Concrete Modulus of Rupture fr (ACI 318-19 19.2.3)
fr = 7.5 x lambda x sqrt(f'c) psi (ACI 19.2.3.1), the tensile stress at which plain concrete first cracks, setting the cracking moment behind deflection and minimum-reinforcement checks. 4000 psi normalweight -> 474 psi (~0.119 f'c); all-lightweight (lambda 0.75) -> 356 psi. A design aid; the engineer of record's stamped design governs.
Formula and source
fr = 7.5 x lambda x sqrt(f'c) psi; lambda = 1.0 normalweight, 0.75 all-lightweight.
The ACI 318-19 19.2.3.1 modulus of rupture of concrete, by name.
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