Building Materials, Third Edition

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Compressive Strength = Maximum Load
Cross-sectional Area
The average of the three values is taken as the compressive strength of concrete of the batch,
provided the individual variation is not more than ± 15 per cent of the average.

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The flexural tensile strength test is performed to estimate the tensile load at which concrete
may crack. This is an indirect test for assessing the tensile strength of concrete. The test consists
in determining the tensile strength at failure or the modulus of rupture. The ingredients of
concrete are mixed as explained in the compression strength test. The concrete is filled in the
mould of size 150 × 150 × 700 mm and compacted with the tamping bar weighing 2 kg, 400 mm
long and with a ramming face 25 mm square. The specimen to be tested is placed in the testing
machine on two 38 mm diameter rollers with a c/c distance of 600 mm. The load is applied
through two similar rollers mounted at the third points, i.e., spaced at 200 mm c/c. The
specimen are stored in water at a temperature of 27± 3°C for 48 hours before testing and are
tested in wet condition. The load is applied without shock and increasing continuously at a rate
of 0.7 N/mm^2 /minute until the specimen fails.

Modulus of rupture =

e

2

p

bd

(a>200 mm)

or, = 2

3pa

bd

(280 mm<a>170 mm)

where, a is the distance between the line of fracture and the nearest support, b and d are width
and depth of specimen,
is the length of the span on which the specimen is supported, and p
is the maximum load applied to the specimen.

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2@sƒX2SVITAX Some of the other methods to estimate the tensile
strength of concrete are briquette test (direct method) and split tensile strength test (indirect
method). Direct methods may not reflect the correct tensile strength because of the practical
difficulties involved (e.g., application of uniaxial tensile load) in the test. This has lead to the
development of a number of indirect methods to determine tensile strength of which splitting
tests are most common. The test consists in applying a compressive force to the concrete
specimen in a way that the specimen fails due to induced tensile stresses in the specimen.
The specimen is made of cylindrical shape with the diameter not less than four times the
maximum size of coarse aggregate and not less than 150 mm. The length of cylinder varies
from one to two diameters. Normally the test cylinder is 150 mm diameter and 300 mm long.
The test consists of applying compressive line loads along the opposite generators of the