Rebound Hammer Test also called Schmidt hammer Test is a type of non-destructive method test used to determine the compressive strength of concrete.
RBT is used to determine the compressive strength of the concrete as per IS 13311 (part 2)-1992. RBT is done by the apparatus called rebound hammer.
Rebound Hammer was invented by Ernst O. Schmidt in 1954, being a civil engineer in Zurich, Switzerland he founded a company named poceq.
Poceq worked for the development and advancement of this technology.
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What is Rebound Hammer?
Rebound Hammer is an instrument used to determine the compressive strength of concrete. It is generally used to determine the compressive strength of the pre-existing concrete.
Rebound Hammer consists of the cylindrical housing in which plunger is attached with spring-mass which rebounds back when it hits against the concrete surface.
Rebound Hammer is used for various purposes in civil engineering. They are portable and can be used easily. It is basically used for the quality of hardened concrete.
They used in the evaluation of the concrete structures and helps in the total estimate of the pre-existing concrete in the structures.
What is Rebound Hammer Test?
The rebound Hammer Test is a non-destructive method of test used to determine the quality of hardened concrete.
These are generally used to determine the compressive strength of the concrete. RBT is done by the rebound hammer, which has a cylindrical casing and has a spring-controlled mass.
It is done by pressing the plunger against the surface of the concrete surface. Spring-controlled mass is rebound back when it hits a concrete surface.
This rebound, measured on a graduated scale of the rebound hammer. The concrete which has high strength and high stiffness will show more rebound value on a graduated scale.
There are several factors like steel reinforcement, the proximity of aggregate, surface texture, age of the concrete, carbonization, air voids that affects the accuracy of the RBT.
One must have done care exercise on the site for accurate readings of the RBT.
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Types of Concrete Test Hammers?
Concrete hammers are classified according to the energies they generated after the impact:-
- Type L
- Type N
These concrete hammers generate less impact energy than Type N 0.54ft-lbf (0.745 Nm). These are suitable for concrete less than 4 inches and mostly used in concrete of lower strengths i.e 725psi (5mPa).
Type L concrete hammers, generally used for the thin cross-sections of structural elements like pipes or used to determine the strength of newly cast concrete for post-tensioning use.
These hammers provide less cosmetic damages to the concrete structures which makes them ideal for the new cast and thin cross-sectional elements.
These hammers are widely used in Rebound Hammer Test as it allows testing wide ranges of concrete and generates 1.63ft-lbf (2.207Nm) energy after the impact.
These hammers are used in concrete more than 4 inches with concrete strengths of 1,450 to 10,152psi (10-70mPa).
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Principle of RBT
RBT is based on the principle that the rebound on the elastic mass depends on the hardness of the concrete with which the spring attached mass strikes.
The plunger of the rebound hammer is pressed against the concrete, the rebound is measured in the graduated scale.
Procedure for Rebound Hammer Test
For RBT the calibration of the rebound hammer should be checked by testing against the steel surface having a Brinell hardness number of about 5000N/mm2.
After the calibration of the rebound hammer is tested on the test anvil for accuracy. The rebound is set perpendicular to the surface of the concrete structure. For which hardness is to be checked.
The test can be done horizontally for the vertical objects and can be done in upward or downward directions.
The rebound number can be different for the same concrete structures. Because of several factors like air voids, carbonization, etc.
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Correlation between compressive strength and rebound number
The correlation between compressive strength and rebound number can be determined by doing a compression test and rebound hammer test simultaneously.
For obtaining correlation between the compressive strength and rebound number, There are different sizes of specimen for the different magnitude of energies generated from the hammer in RBT.
For obtaining a rebound number in concrete, the specimen concrete cube should not be less than 150 mm with an impact energy of 2.2 Nm.
To determine compressive strength, 150 mm cube of concrete specimen taken to compression Testing Machine with a fixed load of 7 N/mm2.
At the time of higher impact energies like 30 Nm. The specimen size of concrete cubes should not be less than 300 mm.
The concrete must be taken out from the curing pond before 24 hours of testing to avoid any errors in the reading. Readings must be taken from the two vertical faces of a specimen,
9 readings should be taken in RBT. The distance of the point of impact in the rebound hammer test must not be less than 20 mm. reading must not be taken from the same point.
The distance from the point of impact should not be less than 20 mm from the edges of the specimen.
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Interpretation of Rebound Hammer Test Results
Interpretation of Rebound Hammer Test results depends upon the compressive strength and average rebound number as the value of rebound number increases strength increases.
|Average Rebound Number||Quality of Concrete|
|> 40||Very good Strength|
|0||Very Poor Concrete|
Advantages and Disadvantages of Rebound Hammer Test
Advantages of Rebound Hammer Test:-
- It is used for the maintenance and rehabilitation of old historical structures.
- The instrument is economical and cost-effective to use.
- It does not require a skilled person to operate the instrument.
- It is used to determine the quality and efficiency of the existing structures.
Disadvantages of Rebound Hammer Test:-
- It does not find the problems accurately.
- Maintenance is more in this instrument due to the spring mechanism.
- The test results do not directly show the results and flaws in the structure.
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Factors Affecting The Rebound Hammer Test Readings
- Type of cement.
- The physical texture of the test surface.
- Age of the specimen.
- Moisture contains in the specimen.
- Type of aggregates.
Type of Cement
The type of cement plays a major role in the formation and testing of the concrete. If high alumina cement is used, it increases the compressive strength of the concrete compared to OPC.
If super sulfated cement is used, it decreases the compressive strength by 50% compared to OPC.
The Physical Texture of the Test Surface
The smoothness of the test surface plays an important role in the rebound hammer test as it affects the readings of the rebound hammer.
If the test surface is not smooth it will affect the rebound of the hammer which ultimately affects the final results of the RBT.
Age of the Specimen
The age of the specimen also affects the final results of the rebound hammer test as it helps to determine the usability of the old structures.
Moisture Contains in the Specimen
The rebound Hammer Test works best on the dry surface concrete as dry surface concrete prevents the overestimation of the strength of the specimen.
The trowel finished surface is not preferable in the rebound hammer test as it increases the percentage error by 20%, the molded specimen is preferred in the RBT. The specimen should be dried prior to 24 hours of the RBT.
Type of Aggregates
The type of aggregates will affect the correlation of the compressive strength and the rebound number. The regular aggregate like gravel and other crushed aggregates does not affect the compressive strength.
The use of lighter aggregates affects the structural ability and strength of the concrete. In these aggregates, we have to specially calibrated rebound hammer.
The RBT is always conducted on the non-carbonated concrete on which the carbon layer is not present, as the carbonized concrete surface has 50% more strength than actual strength in RBT.
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