Profesional

Source: Standard Handbook for Civil Engineers

8

Anne M. Ellis

S.K. Ghosh

David A. Fanella

Earth Tech., Inc.
Alexandria, VA

President
S.K. Ghosh Associates Inc.
Northbrook, IL

Dir. of Engineering
S.K. Ghosh Associates Inc.
Northbrook, IL

CONCRETE DESIGN
AND CONSTRUCTION

C

oncrete made with portland cement is
widely used as a construction material
because ofits many favorable characteristics. One of the most important is
a large strength-cost ratio in many applications.
Another is that concrete, while plastic, may be cast
in forms easily at ordinary temperatures to produce almost any desired shape. The exposed face
may be developed into a smooth or rough hard
surface, capable of withstanding the wear of truck
or airplane traffic, or it may betreated to create
desired architectural effects. In addition, concrete
has high resistance to fire and penetration of water.
But concrete also has disadvantages. An important one is that quality control sometimes is not so
good as for other construction materials because
concrete often is manufactured in the field under
conditions where responsibility for its production cannot be pinpointed.Another disadvantage is
that concrete is a relatively brittle material—its tensile
strength is small compared with its compressive
strength. This disadvantage, however, can be offset
by reinforcing or prestressing concrete with steel. The
combination of the two materials, reinforced concrete, possesses many of the best properties of each
and finds use in a wide variety of constructions,
includingbuilding frames, floors, roofs, and walls;
bridges; pavements; piles; dams; and tanks.

8.1

Important Properties of
Concrete

Characteristics of portland cement concrete can be
varied to a considerable extent by controlling its

ingredients. Thus, for a specific structure, it is
economical to use a concrete that has exactly the
characteristics needed, though weak in others. Forexample, concrete for a building frame should have
high compressive strength, whereas concrete for a
dam should be durable and watertight, and strength
can be relatively small. Performance of concrete in
service depends on both properties in the plastic
state and properties in the hardened state.

8.1.1

Properties in the Plastic State

Workability is an important property for manyapplications of concrete. Difficult to evaluate,
workability is essentially the ease with which the
ingredients can be mixed and the resulting mix
handled, transported, and placed with little loss in
homogeneity. One characteristic of workability that
engineers frequently try to measure is consistency,
or fluidity. For this purpose, they often make a
slump test.
In the slump test, a specimen of themix is
placed in a mold shaped as the frustum of a
cone, 12 in high, with 8-in-diameter base and
4-in-diameter top (ASTM Specification C143).
When the mold is removed, the change in height
of the specimen is measured. When the test is made
in accordance with the ASTM Specification, the
change in height may be taken as the slump.
(As measured by this test, slump decreases as
temperatureincreases; thus the temperature of the
mix at time of test should be specified, to avoid
erroneous conclusions.)
Tapping the slumped specimen gently on one
side with a tamping rod after completing the test

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CONCRETE DESIGN AND CONSTRUCTION

8.2 n Section Eight
may give additional information on the cohesiveness, workability, and placeability of the mix
(“Concrete Manual,” Bureau of Reclamation,
Government Printing Office, Washington, DC
20402 (www.gpo.gov)). A well-proportioned, workable mix settles slowly, retaining its original identity.
A poor mix...