Structural Concrete - Hassoun

7.2 Development of Bond Stresses 259
Figure 7.2 Bond stresses and development length. (a) Distribution of stress along I d
and (b) radial stress in concrete around the bar.
7.2.2 Tests for Bond Efficiency
Tests to determine the bond stress capacity can be made using the pullout test (Fig. 7.2). This test
evaluates the bond capacity of various types of bar surfaces relative to a specific embedded length.
The distribution of tensile stresses will be uniform around the reinforcing bar at a specific section
and varies along the anchorage length of the bar and at a radial distance from the surface of the bar
(Fig. 7.2). However, this test does not represent the effective bond behavior in the surface of the
bars in flexural members because stresses vary along the depth of the concrete section. A second
type of test can be performed on an embedded rod (Fig. 7.3). In these tests, the tensile force, P,
is increased gradually and the number of cracks and their spacings and widths are recorded. The
bond stresses vary along the bar length between the cracks. The strain in the steel bar is maximum
at the cracked section and decreases toward the middle section between cracks.
Tests on flexural members are also performed to study the bond effectiveness along the surface
of the tension bars. The analysis of bond stresses in the bars of these members was explained earlier,
and they are represented by Eq. 7.2.
Based on this discussion, it is important to choose an appropriate length in each reinforcing
bar to develop its full yield strength without a failure in the bond strength. This length is called
the development length, l d . If this length is not provided, the bond stresses in the tension zone of a
beam become high enough to cause cracking and splitting in the concrete cover around the tension
bars (Fig. 7.4). If the split continues to the end of the bar, the beam will eventually fail. Note that
small spacings between tensile bars and a small concrete cover on the sides and bottom will reduce
the bond capacity of the reinforcing bars (Fig. 7.4).