Shear strength and bond strength

Shear strength

Resistance of concrete to sliding of one part over another. Pure shear is about 20 to 30% of $f_c$; in beams the failure is by diagonal tension. IS 456 (Cl. 40) uses the nominal shear stress $\tau_v = V_u/bd$, compared with the permitted $\tau_c$ (Table 19, depends on grade and tension steel %) and $\tau_{c,max}$ (Table 20); stirrups carry the excess.

Bond strength

Bond is the adhesion and friction between the reinforcement and concrete, and mechanical interlock with the ribs of deformed bars, which transfers force between the two materials.

• Design bond stress $\tau_{bd}$ (IS 456, Cl. 26.2.1.1): 1.2 N/mm$^2$ for M20 plain bars in tension; increased by 60% for deformed bars (1.92 N/mm$^2$), and by 25% in compression.
• Development length: $L_d = \dfrac{\phi\,\sigma_s}{4\,\tau_{bd}}$ (Cl. 26.2.1).
• Bond is increased by higher concrete strength, ribs on bars, adequate cover, lower position of the bar (better compaction), hooks and anchorage; it is reduced by rust scale, oil, and top-cast bars with bleeding.