Permissible Bond Stress in Deformed Bars vs Plain Bars

Introduction:
In reinforced concrete structures, the bond between the concrete and steel reinforcement is essential to transfer loads and prevent slippage. Bond stress is the stress developed in the reinforcement due to the adhesion and friction between the reinforcement and surrounding concrete. The permissible bond stress is the maximum stress that can be developed in the reinforcement without causing slip or failure.

Permissible Bond Stress in Deformed Bars:
Deformed bars have ribbed surfaces that provide better adhesion and friction with concrete compared to plain bars. As a result, the permissible bond stress in deformed bars is higher than that in plain bars. The increase in permissible bond stress is attributed to the following factors:

1. Increased Surface Area:
The ribs on the surface of deformed bars increase the surface area in contact with the concrete. This increased surface area provides more adhesion and frictional resistance, allowing for higher bond stress.

2. Mechanical Interlocking:
The ribs on the surface of deformed bars create a mechanical interlocking effect with the surrounding concrete. This effect increases the bond strength between the reinforcement and concrete, allowing for higher bond stress.

3. Reduced Slip:
The ribbed surface of deformed bars reduces the potential for slip between the reinforcement and concrete. This reduction in slip allows for higher bond stress without causing failure.

Permissible Bond Stress in Plain Bars:
Plain bars do not have any surface treatment, resulting in lower adhesion and friction with the surrounding concrete. As a result, the permissible bond stress in plain bars is lower than that in deformed bars.

Conclusion:
In working stress design, the permissible bond stress in deformed bars is higher than that in plain bars. The increase in permissible bond stress is due to the increased surface area, mechanical interlocking, and reduced slip provided by the ribbed surface of deformed bars.