FLEXURAL STRENGTH OF CONCRETE
To determine the Flexure strength (Modulus of rupture) of concrete of a given
Steel prism moulds of the following sizes will be used:
(a) 100 X 100 X 500 when maximum size of aggregate is less than 20 mm.
(b) 150 X 150 X 700 when maximum size of aggregate is up to 30 mm.
The testing machine may be of any reliable type of sufficient capacity for the tests
and capable of applying the load 4kN (400kg/min) for the 150 mm specimen and at a rate
of 1.8 kN (180 kg/min) for the 100 mm specimen .The bed of the testing machine should be
provided with 2 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 c/c is 600mm for 150 mm
specimen or 400 mm for 400 mm specimens .The applied through two similar rollers
mounted at the third point of the supporting span that is spaced at 200or 133 mm c/c. 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 torsion or restrains.
When concrete is subjected to bending tensile and bending compressive stress and
in many cases, direct shear stresses are developed .The most common plain concrete is
subjected to flexure is a highway pavement and the strength concrete for pavement is
commonly evaluated by means of bending tests. Flexure test is intended to give the flexural
strength of concrete in tension. The flexural strength is also easily carried out and more
convenient than the crushing test for use in field, since in this test much small loads are
The flexural strength of the specimen shall be expressed as the modulus of rupture and
shall be calculated to the nearest 0.05 N/mm2
fct = Pl/bd2
fct = 3Pa/bd2
OBSERVATIONS AND CALCULATIONS:
Grade of concrete =
Maximum ratio =
Water content ratio =
Dimension of the specimen selected =
b (mm) =
d (mm) =
a (mm) =
l (mm) =
Load at failure, P (kN) =
Since a>200mm equation (1) can be used.
Modulus of rupture, fct = Pl/bd2
a = distance between line of fracture and the near the support measured on the center line
of the tenth side of the specimen.
b = is the measured width of the specimen in mm.
d = is the measured height of the specimen in mm at the point of failure.
l = is the length in mm of the span on which the specimen was supported.
P = is the maximum load in N (kg) applied on the specimen.
In case a >200 mm for 150mm specimen, equation (1) can be used and a >133 mm for 100
In case 170<a<200 mm for 150 mm specimen, equation (2) can be used and
110< a<133 mm for 100 mm specimen.
1) With 20 mm maximum size of aggregate cast 3 prism of size150X150X700 mm. Take
mix proportion as1: 1.45:3.64, W/C ratio =0.43, for this take
Coarse aggregate =21.84 kg (218.4N), Fine aggregate= 8.7 kg (87N), Cement = 6 kg (60N),
2) The mould shall be filled in two layer to be rammed more than 100 times by a standard
steel ramming road 25mm2
at one end and 400 mm long having a weight of 20 N (2kg).
Suitable vibrator may also do ramming of each layer.
3) After casting the mould shall be covered with wet cloths or gunny bags, remolded after
24 hours and wet cured for 27 days. Specimens shall be tested immediately after removal of
water whilst they are still in wet condition. The dimensions of the each specimen shall be
noted before testing.
4) The bearing surface of the supporting and loading rollers shall be whipped clean and or
any loose sand or other material removed from the surface of specimens where they are to
make contact with the rollers .The specimen shall be then placed in the machine in such a
manner that the load shall be applied to the upper most surface as cast in the mould along
two lines spaced 200 mm or 133 mm apart .The axis of the specimen shall be carefully
aligned with the axis of loading device. No packing shall be used between the bearing
surfaces of the specimen and the rollers .The load shall be applied with shock and
increasing continuously at a rate such that the extreme fiber stress increase at approximately
)/minute that is at a rate of loading of 4 kN (400kg)/minute for 150
mm specimens and at a rate of 1.8 kN (180 kg/minute). For 100 mm specimens, the load
shall be increased until the specimen fails and the maximum load applied to the test shall be
recorded. The appearance of the fracture faces concrete and any unique features in the type
of failure shall be noted.
Modulus of rupture of concrete of the given mix = N/mm2