What is Bond, Bond stress, Anchorage length & bond, Development

Bond:-

The most important assumption made in the theory of RCC is that there is a perfect bond between the concrete and steel. Perfect bond in the sense the bond should act in such a way that there is no slip b/w steel and concrete.

This bond helps in transferring force from steel to concrete and concrete to steel.

Bond stress:

The stress which is acting on the outer interface of steel to the surrounding concrete is called bond stress. This stress helps in keeping bond between  reinforcement and concrete together. Bond stress resists any force that tries to pull out the rods from the concrete.

When you try to pull out the reinforcement bar from hardened concrete, then this Bond stress resists the bar to come out.

By the way different grades of concrete has different bond stress.

well, these bonds are classified into two types:-

1. Anchorage bond (Development length)
2. Flexular bond.

Anchorage bond:- 

This bond is seen  when a bar carrying certain force is removed. In such cases,  it is necessary to transfer this force in the bar to the surrounding concrete over a certain length.

This length of bar  required to transfer the force in the bar to the surrounding concrete through bond is called “Development length (D.L.)” 

Development length (DL) is determined by performing Pull out Test.

It is clear from the above image, that reinforcing bar is embedded in concrete and subjected to a pull T

T = Design Stress x area of bar = 0.87 Fy x (Pi/4) x d2

d= Dia of bar

This force must be transferred from steel to concrete through bond acting over the interface (perimeter) of the bar over a length (D.L.)

If Tbd = Avg Design bond stress

Then Ultimate Bond force = Pull out force

Tbd (Pi x d) x D.L. = 0.87 Fy [(Pi/4 x d^2)]

D.L = [0.87 x Fy x d] / [4 Tbd] 

hence all bars should extend to a distance of (DL) beyond the section where they are required to take full design force.

it is not possible to provide straight bars at all the times due to lack of space at supports. In such scenarios we provide them as hooks and bends.

The anchorage value (hook length) = 16d

The anchorage value (Bend length) =4d (45degree angle)