EK543

When the pull on the bar is small, high stresses develop near the loaded end of the bar; some slip of the bar occurs as adhesion between the bar and the concrete in this region breaks down and as steel ribs begin to crush part of the concrete between the ribs. Failure can occur at this early stage if concrete surrounding the bar is unconfined. If some degree of confinement is present, a rise in load tends to shift the bond stress diagram deeper along the bar engaging additional ribs to resist the additional load. In some instances, especially when the level of confinement is low around the loaded end of the bar, bond resistance near the
surface can be reduced to zero due to extensive cracking and cone breakouts formed in the concrete. As load is increased to maximum levels, shifting of the bond stress diagram continues as bond stress peaks
move deeper along the confined region of the bar. Failure occurs when there is no capacity left provided by the interlocking of the steel ribs and the concrete.