How is Flexural Strength calculated?

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Flexural strength.

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.


How to understand Flexural strength?

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.


What is the necessity for testing 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.


How to calculate Flexural strength?

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.

  • The data such as maximum applied load, known as "P", material span length between points in the test setup, known as "L", width of the material specimen, known as "b"; and average depth of the specimen, known as "d” are collected from the load experiments.
  •  Change maximum applied load in pounds; material span length in inches; width of the material specimen in inches; and average depth of the specimen in inches.
  •  Replace or substitute the numerical values for this data in the equation R = P*L/b*d^2 for manipulating flexural strength. In the equation, R is the Flexural strength, in units of lbs. per square inch and quantity ‘d’ squared.
  •  Further explanation is given by, multiplying P by L and then dividing that quantity by the quantity of ‘b’ multiplied by‘d’ square. The final result is the calculated flexural strength.


What are the Specifications of flexural strength?

  • Appropriate beam specimens should be done in the field.
  •  Pavement concretes should be rigid (1/2 to 2 ½ inch slouch).
  •  After rodding is done, the molds are tapped in order to release bubbles and along the sides are shoveled to merge, to get an elevated slouch.
  •  In any case, the beam surfaces should not be left dry. In order to achieve it, it should be dipped in saturated lime water for at least 20 hours prior to testing.
  • For the higher variability of flexural strength results, conditions and examinations of noticeable low strengths should be taken into account.
  •  Standard deviation for projects with good control range is about 40 to 80 psi. Values higher 100 psi shows testing problems, and there is high possibility that testing issues, or moisture variance inside a beam, will result in low strength.


What are the issues with flexural strength?

  • Flexural tests are tremendously responsive to specimen preparation, handling, and curing procedure.
  •  Beam specimens are extremely weighty, and letting a beam to dry will give lower strengths. So, beams must be treated and tested in a typical method when it is still wet.
  •  Drying less time can produce a quick drop in flexural strength.
  •  Cylinder strengths are also used for concrete structures. Majority of state highway agencies have used flexural strength, however are currently changing to compressive strength for job control on concrete paving.
  •  For the purpose of designing, flexure can be used, however the equivalent compressive strength must be used to order and acknowledge the concrete. 
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