What is BOND?

✓ Bond is a grip between Concrete and Steel. The above two materials shear the … loads. Once the grip is absent, the reinforcement provided would serve the purpose.

What is BOND STRESS?

✓ The internal griping capacity generated in per unit area against … loads are called Bond Stress. So, griping stress is called Bond Stress.

Types of Bond :

✓ There are two types of Bond.

  • Direct Bond
  • Local Bond
Intensity of Bond Stress :

✓ The intensity of Bond Stress is defined as the ratio of load bearing capacity of bar and the contact area of bars in concrete.

τbd = (P/πφL) or (T/πφL)

       = π(φ)2σs/πφL

Considering Ld as Bond length,

τbd = φσs/4Ld

so, Ld = φσs/τbd

This Bond Length also called development length.

Hence,

φ = Nominal diameter of the bar

σs = Stress in bar at the section considered at design load

τbd = Design bond stress


Cl.26.2.1.1(limit)

GRADE OF CONCRETE     DESIGN BOND STRESS

M20                      1.2

M25                      1.4

M30                      1.5

M35                      1.7

M40                      1.9

For deformed bar,

τ'bd = 1.6τbd

For bars in compression,

τ'bd = 1.25τbd

(Working)
M10 — ……

M15 — 0.6

M20 — 0.8

M25 — 0.9

M30 — 1.0

M35 — 1.1

M40 — 1.2

  • LOCAL BOND

The griping capacity between steel and concrete against bending is called Local Bond.

Moment, M = T × Lever Arm

At section (1-1),

say, M1 = T1 × jd ………(i)

so, T1 = M1/jd ………(i)

At section (2-2),

M2 = M1+ dM

M2 = (T1 + ΔT) × jd

(T1 + ΔT) = M2/j.dz


so, ΔT = (ii) – (i)

so, ΔT = dM/jd ………(iii)

Bond griping force for this ‘dx’ = (π.φ.dx.τbd) = ΔT ………(iv)

so, dM/jd = (π.φ.dx.τbd)

so, τbd = (dM/dx) × 1/(jd×πφ)

so, τbd = V/(πφ×jd) ………(v)

From the equation (v) the perimeter bars, required to limit bond stress to a particular can be computed.

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The above equation does not depends on σs value but on φ value i.e. diameter of bar mainly.

Factors effecting the properties of such discussed bonds in previous pages :
  • Sufficient cover to reinforcement
  • Richness (τbd) of concrete
  • Roughness of steel (Cl.26.2.1.1)
  • Using twisted bars, welding the stirrup bars with main bar.