Flexural strength is a mechanical limitation for brittle material such as concrete which is defined as a material's capacity to defend against bend or twist under load. Flexural strength is the measure of the tensile strength of concrete. It is measured in terms of stress, whose symbol is σ. Flexural strength is a measure of an unreinforced concrete beam or slab to resist failure in bending. It is measured by loading 6 x 6-inch (150 x 150 mm) concrete beams whose length is at least three times the depth. The flexural strength is expressed as Modulus of Rupture (MR) in psi (MPa). It is also known as bend strength, or fracture strength. The oblique bending test is most often done, using a three point flexural test technique, in which a rod specimen having either a circular or rectangular cross-section is bent until fracture or rupture. The flexural strength symbolizes the highest stress in the material at its moment of rupture or fracture. Three point flexural tests gives values for the modulus of elasticity in bending. Flexural MR is about 12 to 20 percent of compressive strength. Though, the greatest correlation for particular materials is got from the laboratory experiments for given materials and mix design. The MR predicted by third-point loading is inferior sometimes as much as 15% than the MR predicted by center-point loading.
In case, if an entity or an object which is made of a single material, like a wooden beam or a steel rod is bent, it undergoes a series of stresses across its profundity. The stress will be at its utmost compressive stress value, on the concave face (inner surface) of the bend and the stress will be at its utmost tensile value, around the convex face (outer surface). These inner and outer edges of the beam or rod are known as the 'Extreme fibers'. Majority of materials are unsuccessful or they fail under tensile stress before they fail under compressive stress. As a result, the maximum tensile stress value that can be sustained before the beam or rod fails is considered to be its flexural strength.
A laboratory mix design, based on flexure may be needed, or cement content may be selected from early experience to offer the required design MR, as the designers of pavements make use of a hypothesis based on flexural strength. Few of them may also utilize MR for field control and acceptance of pavements. Only a small number of them use flexural testing for structural concrete. Generally, agencies which do not use flexural strength for field control, find compressive strength to be useful, easy and constant to evaluate the worth of the concrete.
The data required to calculate flexural strength are measured by experimentation, with rectangular samples of the material placed under load in a 3 or 4 point testing setup.