Bond Between Concrete and Steel † permissible stress for the bond
between concrete and steel can be taken as one tenth of the
compressive concrete stress, plus 0„175 N/mm2*. Given the stresses in
concrete and steel, it is possible to calculate sufficient grip length.
e.g. concrete working stress of 5 N/mm^2
steel working stress of 125 N/mm^2
sectional area of reinf. bar = 3„142 r^2 or 0„7854 d^2
tensile strength of bar = 125¾0„7854 d^2
circumference of bar = 3„142 d
area of bar in contact = 3„142¾d¾L
Key: r = radius of steel bar
d = diameter of steel bar
L = Length of bar in contact
- Conc. bond stress = (0„10¾5 N/mm^2 ) + 0„175 = 0„675 N/mm^2
Total bond stress = 3„142 d¾L¾0„675 N/mm^2
Thus, developing the tensile strength of the bar:
(^125) ¾ 0.7854 d^2 = 3„142 d¾L¾0„675
98„175 d = 2„120 L
L = 46 d
As a guide to good practice, a margin of 14 d should be added to L.
Therefore the bar bond or grip length in this example is equivalent
to 60 times the bar diameter.
Grip Length of Reinforcement