Below equation indicates that the unit bond stress is proportional to the shear at a particular section. Whereu = Local average bond stress per unit of bar surface area
∑o = Sum of the perimeters of all the bars
Actual Distribution of Flexural Bond Stress
- The actual distribution of bond stress along deformed reinforcing bars is more complex than that represented by the above equation.
- The above beam is subject to pure bending moment, while in case of loads producing shear and moment, the situation is not so simple as predicated by the above eq. except in the general sense that bond stresses are highest in the regions of high shear.
Nature of Bond Failure
- In case of plain bar, longitudinal splitting of bar with the concrete occurs if the pull forces are greater than the adhesion forces and friction forces.
- In deformed bar, a “bond failure” in normal weight concrete is always a splitting failure. In splitting failure, the concrete splits into segments due to wedging action of the lugs against the concrete.
- Recent studies have hypothesized that the action of splitting arises from a stress condition analogous to a concrete cylinder surrounding a reinforcing bar and acted upon by the outward radial components of the bearing forces from the bar.
- The cylinder would have an inner diameter equal to the bar diameter and a thickness C equal to the smaller of Cb, the clear bottom cover, or Cs, half of the clear spacing to the next adjacent bar.
- The tensile strength of this concrete cylinder determines the strength against splitting.
- When Cs > Cb, longitudinal cracks through the bottom cover form first.
- If Cs is only nominally greater than Cb, the secondary splitting will be side splitting along the plane of the bars.
- If Cs is significantly greater than Cb, the secondary splitting will also be though the bottom cover to create a V-notch failure.
- If Cs < Cb a side-split type of failure occurs