is considered unsafe. For this reason, the compressive
strength of concrete is checked on all projects.
The strength of the concrete is checked by the use
of cylindrical molds that are 6 inches in diameter and
12 inches in height. Concrete samples must be taken
on the jobsite from the concrete that is being placed.
After being cured for a time period that ranges
between 7 to 28 days, the cylinders are broken to
failure by a laboratory crushing machine that
measures the force required for the concrete to fail.
For further information on concrete strength and
testing, refer to Engineering Aid 3, NAVEDTRA
10696, and NAVFAC MO 330. (The MO 330 should
be maintained in a battalions tech library.)
PURPOSES AND TYPES OF
Reinforced concrete was designed on the principle
that steel and concrete act together in resisting force.
Concrete is strong in compression but weak in
tension. The tensile strength is generally rated about
10 percent of the compression strength. For this
reason, concrete works well for columns and posts that
are compression members in a structure. But, when it
is used for tension members, such as beams, girders,
foundation walls, or floors, concrete must be
reinforced to attain the necessary tension strength.
Steel is the best material for reinforcing concrete
because the properties of expansion for both steel and
concrete are considered to be approximate] y the same;
that is, under normal conditions, they will expand and
contract at an almost equal rate.
NOTE: At very high temperatures, steel expands
more rapidly than concrete and the two materials will
Another reason steel works well as a
reinforcement for concrete is because it bonds well
with concrete. This bond strength is proportional to
the contact surface of the steel to the concrete. In other
words, the greater the surface of steel exposed to the
adherence of concrete, the stronger the bond. A
deformed reinforcing bar adheres better than a plain,
round, or square one because it has a greater bearing
surface. In fact, when plain bars of the same diameter
are used instead of deformed bars, approximately 40
percent more bars must be used.
The rougher the surface of the steel, the better it
adheres to concrete. Thus steel with a light, firm layer
of rust is superior to clean steel; however, steel with
loose or scaly rust is inferior. Loose or scaly rust can
be removed from the steel by rubbing the steel with
burlap or similar material. This action leaves only the
firm layer of rust on the steel to adhere to the concrete.
NOTE: Reinforcing steel must be strong in
tension and, at the same time, be ductile enough to be
shaped or bent cold.
Reinforcing steel can be used in the form of bars
or rods that are either plain or deformed or in the form
of expanded metal, wire, wire fabric, or sheet metal.
Each type is useful for different purposes, and
engineers design structures with those purposes in
Plain bars are round in cross section. They are
used in concrete for special purposes, such as dowels
at expansion joints, where bars must slide in a metal
or paper sleeve, for contraction joints in roads and
runways, and for column spirals. They are the least
used of the rod type of reinforcement because they
offer only smooth, even surfaces for bonding with
Deformed bars differ from the plain bars in that
they have either indentations in them or ridges on
them, or both, in a regular pattern. The twisted bar, for
example, is made by twisting a plain, square bar cold.
The spiral ridges, along the surface of the deformed
bar, increase its bond strength with concrete. Other
forms used are the round and square corrugated bars.
These bars are formed with projections around the
surface that extend into the surrounding concrete and
prevent slippage. Another type is formed with
longitudinal fins projecting from the surface to
prevent twisting. Figure 7-1 shows a few of the types
of deformed bars available. In the United States,
deformed bars are used almost exclusively; while in
Europe, both deformed and plain bars are used.
Figure 7-1.Various types of deformed bars.