Civil Engineering Reference
In-Depth Information
in bar tension due to the fact that the uncracked concrete is now resisting tension. Thus
the bond stress in the surrounding concrete, which was zero at the crack, will drastically
change within this small distance as the tension in the bar changes.
In the past it was common to compute the maximum theoretical bond stresses at
points in the members and to compare them with certain allowable values obtained by
tests. It is the practice today, however, to look at the problem from an ultimate standpoint,
where the situation is a little different. Even if the bars are completely separated from the
concrete over considerable parts of their length, the ultimate strength of the beam will not
be affected if the bars are so anchored at their ends that they cannot pull loose.
The bonding of the reinforcing bars to the concrete is due to several factors, including
the chemical adhesion between the two materials, the friction due to the natural roughness
of the bars, and the bearing of the closely spaced rib-shaped deformations on the bar sur-
faces against the concrete. The application of the force
P
to the bar shown in Figure 7.5 is
considered in the discussion that follows.
When the force is first applied to the bar, the resistance to slipping is provided by the
adhesion between the bar and the concrete. If plain bars were used, it would not take much
tension in the bars to break this adhesion, particularly adjacent to a crack in the concrete. If
this were to happen for a smooth surface bar, only friction would remain to keep the bar
from slipping. There is also some Poisson's effect due to the tension in the bars. As they are
tensioned they become a little smaller, enabling them to slip more easily. If we were to use
straight, plain, or smooth reinforcing bars in beams, there would be very little bond strength
and the beams would only be a little stronger than if there were no bars. The introduction of
deformed bars was made so that in addition to the adhesion and friction there would also be
a resistance due to the bearing of the concrete on the lugs or ribs (or deformations) of the
bars as well as the so-called shear-friction strength of the concrete between the lugs.
Deformed bars are used in almost all work. However, plain bars or plain wire fabrics
are sometimes used for lateral reinforcement in compression members (as ties or spirals
as described in Chapter 9), for members subject to torsion, and for confining reinforcing
in splices (ACI R3.5.4).
As a result of these facts, reinforcing bars are made with rib-type deformations. The
chemical adhesion and friction between the ribs are negligible, and thus bond is primarily
supplied by bearing on the ribs. Based on testing, the crack patterns in the concrete show
that the bearing stresses are inclined to the axis of the bars from about 45
to 80
(the
angle being appreciably affected by the shape of the ribs.)
1
Figure 7.5
Bearing forces on bar and bearing
of bar ribs on concrete.
1
Goto, Y., 1971, “Cracks Formed on Concrete Around Deformed Tensioned Bar,”
ACI Journal, Proceedings
,
68, p. 244.
