Page 110 - Design of Reinforced Masonry Structures

3.4 CHAPTER THREE

tallow, salts of wood resins, and various chemicals have been employed as admixtures.
Admixtures are commercially produced; accordingly, their composition is available
from the manufacturers upon request. Admixtures containing chlorides accelerate the
corrosion of steel reinforcing, ferrous ties and anchors, metal doors, and metal window
frames. Therefore, admixtures containing chlorides are not permitted for use in mortar.
Specification for Masonry Structures [3.4] does not permit use of admixtures containing
more than 0.2 percent chloride ions.
Specification for Masonry Structures [3.4] requires that mortar be prepared at the jobsite
by mixing cementitious materials and aggregates with a sufficient amount of water for 3 to
5 min in a mechanical batch mixer so as to produce workable consistency. Unless accept-
able, mortar should not prepared by hand mixing. Workability of mortar should be main-
tained by remixing or retemping mortar that has begun to harden or stiffen due to hydration
of cement. Mortar not used within 2½ h of initial mixing must be discarded. Temperature
and wind velocity at the time of construction have a significant influence on useful life of
mortar. If the ambient temperature exceeds 100 or 90°F with a wind velocity of 8 mph, then
the mortar must be discarded if not used within 2 h of initial mixing.

3.2.3 Workability of Mortar
A mason’s main requirement for mortar is workability. Without good workability, the
chances for well-filled mortar joints in the masonry are very low [3.11]. Workable mortar
can be spread easily with a trowel into the separations and crevices of the masonry unit.
It also supports the weight of the masonry units when placed and facilitates alignment.
It adheres to vertical masonry surfaces and readily extrudes from the mortar joints when
mason applies pressure to bring the unit into alignment.
Workability is the result of the ball bearing effect of aggregate particles lubricated by
the cementing paste. It is a combination of several properties such as plasticity, consis-
tency, cohesion, and adhesion, all of which cannot be precisely measured in the laboratory.
Although largely determined by aggregate grading, material proportions, and air content,
the final adjustment to workability depends on water content [3.1]. However, the addition
of water into a mortar mix is not governed by any specific volumetric requirements. The
addition of water is left to the discretion of the mason contractor. The amount of water
required to produce a workable mortar depends on the mortar type, the moisture content of
the sand, the consistency desired, and the absorption rate of the masonry units. The mason
best assesses the workability by observing the response of mortar to the trowel. Adding too
little or too much water would produce mortar that is not workable. A workable mortar is
obtained by adding just enough water to the mortar mix to produce a mortar that is “sticky”
and adheres well to the specified masonry units [3.1].

3.2.4 Methods of Specifying Mortar
To satisfy minimum code requirements, mortar must conform to ASTM C270-05a which
provides two methods of specifying mortar: (1) by proportions and (2) by properties. The
proportions specification tells the contractor to mix the constituents in the volumetric pro-
portions given in ASTM C270-05a Table C-1 (see Table A.1); no physical requirements are
placed on the mortar itself. The properties specification instructs the contractor to develop a
mortar mix that will yield the specified properties under laboratory testing conditions prior to
construction. These are given in ASTM C270-05a Table C-2 (see Table A.2). The results are
submitted to the owner’s representative, and the proportions of ingredients as determined