Test: Bond Stresses - Civil Engineering (CE) MCQ

According to Provisions laid down in IS 456: 2000,
The bond stress for different grades of concrete as per different established provisions are as follows:

The bond stresses value tabulated above is for the plain bar in tension.
Important Point
The value of bond stress is increased by 60% when deformed bars are used as having a better interlocking bond formation in concrete.
The value of bond stress is increased by 25% when the bar is in compression.

The magnitude of bond stresses developed between concrete and steel and its variation in the transfer zone of pretensioned beam, the deformation of concrete shrinkage depends on environmental conditions, age of concrete, size of concrete, concrete composition, etc.

Bond stress: 

• When the steel bars are embedded in concrete, the concrete after setting adheres to the surface of the bars and thus resist any force that tends to pull or push this rod. The intensity of this adhesive force is called bond stress. 
• It is the longitudinal shear stress acting on the surface between the steel and 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.

Bond strength is affected by many factors. The following factors attempt to increase the bond strength:

• Using ribbed/deformed bars instead of plain bars.
• Using smaller diameter bars
• use of higher concrete grade
• Increasing the cover around the bar
• Increasing the length of embedment, hooks, bends, etc.
• Using mechanical anchorages
• Proper curing

Additional Information
The permissible bond stress for the different grades of concrete are tabulated below: 
As per IS 456: 2000, Clause 26.2.1.1,
In limit state method of design, the design bond strength for plain bars in tension are given below in the table:

Note:

• The above value bond stress is for the plain bar in tension.
• The value of bond stress is increased by 60 % when deformed bars are used.
• The value of bond stress is increased by 25 % when the bar is in compression.

Development length:
(i) The cal culated tension or compression in any bar at any section shall be developed on each side of the section by an appropriate development length or end anchorage or a combination thereof.
(ii) Development length can be calculated as:

Where, ϕ = Diameter of bar
τ_bd = Design bond stress = Permissible value of average bond stress
The value of bond stress is increased by 60% for a deformed bar in tension and a further increase of 25% is made for bars in compression.
Calculation:
Given,
ϕ = 12 mm
τ_bd = 1.5 N/mm² 
So, τ_bd = 1.5 × 1.6 N/mm²
Development length, 
 = 543.75 mm ≈ 544 mm

Bond stress: 

• When the steel bars are embedded in concrete, the concrete after setting, adheres to the surface of the bars and thus resist any force that tends to pull or push this rod. The intensity of this adhesive force is called bond stress. 
• It is the longitudinal shear stress acting on the surface between the steel and 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.

Explanation:
If the bond stress developed in a reinforced concrete beam is more than the permissible value, then it can be brought down by,

• Increasing the depth of beam
• Decreasing the diameter of the bar
• Increasing the number of bars

Additional Information
The permissible bond stress for the different grades of concrete are tabulated below:
As per IS 456: 2000, Clause 26.2.1.1,
In limit state method of design, the design bond strength for plain bars in tension are given below in the table:

Note:

• The above value bond stress is for the plain bar in tension.
• The value of bond stress is increased by 60 % when deformed bars are used.
• The value of bond stress is increased by 25 % when the bar is in compression.

Bond stress: 

• When the steel bars are embedded in concrete, the concrete after setting adheres to the surface of the bars and thus resist any force that tends to pull or push this rod. The intensity of this adhesive force is called bond stress. 
• It is the longitudinal shear stress acting on the surface between the steel and 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.

Bond strength is affected by many factors. The following factors attempt to increase the bond strength:

• Using ribbed/deformed bars instead of plain bars.
• Using smaller diameter bars
• use of higher concrete grade
• Increasing the cover around the bar
• Increasing the length of embedment, hooks, bends, etc.
• Using mechanical anchorages
• Proper curing

Additional Information
The permissible bond stress for the different grades of concrete are tabulated below: 
As per IS 456: 2000, Clause 26.2.1.1,
In limit state method of design, the design bond strength for plain bars in tension are given below in the table:

Note:

• The above value bond stress is for the plain bar in tension.
• The value of bond stress is increased by 60 % when deformed bars are used.
• The value of bond stress is increased by 25 % when the bar is in compression.

The bond stress is zero at the end but builds up rapidly to a maximum over a very short length and this value decreases as the stress in the wire builds up at a distance equal to the transmission length, the bond stress is almost zero while the stress in steel and concrete reach their maximum values.

Design bond stress:

• Bond stress is the result of the bonding between the concrete surface and the reinforcement steel.
• It varies depending upon the type of concrete and type of reinforcement used.
• If plane rounded steel is used as reinforcement then the bond stress will be less, if the same concrete is used with HYSD steel as reinforcement then the bond stress is higher.
• The bond between steel and concrete is mainly due to pure adhesive resistance, frictional resistance, and mechanical resistance.

According to IS 456 ∶ 2000, the design bond stress in the limit state method for plain bars in tension shall be as below:

Note:

• For deformed bars conforming to IS 1786, these values shall be increased by 60 percent.
• For bars in compression. the values of bond stress for bars in tension shall be increased by 25 percent.

Calculation:
Design bond stress for deformed bars (conforming to IS 1786) for M30 concrete
= 1.5 × 1.60 = 2.4 N/mm²

The magnitude of the average bond stress is considerably less than the maximum local bond stress but according to the investigations of R_os, the average bond stress varied from 3.25 to 1n/mm² for round wires of 1.5 to 5mm diameter in the case of wires initially tensioned to a stress of 1200n/mm² and tensile stress shall not be allowed at any loading stage up to cracking in case of members assembled out of precast blocks.

Based on tests conducted according to Marshall using wires of 2 and 5mm diameter stressed to 1575 and 1100n/mm² respectively in conjunction with a concrete having cube strength of 80n/mm², the values of maximum bond stress and constant ψ found to be 7.42n/mm² and 0.00725 respectively.