Failure to inspect form work during and after
concrete placement to detect abnormal
deflections or other signs of imminent failure.
There are many reasons why forms fail. It is the
responsibility of the Builder to ensure that the forms
are correctly constructed according to design, and that
proper techniques are followed.
LEARNING OBJECTIVE: Upon completing
this section, you should be able to determine
the types of ties for and placement of
Concrete is strong under compression, but
relatively weak under tension. The reverse is true for
steel. Therefore, when the two are combined, one
makes up for the deficiency of the other. When steel
is embedded in concrete in a manner that assists it in
carrying imposed loads, the combination is known as
reinforced concrete. The steel may consist of welded
wire fabric or expanded metal mesh, but, more often,
it consists of reinforcing bars, or more commonly
WELDED WIRE FABRIC
Welded wire fabric, often referred to as wire
mesh, comes in rolls and sheets. These must be cut
to tit your individual application. The individual
sections of fabric must be tied together, or lapped,
to form a continuous sheet of fabric.
Specifications and designs are usually used when
wire fabric is being lapped. However, as a rule of
thumb, one complete lap is usually sufficient with a
minimum of 2 inches between laps. Whenever the
rule of thumb is not allowed, use the end lap or side
In the end lap method, the wire mesh is lapped by
overlapping one full mesh measured from the end of
the longitudinal wires in one piece to the end of
longitudinal wires in the adjacent piece. The two
pieces are then tied at 1 1/2-foot centers with a snap
tie. In the side lap method, the two longitudinal side
wires are placed one alongside and overlapping the
other and then are tied with a snap tie every 3 feet.
Before placing reinforcing steel in forms, all form
oiling should be completed. As mentioned earlier, oil
or other coating should not contact the reinforcing
steel in the formwork. Oil on reinforcing bars reduces
the bond between the bars and the concrete. Use a
piece of burlap to clean the bars of rust, scale, grease,
mud, or other foreign matter. A light film of rust or
mill scale is not objectionable.
Rebars must be tied together for the bars to
remain in a desired arrangement during pouring.
Tying is also a means of keeping laps or splices in
place. Laps allow bond stress to transfer the load
from one bar, first into the concrete and then into the
Methods of Tying
Several types of ties can be used with rebar.
Some are more effective than others. The views in
figure 7-20 illustrate the six types used by the
Seabees: (A) snap, or simple, tie, (B) wall tie, (C)
double-strand tie, (D) saddle tie, (E) saddle tie with
twist, and (F) cross, or figure-eight, tie. As a Builder,
you will probably be concerned only with the snap
Figure 7-20.-Types of ties.