Concrete f'c from Modulus of Rupture (ACI 318-19 19.2.3)
The inverse of the modulus-of-rupture tile: back out the equivalent f'c from a flexural-beam (modulus-of-rupture) test, f'c = (fr / (7.5 x lambda))^2. A 474 psi normalweight rupture strength implies ~4,000 psi. The code fr is a conservative lower bound, so the implied f'c is a lower-bound equivalent, not a cylinder-break value. A design aid; the engineer of record's stamped design governs.
Formula and source
f'c = (fr / (7.5 x lambda))^2, the ACI 318-19 19.2.3.1 relation fr = 7.5 x lambda x sqrt(f'c) solved for the compressive strength; lambda = 1.0 normalweight, 0.75 all-lightweight.
The ACI 318-19 19.2.3.1 modulus of rupture of concrete, solved for f'c, by name.
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