Table of Contents

Objective

To determine the flexural strength of concrete, which comes into play when a road slab with insufficient sub-grade support is subjected to wheel loads and/or changes in volume due to temperature/shrinkage.

standard reference

IS: 516-1959 – Methods for testing the strength of concrete

tools and equipment

beam mold Size 15 x 15 x 70 cm (when total size is less than 38 mm) or size 10 x 10 x 50 cm (when total size is less than 19 mm)

tamping bar (tamping sections 40 cm long, weighing 2 kg and having a size of 25 mm x 25 mm)

Flexural Test Machine– Two steel rollers of diameter 38 mm shall be provided in the bed of the test machine on which the specimen is to be supported, and these rollers shall be mounted in such a way that the center to center distance is 60 cm for 15.0 cm samples . or 40 cm for 10.0 cm samples. The load shall be applied by means of two identical rollers which are mounted at the third points of the supporting span, at a distance of 20 or 13.3 cm from the center to the centre. The load shall be divided equally between the two loading rollers, and all rollers shall be mounted in such a way that the load is subjected to sampling axially and without any torsional stresses or obstacles.

Process

Prepare the test sample by pouring concrete into the mold in 3 layers of approximately equal thickness. Tamp each layer 35 times using the tamping bar as described above. The tamping should be distributed evenly over the entire crosssection of the beam mold and throughout the depth of each layer.
Clean the bearing surfaces of the supporting and loading rollers, and remove any loose sand or other material from the specimen surfaces where they are to make contact with the rollers.
Circular rollers fabricated from steel having a cross section of 38 mm diameter shall be used to support and provide loading points to the specimens. The length of the rollers should be at least 10 mm longer than the width of the test specimen. A total of four rollers will be used, of which three will be able to rotate with their axes. The distance between the outer rollers (ie span) will be 3D and the distance between the inner rollers will be D, The inner rollers should be evenly spaced between the outer rollers, so that the whole system is organized.
The sample stored in water shall be tested immediately upon removal from the water; while they are still wet. The test specimen shall be placed in a correctly centered machine at right angles to the rollers with the longitudinal axis of the specimen. For molded samples, the direction of filling of the mold shall be normal to the direction of loading.

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

calculation

Flexural strength or modulus of rupture (F_b) is given by

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

F_b= 3pa/bd² (When a <20.0 सेमी लेकिन> for 17.0 15.0 cm sample or <13.3 सेमी लेकिन> 11.0 cm for a 10.0 cm sample.)

Where,

A = the distance between the line of fracture and the nearest support, measured at the center line of the tensile side of the specimen

b = width of the sample (cm)

D = failure point depth (cm)

L = Supported Length (cm)

p = max. Weight (kg)

reports

The flexural strength of concrete is reported to be two important figures.

safety precautions:

Use hand gloves, safety shoes during the test.
After the test, switch off the machine.
Grease all exposed metal parts.
Place the guide rods firmly on the base and top plate.
The equipment should be thoroughly cleaned before and after testing.