Building Design and Construction Handbook - Merritt - Ventech!

4.20 SECTION FOUR
sometimes encountered, however, in determining just how much material constitutes
a cubic foot: a bag of cement (94 lb) by agreement is called a cubic foot in proportioning
mortars or concretes, but an actual cubic foot of lime putty may be used
in proportioning mortars. Since hydrated limes are sold in 50-lb bags (Art. 4.5.3),
each of which makes somewhat more than a cubic foot of putty, weights of 40, 42,
and 45 lb of hydrated lime have been used as a cubic foot in laboratory studies;
but on the job, a bag is frequently used as a cubic foot. Masonry cements are sold
in bags containing 70 to 80 lb (Art. 4.8), and a bag is considered a cubic foot.
4.16.2 Properties of Mortars
Table 4.4 lists types of mortars as a guide in selection for unit masonry.
Workability is an important property of mortars, particularly of those used in
conjunction with unit masonry of high absorption. Workability is controlled by the
character of the cement and amount of sand. For example, a mortar made from 3
parts sand and 1 part slaked lime putty will be more workable than one made from
2 parts sand and 1 part portland cement. But the 3:1 mortar has lower strength. By
proper selection or mixing of cementitious materials, a satisfactory compromise
may usually be obtained, producing a mortar of adequate strength and workability.
Water retention—the ratio of the flow after 1-min standard suction to the flow
before suction—is used as an index of the workability of mortars. A high value of
water retention is considered desirable for most purposes. There is, however, a wide
variation in water retention of mortars made with varying proportions of cement
and lime and with varying limes. The ‘‘Standard Specification for Mortar for Unit
Masonry,’’ ASTM C270, requires mortar mixed to an initial flow of 100 to 115, as
determined by the test method of ASTM C109, to have a flow after suction of at
least 75%.
Strength of mortar is frequently used as a specification requirement, even though
it has little relation to the strength of masonry. (See, for example, ASTM C270,
TABLE 4.4 Types of Mortar
Mortar
type
Portland
cement
Masonry
cement
Parts by volume
Hydrated lime or
lime putty
Aggregate measured in
damp, loose condition
Min avg
compressive
strength of
three 2-in
cubes at 28
days, psi
M 1 1 2500
1
1
⁄4
S 1
⁄2 1 1800
1 Over 1 ⁄4 to 1 ⁄2 Not less than 21 ⁄4
N 1 and not more than 750
1 Over 1 ⁄2 to 11 ⁄4 3 times the sum of
O 1 the volumes of the 350
1 Over 11 ⁄4 to 21 ⁄2 cements and limes
K 1 Over 21 ⁄2 to 4 used 75
PL 1 1 1
⁄4 to ⁄2 2500
PM 1 1 2500