Bond between steel and concrete

Bond between steel and concrete is the grip which allows stress transfer from concrete to the reinforcing bar, so that the two act together in a member. It has three parts: chemical adhesion of cement paste to steel, friction caused by shrinkage-induced gripping, and mechanical interlock between ribs of deformed bars and concrete (the biggest contribution).

The bond stress is $\tau_b = \dfrac{T}{\pi \phi L}$, and the design bond stress for M20 is 1.2 N/mm$^2$ for plain bars (IS 456, Cl. 26.2.1.1), 60% more for deformed bars. The development length $L_d = \phi\sigma_s/(4\tau_{bd})$ provides the required anchorage. Bond is better with higher $f_{ck}$, deformed bars, adequate cover, and well-compacted concrete; it is reduced by oil, loose rust and by bleeding under horizontal top bars. It is tested by the pull-out test.