Schmidt hammer or Rebound Hammer test of Concrete
Rebound hammer is also known as the Schidmt hammer test. It consists of Spring loaded hammer Elastic mass which having ended with a plunger which can be easily operated with the hands.
While testing, these hammer is impacted on the hard concrete surface. During the test, the hammer have to hit the concrete at defined energy, after that we have to note the rebound number with the help of writer.
And the rebound number correlated with the Compressive strength of concrete.
These test is a Non-destructive test, it means the Compressive strength of concrete can be determined without causing any damage to the concrete surface.
In Rebound hammer test, the concrete can be tested directly on the site instead of going to the lab without any damage.
The Rebound hammer test was first covered by IS 13311 (Part 2) – 1992. The Swiss engineer Ernst Schmidt is the name of the scientist who first introduced Rebound hammer test.
Later these test was also called as Schmidt hammer test or Swiss Hammer Test.
About Schmidt hammer test:
For understanding, Take a rubber band and stretch it oppositely, Rubber-band has an elastic property which it regains or rebound to its original position Similarly, Schmidt hammer is impacted on a concrete surface and then it rebounds to its original position. For more details about Hammer refer the following figure of Schmidt Rebound Hammer.
The impact of rebound is evaluated using a graph of the hardness of the concrete & rebound number, and then the compressive strength is calculated.
Operation of the Schmidt Rebound hammer:
The operation of Schmidt hammer is depicted in the following diagram and you can also refer the above figure 1 for clear understanding.
From the above given image, the first image of the Rebound hammer is set the right angle to the concrete surface and in the second hammer is pushed to the concrete surface at defined energy. And in the third, the hammer is released, so in the fourth image, the hammer rebounds and shows rebound number in the graduated scale.
Procedure for Schmidt hammer test or Rebound Hammer Test:
Before starting the procedure calibration of the hammer is required. To do this, the Schmidt hammer is tested against the Steel test anvil which has a Brinell hardness number of about 5000MPa.
- Clean the surface of concrete, ensure the concrete is clean and dirt free.
- As mentioned in the Operation of Rebound hammer, it involves four stages in the first stage the hammer is held at right angles to the Concrete Surface of structure for finding the rebound number.
- Ensure that the hammer is placed exactly right angle to the concrete surface other angles may show a discrepancy in results.
- The procedure is same for different positions but ensure that you are holding the hammer in right angles to the test specimen. Suppose to test the strength of column the Horizontal Position is implemented. Similarly for testing beam strength Vertical Upward Position is implemented.
- After Holding Hammer in position then the plunger is made to hit the concrete surface with impact energy.
- Approximate Impact energy levels are mentioned in the below table. The impact energy is different for different applications of concrete.
- After pushing plunger towards the concrete surface, it rebounds back and shows a reading in a graduated scale which is noted as a rebound number.
- 5-6 readings of rebound number are measured and average the results for rebound index.
- After finding the rebound index, the value is correlated with the Compressive strength by using the below graph.
Rebound Hammer Test Graph:
As you can see there are different lines are interpolated in the graph. The graph is different for different positions which is clearly depicted.
Suppose, if the average rebound number for horizontal position (concrete column) is 38(rebound index) as per graph the Compressive strength is 36N/mm2
The below table gives Quality of concrete for different values of rebound number.
Advantages of Rebound hammer:
- It is a simple and instant test which can be performed on site.
- Rebound hammer test does not require any calculation. Determination of Compressive strength, can be find by the rebound number and correlate it in graph.
- It is a Non-destructive test which is very helpful for determining the strength of old concrete structures.
- There is difference in the price of rebound hammer is low and the price of Universal testing machine is high.
- This method is not only used to find out the Compressive strength of concrete it also helps in comparing one particular element to other and to find the concrete uniformity and quality as well.
Disadvantages of Rebound hammer:
- Whil testing the Rebound Hammer should be precisely pushed in right angle to the surface, if it is used in other intermediate angles it will show wrong results.
- In this test, we cannot determine the Flaws easily as well as the periodic maintenance is must required.
- Rebound hammer test results may varies due to various factors such as the type of cement & aggregate, atmospheric moisture and presence of moisture content in concrete.
Video Explanation of Rebound Hammer or Schmidt Hammer Test :
Also read our Blogs on other cement tests... .
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