Important Tests on Concrete | Tests on Hardened Concrete
Concrete testing forms an important part of understanding the behaviour of fresh concrete and the strength of concrete in the case of hardened concrete. Important tests on hardened concrete are covered further in this blog as a continuation of the previous blog on tests on fresh concrete.
Compression Test on Concrete
The compression test is one of the most commonly performed on hardened concrete. It is easy to perform and tests almost all desirable qualities of the concrete. Important points related to the test are discussed further.
Metal moulds that are cubical or cylindrical in shape are used for testing. The cube specimen is of the size 15 * 15 * 15 cm if the largest nominal size of the aggregate does not exceed 20mm. The cylindrical specimen should have a diameter of 15 cm and a height of 30 cm.
Further, a steel bar 16mm in diameter, 0.6m long and bullet-pointed at the lower end serves as a tamping bar.
Mould oil is applied on the contact surfaces of the mould in order to ensure that no water escapes through voids in the mould while filling. The specimens are filled as soon as possible after mixing. It is filled in layers 5 cm deep. Tamping is required as per the below conditions.
For cubical specimens, 35 strokes per layer for 15cm cubes and 25 strokes per layer for 10cm cubes.
For cylindrical specimens, 30 strokes per layer.
Vibrators can also be used to compact each layer.
Specimens are stored in the moist air of at least 90% relative humidity at a temperature of 27°C for 24 hours. After this period, the specimens are marked and removed from the moulds and unless required for the test within 24 hours, immediately submerged in clean fresh water. The freshwater is to be replaced every seven days. The specimens are not allowed to dry at any time until they have been tested.
The specimens are tested using a compression testing machine. The following procedure is used for performing the test.
The specimen is removed from the water and excess water is wiped out
Dimensions of the specimen are noted
The specimen is placed in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast
The specimen is aligned centrally on the base plate of the machine
The movable portion f the machine is rotated gently so that it touches the top surface of the specimen
The load is applied gradually without shock and continuously at the rate of 140 kg/cm^2/minute (kg per centimetre square per minute) till the specimen fails
The maximum load at which the specimen fails is noted
The maximum load divided by the surface area of the specimen gives the compressive strength of the cast concrete cube.
Difference Between Cube Strenght and Cylinder Strenght
The cylindrical specimen is less affected by the end restraints caused by plates and hence it gives more uniform results than the cube. Cylindrical specimens are cast and tested in the same direction, unlike the cubes which are cast in one direction but tested in another direction. Therefore, cylindrical specimens represent the actual scenario in structural members.
Points in favour of cubes are that the cube represents the shape of structural members. Also, cube specimens don't require capping which seems to influence strength to some extent in the case of cylindrical specimens.
Tensile Strength of Concrete: Flexural Test on Concrete
Direct measurement of the tensile strength of concrete is not possible therefore methods have been developed to measure the flexural tension of concrete. There are two systems of loading in measuring the flexural tension i.e., centre-point loading and third-point loading.
In centre-point loading, maximum point fibre stress will come below the point of loading where the bending moment is maximum. In a third-point loading system, the load is applied symmetrically at two points and the crack can appear within the middle third where the bending moment is maximum. IS 516-1959 recommends this third-point loading system.
The standard size of the specimen used is 15 * 15 * 70 cm. A tamping bar made of steel with 40cm length and having a ramming face of 25 mm square is also required for compacting purposes.
Test specimens are prepared by filling in layers and compacting each layer as done in compression testing. Then the specimens are stored in water at a temperature of 24°C for 48 hours before testing. They are tested immediately after removing them from the water whilst they are still wet. The following procedure is followed for testing.
Dimensions of the specimen are measured before testing
Bearing surfaces of the supporting and loading rulers are cleaned
The specimen is placed in such a way that load is applied to the uppermost surface as cast in the mould
The axis of the specimen is carefully aligned with the axis of the loading machine and no packing is used between the specimens and rollers
Load is applied without any shock at a rate of 400 kg/min for 15 cm specimens and at a rate of 180 kg/min for the 10 cm specimen
The test is carried out until the specimen fails and the maximum load is recorded
The flexural strength of the specimen is expressed as the modulus of rupture fb which if 'a' equals the distance between the line of fracture and the nearer support, measured on the tensile side of the specimen,
fb = (P * l) / (b * d^2),
where 'a' is greater than 20 cm for a 15 cm specimen or greater than 13.3 cm for a 10 cm specimen.
fb = (3P * a) / (b * d^2),
where 'a' is less than 20 cm but greater than 17 cm for 15 cm specimen or less than 13.3 cm but greater than 11 cm for a 10 cm specimen.
b - width of the specimen (cm)
d - depth of the specimen (cm)
l - length of the span on which specimen was supported (cm)
P - maximum load (kg)
If 'a' is less than 17 cm for a 15 cm specimen or less than 11 cm for a 10 cm specimen, the test results are to be discarded.
Other indirect methods have been developed to measure the tensile strength of the concrete. Some of the methods are listed below.
Cylinder splitting tensile test
Ring tension test
Double punch test
These are the important tests performed on hardened concrete to examine its strength and behaviour. There are special types of testing methods called the non-destructive testing methods for accessing the strength of concrete without destroying it. These methods are used to access the strength of old structures for repair and rehabilitation purposes.
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