MASONRY - Materials for Civil and Construction Engineers - page 307

T ABLE 8.1 Weight Classifications and Allowable Maximum Water Absorp-
tion of Concrete Masonry Units (ASTM C90 and C129) (Copyright ASTM,
reprinted with permission).

Maximum Water

Absorption kg/m 3 (lb/ft 3 )

Weight

Unit Weight Mg/m 3 (pcf)

Classification (Average of 3 units)

Lightweight 1.68 (105) 288 (18)

Medium Weight 1.68-2.00 (105-125) 240 (15)

Normal Weight 2.00 (125) or more 208 (13)

with dry unit weights as shown in Table 8.1. Well-graded sand, gravel, and
crushed stone are used to manufacture normal-weight units. Lightweight ag-
gregates such as pumice, scoria, cinders, expanded clay, and expanded shale
are used to manufacture lightweight units. Lightweight units are the most
common concrete units used in masonry construction because they are easy
to handle and transport, and the weight of the structure is reduced. Light-
weight units have higher thermal and fire resistance properties and lower
sound resistance than normal weight units.
Concrete masonry units are manufactured using a relatively dry (zero-
slump) concrete mixture consisting of portland cement, aggregates, water, and
admixtures. Type I cement is usually used to manufacture concrete masonry
units; however, Type III is sometimes used to reduce the curing time. Air-
entrained concrete is sometimes used to increase the resistance of the masonry
structure to freeze and thaw effects and to improve workability, compaction,
and molding characteristics of the units during manufacturing. The units are
molded under pressure, then cured, usually using low-pressure steam curing.
After manufacturing, the units are stored under controlled conditions so that
the concrete continues curing.
Concrete masonry units can be classified as load bearing (ASTM C90)
and non-load bearing (ASTM C129). Load-bearing units must satisfy a higher
minimum compressive strength requirement than non-load-bearing units,
as shown in Table 8.2. The compressive strength of individual concrete
masonry units is determined by capping the unit and applying load in the
direction of the height of the unit until failure (ASTM C140). A full-size unit
is recommended for testing, although a portion of a unit can be used if the
capacity of the testing machine is not large enough. The gross area com-
pressive strength is calculated by dividing the load at failure by the gross
cross-sectional area of the unit. The net area compressive strength is calcu-
lated by dividing the load at failure by the net cross-sectional area. The net
cross-sectional area is calculated by dividing the net volume of the unit by
its average height. The net volume is determined using the water displace-
ment method according to ASTM C140.