Theory:
Flexural strength is one measure of the tensile strength of concrete. It is a measure of an unreinforced concrete beam to resist failure in bending. It is measured by loading 150 x 150 mm concrete beams with a span length of 700 mm. This test is performed by three point loading experiment. The Third point loading test applies the forces at the 1/3 and 2/3 points equally from the top side by distributing a single centred force through a steel beam to two points rather than one. The beam is supported at two points from below near the ends. The bending moment is lower in a third point test than in a centre point test. Highway designer use a theory based on flexural strength for design of pavements. However, there is verylimited use of flexural testing for structural concrete.
Figure 9 shows a typical test
arrangement for flexural strength test
Flexural strength tests are extremely sensitive to specimen preparation, handling, and curing procedure. Beams are very heavy and can be damaged when handled and transported from the jobsite to the lab. Allowing a beam to dry will yield lower strengths. The beams must be cured in a standard manner, and should be tested while wet. Meeting all these requirements on a job site is extremely difficult and hence often results in unreliable and generally low MR values. A short period of drying can produce a sharp drop in flexural strength.
Objective:
To determine flexural strength of cubic concrete specimens.
Reference:
IS: 516 - 1959,
IS: 1199-1959,
SP: 23-1982,
IS: 10086-1982.
Apparatus:
Flexural testing beam moulds, tamping rod, metallic sheet, universal testing machine.
Material:
Cement, sand, aggregate and water, grease
Procedure:
1. Sampling of Materials: Samples of aggregates for each batch of concrete shall be of the desired grading and shall be in an air-dried condition. The cement samples, on arrival at the laboratory, shall be thoroughly mixed dry either by hand or in a suitable mixer in such a manner as to ensure the greatest possible
blending and uniformity in the material.
2. Proportioning: The proportions of the materials, including water, in concrete
mixes used for determining the suitability of the materials available, shall be similar in all respects to those to be employed in the work.
3. Weighing: The quantities of cement, each size of aggregate, and water for each batch shall be determined by weight, to an accuracy of 0.1 percent of the total
weight of the batch.
4. Mixing of Concrete: The concrete shall be mixed by hand, or preferably, in a
laboratory batch mixer, in such a manner as to avoid loss of water or other materials. Each batch of concrete shall be of such a size as to leave about 10
percent excess after moulding the desired number of test specimens.
5. Mould: The standard size shall be 15 × 15 × 70 cm. Alternatively, if the largest
nominal size of the aggregate does not exceed 19 mm, specimens 10 × 10 × 50
cm may be used.
6. Compacting: The test specimens shall be made as soon as practicable after
mixing, and in such a way as to produce full compaction of the concrete with
neither segregation nor excessive laitance.
7. Curing: The test specimens shall be stored in a place, free from vibration, in
moist air of at least 90 percent relative humidity and at a temperature of 27° ±
2°C for 24 hours ± ½ hour from the time of addition of water to the dry ingredients.
8. Placing the Specimen in the Testing Machine: The bearing surfaces of the
supporting and loading rollers shall be wiped clean, and any loose sand or
other material removed from the surfaces of the specimen where they are to
make contact with the rollers
9. The specimen shall then be placed in the machine in such a manner that the
load shall be applied to the uppermost surface as cast in the mould, along two
lines spaced 20.0 or 13.3 cm apart.
10. The axis of the specimen shall be carefully aligned with the axis of the loading device. No packing shall be used between the bearing surfaces of the specimen
and the rollers
Apparatus:
Flexural testing beam moulds, tamping rod, metallic sheet, universal testing machine.
Material:
Cement, sand, aggregate and water, grease
Procedure:
1. Sampling of Materials: Samples of aggregates for each batch of concrete shall be of the desired grading and shall be in an air-dried condition. The cement
samples, on arrival at the laboratory, shall be thoroughly mixed dry either by
hand or in a suitable mixer in such a manner as to ensure the greatest possible
blending and uniformity in the material.
2. Proportioning: The proportions of the materials, including water, in concrete
mixes used for determining the suitability of the materials available, shall be
similar in all respects to those to be employed in the work.
3. Weighing: The quantities of cement, each size of aggregate, and water for each batch shall be determined by weight, to an accuracy of 0.1 percent of the total
weight of the batch.
4. Mixing of Concrete: The concrete shall be mixed by hand, or preferably, in a
laboratory batch mixer, in such a manner as to avoid loss of water or other
materials. Each batch of concrete shall be of such a size as to leave about 10
percent excess after moulding the desired number of test specimens.
5. Mould: The standard size shall be 15 × 15 × 70 cm. Alternatively, if the largest
nominal size of the aggregate does not exceed 19 mm, specimens 10 × 10 × 50
cm may be used.
6. Compacting: The test specimens shall be made as soon as practicable after
mixing, and in such a way as to produce full compaction of the concrete with
neither segregation nor excessive laitance.
7. Curing: The test specimens shall be stored in a place, free from vibration, in
moist air of at least 90 percent relative humidity and at a temperature of 27° ±
2°C for 24 hours ± ½ hour from the time of addition of water to the dry
ingredients.
8. Placing the Specimen in the Testing Machine: The bearing surfaces of the
supporting and loading rollers shall be wiped clean, and any loose sand or
other material removed from the surfaces of the specimen where they are to
make contact with the rollers
9. The specimen shall then be placed in the machine in such a manner that the
load shall be applied to the uppermost surface as cast in the mould, along two
lines spaced 20.0 or 13.3 cm apart.
10. The axis of the specimen shall be carefully aligned with the axis of the loading device. No packing shall be used between the bearing surfaces of the specimen and the rollers.
11. The load shall be applied without shock and increasing continuously at a rate such that the extreme fibre stress increases at approximately 7 kg/sq cm/min, that is, 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.
12. The load shall be increased until the specimen fails, and the maximum load
applied to the specimen during the test shall be recorded. The appearance of
the fractured faces of concrete and any unusual features in the type of failure
shall be noted.
Observation:
• Length of Specimen (l): _______ mm
• Width of the specimen (b): _______ mm
• Depth of the specimen (d): _______ mm
a’ equals the distance between the line of fracture and the nearer support, measured on the centre line of the tensile side of the specimen.
** When a> 200 mm for a 150 mm specimen, the flexural strength of the specimen
expressed as the modulus of rupture, fb, is calculated from:
Fb =(p x l)/a x d^2
But, if 200> a> 170 mm for a 150 mm specimen fb, is calculated from:
Fb = (P × l)/(a × d^2)
Results:
• The average 7 days modulus of rupture of concrete sample is : _______ MPa
• The average 28 days modulus of rupture of concrete sample is : _______ MPa
Precautions:
(Discuss about the precautions to be taken while conducting this experiment)
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Questions:
1. What is the relationship of flexural strength of concrete with its compressive strength?
2. What is the significance of moment of inertia with respect to bending stress?
3. What is cracking load?
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