Civil Engineering Reference
In-Depth Information
Twisted square bar, formerly used to increase bond between concrete and steel.
(Courtesy of Clemson University Communications Center.)
Equal and opposite forces develop between the reinforcing bars and the concrete as
shown in Figure 7.5. These internal forces are caused by the wedging action of the ribs
bearing against the concrete. They will cause tensile stresses in a cylindrical piece of con-
crete around each bar. It's rather like a concrete pipe filled with water which is pressing
out against the pipe wall, causing it to be placed in tension. If the tension becomes too
high, the pipe will split.
In a similar manner, if the bond stresses in a beam become too high, the concrete will
split around the bars and eventually the splits will extend to the side and/or bottom of the
beam. If either of these types of splits runs all the way to the end of the bar, the bar will
slip and the beam will fail. The closer the bars are spaced together and the smaller the
cover, the thinner will be the concrete cylinder around each bar and the more likely that a
bond-splitting failure will occur.
Figure 7.6 shows examples of bond failures that may occur for different values of
concrete cover and bar spacing. These are as shown by MacGregor.
2
Splitting resistance along bars depends on quite a few factors, such as the thickness
of the concrete cover, the spacing of the bars, the presence of coatings on the bars, the
types of aggregates used, the transverse confining effect of stirrups, and so on. Because
Figure 7.6
Types of bond failures.
2
MacGregor, J. G., 1997,
Reinforced Concrete Mechanics and Design
, 3rd ed. (Upper Saddle River, NJ:
Prentice-Hall), p. 288.
