RAM Concept uses a parabolic-plastic stress-strain curve for concrete based on the Portland Cement Association’s parabolic stress-strain curve [see PCA’s Notes on ACI 318-99 Building Code Requirements for Structural Concrete, Figure 6-8]. This curve is used for both strength and service cross section analyses. The curve is totally defined by two parameters:

• f’_c = Concrete Cylinder Strength
• E_c = Concrete Elastic Modulus (tangent at zero strain)

A third parameter, the strain at which the concrete behavior changes from parabolic to linear, is calculated:

ε₀ = 2 (0.85 f’_c)/ E_c

For ε_c < 0 (tension)

f_c = 0

For 0 < ε_c < ε₀ (parabolic range)

f_c = 0.85 f’_c [2( ε_c /ε₀ ) – (ε_c /ε₀ )²]

For ε_c ≥ ε₀ (plastic range)

f_c = 0.85 f’_c