Bond stress and Development length

When the steel bars are embedded in concrete, the concrete after setting, adheres to the surface of the bars and resist any force that tends to pull or push this rod. The intensity of the adhesive force is called bond stress. It is the longitudinal shear stress acting on the surface between the steel and the concrete. The term bond  is used to describe the means by which slip between the steel and concrete is prevented. The bond is provided by anchoring the bars properly and extending the bars beyond the point of maximum shear.
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Bond stress is defined as the shear force per unit of Nominal surface area of a reinforcement bar acting parallel to the bar on the interface between the bars and surrounding concrete.
The bond between steel and concrete is mainly due to pure adhesive resistance, frictional resistance and mechanical resistance.
The bond stress developed in concrete around the steel reinforcement is given by

Design bond stress

• For deformed bars these values shall be increased by 60 percent.
• For bars in compression can be increased by 25 percent.

Development length ( L_d) 

Whenever some reinforcing bar is to anchored or two bars have to be given an overlap, it is essential that they must get sufficient length of embedment or overlap as the case may be, so that no slippage takes place. The length of embedment or overlap ( also known as development length or anchorage) is given by

Some Important Points:-

1. The deformed bars may be used without end anchorages (i.e. hooks) provided development length required is satisfied. The hooks should normally be provided for plain bars in tension.
2. When hooks are formed in deformed bars, the internal radius of the bend should be at least three times the diameter of the bar. The length of the straight bar beyond the end of the curve should be at least four times the diameter of bar.
3. The anchorage value of a standard U-type hook alone is equal to 16d, where d is diameter of a reinforcement bar.
4. The length of lap for reinforcement bars in tension shall not be less than L_d or 30d, whichever is greater
5. The length of lap for reinforcement bars in compression shall not be less than L_d or 24d, whichever is greater
6. When bars of two different diameters are to be spliced, the length of lap shall be calculated on the basis of diameter of the smaller bar.
7. The bond length  used for splicing bar in tension, for M15 grade concrete is equal to 58d, where d is diameter of reinforcing bar
8. The measurement of anchorage lengths of inclined bars, used to act as shear reinforcement is taken in tension zone from the end of the sloping portion of the bar and in compression zone from mid-depth of the beam.
9. The minimum spacing between horizontal parallel reinforcement of the same diameter should not be less than the diameter of bar.