What is Flexural Strength of Concrete? 5 Test Steps, & Problems

Flexural strength of concrete is the measure of the tensile strength of concrete and to resist failure in the bending it is a measure of an un-reinforced concrete beam or slab.

With a span length at least three times the depth, flexural strength of concrete is measured by loading 6 x 6 inch (150 x 150mm) concrete beams. As Modulus of Rupture in MPa, the flexural strength is expressed and by standard test methods ASTM C78 (third-point loading) or ASTM C293 (center-point loading) it is determined.

The specimen size and type of loading does impact the measured flexural strength and comparisons or requirements should be based on the loading configuration and same beam size. It is also observed that with larger beam specimens, a lower flexural strength of concrete will be measured.

Depending on the type, size, and volume of coarse aggregate flexural MR (Modulus of Rupture) is about 10 to 20 percent of compressive strength and for given materials and mix design, the best correlation for a specific material is obtained by laboratory tests. The modulus of rupture determined by third point loading is lower than the MR determined by center point loading sometimes as much as 15 percent.

Why Test Flexural Strength of Concrete?

The designers of pavements use a theory that is based on flexural strength so laboratory mix design based on flexure may be required or to yield the needed design MR, cement content may be selected from past experience. Some also use MR for acceptance of pavements and field control and few use this test for structural concrete.

Test for Flexural Strength of Concrete:

Apparatus:

There are the following apparatus used for determining the flexural strength of concrete as given below;

1. Beam Mould
2. Tamping Bar
3. Flexural Testing Machine

Procedure:

There are the following steps in the procedure of flexural strength of concrete as given below;

1. By filling the concrete into the mold in 3 layers of approximately equal thickness, prepare the test specimen and by using the tamping bar, tamp each layer 35 times. Over the entire cross-section of the beam mold and throughout the depth of each layer, tamping should be distributed uniformly.
2. Then clean the bearing surfaces of the supporting and loading rollers, and from the surfaces of the specimen where they are to make contact with the rollers remove any loose sand or other material.
3. For providing support and loading points to the specimens, circular rollers manufactured out of steel having cross-section with a diameter of 38 mm will be used. The length of the rollers shall be at least 10 mm more than the width of the test specimen and a total of four rollers shall be used. The distance between the inner rollers shall be d and the distance between the outer rollers (i.e. span) shall be 3d and the inner rollers shall be equally spaced between the outer rollers.
4. The specimen stored in water shall be tested instantly on removal from water and at right angles to the rollers, the test specimen shall be placed in the machine correctly centered with the longitudinal axis of the specimen. The mold filling direction shall be normal to the direction of loading for molded specimens.
5. For the 15 cm specimens and at a rate of 180 kg/min for the 10 cm specimens, the load shall be applied at a rate of loading of 400 kg/min.

Calculation:

The flexural strength f_b is given by;

F_b = pl/bd² (when a > 13.0cm for 10cm specimen or a > 20.0cm for 15.0cm specimen)

Or

F_b = 3pa/bd²   (when a < 13.3 cm but > 11.0cm for 10.0cm specimen or a < 20.0cm but > 17.0 for 15.0cm specimen)

Where,

a is the distance between the nearer support and the line of fracture.

b is the width of specimen in cm.

d is the failure point depth in cm.

l is the supported length in cm.

p is the max. Load in kg.

Safety & Precautions:

1. At the time of the test, use hand gloves.
2. Switch off the machine after the test.
3. Grease all the exposed metal parts.
4. To the base & top plate, keep the guide rods firmly fixed.
5. Before testing & after testing equipment should be cleaned.

Problems With Flexural:

To specimen preparation, handling, and curing procedure flexural tests are extremely sensitive and beam specimens are very heavy and there will be yield lower strengths by allowing a beam to dry. Beams must be tested while wet and must be cured in a standard manner. A sharp drop in flexural strength is produced due to a short period of drying.

For control and acceptance of concrete, the concrete industry and inspection agencies are much more familiar with traditional cylinder compression tests and for design purposes flexural can be used.