Page 114 - Masonry and Concrete

Understanding Masonry
113
UNDERSTANDING MASONRY

absorption, pore structure, and surface characteristics of the units, the
water retention of the mortar, and curing conditions such as temperature,
relative humidity, and wind combine to influence the completeness and
integrity of the mortar-to-unit bond. Voids at the mortar-to-unit interface
offer little resistance to water infiltration and increase the chance of sub-
sequent disintegration and failure if repeated freezing and thawing
occurs.
Although a certain amount of surface absorption is desirable to
increase the depth of penetration of the mortar paste, excessive suc-
tion reduces the amount of water available for cement hydration at the
unit surface. Moist curing of masonry after construction assures com-
plete hydration of the cement and improves the bond of mortar to
high-suction brick and to dry, absorptive concrete masonry units. Clay
brick with low absorption, dense stone, and nonabsorptive glass block
provide little or no absorption of mortar paste into surface pores.
These types of units require a relatively stiff, low-water-content mor-
tar. Unit texture also affects bond. Coarse concrete masonry units and
the wire cut surfaces of extruded clay brick produce a better mechani-
cal bond than molded brick or the die-formed surfaces of extruded
brick. Loose sand particles, dirt, coatings, and other contaminants also
adversely affect mortar bond.
Workmanship is also critical in mortar bond. Full mortar joints
must assure complete coverage of all contact surfaces, and maximum
extent of bond is necessary to reduce water penetration. Once a unit
has been placed and leveled, additional movement will break or seri-
ously weaken the bond. Mortars with high water retention allow more
time for placing units before evaporation or unit suction alters the
moisture content of the mortar.
Masonry compressive strength depends on both the unit and the
mortar. As with concrete, the strength of mortar is determined by the
cement content and the water/cement ratio of the mix. Since water
content is adjusted to achieve proper workability and flow, and since
bond strength is ultimately of more importance in masonry construc-
tion, higher compressive strengths are sometimes sacrificed to
increase or alter other characteristics. For loadbearing construction,
building codes generally provide minimum allowable working
stresses, and required compressive strengths may easily be calculated
using accepted engineering methods. Strengths of standard mortar

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