The procedures for conducting the flexural-strength
test are as follows:
1. Assemble the loading device, as shown in figure
13-24. Turn the test beam so that the finished surface is
to the side and centered in the loading assembly. Operate
the testing apparatus until the loading blocks are brought
into contact with the upper surface of the beam. Be sure
to secure full contact between the loading (and
supporting) surfaces and the beam. If the surface of the
specimen is so rough that full contact is not secured,
grind the specimen to secure full contact.
2. Apply the test load at a rate such that the increase
in extreme fiber stress in the beam is between 125 and
175 pounds per square inch per minute. The extreme
fiber stress corresponding to any load maybe estimated
from the equation given in Step 4a below. Obtain
readings on the proving-ring dial, and convert them to
corresponding total loads in pounds by applying the
proving-ring constant. Aside from the reading used to
control the rate of application of the load, the only
reading necessary is the one that corresponds to the
maximum load applied to the beam.
3. After the specimen has broken, obtain dimen-
sions of the cross section at which failure occurred to
the nearest 0.1 inch. These dimensions represent the
average width and average depth of the section in
failure.
4. The flexural strength, expressed in terms of
modulus of rupture, is given in psi, and can be
calculated as follows:
a. If the specimen broke within the middle third
of the span length, use the following equation:
Where:
R
= modulus of rupture (in psi)
P
= maximum applied load (in pounds)
L
= span length (in inches)
b
= average width of specimen (in inches)
d
= average depth of specimen (in inches)
b. If the specimen broke outside the middle
third of the span length by not more than 5 percent of
the span length, calculate the modulus of rupture as
follows:
Where:
R
=
P
=
a
=
b
=
d
=
c.
modulus of rupture (in psi)
maximum applied load (in pounds)
distance (in inches) between the line of
fracture and the nearest support, measured
along the center line of the bottom surface
of the beam
average width of specimen (in inches)
average depth of specimen (in inches)
If the specimen broke outside the middle
third of the span length by more than 5 percent of the
span length, discard the results of the test.
5. The report of the test for flexural strength should
include the following information:
a.
b.
c.
d.
e.
f.
g.
h.
Identification number
Average width to the nearest 0.1 inch
Average depth to the nearest 0.1 inch
Span length
Maximum applied load
Modulus of rupture to the nearest 5 psi
Defects in specimen
Age of specimen
Values of the modulus of rupture vary widely,
depending on the concrete tested. Specification relative
to concrete pavements frequently require modulus of
rupture in excess of 600 to 650 psi (28-day curing,
third-point loading). The flexural strength (modulus of
rupture) generally may be expected to be approximately
15 percent of the compressive strength for comparable
conditions of age and curing.
An approximate relationship between modulus of
rupture and compressive strength can be calculated from
the following formula:
Where:
f ´c
= compressive strength (in psi)
R = modulus of rupture (in psi).
13-33
