Concrete modulus of rupture – what property does it represent? In structural concrete testing, the modulus of rupture obtained from a flexural test is a measure of which material property?

Introduction:
Because concrete is weak in tension, direct tensile testing is difficult and not commonly performed on site. Instead, flexural testing on beams provides an indirect measure known as the modulus of rupture, which is used for pavement design and serviceability assessments.

Given Data / Assumptions:

• Standard third-point or center-point loading of a concrete beam specimen.
• Linear elastic assumptions up to fracture initiation.
• Nominal specimen dimensions and span satisfy code requirements.

Concept / Approach:

The modulus of rupture is the extreme fiber stress at failure under bending, computed from simple beam theory using the measured failure load. It reflects the tensile strength in flexure, which is often higher than direct tensile strength due to stress gradients and size effects. It should not be confused with compressive strength or with the characteristic cylinder/cube strength used in design.

Step-by-Step Solution:

Verification / Alternative check:

Empirical relationships link modulus of rupture to compressive strength (e.g., proportional to sqrt(fck)), supporting its use for pavement design and crack control checks.

Why Other Options Are Wrong:

Direct tensile strength (A) is measured by split-cylinder or direct tension tests; compressive strength (B) uses cubes/cylinders; characteristic strength (D) refers to compressive fractile; shear capacity (E) is a different mechanism involving aggregate interlock and dowel action.

Common Pitfalls:

Over-reliance on modulus of rupture for structural members; ignoring size, rate, and moisture conditioning effects on measured flexural strength.

Final Answer:

the tensile strength of the concrete under bending