Page 10 - Masonry and Concrete

Understanding Concrete
9
UNDERSTANDING CONCRET

Maximum Aggregate Size, in. Air Content, % by Volume
3
⁄8 6 to 10
1
⁄2 5 to 9
3
⁄4 4 to 8
1 1
1 3 ⁄2 to 6 ⁄2
1
1 ⁄2 3 to 6
FIGURE 2-1

Recommended air content for various maximum aggregate sizes. (From
Waddell, Concrete Manual, International Conference of Building Officials,
1989. Based on Uniform Building Code Table 26-B).

Consistency is the aspect of workability related to the flow charac-
teristics of fresh concrete. It is an indication of the fluidity or wetness
of a mix and is measured by the slump test. Fresh concrete is placed in
a metal cone. When the cone is removed, the concrete slumps a certain
amount, depending on how fluid it is. A wet, soft mix slumps more
than a drier, stiffer one. A high-slump concrete is one that is very fluid,
and a low-slump concrete is drier and more stiff. A high-slump mix
may cause excessive bleeding, shrinkage, cracking, and dusting of the
hardened concrete. There is a certain range of consistency which is
appropriate for each type of work. Workability is at a maximum in con-
crete of medium consistency with a slump between 3 and 6 in (Figure
2-2). Both very dry (low slump) and very wet (high slump) mixes are
less workable.
Cohesiveness is the element of workability which indicates
whether a mix is harsh, sticky, or plastic. Plasticity is a desirable prop-
erty in concrete, indicating that a mix can be molded and hold a shape
when formed. A harsh mix lacks plasticity and the ingredients may
tend to separate. Harshness can be caused by either an excess or defi-
ciency of mixing water (high- or low-slump mixes), a deficiency of
cement (lean mixes), or a deficiency of fine aggregate particles. Harsh-
ness may also be caused by an excess of rough, angular, flat, or elon-
gated aggregate particles. Harsh mixes can sometimes be improved by
air entrainment or by increasing the fine aggregate or cement content,

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