.
For the majority of masonry construction, the single most impor-
tant property of mortar is bond strength and integrity. For durability,
weather resistance, and resistance to loads, it is critical that the bond
between units and mortar be strong and complete. The term mortar
bond refers to a property that includes

■ Extent of bond or area of contact between unit and mortar

■ Bond strength or adhesion of the mortar to the units

The mechanical bond between the mortar and the individual bricks,
blocks, or stones holds the construction together, provides resistance to
tensile and flexural stress, and resists the penetration of moisture. The
strength and extent of the bond are affected by many variables of mater-
ial and workmanship. The mortar must have good workability to spread
easily and wet the unit surfaces. The unit surfaces must be rough enough
to provide physical interlocking, and sufficient absorption to draw the
wet mortar into these surface irregularities. The moisture content,

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