Flexural strength (modulus of rupture) is measured on a plain concrete beam under symmetrical third-point (two-point) loading; the result is computed from the failure load, span and section, with a correction/rejection rule depending on whether fracture occurs within the middle third. It is the design parameter for pavement-quality concrete and rigid pavements (IRC 58).
Key Requirements
•Use third-point (two-point) loading so the middle third is under near-constant moment with zero shear
•Supports and load rollers parallel, free to rotate, accurately spaced over the specified span
•If fracture is within the middle third: fb = P·L / (b·d²)
•If fracture is outside the middle third (within the permitted distance): apply the modified formula fb = 3·P·a / (b·d²); beyond that, discard the result
•Apply the controlled rate of loading on a calibrated machine
Formulas
f_b = P·L / (b·d²) (fracture within middle third)
Modulus of rupture, third-point loading, fracture in middle third