Flexural strength test of concrete

Flexural test evaluates the tensile strength of concrete indirectly. It tests the ability of unreinforced concrete beam or slab to withstand failure in bending.

The results of flexural test on concrete expressed as a modulus of rupture which denotes as (MR) in MPa or psi.

AIM: To determine the Flexural Strength of Concrete.

APPARATUS:

1. Beam mould 

·         15 x 15x 70 cm (when size of aggregate is less than 38 mm)

·         10 x 10 x 50 cm (when size of aggregate is less than 19 mm)

2. Tamping bar 

·         40 cm long,

·         weight 2 kg

·         tamping section size of 25 mm x 25 mm

3. Flexural test machine: The bed of the testing machine shall be provided with two steel rollers, 38 mm in diameter, on which the specimen is to be supported, and these rollers shall be so mounted that the distance from centre to centre is 60 cm for 15.0 cm specimens or 40 cm for 10.0 cm specimens. The load shall be applied through two similar rollers mounted at the third points of the supporting span that is, spaced at 20 or 13.3 cm centre to centre. The load shall be divided equally between the two loading rollers, and all rollers shall be mounted in such a manner that the load is applied axially and without subjecting the specimen to any torsional stresses or restraints.

FIGURE:

PROCEDURE:

1. Prepare the test specimen by filling the concrete into the mould in 3 layers of approximately equal thickness. Tamp each layer 35 times using the tamping bar as specified above. Tamping should be distributed uniformly over the entire crossection of the beam mould and throughout the depth of each layer.
2. Clean the bearing surfaces of the supporting and loading rollers , and remove any loose sand or other material from the surfaces of the specimen where they are to make contact with the rollers.
3. Circular rollers manufactured out of steel having cross section with diameter 38 mm will be used for providing support and loading points to the specimens. The length of the rollers shall be at least 10 mm more than the width of the test specimen. A total of four rollers shall be used, three out of which shall be capable of rotating along their own axes. The distance between the outer rollers (i.e. span) shall be 3d and the distance between the inner rollers shall be d. The inner rollers shall be equally spaced between the outer rollers, such that the entire system is systematic.
4. The specimen stored in water shall be tested immediately on removal from water; whilst they are still wet. The test specimen shall be placed in the machine correctly centered with the longitudinal axis of the specimen at right angles to the rollers. For moulded specimens, the mould filling direction shall be normal to the direction of loading.

5. The load shall be applied at a rate of loading of 400 kg/min for the 15.0 cm specimens and at a rate of 180 kg/min for the 10.0 cm specimens.

CALCULATION:

The Flexural Strength or modulus of rupture (f_b) is given by

f_b = pl/bd²  

(when a > 20.0cm for 15.0cm specimen or > 13.0cm for 10cm specimen)

or

f_b = 3pa/bd²  

(when a < 20.0cm but > 17.0 for 15.0cm specimen or < 13.3 cm but > 11.0cm for 10.0cm specimen.)

Where,

a = the distance between the line of fracture and the nearer support, measured on the center line of the tensile side of the specimen

b = width of specimen (cm)

d = failure point depth (cm)

l = supported length (cm)

p = max. Load (kg)

PRECAUTIONS :

• Use hand gloves and safety shoes at the time of test.
• After test, switch off the machine.
• Keep all the exposed metal parts greased.
• Keep the guide rods firmly fixed to the base & top plate.

Equipment should be cleaned thoroughly before testing & after testing.