Page 9 - Masonry and Concrete

Understanding Concrete
8 CHAPTER TWO

2.1 Basic Properties of Concrete

The term fresh concrete refers to the wet mix of ingredients before they
begin to cure. When the material begins to set but is not fully cured, it
is called green concrete. After it has fully cured, it is called hardened
concrete. Fresh concrete must be workable, and hardened concrete
must be strong and durable. The quality of the ingredients, the pro-
portions in which they are mixed, and the way the concrete is han-
dled, placed, and cured affect these properties.

2.1.1 Properties of Fresh Concrete
Concrete workability is the relative ease with which a fresh mix can be
handled, placed, compacted, and finished without segregation or sep-
aration of the individual ingredients. Good workability is required to
produce concrete that is both economical and high in quality. Fresh
concrete has good workability if it can be formed, compacted, and fin-
ished to its final shape and texture with minimal effort and without
segregation of the ingredients. Concrete with poor workability does
not flow smoothly into forms or properly envelop reinforcing steel and
embedded items, and it is difficult to compact and finish. Depending
on the application, though, a mix that has good workability for one
type or size of element may be too stiff or harsh for another, so the term
is relative. Each mix must be suitable for its intended use, achieving a
balance among required fluidity, strength, and economy. Workability
is related to the consistency and cohesiveness of the mix and is
affected by cement content, aggregates, water content, and admixtures.
Concrete workability is increased by air entrainment. Entrained air
is different from entrapped air. Entrapped air usually accounts for
about 1 to 2% of the volume of fresh concrete and its inclusion is not
intentional. Small amounts of air are inadvertently entrapped in the
concrete mixing process. Air content can be intentionally increased by
a controlled process called air entrainment, which uses either a spe-
cial cement or a chemical admixture to introduce evenly distributed,
microscopic air bubbles. In fresh concrete, the tiny air bubbles act
almost like ball bearings or a lubricant in the mix, and in hardened
concrete they increase winter durability. Too much air reduces the
strength of concrete, though, so air content is generally recommended
to be within the ranges shown in Figure 2-1.

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