Concrete Stress-Strain Diagram for Nonlinear Method

\frac{σ_{c}}{f_{c m}} = \frac{k \cdot η - η^{2}}{1 + (k - 2) \cdot η}

η	=ε_c/ε_c1
ε_c1	Compressive strain for maximum value of concrete compressive stress
k	1.05·E_cm ·\|ε_c1 \|/f_cm

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