NZS 3101 Concrete Crack Width

Based on NZS 3101:2006 Clauses 7.6.3 and Appendix B, this tool estimates flexural crack width under service loads. For interior members with minor exposure.

Geometry & Materials

Concrete strength f'c (MPa)

Steel yield strength f_y (MPa)

Section width b_w (mm)

Section depth h (mm)

Effective cover c (mm)

Reinforcement & Loading

Total steel area A_s (mm²)

Bar diameter d_b (mm)

Number of bars

Service moment M* (kNm)

Load duration

Effective depth d = h - c (to centroid of tension steel).

Results

Crack Width Check

Effective depth d -

Modular ratio n = E_s/E_c -

Steel stress f_s -

Calculated crack width w -

Allowable crack width -

w ≤ w_allow

Utilization -

Stress ratio f_s/f_y -

Duration factor β -

Formula Summary

E_c = 4700√f'c (MPa)
n = E_s/E_c = 200000/E_c
f_s = M* / (A_s · d) · (d - x)/x
x = [-nA_s + √(nA_s)² + 2nb_wnA_s] / (nb_w)
w = 2β f_s / E_s · (c + d_b/2)

Notes

This tool uses the simplified crack width method per NZS 3101 Appendix B.

Allowable crack widths: interior members = 0.3mm, exterior members = 0.2mm.

Duration factor: β = 1.0 for short-term, β = 1.5 for long-term loading.

For more accurate analysis, use the full tension stiffening model or refer to NZS 3101 Clause 7.6.3 for crack control provisions.